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<h3>Reference Clock Audio Drivers</h3>
<img src="pic/radio2.jpg" alt="jpg" align="left">ICOM R-72 shortwave receiver and Sure audio mixer
- <p>Last update: <csobj format="ShortTime" h="25" locale="00000409" region="0" t="DateTime" w="61">18:36</csobj> UTC <csobj format="LongDate" h="25" locale="00000409" region="0" t="DateTime" w="246">Thursday, July 28, 2005</csobj></p>
+ <p>Last update: <csobj format="ShortTime" h="25" locale="00000409" region="0" t="DateTime" w="61">04:06</csobj> UTC <csobj format="LongDate" h="25" locale="00000409" region="0" t="DateTime" w="285">Saturday, January 06, 2007</csobj></p>
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<h4>Related Links</h4>
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<h4 id="sound">Sound Card Drivers</h4>
<p>There are some applications in which the computer time can be disciplined to an audio signal, rather than a serial timecode and communications port or special purpose bus peripheral. This is useful in such cases where the audio signal is sent over a telephone circuit, for example, or received directly from a shortwave receiver. In such cases the audio signal can be connected via an ordinary sound card or baseboard audio codec. The suite of NTP reference clock drivers currently includes three drivers suitable for these applications. They include a driver for the Inter Range Instrumentation Group (IRIG) signals produced by many radio clocks and timing devices, another for the Canadian time/frequency radio station CHU and a third for the NIST time/frequency radio stations WWV and WWVH. The radio drivers are designed to work with ordinary inexpensive shortwave radios and may be one of the least expensive ways to build a good primary time server.</p>
- <p>All three drivers make ample use of sophisticated digital signal processing algorithms designed to efficiently extract timing signals from noise and interference. The radio station drivers in particular implement optimum linear demodulation and decoding techniques, including maximum likelihood and soft-decision methods. The documentation page for each driver contains an in-depth discussion on the algorithms and performance expectations. In some cases the algorithms are further analyzed, modelled and evaluated in a technical report.</p>
+ <p>All three drivers make ample use of sophisticated digital signal processing algorithms designed to efficiently extract timing signals from noise and interference. The radio station drivers in particular implement optimum linear demodulation and decoding techniques, including maximum-likelihood and soft-decision methods. The documentation page for each driver contains an in-depth discussion on the algorithms and performance expectations. In some cases the algorithms are further analyzed, modelled and evaluated in a technical report.</p>
<p>Currently, the audio drivers work with with Sun operating systems and audio codecs, including SunOS 4.1.3 and Solaris from 2.6 and probably all others in between. They also work with FreeBSD from 4.1 with compatible sound card. In fact, the interface is quite generic and support for other systems, in particular the various Unix generics, should not be difficult. Volunteers are solicited.</p>
<p>The audio drivers include a number of common features designed to groom input signals, suppress spikes and normalize signal levels. An automatic gain control (AGC) feature provides protection against overdriven or underdriven input signals. It is designed to maintain adequate demodulator signal amplitude while avoiding occasional noise spikes. In order to assure reliable operation, the signal level must be in the range where the audio gain control is effective. In general, this means the input signal level must be such as to cause the AGC to set the gain somewhere in the middle of the range from 0 to 255, as indicated in the timecode displayed by the <tt>ntpq</tt> program.</p>
- <p>The drivers operate by disciplining a logical clock based on the codec sample clock to the audio signal as received. This is done by stuffing or slipping samples as required to maintain exact frequency to the order of 0.1 PPM. In order for the driver to reliably lock on the audio signal, the sample clock frequency tolerance must be less than 250 PPM (.025 percent) for the IRIG driver and half that for the radio drivers. The largest error observed so far is about 60 PPM, but it is possible some sound cards or codecs may exceed that value.</p>
+ <p>The IRIG and WWV drivers operate by disciplining a logical clock based on the codec sample clock to the audio signal as received. This is done by stuffing or slipping samples as required to maintain exact frequency to the order of 0.1 PPM. In order for the driver to reliably lock on the audio signal, the sample clock frequency tolerance must be less than 250 PPM (.025 percent) for the IRIG driver and half that for the WWV driver. The largest error observed so far is about 60 PPM, but it is possible some sound cards or codecs may exceed that value. In any case, the configuration file command tinker codec command can be used to change the systematic offset in units of 125 PPM.</p>
<p>The drivers include provisions to select the input port and to monitor the input signal. The <tt>fudge flag 2</tt> selects the microphone port if set to zero or the line-in port if set to one. It does not seem useful to specify the compact disc player port. The <tt>fudge flag 3</tt> enables the input signal monitor using the previously selected output port and output gain. Both of these flags can be set in the configuration file or remotely using the <tt>ntpdc</tt> utility program.</p>
<h4 id="short">Shortwave Radio Drivers</h4>
<p>The WWV/H and CHU audio drivers require an external shortwave radio with the radio output - speaker or headphone jack - connected to either the microphone or line-in port on the computer. There is some degree of art in setting up the radio and antenna and getting the setup to work. While the drivers are highly sophisticated and efficient in extracting timing signals from noise and interference, it always helps to have as clear a signal as possible.</p>
<li>The lower frequency bands are best for shorter distances, while the higher bands are best for longer distances.
<li>The optimum frequencies are higher at the peak of the 11-year sunspot cycle and lower at the trough. The current sunspot cycle should peak in the first couple of years beginning the century.
</ul>
- <p>The best way to choose a frequency is to listen at various times over the day and determine the best highest (daytime) and lowest (nighttime) frequencies. Then, assuming one is available, choose the highest frequency between these frequencies. This strategy assumes that the high frequency is more problematic than the low, that the low frequency probably comes with severe multipath and static, and insures that probably twice a day the chosen frequency will work. For instance, on the east coast the best compromise CHU frequency is probably 7335 kHz and the best WWV frequency is probably 15 MHz.</p>
+ <p>The best way to choose a frequency is to listen at various times over the day and determine the best highest (daytime) and lowest (nighttime) frequencies. Then, assuming one is available, choose the highest frequency between these frequencies. This strategy assumes that the high frequency is more problematic than the low, that the low frequency probably comes with severe multipath and noise, and insures that probably twice a day the chosen frequency will work. For instance, on the east coast the best compromise CHU frequency is probably 7335 kHz and the best WWV frequency is probably 15 MHz.</p>
<h4>Autotune Modes</h4>
<p>The shortwave drivers include support for an optional autotune function compatible with ICOM receivers and transceivers. The <tt>mode</tt> keyword of the <tt>server</tt> configuration command specifies the ICOM ID select code in decimal. A missing or zero argument disables the CI-V interface. Since all ICOM select codes are less than 128, the high order bit of the code is used by the driver to specify the baud rate. If this bit is not set, the rate is 9600 bps for the newer radios; if set, the rate is 1200 bps for the older radios. Following are the ID select codes for the known radios.</p>
<table width="100%" cols="6">
<td>726</td>
<td>0x30</td>
<td>48</td>
- <td>R71</td>
- <td>0x1A</td>
- <td>26</td>
+ <td>7000</td>
+ <td>0x70</td>
+ <td>113</td>
</tr>
<tr>
<td>735</td>
<td>0x04</td>
<td>4</td>
- <td>R72</td>
- <td>0x32</td>
- <td>50</td>
+ <td>R71</td>
+ <td>0x1A</td>
+ <td>26</td>
</tr>
<tr>
<td>746</td>
<td>0x66</td>
<td>102</td>
- <td>R75</td>
- <td>0x5a</td>
- <td>90</td>
+ <td>R72</td>
+ <td>0x32</td>
+ <td>50</td>
</tr>
<tr>
<td>751</td>
<td>0x1c</td>
<td>28</td>
- <td>R7000</td>
- <td>0x08</td>
- <td>8</td>
+ <td>R75</td>
+ <td>0x5a</td>
+ <td>90</td>
</tr>
<tr>
<td>756PROII</td>
<td>0x64</td>
<td>100</td>
- <td>R7100</td>
- <td>0x34</td>
- <td>52</td>
+ <td>R7000</td>
+ <td>0x08</td>
+ <td>8</td>
</tr>
<tr>
<td>761</td>
<td>0x1e</td>
<td>30</td>
- <td>R8500</td>
- <td>0x4a</td>
- <td>74</td>
+ <td>R7100</td>
+ <td>0x34</td>
+ <td>52</td>
</tr>
<tr>
<td>765</td>
<td>0x2c</td>
<td>44</td>
+ <td>R8500</td>
+ <td>0x4a</td>
+ <td>74</td>
+ </tr>
+ <tr>
+ <td></td>
+ <td></td>
+ <td></td>
<td>R9000</td>
<td>0x2a</td>
<td>42</td>
</table>
<h4 id="setup">Setup and Debugging Aids</h4>
<p>The audio drivers include extensive setup and debugging support to help hook up the audio signals and monitor the driver operations. The documentation page for each driver describes the various messages that can be produced either in real time or written to the <tt>clockstats</tt> file for later analysis. Of particular help in verifying signal connections and compatibility is a provision to monitor the signal via headphones or speaker.</p>
- <p>Connecting radios and IRIG devices to the computer and verifying correct configuration is somewhat of a black art. The signals have to be connected to the correct ports and the signal level maintained within tolerances. Some radios have recorder outputs which produce a line level signal not affected by the volume control. These signals can be connected to the line-in port on the computer. If the level is too low, connect to the microphone-in port instead. If the radio does not have a recorder output, connect the headphone or speaker output to the line-in port and adjust the volume control so the driver indicates comfortably above the minimum specified and the AGC level somewhere in the middle of the range 0-255. IRIG signals are usually much larger than radio outputs, usually in the range to several volts and may even overload the line-in port. In such cases an attenuator must be used to reduce the signal level below the overload point.</p>
- <p>It is very easy to underdrive or overdrive the audio codec, in which case the drivers will not synchronize to the signal. The drivers use <tt>fudge flag2</tt> to enable audio monitoring of the input signal. This is useful during setup to confirm the signal is actually reaching the audio codec and generally free of hum and interference. This feature is not intended for regular use, since it does increase the processor load on the system. Note that the speaker volume must be set before the driver is started.</p>
+ <p>Connecting radios and IRIG devices to the computer and verifying correct configuration is somewhat of a black art. The signals have to be connected to the correct ports and the signal level maintained within tolerances. Some radios have recorder outputs which produce a microphone-level signal not affected by the volume control. These signals can be connected to the microphone port on the computer. If the radio does not have a recorder output, connect the headphone or speaker output to the line-in port and adjust the volume control so the driver indicates comfortably above the minimum specified and the AGC level somewhere in the middle of the range 0-255. IRIG signals are usually much larger than radio outputs, usually in the range to several volts and may even overload the line-in port. In such cases the signal is designed to drive a cable termineted with a 50-ohm resistor, which results in a level the line-in port can handle..</p>
+ <p>It is very easy to underdrive or overdrive the audio codec, in which case the drivers will not synchronize to the signal. The drivers use <tt>fudge flag2</tt> to enable audio monitoring of the input signal. This is useful during setup to confirm the signal is actually reaching the audio codec and generally free of noise and interference. Note that the monitorr volume must be set before the driver is started.</p>
<p>The drivers write a synthesized timecode to the <tt>clockstats</tt> file each time the clock is set or verified and at other times if verbose monitoring is enabled. The format includes several fixed-length fields defining the UTC time to the millisecond, together with additional variable-length fields specific to each driver. The data include the intervals since the clock was last set or verified, the audio gain and various state variables and counters specific to each driver.</p>
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<p>The WWV signal format is described in NIST Special Publication 432 (Revised 1990). It consists of three elements, a 5-ms, 1000-Hz pulse, which occurs at the beginning of each second, a 800-ms, 1000-Hz pulse, which occurs at the beginning of each minute, and a pulse-width modulated 100-Hz subcarrier for the data bits, one bit per second. The WWVH format is identical, except that the 1000-Hz pulses are sent at 1200 Hz. Each minute encodes nine BCD digits for the time of century plus seven bits for the daylight savings time (DST) indicator, leap warning indicator and DUT1 correction.</p>
<h4>Program Architecture</h4>
<p>As in the original program, the clock discipline is modelled as a Markov process, with probabilistic state transitions corresponding to a conventional clock and the probabilities of received decimal digits. The result is a performance level which results in very high accuracy and reliability, even under conditions when the minute beep of the signal, normally its most prominent feature, can barely be detected by ear using a communications receiver.</p>
- <p>The analog audio signal from the shortwave radio is sampled at 8000 Hz and converted to digital representation. The 1000/1200-Hz pulses and 100-Hz subcarrier are first separated using two IIR filters, a 600-Hz bandpass filter centered on 1100 Hz and a 150-Hz lowpass filter. The minute synch pulse is extracted using an 800-ms synchronous matched filter and pulse grooming logic which discriminates between WWV and WWVH signals and noise. The second synch pulse is extracted using a 5-ms FIR matched filter and 8000-stage comb filter.</p>
- <p>The phase of the 100-Hz subcarrier relative to the second synch pulse is fixed at the transmitter; however, the audio stage in many radios affects the phase response at 100 Hz in unpredictable ways. The driver adjusts for each radio using two 170-ms synchronous matched filters. The I (in-phase) filter is used to demodulate the subcarrier envelope, while the Q (quadrature-phase) filter is used in a tracking loop to discipline the codec sample clock and thus the demodulator phase.</p>
- <p>A bipolar data signal is determined from the matched filter I and Q channels using a pulse-width discriminator. The discriminator samples the I channel at 15 ms (<i>n</i>), 200 ms (<i>s</i><sub>1</sub>) and 500 ms (<i>s</i><sub>0</sub>), and the envelope (RMS I and Q channels) at 200 ms (<i>e</i><sub>1</sub>) and the end of the second (<i>e</i><sub>0</sub>). The bipolar data signal is expressed <i>s</i><sub>1</sub> - 2<i>s</i><sub>0 </sub>- <i>n.</i> Note that, since the signals <i>s</i><sub>0</sub> and <i>s</i><sub>1</sub> include the noise <i>n</i>, this term cancels out. The data bit SNR is calculated as 20 log<sub>10</sub>(<i>e</i><sub>1</sub> / <i>e</i><sub>0</sub>). If the driver has not synchronized to the minute pulse, or if the data bit amplitude <i>e</i><sub>1</sub> or SNR are below thresholds, the bit is considered invalid and the bipolar signal is forced to zero.</p>
+ <p>The analog audio signal from the shortwave radio is sampled at 8000 Hz and converted to digital representation. The 1000/1200-Hz pulses and 100-Hz subcarrier are first separated using two IIR filters, a 600-Hz bandpass filter centered on 1100 Hz and a 150-Hz lowpass filter. The minute pulse is extracted using an 800-ms synchronous matched filter and pulse grooming logic which discriminates between WWV and WWVH signals and noise. The second pulse is extracted using a 5-ms FIR matched filter and 8000-stage comb filter.</p>
+ <p>The phase of the 100-Hz subcarrier relative to the second pulse is fixed at the transmitter; however, the audio stage in many radios affects the phase response at 100 Hz in unpredictable ways. The driver adjusts for each radio using two 170-ms synchronous matched filters. The I (in-phase) filter is used to demodulate the subcarrier envelope, while the Q (quadrature-phase) filter is used in a tracking loop to discipline the demodulator phase.</p>
+ <p>A bipolar data signal is determined from the matched filter I and Q channels using a pulse-width discriminator. The discriminator samples the I channel at 15 ms (<i>n</i>), 200 ms (<i>s</i><sub>0</sub>) and 500 ms (<i>s</i><sub>1</sub>), and the envelope (RMS I and Q channels) at 200 ms (<i>e</i><sub>1</sub>) and the end of the second (<i>e</i><sub>0</sub>). The bipolar data signal is expressed <i>s</i><sub>0</sub> - 2<i>s</i><sub>1 </sub>- <i>n</i>, where positive values correspond to zero and negative values correspond to one. Note that, since the signals <i>s</i><sub>0</sub> and <i>s</i><sub>1</sub> include the noise <i>n</i>, the noise component cancels out. The data bit SNR is calculated as 20 log<sub>10</sub>(<i>e</i><sub>1</sub> / <i>e</i><sub>0</sub>). If the driver has not synchronized to the minute pulse, or if the data bit amplitude <i>e</i><sub>1</sub> or SNR are below thresholds, the bit is considered invalid and the bipolar signal is forced to zero.</p>
<p>The bipolar signal is exponentially averaged in a set of 60 accumulators, one for each second, to determine the semi-static miscellaneous bits, such as DST indicator, leap second warning and DUT1 correction. In this design a data average value larger than a positive threshold is interpreted as +1 (hit) and a value smaller than a negative threshold as a -1 (miss). Values between the two thresholds, which can occur due to signal fades, are interpreted as an erasure and result in no change of indication.</p>
<p>The BCD digit in each digit position of the timecode is represented as four data bits. The bits are correlated with the bits corresponding to each of the valid decimal digits in this position. If any of the four bits are invalid, the correlated value for all digits in this position is assumed zero. In either case, the values for all digits are exponentially averaged in a likelihood vector associated with this position. The digit associated with the maximum over all averaged values then becomes the maximum likelihood selection for this position and the ratio of the maximum over the next lower value represents the digit SNR.</p>
<p>The decoding matrix contains nine row vectors, one for each digit position. Each row vector includes the maximum likelihood digit, likelihood vector and other related data. The maximum likelihood digit for each of the nine digit positions becomes the maximum likelihood time of the century. A built-in transition function implements a conventional clock with decimal digits that count the minutes, hours, days and years, as corrected for leap seconds and leap years. The counting operation also rotates the likelihood vector corresponding to each digit as it advances. Thus, once the clock is set, each clock digit should correspond to the maximum likelihood digit as transmitted.</p>
<p>Each row of the decoding matrix also includes a compare counter and the most recently determined maximum likelihood digit. If a digit likelihood exceeds the decision level and compares with previous digits for a number of successive minutes in any row, the maximum likelihood digit replaces the clock digit in that row. When this condition is true for all rows and the second epoch has been reliably determined, the clock is set (or verified if it has already been set) and delivers correct time to the integral second. The fraction within the second is derived from the logical master clock, which runs at 8000 Hz and drives all system timing functions.</p>
<p>The logical master clock is derived from the audio codec clock. Its frequency is disciplined by a frequency-lock loop (FLL) which operates independently of the data recovery functions. At averaging intervals determined by the measured jitter, the frequency error is calculated as the difference between the most recent and the current second epoch divided by the interval. The sample clock frequency is then corrected by this amount. When first started, the frequency averaging interval is eight seconds, in order to compensate for intrinsic codec clock frequency offsets up to 125 PPM. Under most conditions, the averaging interval doubles in stages from the initial value to over 1000 seconds, which results in an ultimate frequency precision of 0.125 PPM, or about 11 ms/day.</p>
<p>It is important that the logical clock frequency is stable and accurately determined, since in most applications the shortwave radio will be tuned to a fixed frequency where WWV or WWVH signals are not available throughout the day. In addition, in some parts of the US, especially on the west coast, signals from either or both WWV and WWVH may be available at different times or even at the same time. Since the propagation times from either station are almost always different, each station must be reliably identified before attempting to set the clock.</p>
- <p>Reliable station identification requires accurate discrimination between very weak signals in noise and noise alone. The driver very aggresively soaks up every scrap of signal information, but has to be careful to avoid making pseudo-sense of noise alone. The signal quality metric depends on the minute pulse amplitude and SNR measured in second 0 of the minute, together with the data subcarrier amplitude and SNR measured in second 1. If all four values are above defined thresholds a hit is declared, otherwise a miss. The number of hits declared in the last six minutes by each station represents the high order bits of the metric value, while the current minute pulse amplitude repressents the low order bits. The resulting value is then scaled from zero to 100 for use as a quality indicator. It is used by the autotune function described below and reported in the timecode string.</p>
+ <p>Reliable station identification requires accurate discrimination between very weak signals in noise and noise alone. The driver very aggressively soaks up every scrap of signal information, but has to be careful to avoid making pseudo-sense of noise alone. The signal quality metric depends on the minute pulse amplitude and SNR measured in second 0 of the minute, together with the data subcarrier amplitude and SNR measured in second 1. If all four values are above defined thresholds a hit is declared, otherwise a miss. The number of hits declared in the last six minutes by each station represents the high order bits of the metric value, while the current minute pulse amplitude represents the low order bits. The resulting value is then scaled from zero to 100 for use as a quality indicator. It is used by the autotune function described below and reported in the timecode string.</p>
<h4>Performance</h4>
- <p>It is the intent of the design that the accuracy and stability of the indicated time be limited only by the characteristics of the ionospheric propagation medium. Conventional wisdom is that synchronization via the HF medium is good only to a millisecond under the best propagation conditions. The performance of the NTP daemon disciplined by the driver is clearly better than this, even under marginal conditions. Ordinarily, with marginal to good signals and a frequency averaging interval of 1024 s, the frequency is stabilized within 0.1 PPM and the time within 0.5 ms. The frequency stability characteristic is highly important, since the clock may have to free-run for several hours before reacquiring the WWV/H signal.</p>
- <p>The expected accuracy over a typical day was determined using the DSP93 and an oscilloscope and cesium oscillator calibrated with a GPS receiver. With marginal signals and allowing 15 minutes for initial synchronization and frequency compensation, the time accuracy determined from the WWV/H second synch pulse was reliably within 125 <font face="Symbol">m</font>s. In the particular DSP93 used for program development, the uncorrected CPU clock frequency offset was 45.8±0.1 PPM. Over the first hour after initial synchronization, the clock frequency drifted about 1 PPM as the frequency averaging interval increased to the maximum 1024 s. Once reaching the maximum, the frequency wandered over the day up to 1 PPM, but it is not clear whether this is due to the stability of the DSP93 clock oscillator or the changing height of the ionosphere. Once the frequency had stabilized and after loss of the WWV/H signal, the frequency drift was less than 0.5 PPM, which is equivalent to 1.8 ms/h or 43 ms/d. This resulted in a step phase correction up to several milliseconds when the signal returned.</p>
- <p>The measured propagation delay from the WWV transmitter at Boulder, CO, to the receiver at Newark, DE, is 23.5±0.1 ms. This is measured to the peak of the pulse after the second synch comb filter and includes components due to the ionospheric propagation delay, nominally 8.9 ms, communications receiver delay and program delay. The propagation delay can be expected to change about 0.2 ms over the day, as the result of changing ionosphere height. The DSP93 program delay was measured at 5.5 ms, most of which is due to the 400-Hz bandpass filter and 5-ms matched filter. Similar delays can be expected of this driver.</p>
- <h4>Program Operation</h4>The driver begins operation immediately upon startup. It first searches for one or both of the stations WWV and WWVH and attempts to acquire minute synch. This may take some fits and starts, as the driver expects to see several consecutive minutes with good signals and low jitter. If the autotune function is active, the driver will rotate over all five frequencies and both WWV and WWVH stations until at least three good minutes are found.<p>When a minute synch candidate has been found, the driver acquires second synch, which can take up to several minutes, depending on signal quality. At the same time the driver accumulates likelihood values for the unit (seconds) digit of the nine digits of the timecode, plus the seven miscellaneous bits included in the WWV/H transmission format. When a good unit digit has been found, the driver accumlates likelihood values for the remaining eight digits of the timecode. When three repetitions of all nine digits have decoded correctly, which normally takes 15 minutes with good signals, and up to 40 minutes when buried in noise, and the second synch has been acquired, the clock is set (or verified) and is selectable to discipline the system clock.</p>
- <p>Once the clock is set, it continues to provide correct timecodes, even if all signals are losst. The time is considered correct as long as the second synch amplitude and SNR are above specified thresholds and jitter is below threshold. As long as the clock is set or verified, the system clock offsets are provided once each minute to the reference clock interface, where they are processed using the same algorithms used with other local reference clocks and remote servers. Using these algorithms, the system clock can in principle be disciplined to a much finer resolution than the 125-<font face="Symbol">m</font>s sample interval would suggest, although the ultimate accuracy is probably limited by propagation delay variations as the ionspheric height varies throughout the day and night.</p>
+ <p>It is the intent of the design that the accuracy and stability of the indicated time be limited only by the characteristics of the ionospheric propagation medium. Conventional wisdom is that manual synchronization via oscilloscope and HF medium is good only to a millisecond under the best propagation conditions.</p>
+ <p>The performance of the NTP daemon disciplined by this driver is clearly better than this, even under marginal conditions. The figure below shows the measured offsets over a typical day near the bottom of the sunspot cycle ending in October, 2006. Variations up to ±0.4 ms can be expected due to changing ionospheric layer height and ray geometry over the day and night.</p>
+ <div align="center">
+ <img src="../pic/offset1211.gif" alt="gif"></div>
+ <p>The accuracy with this driver has been determined for a Pentium 4 running FreeBSD 6.1. For these measurements the computer clock was disciplined within a few microseconds of UTC using a PPS signal and GPS receiver and the measured offsets determined from the peerstats data.</p>
+ <p>The predicted propagation delay from the WWV transmitter at Boulder, CO, to the receiver at Newark, DE, is 9.7±0.1 ms. In addition, the receiver contributes 4.7 ms, codec 0.2 ms and 600-Hz bandpass filter 1.1 ms. With these values, the systematic error, which can be eliminated by the <tt>fudge time1</tt> parameter, is about 0.4 ms and varies ±0.4 ms over the day as the result of changing ionospheric height and ray geometry.</p>
+ <h4>Program Operation</h4>
+ The driver begins operation immediately upon startup. It first searches for one or both of the stations WWV and WWVH and attempts to acquire minute synch. This may take some fits and starts, as the driver expects to see several consecutive minutes with good signals and low jitter. If the autotune function is active, the driver will rotate over all five frequencies and both WWV and WWVH stations until at least three good minutes are found.<p>When a minute synch candidate has been found, the driver acquires second synch, which can take up to several minutes, depending on signal quality. At the same time the driver accumulates likelihood values for the unit (seconds) digit of the nine digits of the timecode, plus the seven miscellaneous bits included in the WWV/H transmission format. When a good unit digit has been found, the driver accumulated likelihood values for the remaining eight digits of the timecode. When three repetitions of all nine digits have decoded correctly, which normally takes 15 minutes with good signals, and up to 40 minutes when buried in noise, and the second synch has been acquired, the clock is set (or verified) and is selectable to discipline the system clock.</p>
+ <p>Once the clock is set, it continues to provide correct timecodes, even if all signals are lost. The time is considered correct as long as the second synch amplitude and SNR are above specified thresholds and jitter is below threshold. As long as the clock is set or verified, the system clock offsets are provided once each minute to the reference clock interface, where they are processed using the same algorithms used with other local reference clocks and remote servers. Using these algorithms, the system clock can in principle be disciplined to a much finer resolution than the 125-<font face="Symbol">m</font>s sample interval would suggest, although the ultimate accuracy is probably limited by propagation delay variations as the ionospheric height varies throughout the day and night.</p>
<p>The codec clock frequency is disciplined during times when WWV/H signals are available. The algorithm refines the frequency offset using increasingly longer averaging intervals to 1024 s, where the precision is about 0.1 PPM. With good signals, it takes well over two hours to reach this degree of precision; however, it can take many more hours than this in case of marginal signals. Once reaching the limit, the algorithm will follow frequency variations due to temperature fluctuations and ionospheric height variations.</p>
<p>It may happen as the hours progress around the clock that WWV and WWVH signals may appear alone, together or not at all. When the driver has mitigated which station and frequency is best, it sets the reference identifier to the string WV<i>f</i> for WWV and WH<i>f</i> for WWVH, where <i>f</i> is the frequency in megahertz. If the propagation delays have been properly set with the <tt>fudge time1</tt> (WWV) and <tt>fudge time2</tt> (WWVH) commands in the configuration file, handover from one station to the other is seamless.</p>
- <p>Once the clock has been set for the first time, it will appear reachable and selectable to discipline the system clock. Operation continues as long as the signal quality from at least one station on at least one frequency is acceptable. A consequence of this design is that, once the clock is set, the time and frequency are disciplined only by the second synch pulse and the clock digits themselves are driven by the clock state machine. If for some reason the state machine drifts to the wrong second, it would never reresynchronize. To protect against this most unlikely situation, if after two days with no signals, the clock is considered unset and resumes the synchronization procedure from the beginning.</p>
+ <p>Once the clock has been set for the first time, it will appear reachable and selectable to discipline the system clock. Operation continues as long as the signal quality from at least one station on at least one frequency is acceptable. A consequence of this design is that, once the clock is set, the time and frequency are disciplined only by the second synch pulse and the clock digits themselves are driven by the clock state machine. If for some reason the state machine drifts to the wrong second, it would never resynchronize. To protect against this most unlikely situation, if after two days with no signals, the clock is considered unset and resumes the synchronization procedure from the beginning.</p>
<p>However, as long as the clock has once been set correctly and allowed to converge to the intrinsic codec clock frequency, it will continue to read correctly even during the holdover interval, but with increasing dispersion. Assuming the clock frequency can be disciplined within 1 PPM, it can coast without signals for several days without exceeding the NTP step threshold of 128 ms. During such periods the root dispersion increases at 5 <font face="Symbol">m</font>s per second, which makes the driver appear less likely for selection as time goes on. Eventually, when the dispersion due all causes exceeds 1 s, it is no longer suitable for synchronization.</p>
<p>To work well, the driver needs a shortwave receiver with good audio response at 100 Hz. Most shortwave and communications receivers roll off the audio response below 250 Hz, so this can be a problem, especially with receivers using DSP technology, since DSP filters can have very fast rolloff outside the passband. Some DSP transceivers, in particular the ICOM 775, have a programmable low frequency cutoff which can be set as low as 80 Hz. However, this particular radio has a strong low frequency buzz at about 10 Hz which appears in the audio output and can affect data recovery under marginal conditions. Although not tested, it would seem very likely that a cheap shortwave receiver could function just as well as an expensive communications receiver.</p>
<h4>Autotune</h4>
<p>If specified, the driver will attempt to open the device <tt>/dev/icom</tt> and, if successful will activate the autotune function and tune the radio to each operating frequency in turn while attempting to acquire minute synch from either WWV or WWVH. However, the driver is liberal in what it assumes of the configuration. If the <tt>/dev/icom</tt> link is not present or the open fails or the CI-V bus or radio is inoperative, the driver quietly gives up with no harm done.</p>
<p>Once acquiring minute synch, the driver operates as described above to set the clock. However, during seconds 59, 0 and 1 of each minute it tunes the radio to one of the five broadcast frequencies to measure the minute synch pulse amplitude and SNR in second 0 and data pulse amplitude and SNR in second 1 to update the signal metric. In principle, the data pulse in second 58 is usable, but the AGC in most radios is not fast enough for a reliable measurement. Each of the five frequencies are probed in a five-minute rotation to build a database of current propagation conditions for all signals that can be heard at the time. At the end of each probe a mitigation procedure scans the database and retunes the radio to the best frequency and station found. For this to work well, the radio should be set for a fast AGC recovery time. This is most important while tracking a strong signal, which is normally the case, and then probing another frequency, which may have much weaker signals.</p>
<p>At the end of each probe, the frequency and station with the maximum metric is chosen, with ties going first to the highest frequency and then to WWV in order. A station is considered valid only if the metric is above a specified threshold' if below, the rotating probes continue until a valid station is found.</p>
- <dl>
- </dl>
+ <p>The behavior of the autotune function over a typical day is shown in the figure below.</p>
+ <div align="center">
+ <img src="../pic/freq1211.gif" alt="gif"></div>
+ <p>As expected, the lower frequencies prevail when the ray path is in darkness (0100-1300 UTC) and the higher frequencies when the path is in sunlight (1300-0100 UTC). Note two periods in the figure show zero frequency when signals are below the minimum for all frequencies and stations.</p>
<h4>Diagnostics</h4>
<p>The autotune process produces diagnostic information along with the timecode. This is very useful for evaluating the performance of the algorithms, as well as radio propagation conditions in general. The message is produced once each minute for each frequency in turn after minute synch has been acquired.</p>
<p><tt>wwv5 status agc epoch secamp/secsnr datamp/datsnr wwv wwvh</tt></p>
- <p>where the fields after the <tt>wwv5</tt> identifier are: <tt>status</tt> contains status bits, <tt>agc</tt> audio gain, <tt>epoch </tt>second epoch, <tt>secamp/secsnr </tt>second pulse ampliture/SNR, and <tt>wwv</tt> and <tt>wwvh</tt> are two sets of fields, one each for WWV and WWVH. Each of the two fields has the format</p>
+ <p>where the fields after the <tt>wwv5</tt> identifier are: <tt>status</tt> contains status bits, <tt>agc</tt> audio gain, <tt>epoch </tt>second epoch, <tt>secamp/secsnr </tt>second pulse amplitude/SNR, and <tt>wwv</tt> and <tt>wwvh</tt> are two sets of fields, one each for WWV and WWVH. Each of the two fields has the format</p>
<p><tt>ident score metric minamp/minsnr</tt></p>
<p>where <tt>ident </tt>encodes the station (<tt>WV</tt> for WWV, <tt>WH</tt> for WWVH) and frequency (2, 5, 10, 15 or 20), <tt>score</tt> 32-bit shift register recording the hits (1) and misses (0) of the last 32 probes (hits and misses enter from the right), <tt>metric</tt> is described above, and <tt>minamp/minsnr</tt> minute pulse ampliture/SNR. An example is:</p>
<p><tt>wwv5 000d 111 5753 3967/20.1 3523/10.2 WV20 bdeff 100 8348/30.0 WH20 0000 1 22/-12.4</tt></p>
When enabled by the <tt>filegen</tt> facility, every received timecode is written to the <tt>clockstats</tt> file in the following format:
<p><tt>sq yyyy ddd hh:mm:ss ld du lset agc ident metric errs freq avg<br>
- s</tt> synch indicator (<tt>?</tt> or space)
+ s</tt> synch indicator (<tt>?</tt> or space)
<tt>q </tt>quality character (see below)
<tt>yyyy </tt>Gregorian year
<tt>ddd </tt>day of year
Audio Device: <tt>/dev/audio</tt> and <tt>/dev/audioctl</tt>
<p>Note: This driver supersedes an older one of the same name, address and ID which required replacing the original kernel audio driver with another which worked only on older Sun SPARC architectures and SunOS operating systems. The new driver requires no modification of the operating system and works on FreeBSD, SunOS and Solaris. While it is generic and likely portable to other systems, it is somewhat slower than the original, since the extensive signal conditioning, filtering and decoding is done in user space, not kernel space.</p>
<h4>Description</h4>
- <p>This driver supports the Inter-Range Instrumentation Group (IRIG) standard time distribution signal using the audio codec native to some workstations. This signal is generated by several radio clocks, including those made by Arbiter, Austron, Bancomm, Odetics, Spectracom and TrueTime, among others, although it is often an add-on option. The signal is connected via an optional attenuator box and cable to either the microphone or line-in port. The driver receives, demodulates and decodes the IRIG-B and IRIG-E signal formats using internal filters designed to reduce the effects of noise and interference.</p>
+ <p>This driver supports the Inter-Range Instrumentation Group (IRIG) standard time distribution signal using the audio codec native to most workstations and PCs. This signal is generated by several radio clocks, including those made by Arbiter, Austron, Bancomm, Odetics, Spectracom, Symmetricom and TrueTime, among others, although it is often an add-on option. The signal is connected via an optional attenuator and cable to either the microphone or line-in port. The driver receives, demodulates and decodes the IRIG-B and IRIG-E signal formats using internal filters designed to reduce the effects of noise and interference.</p>
<p>This driver incorporates several features in common with other audio drivers such as described in the <a href="driver7.html">Radio CHU Audio Demodulator/Decoder</a> and the <a href="driver36.html">Radio WWV/H Audio Demodulator/Decoder</a> pages. They include automatic gain control (AGC), selectable audio codec port and signal monitoring capabilities. For a discussion of these common features, as well as a guide to hookup, debugging and monitoring, see the <a href="../audio.html">Reference Clock Audio Drivers</a> page.</p>
<p>The IRIG signal format uses an amplitude-modulated carrier with pulse-width modulated data bits. For IRIG-B, the carrier frequency is 1000 Hz and bit rate 100 b/s; for IRIG-E, the carrier frequenchy is 100 Hz and bit rate 10 b/s. While IRIG-B provides the best accuracy, generally within a few tens of microseconds relative to IRIG time, it can also generate a significant load on the processor with older workstations. Generally, the accuracy with IRIG-E is about ten times worse than IRIG-B, but the processor load is ten times less.</p>
- <p>The program processes 8000-Hz <font face="symbol">m</font>-law companded samples using separate signal filters for IRIG-B and IRIG-E, a comb filter, envelope detector and automatic threshold corrector. Cycle crossings relative to the corrected slice level determine the width of each pulse and its value - zero, one or position identifier. The data encode 20 BCD digits which determine the second, minute, hour and day of the year and sometimes the year and synchronization condition. The comb filter exponentially averages the corresponding samples of successive baud intervals in order to reliably identify the reference carrier cycle. A type-II phase-lock loop (PLL) performs additional integration and interpolation to accurately determine the zero crossing of that cycle, which determines the reference timestamp. A pulse-width discriminator demodulates the data pulses, which are then encoded as the BCD digits of the timecode. The timecode and reference timestamp are updated once each second with IRIG-B (ten seconds with IRIG-E) and local clock offset samples saved for later processing. At poll intervals of 64 s, the saved samples are processed by a trimmed-mean filter and used to update the system clock.</p>
+ <p>The program processes 8000-Hz <font face="symbol">m</font>-law companded samples using separate signal filters for IRIG-B and IRIG-E, a comb filter, envelope detector and automatic threshold corrector. Cycle crossings relative to the corrected slice level determine the width of each pulse and its value - zero, one or position identifier. The data encode 20 BCD digits which determine the second, minute, hour and day of the year and sometimes the year and synchronization condition. The comb filter exponentially averages the corresponding samples of successive baud intervals in order to reliably identify the reference carrier cycle. A type-II phase-lock loop (PLL) performs additional integration and interpolation to accurately determine the zero crossing of that cycle, which determines the reference timestamp. A pulse-width discriminator demodulates the data pulses, which are then encoded as the BCD digits of the timecode. The timecode and reference timestamp are updated once each second with IRIG-B (ten seconds with IRIG-E) and local clock offset samples saved for later processing. At poll intervals of 64 s, the saved samples are processed by a median filter and used to update the system clock.</p>
<p>Infinite impulse response (IIR) filters are used with both IRIG-B and IRIG-E formats. An 800-Hz highpass filter is used for IRIG-B and a 130-Hz lowpass filter for IRIG-E. These are intended for use with noisy signals, such as might be received over a telephone line or radio circuit, or when interfering signals may be present in the audio passband. The driver determines which IRIG format is in use by sampling the amplitude of each filter output and selecting the one with maximum signal. An automatic gain control feature provides protection against overdriven or underdriven input signal amplitudes. It is designed to maintain adequate demodulator signal amplitude while avoiding occasional noise spikes. In order to assure reliable capture, the decompanded input signal amplitude must be greater than 100 units and the codec sample frequency error less than 250 PPM (.025 percent).</p>
<p>The program performs a number of error checks to protect against overdriven or underdriven input signal levels, incorrect signal format or improper hardware configuration. The specific checks are detailed later in this page. Note that additional checks are done elsewhere in the reference clock interface routines.</p>
<p>Unlike other drivers, which can have multiple instantiations, this one supports only one. It does not seem likely that more than one audio codec would be useful in a single machine. More than one would probably chew up too much CPU time anyway.</p>
<h4>IRIG-B Timecode Format</h4>
- <p>The 100 elements of the IRIG timecode are numbered from 0 through 99. Position identifiers occur at elements 0, 9, 19 and every ten thereafter to 99. The control function (CF) elements begin at element 50 (CF 1) and extend to element 78 (CF 27). The straight-binary-seconds (SBS) field, which encodes the seconds of the UTC day, begins at element 80 (CF 28) and extends to element 97 (CF 44). The encoding of elements 50 (CF 1) through 78 (CF 27) is device dependent. This driver presently decodes the CF elements, but does nothing with them.</p>
+ <p>The 100 elements of the IRIG timecode are numbered from 0 through 99. Position identifiers occur at elements 0, 9 and every ten thereafter to 99. The control function (CF) elements begin at element 50 (CF 1) and extend to element 78 (CF 27). The straight-binary-seconds (SBS) field, which encodes the seconds of the UTC day, begins at element 80 (CF 28) and extends to element 97 (CF 44). The encoding of elements 50 (CF 1) through 78 (CF 27) is device dependent. This driver presently decodes the CF elements, but does nothing with them.</p>
<p>Where feasible, the IRIG signal source should be operated with signature control so that, if the signal is lost or mutilated, the source produces an unmodulated signal, rather than possibly random digits. The driver will automatically reject the data and declare itself unsynchronized in this case. Some devices, in particular Spectracom radio/satellite clocks, provide additional year and status indication in the format:</p>
<pre>
Element CF Function
</pre>
Other devices set these elements to zero.
<h4>Performance and Horror Stories</h4>
- <p>The <font face="symbol">m</font>-law companded data format allows considerable latitude in signal levels; however, an automatic gain control (AGC) function is implemented to further compensate for varying input signal levels and to avoid signal distortion. For proper operation, the IRIG signal source should be configured for analog signal levels, NOT digital TTL levels.</p>
- <p>The accuracy of the system clock synchronized to the IRIG-B source with this driver and the <tt>ntpd</tt> daemon is 10-20 <font face="symbol">m</font>s with a Sun UltraSPARC II running Solaris 2.6 and maybe twice that with a Sun SPARC IPC running SunOS 4.1.3. Be however acutely aware that the accuracy with Solaris 2.8 and presumably beyond has seriously degraded to the order of several milliseconds. The Sun kernel driver has a sawtooth modulation with amplitude over 5 ms peak-peak and period 5.5 s. The crafty IRIG driver uses a transverse filter to remove the modulation and something called a botttom-fisher to remove incidental positive spikes especially prevalent with Sun Blade 1000 and possibly other systems. The result is nominal accuracy and jitter something less than 0.5 ms, but the this is still far inferior to the performance with older systems.</p>
+ <p>The <font face="symbol">m</font>-law companded data format allows considerable latitude in signal levels; however, an automatic gain control (AGC) function is implemented to further compensate for varying input signal levels and to avoid signal distortion. For proper operation, the IRIG signal source should be configured for analog signal levels, NOT digital TTL levels. In most radios the IRIG signal is driven ±10 V behind 50 Ohms. In such cases the cable should be terminate at the line-in port with a 50-Ohm resistor to avoid overloading the codec.</p>
+ <p>The accuracy with this driver has been determined for a Pentium 4 running FreeBSD 6.1. For these measurements the computer clock was disciplined within a few microseconds of UTC using a PPS signal and GPS receiver and the measured offsets determined from the peerstats data. The systematic error, which can be eliminated by the <tt>fudge time1</tt> parameter, is <font face="symbol">-</font>98 <font face="symbol">m</font>s with standard deviation 9 <font face="symbol">m</font>s and maximum deviation 30 <font face="symbol">m</font>s. However, be acutely aware that the accuracy with Solaris 2.8 and presumably beyond has been seriously degraded to the order of several milliseconds. The Sun kernel driver has a sawtooth modulation with amplitude over 5 ms P-P and period 5.5 s. The crafty IRIG driver uses a transverse filter to remove the modulation and something called a botttom-fisher to remove incidental positive spikes especially prevalent with Sun Blade 1000 and possibly other systems. The result is nominal accuracy and jitter something less than 0.5 ms, but this is still far inferior to the performance with older systems.</p>
<p>The processor resources consumed by the daemon can be significant, ranging from about 1.2 percent on the faster UltraSPARC II to 38 percent on the slower SPARC IPC. However, the overall timing accuracy is limited by the resolution and stability of the CPU clock oscillator and the interval between clock corrections, which is 64 s with this driver. This performance, while probably the best that can be achieved by the daemon itself, can be improved with assist from the PPS discipline as described elsewhere in this documentation.</p>
- <h4>Autotune</h4>
- <p>The driver includes provisions to automatically tune the radio in response to changing radio propagation conditions throughout the day and night. The radio interface is compatible with the ICOM CI-V standard, which is a bidirectional serial bus operating at TTL levels. The bus can be connected to a serial port using a level converter such as the CT-17.</p>
- <p>Each ICOM radio is assigned a unique 8-bit ID select code, usually expressed in hex format. To activate the CI-V interface, the <tt>mode</tt> keyword of the <tt>server</tt> configuration command specifies a nonzero select code in decimal format. A table of ID select codes for the known ICOM radios is given on the <a href="../audio.html">Reference Clock Audio Drivers</a> page. A missing <tt>mode</tt> keyword or a zero argument leaves the interface disabled.</p>
- <p>If specified, the driver will attempt to open the device <tt>/dev/icom</tt> and, if successful will activate the autotune function and tune the radio to each operating frequency in turn while attempting to acquire minute sync from CHU. However, the driver is liberal in what it assumes of the configuration. If the <tt>/dev/icom</tt> link is not present or the open fails or the CI-V bus or radio is inoperative, the driver quietly gives up with no harm done.</p>
<h4>Monitor Data</h4>
+
The timecode format used for debugging and data recording includes data helpful in diagnosing problems with the IRIG signal and codec connections. With debugging enabled (-d on the ntpd command line), the driver produces one line for each timecode in the following format:
<p><tt>00 1 98 23 19:26:52 721 143 0.694 47 20 0.083 66.5 3094572411.00027</tt></p>
<p>The first field containes the error flags in hex, where the hex bits are interpreted as below. This is followed by the IRIG status indicator, year of century, day of year and time of day. The status indicator and year are not produced by some IRIG devices. Following these fields are the carrier amplitude (0-8100), codec gain (0-255), field phase (0-79), time constant (2-20), modulation index (0-1), carrier phase error (0±0.5) and carrier frequency error (PPM). The last field is the on-time timestamp in NTP format. The fraction part is a good indicator of how well the driver is doing. With an UltrSPARC 30, this is normally within a few tens of microseconds relative to the IRIG-B signal and within a few hundred microseconds with IRIG-E.</p>
<h4>Description</h4>
<p>This driver synchronizes the computer time using data encoded in radio transmissions from Canadian time/frequency station CHU in Ottawa, Ontario. It replaces an earlier one, built by Dennis Ferguson in 1988, which required a special line discipline to preprocessed the signal. The new driver includes more powerful algorithms implemented directly in the driver and requires no preprocessing.</p>
<p>CHU transmissions are made continuously on 3330 kHz, 7335 kHz and 14670 kHz in upper sideband, compatible AM mode. An ordinary shortwave receiver can be tuned manually to one of these frequencies or, in the case of ICOM receivers, the receiver can be tuned automatically as propagation conditions change throughout the day and night. The performance of this driver when tracking the station is ordinarily better than 1 ms in time with frequency drift less than 0.5 PPM when not tracking the station.</p>
- <p>While there are currently no known commercial CHU receivers, a simple but effective receiver/demodulator can be constructed from an ordinary shortwave receiver and Bell 103 compatible, 300-b/s modem or modem chip, as described on the <a href="../pps.html">Pulse-per-second (PPS) Signal Interfacing</a> page. The driver can use the modem to receive the radio signal and demodulate the data or, if available, the driver can use the audio codec of the Sun workstation or another with compatible audio interface. In the latter case, the driver implements the modem using DSP routines, so the radio can be connected directly to either the microphone or line input port.</p>
+ <p>While there are currently no known commercial CHU receivers, a simple but effective receiver/demodulator can be constructed from an ordinary shortwave receiver and Bell 103 compatible, 300-b/s modem or modem chip, as described on the <a href="../pps.html">Pulse-per-second (PPS) Signal Interfacing</a> page. The driver can use the modem to receive the radio signal and demodulate the data or, if available, the driver can use the audio codec of the Sun workstation or another machine with compatible audio interface. In the latter case, the driver implements the modem using DSP routines, so the radio can be connected directly to either the microphone or line input port.</p>
<p>This driver incorporates several features in common with other audio drivers such as described in the <a href="driver36.html">Radio WWV/H Audio Demodulator/Decoder</a> and the <a href="driver6.html">IRIG Audio Decoder</a> pages. They include automatic gain control (AGC), selectable audio codec port and signal monitoring capabilities. For a discussion of these common features, as well as a guide to hookup, debugging and monitoring, see the <a href="../audio.html">Reference Clock Audio Drivers</a> page.</p>
- <p>Ordinarily, the driver poll interval is set to 14 (about 4.5 h), although this can be changed with configuration commands. As long as the clock is set or verified at least once during this interval, the NTP algorithms will consider the source reachable and selectable to discipline the system clock. However, if this does not happen for eight poll intervals, the algorithms will consider the source unreachable and some other source will be chosen (if available) to discipline the system clock.</p>
+ <p>Ordinarily, the driver timeout interval is set to 1440 minutes (one day). If no signals are heard before the timeout, the driver resets all state variables and starts over. As long as the clock is set or verified at least once during this interval, the NTP algorithms will consider the source reachable and selectable to discipline the system clock. However, if this does not happen for eight poll intervals, the algorithms will consider the source unreachable and some other source will be chosen (if available) to discipline the system clock.</p>
<p>The decoding algorithms process the data using maximum-likelihood techniques which exploit the considerable degree of redundancy available in each broadcast message or burst. As described below, every character is sent twice and, in the case of format A bursts, the burst is sent eight times every minute. In the case of format B bursts, which are sent once each minute, the burst is considered correct only if every character matches its repetition in the burst. In the case of format A messages, a majority decoder requires at least six repetitions for each digit in the timecode and more than half of the repetitions decode to the same digit. Every character in every burst provides an independent timestamp upon arrival with a potential total of over 60 timestamps for each minute.</p>
<p>A timecode in the format described below is assembled when all bursts have been received in the minute. The timecode is considered valid and the clock set when at least one valid format B burst has been decoded and the above requirements are met. The <tt>yyyy</tt> year field in the timecode indicates whether a valid format B burst has been received. Upon startup, this field is initialized at zero; when a valid format B burst is received, it is set to the current Gregorian year. The <tt>q</tt> quality character field in the timecode indicates whether a valid timecode has been determined. If any of the high order three bits of this character are set, the timecode is invalid.</p>
<p>Once the clock has been set for the first time, it will appear reachable and selectable to discipline the system clock, even if the broadcast signal is lost. Since the signals are almost always available during some period of the day and the NTP clock discipline algorithms are designed to work well even in this case, it is unlikely that the system clock could drift more than a few tens of milliseconds during periods of signal loss. To protect against this most unlikely situation, if after four days with no signals, the clock is considered unset and resumes the synchronization procedure from the beginning.</p>
<h4>Debugging Aids</h4>
<p>The most convenient way to track the program status is using the <tt>ntpq</tt> program and the <tt>clockvar</tt> command. This displays the last determined timecode and related status and error counters, even when the program is not discipline the system clock. If the debugging trace feature (<tt>-d</tt> on the <tt>ntpd</tt> command line)is enabled, the program produces detailed status messages as it operates. If the <tt>fudge flag 4</tt> is set, these messages are written to the <tt>clockstats</tt> file. All messages produced by this driver have the prefix <tt>chu</tt> for convenient filtering with the Unix <tt>grep</tt> command.</p>
<p>With debugging enabled the driver produces messages in the following formats:</p>
- <p>A format <tt>chuA</tt> message is produced for each format A burst received in seconds 32 through 39 of the minute:</p>
- <p><tt>chuA n b s code</tt></p>
- <p>where <tt>n</tt> is the number of characters in the burst (0-11), <tt>b</tt> the burst distance (0-40), <tt>s</tt> the synchronization distance (0-40) and <tt>code</tt> the burst characters as received. Note that the hex digits in each character are reversed and the last ten digits inverted, so the burst</p>
- <p><tt>11 40 1091891300ef6e76ecff</tt></p>
- <p>is interpreted as containing 11 characters with burst distance 40. The nibble-swapped timecode shows DUT1 +0.1 second, year 1998 and TAI -UTC 31 seconds.</p>
- <p>A format <tt>chuB</tt> message is produced for each format B burst received in second 31 of the minute:</p>
- <p><tt>chuB n b f s m code</tt></p>
- <p>where <tt>n</tt> is the number of characters in the burst (0-11), <tt>b</tt> the burst distance (0-40), <tt>f</tt> the field alignment (-1, 0, 1), <tt>s</tt>the synchronization distance (0-16), <tt>m</tt>the burst number (2-9) and <tt>code</tt> the burst characters as received. Note that the hex digits in each character are reversed, so the burst</p>
- <p><tt>10 38 0 16 9 06851292930685129293</tt></p>
- <p>is interpreted as containing 11 characters with burst distance 38, field alignment 0, synchronization distance 16 and burst number 9. The nibble-swapped timecode shows day 58, hour 21, minute 29 and second 39.</p>
+ <p>A single message beginning with <tt>chuB</tt> is produced for each format A burst received in second 31, while six messages beginning with <tt>chuA</tt> are produced for each format A burst received in seconds 32 through 37 of the minute. The following four numbers are</p>
+ <p><tt>stat sig n b</tt></p>
+ <p>where <tt>stat</tt> is the status code, <tt>sig</tt> the signal level, <tt>n</tt> the number of characters in the burst (0-11) and <tt>b</tt> the burst distance (0-40). Good bursts will have signal levels of a few hundred or more and the other numbers near the top of the range specified. See the source for the interpretation of the remaining data in the burst.</p>
<p>If the CI-V interface for ICOM radios is active, a debug level greater than 1 will produce a trace of the CI-V command and response messages. Interpretation of these messages requires knowledge of the CI-V protocol, which is beyond the scope of this document.</p>
<h4>Monitor Data</h4>
+
When enabled by the <tt>filegen</tt> facility, every received timecode is written to the <tt>clockstats</tt> file in the following format:
<pre>
- sq yy ddd hh:mm:ss.fff ld dut lset agc rfrq bcnt dist tsmp
+ sq yy ddd hh:mm:ss.fff l ds du lset agc rfrq bcnt dist tsmp
s sync indicator
q quality character
mm minute of hour
ss second of minute
fff millisecond of second
- l leap second warning
- d DST state
+ lw leap second warning
+ dst DST state
dut DUT sign and magnitude in deciseconds
lset minutes since last set
agc audio gain (0-255)
- rfrq radio frequency
- bcnt burst count
- dist decoding distance
+ ident CHU identifier code
+ dist decoder distance
tsmp timestamps captured
</pre>
The fields beginning with <tt>year</tt> and extending through <tt>dut</tt> are decoded from the received data and are in fixed-length format. The <tt>agc</tt> and <tt>lset</tt> fields, as well as the following driver-dependent fields, are in variable-length format.
<p>It is important to note that one or more of the above alarms does not necessarily indicate a clock error, but only that the decoder has detected a condition that may in future result in an error.</p>
<dt><tt>yyyy ddd hh:mm:ss.fff</tt>
<dd>The timecode format itself is self explanatory. Note that the Gregorian year is decoded directly from the transmitted timecode.
- <dt><tt>l</tt>
+ <dt><tt>lw</tt>
<dd>The leap second warning is normally space, but changes to <tt>L</tt> if a leap second is to occur at the end of the month of June or December.
- <dt><tt>d</tt>
- <dd>The DST code for Canada encodes the state for all provinces.
- <dt><tt>dut</tt>
- <dd>The DUT sign and magnitude shows the current UT1 offset relative to the displayed UTC time, in deciseconds.
- <dt><tt>lset</tt>
+ <dt><tt>dst</tt>
+ <dd>The DST code for Canada encodes the state for all provinces. It is encoded as two hex characters.<dt><tt>dut</tt>
+ <dd>The DUT sign and magnitude shows the current UT1 offset relative to the displayed UTC time, in deciseconds. It is encoded as one digit preceeded by sign.<dt><tt>lset</tt>
<dd>Before the clock is set, the interval since last set is the number of minutes since the program was started; after the clock is set, this is number of minutes since the time was last verified relative to the broadcast signal.
<dt><tt>agc</tt>
<dd>The audio gain shows the current codec gain setting in the range 0 to 255. Ordinarily, the receiver audio gain control or IRIG level control should be set for a value midway in this range.
- <dt><tt>rfrq</tt>
- <dd>The current radio frequency, if the CI-V interface is active, or 'X' if not.
- <dt><tt>bcnt</tt>
- <dd>The number of format A bursts received during the most recent minute bursts were received.
+ <dt><tt>ident</tt>
+ <dd>The CHU identifier followed by the current radio frequency code, if the CI-V interface is active, or 'X' if not. The radio frequncy is encoded as 0 for 3330 kHz, 1 for 7335 kHz and 2 for 14670 kHz.
<dt><tt>dist</tt>
- <dd>The minimum decoding distance determined during the most recent minute bursts were received.
+ <dd>The minimum decoding distance determined during the most recent minute bursts were received. The values range from 0 to 100, with the higher values indicating good signals.
<dt><tt>tsmp</tt>
- <dd>The number of timestamps determined during the most recent minute bursts were received.
- </dl>
+ <dd>The number of timestamps determined during the most recent minute bursts were received. The values range from 0 to 60, with the higher values indicating good signals.</dl>
<h4>Modes</h4>
<p>The <tt>mode</tt> keyword of the <tt>server</tt> configuration command specifies the ICOM ID select code. A missing or zero argument disables the CI-V interface. Following are the ID select codes for the known radios.</p>
<table width="100%" cols="6">
projects / thirdparty / ntp.git / commitdiff
