+* Audio documentation updates from Dave Mills.
* New config file parsing code, from Sachin Kamboj.
* fuzz bit cleanup from Dave Mills.
* replay cleanup from Dave Mills.
<body>
<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">04:06</csobj> UTC <csobj format="LongDate" h="25" locale="00000409" region="0" t="DateTime" w="285">Saturday, January 06, 2007</csobj></p>
+ <p>Last update: <csobj format="ShortTime" h="25" locale="00000409" region="0" t="DateTime" w="61">19:40</csobj> UTC <csobj format="LongDate" h="25" locale="00000409" region="0" t="DateTime" w="223">Friday, April 13, 2007</csobj></p>
<br clear="left">
<h4>Related Links</h4>
<script type="text/javascript" language="javascript" src="scripts/links8.txt"></script>
<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 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>
+ <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 <tt>tinker codec</tt> 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> command 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> command 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>
<p>The most important factor affecting the radio signal is the antenna. It need not be long - even 15 feet is enough if it is located outside of a metal frame building, preferably on the roof, and away from metallic objects. An ordinary CB whip mounted on a PVC pipe and wooden X-frame on the roof should work well with most portable radios, as they are optimized for small antennas.</p>
<li>Transitions between daytime and nightime conditions generally occur somewhat after sunrise and sunset at the midpoint of the path from transmitter to receiver.
<li>Ambient noise (static) on the lower frequencies follows the thunderstorm season, so is higher on summer afternoons and evenings.
<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 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>
+ <li>The optimum frequencies are higher at the peak of the 11-year sunspot cycle and lower at the trough. The current sunspot cycle began at the minimum in late 2006 and should reach its peak in 2012.</ul>
+ <p>The best way to choose a frequency is to listen at various times over the day and determine the highest (daytime) and lowest (nighttime) frequencies that work well. Choose the frequency that works for the most number of hours in the day, usually the highest frequency. For instance, on the east coast the best compromise CHU frequency is 7335 kHz and the best WWV frequency is 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">
Autotune Port: <tt>/dev/icom</tt>; 1200/9600 baud, 8-bits, no parity<br>
Audio Device: <tt>/dev/audio</tt> and <tt>/dev/audioctl</tt>
<h4>Description</h4>This driver synchronizes the computer time using shortwave radio transmissions from NIST time/frequency stations <a href="http://www.bldrdoc.gov/timefreq/stations/wwv.html">WWV</a> in Ft. Collins, CO, and <a href="http://www.bldrdoc.gov/timefreq/stations/wwvh.htm">WWVH</a> in Kauai, HI. Transmissions are made continuously on 2.5, 5, 10 and 15 MHz from both stations and on 20 MHz from WWV. 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 by the driver as propagation conditions change throughout the day and season. The radio is connected via an optional attenuator and cable to either the microphone or line-in port of a workstation or PC.
- <p>The driver requires an audio codec or sound card with sampling rate 8 kHz and <font face="symbol">m</font>-law companding to demodulate the data. This is the same standard as used by the telephone industry and is supported by most hardware and operating systems, including Solaris, FreeBSD and Linux, among others. In this implementation only one audio driver and codec can be supported on a single machine. In order to assure reliable signal capture, the codec frequency error must be less than 125 PPM (.0125 percent). If necessary, the <tt>tinker codec</tt> configuration command can be used to bracket the codec frequency to this range.</p>
+ <p>The driver requires an audio codec or sound card with sampling rate 8 kHz and <font face="symbol">m</font>-law companding to demodulate the data. This is the same standard as used by the telephone industry and is supported by most hardware and operating systems, including Solaris, FreeBSD and Linux, among others. In this implementation only one audio driver and codec can be supported on a single machine. In order to assure reliable signal capture, the codec frequency error must be less than 187 PPM (.0187 percent). If necessary, the <tt>tinker codec</tt> configuration command can be used to bracket the codec frequency to this range.</p>
<p>In general and without calibration, the driver is accurate within 1 ms relative to the broadcast time when tracking a station. However, variations up to 0.3 ms can be expected due to diurnal variations in ionospheric layer height and ray geometry. In Newark DE, 2479 km from the transmitter, the predicted two-hop propagation delay varies from 9.3 ms in sunlight to 9.0 ms in moonlight. When not tracking the station the accuracy depends on the computer clock oscillator stability, ordinarily better than 0.5 PPM.</p>
<p>After calibration relative to the PPS signal from a GPS receiver, the mean offset with a 2.4-GHz P4 running FreeBSD 6.1 is generally within 0.1 ms short-term with 0.4 ms jitter. The long-term mean offset varies up to 0.3 ms due to propagation path geometry variations. The processor load due to the driver is 0.4 percent on the P4.</p>
<p>The driver 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>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>
<h4>Master Clock Discipline</h4>
- <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. The maximum value of the 5-ms pulse after the comb filter represents the on-time epoch of the second. At averaging intervals determined by the measured jitter, the frequency error is calculated as the difference between the epoches over the interval divided by the interval. The sample clock frequency is then corrected by this amount divided by a time constant of three.</p>
- <p>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 1024 s, which results in an ultimate frequency resolution of 0.125 PPM, or about 11 ms/day.</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. The maximum value of the 5-ms pulse after the comb filter represents the on-time epoch of the second. At averaging intervals determined by the measured jitter, the frequency error is calculated as the difference between the epoches over the interval divided by the interval itself. The sample clock frequency is then corrected by this amount divided by a time constant of 8.</p>
+ <p>When first started, the frequency averaging interval is 8 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 1024 s, which results in an ultimate frequency resolution of 0.125 PPM, or about 11 ms/day.</p>
<p>The data demodulation functions operate using the subcarrier clock, which is independent of the epoch. However, the data decoding functions are driven by the epoch. The decoder is phase-locked to the epoch in such a way that, when the clock state machine has reliably decoded the broadcast time to the second, the epoch timestamp of that second becomes a candidate to set the system clock.</p>
<p>The comb filter can have a long memory and is vulnerable to noise and stale data, especially when coming up after a long fade. Therefore, a candidate is considered valid only if the 5-ms signal amplitude and SNR are above thresholds. In addition, the system clock is not set until after one complete averaging interval has passed with valid candidates.</p>
<h4>Station Identification</h4>
<p>It is important that the logical clock frequency is stable and accurately determined, since in many 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 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. In principle, the data pulse in second 58 is usable, but the AGC in most radios is not fast enough for a reliable measurement.</p>
- <p>The number of hits declared in the last six minutes for each station represents the high order bits of the metric, while the current minute pulse amplitude represents the low order bits. Only if the metric is above a defined threshold is the station signal considered acceptable. The metric is also used by the autotune function described below and reported in the timecode string.</p>
+ <p>The number of hits declared in the last 6 minutes for each station represents the high order bits of the metric, while the current minute pulse amplitude represents the low order bits. Only if the metric is above a defined threshold is the station signal considered acceptable. The metric is also 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 manual synchronization via oscilloscope and HF medium is good only to a millisecond under the best propagation conditions. The performance of the NTP daemon disciplined by this driver is clearly better than this, even under marginal conditions.</p>
<p>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>
<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 standard serial port using a level converter such as the CT-17. Further details are on the <a href="../audio.html">Reference Clock Audio Drivers</a> page.</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 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 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 signal metric as descrobed above. 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>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 signal metric as described above. 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>The mitigation procedure selects the frequency and station with the highest valid metric, 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 no station is above the metric, the rotating probes continue until a valid station is found.</p>
<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 moonlight (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>
+ <p>As expected, the lower frequencies prevail when the ray path is in moonlight (0100-1300 UTC) and the higher frequencies when the path is in sunlight (1300-0100 UTC). Note three periods in the figure show zero frequency when signals are below the minimum for all frequencies and stations.</p>
<h4>Debugging Aids</h4>
<p>The most convenient way to track the driver 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 driver is not disciplining the system clock. If the debugging trace feature (<tt>-d</tt> on the <tt>ntpd</tt> command line) is enabled, the driver 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>wwv</tt> for convenient filtering with the Unix <tt>grep</tt> command.</p>
<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>
Audio Device: <tt>/dev/audio</tt> and <tt>/dev/audioctl</tt>
<h4>Description</h4>
<p>This driver synchronizes the computer time using the Inter-Range Instrumentation Group (IRIG) standard time distribution signal. 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 of a workstation or PC.</p>
- <p>The driver requires an audio codec or sound card with sampling rate 8 kHz and <font face="symbol">m</font>-law companding to demodulate the data. This is the same standard as used by the telephone industry and is supported by most hardware and operating systems, including Solaris, FreeBSD and Linux, among others. In this implementation, only one audio driver and codec can be supported on a single machine. In order to assure reliable signal capture, the codec frequency error must be less than 125 PPM (.0125 percent). If necessary, the <tt>tinker codec</tt> configuration command can be used to bracket the codec frequency to this range.</p>
- <p>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 terminated at the line-in port with a 50-Ohm resistor to avoid overloading the codec. 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 automatically rejects the data and declare itself unsynchronized in this case. Some devices, in particular Spectracom radio/satellite clocks, provide additional year and status indication; other devices may not.</p>
+ <p>The driver requires an audio codec or sound card with sampling rate 8 kHz and <font face="symbol">m</font>-law companding to demodulate the data. This is the same standard as used by the telephone industry and is supported by most hardware and operating systems, including Solaris, FreeBSD and Linux, among others. In this implementation, only one audio driver and codec can be supported on a single machine. In order to assure reliable signal capture, the codec frequency error must be less than 250 PPM (.025 percent). If necessary, the <tt>tinker codec</tt> configuration command can be used to bracket the codec frequency to this range.</p>
+ <p>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 terminated at the line-in port with a 50-Ohm resistor to avoid overloading the codec. 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 automatically rejects the data and declares itself unsynchronized in this case. Some devices, in particular Spectracom radio/satellite clocks, provide additional year and status indication; other devices may not.</p>
<p>In general and without calibration, the driver is accurate within 500 <font face="symbol">m</font>s relative to the IRIG time. After calibrating relative to the PPS signal from a GPS receiver, the mean offset with a 2.4-GHz P4 running FreeBSD 6.1 is less than 20 <font face="symbol">m</font>s with standard deviation 10 <font face="symbol">m</font>s. Most of this is due to residuals after filtering and averaging the raw codec samples, which have an inherent jitter of 125 <font face="symbol">m</font>s. The processor load due to the driver is 0.6 percent on the P4.</p>
<p>However, be acutely aware that the accuracy with Solaris 2.8 and 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. This distortion is especially prevalent with Sun Blade 1000 and possibly other systems.</p>
<p>The driver 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>
<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:
+ 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 (<tt>-d</tt> on the <tt>ntpd</tt> 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 20 0.3 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. Note that the time of day is for the previous minute, not the current time. The status indicator and year are not produced by some IRIG devices and appear as zeros. Following these fields are the carrier amplitude (0-6000), codec gain (0-255), modulation index (0-1), time constant (4-20), carrier phase error (0±0.5) and carrier frequency error (PPM). The last field is the on-time timestamp in NTP format.</p>
<p>The error flags are defined as follows in hex:</p>
<p>The driver 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>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>
<h4>Technical Overview</h4>
- <p>The driver processes 8-kHz <font face="symbol">m</font>-law companded codec samplesusing 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. The single format B burst is considered correct only if every character matches its repetition in the burst. For the eight format A bursts, a majority decoder requires more than half of the 16 repetitions for each digit decode to the same value. Every character in every burst provides an independent timestamp upon arrival with a potential total of 60 timestamps for each minute.</p>
+ <p>The driver processes 8-kHz <font face="symbol">m</font>-law companded codec samples 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. The single format B burst is considered correct only if every character matches its repetition in the burst. For the eight format A bursts, a majority decoder requires more than half of the 16 repetitions for each digit decode to the same value. Every character in every burst provides an independent timestamp upon arrival with a potential total of 60 timestamps for each minute.</p>
<p>The CHU timecode format is described on the <a href="http://inms-ienm.nrc-cnrc.gc.ca/time_services/chu_e.html">CHU website</a>. A timecode is assembled when all bursts have been received in each minute. The timecode is considered valid and the clock set when at least one valid format B burst has been decoded and the majority decoder declares success. Once the driver has synchronized for the first time, it will appear reachable and selectable to discipline the system clock. It is normal on occasion to miss a minute or two due to signal fades or noise. If eight successive minutes are missed, the driver is considered unreachable and the system clock will free-wheel at the latest determined frequency offset. 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 with long intervals between updates, it is unlikely that the system clock will drift more than a few milliseconds during periods of signal loss.</p>
<h4>Baseband Signal Processing</h4>
<p>The program consists of four major parts: the DSP modem, maximum-likelihood UART, burst assembler and majority decoder. The DSP modem demodulates Bell 103 modem answer-frequency signals; that is, frequency-shift keyed (FSK) tones of 2225 Hz (mark) and 2025 Hz (space). It consists of a 500-Hz bandpass filter centered on 2125 Hz followed by a limiter/discriminator and raised-cosine lowpass filter optimized for the 300-b/s data rate. </p>
<p>The burst distance is incremented by one for each bit in the first block that matches the corresponding bit in the second block and decremented by one otherwise. In a format B burst the second block is bit-inverted relative to the first, so a perfect burst of five 8-bit characters has distance -40. In a format A burst the two blocks are identical, so a perfect burst has distance +40. Format B bursts must be perfect to be acceptable; however, format A bursts, which are further processed by the majority decoder, are acceptable if the distance is at least 28.</p>
<h4>Majority Decoder</h4>
<p>Each minute of transmission includes eight format A bursts containing two timecodes for each second from 32 through 39. The majority decoder uses a decoding matrix of ten rows, one for each digit position in the timecode, and 16 columns, one for each 4-bit code combination that might be decoded at that position. In order to use the character timestamps, it is necessary to reliably determine the second number of each burst. In a valid burst, the last digit of the two timecodes in the burst must match and the value must be in the range 2-9 and greater than in the previous burst.</p>
- <p>As each digit of a valid burst is processed, the value at the row corresponding to the digit position in the timecode and column corresponding to the code found at that position is incremented. At the end of the minute, each row of the decoding matrix encodes the number of occurences of each code found at the corresponding position.</p>
- <p>The maximum over all occurences at each digit position is the distance for that position and the corresponding code is the maximum-likelihood digit. If the distance is not more than half the total number of occurences, the decoder assumes a soft error and discards all information collected during the minute. The decoding distance is defined as the sum of the distances over the first nine digits; the tenth digit varies over the seconds and is uncounted.</p>
+ <p>As each digit of a valid burst is processed, the value at the row corresponding to the digit position in the timecode and column corresponding to the code found at that position is incremented. At the end of the minute, each row of the decoding matrix encodes the number of occurrences of each code found at the corresponding position.</p>
+ <p>The maximum over all occurrences at each digit position is the distance for that position and the corresponding code is the maximum-likelihood digit. If the distance is not more than half the total number of occurrences, the decoder assumes a soft error and discards all information collected during the minute. The decoding distance is defined as the sum of the distances over the first nine digits; the tenth digit varies over the seconds and is uncounted.</p>
<p>The result of the majority decoder is a nine-digit timecode representing the maximum-likelihood candidate for the transmitted timecode in that minute. Note that the second and fraction within the minute are always zero and that the actual reference point to calculate timestamp offsets is backdated to the first second of the minute. At this point the timecode block is reformatted and the year, days, hours and minutes extracted along with other information from the format B burst, including DST state, DUT1 correction and leap warning. The reformatting operation checks the timecode for invalid code combinations that might have been left by the majority decoder and rejects the entire timecode if found.</p>
<p>If the timecode is valid, it is passed to the reference clock interface along with the backdated timestamps accumulated over the minute. A perfect set of eight bursts could generate as many as 80 timestamps, but the maximum the interface can handle is 60. These are processed using a median filter and trimmed-mean average, so the resulting system clock correction is usually much better than would otherwise be the case with radio noise, UART jitter and occasional burst errors.</p>
<h4>Autotune</h4>
<p>where <tt>stat</tt> is the status code, <tt>sig</tt> the character span, <tt>n</tt> the number of characters in the burst (9-11) and <tt>b</tt> the burst distance (0-40). Good bursts will have spans of a 800 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. Note that each character of the burst is encoded as two digits in nibble-swapped order.</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>
+ 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 yyyy ddd hh:mm:ss lw dst du lset agc rfrq bcnt dist tsmp
s sync indicator
projects / thirdparty / ntp.git / commitdiff
