<p>As long as the clock is set or verified, the system clock offsets are provided once each second to the reference clock interface, where they are saved in a buffer. At the end of each minute the buffer samples are groomed by the median filter and trimmed-mean averaging functions. Using these functions, 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>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 is first started, the NTP reference identifier appears as <tt>NONE</tt>. 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, even if the broadcast signal fades to obscurity. A consequence of this design is that, once the clock is set, the time and frequency are disciplined only by the second sync 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>However, as long as the clock has once been set correctly and allowed to converge on the intrinsic codec clock frequency, it will continue to read correctly after a period of signal loss. Assuming the clock frequency can be disciplined within 1 PPM, it can coast without signals for several days without exceeding that 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 appears 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 at all.</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>
+ <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 and for a holdover interval of about 30 minutes after signal loss on both stations on all frequencies. A consequence of this design is that, once the clock is set, the time and frequency are disciplined only by the second sync 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>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 will appear unreachable for synchronization purposes after the holdover intervall. The local clock driver can be used to extend the holdover interval if required.</p>
+ <p>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 appears 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 at all.</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>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 below. 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. A missing <tt>mode</tt> keyword or a zero argument leaves the interface disabled.</p>
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<li class='inline'><a href='refclock.html'>Reference Clock Drivers</a><br>\\r
-<li class='inline'><a href='driver7.html'>Radio CHU Audio Demodulator/Decoder</a><br>\\r
-<li class='inline'><a href='driver36.html'>Radio WWV/H Audio Demodulator/Decoder</a><br>\\r
-<li class='inline'><a href='driver6.html'>IRIG Audio Decoder</a>\\r
+<li class='inline'><a href='drivers/driver7.html'>Radio CHU Audio Demodulator/Decoder</a><br>\
+<li class='inline'><a href='drivers/driver36.html'>Radio WWV/H Audio Demodulator/Decoder</a><br>\
+<li class='inline'><a href='drivers/driver6.html'>IRIG Audio Decoder</a>\
</ul>")
\ No newline at end of file
projects / thirdparty / ntp.git / commitdiff
