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5 <title>Radio WWV/H Audio Demodulator/Decoder</title>
8 <h3>Radio WWV/H Audio Demodulator/Decoder</h3>
13 Address: 127.127.36.<i>u</i> <br>
14 Reference ID: <tt>WWV</tt> or <tt>WWVH</tt> <br>
15 Driver ID: <tt>WWV_AUDIO</tt> <br>
16 Autotune Port: <tt>/dev/icom</tt>; 1200/9600 baud, 8-bits, no
18 Audio Device: <tt>/dev/audio</tt> and <tt>/dev/audioctl</tt>
22 This driver synchronizes the computer time using data encoded in
23 shortwave radio transmissions from NIST time/frequency stations WWV
24 in Ft. Collins, CO, and WWVH in Kauai, HI. Transmissions are made
25 continuously on 2.5, 5, 10, 15 and 20 MHz. An ordinary shortwave
26 receiver can be tuned manually to one of these frequencies or, in
27 the case of ICOM receivers, the receiver can be tuned automatically
28 by the driver as propagation conditions change throughout the day
29 and night. The performance of this driver when tracking one of the
30 stations is ordinarily better than 1 ms in time with frequency
31 drift less than 0.5 PPM when not tracking either station.
33 <p>The demodulation and decoding algorithms used by this driver are
34 based on a machine language program developed for the TAPR DSP93
35 DSP unit, which uses the TI 320C25 DSP chip. The analysis, design
36 and performance of the program running on this unit is described
37 in: Mills, D.L. A precision radio clock for WWV transmissions.
38 Electrical Engineering Report 97-8-1, University of Delaware,
39 August 1997, 25 pp. Available from <a href=
40 "http://www.eecis.udel.edu/~mills/reports.htm">
41 www.eecis.udel.edu/~mills/reports.htm</a>. For use in this driver,
42 the original program was rebuilt in the C language and adapted to
43 the NTP driver interface. The algorithms have been modified
44 somewhat to improve performance under weak signal conditions and to
45 provide an automatic station identification feature.</p>
47 <p>This driver incorporates several features in common with other
48 audio drivers such as described in the <a href="driver7.htm">Radio
49 CHU Audio Demodulator/Decoder</a> and the <a href="driver6.htm">
50 IRIG Audio Decoder</a> pages. They include automatic gain control
51 (AGC), selectable audio codec port and signal monitoring
52 capabilities. For a discussion of these common features, as well as
53 a guide to hookup, debugging and monitoring, see the <a href=
54 "audio.htm">Reference Clock Audio Drivers</a> page.</p>
56 <p>The WWV signal format is described in NIST Special Publication
57 432 (Revised 1990). It consists of three elements, a 5-ms, 1000-Hz
58 pulse, which occurs at the beginning of each second, a 800-ms,
59 1000-Hz pulse, which occurs at the beginning of each minute, and a
60 pulse-width modulated 100-Hz subcarrier for the data bits, one bit
61 per second. The WWVH format is identical, except that the 1000-Hz
62 pulses are sent at 1200 Hz. Each minute encodes nine BCD digits for
63 the time of century plus seven bits for the daylight savings time
64 (DST) indicator, leap warning indicator and DUT1 correction.</p>
66 <h4>Program Architecture</h4>
68 <p>As in the original program, the clock discipline is modelled as
69 a Markov process, with probabilistic state transitions
70 corresponding to a conventional clock and the probabilities of
71 received decimal digits. The result is a performance level which
72 results in very high accuracy and reliability, even under
73 conditions when the minute beep of the signal, normally its most
74 prominent feature, can barely be detected by ear with a shortwave
77 <p>The analog audio signal from the shortwave radio is sampled at
78 8000 Hz and converted to digital representation. The 1000/1200-Hz
79 pulses and 100-Hz subcarrier are first separated using two IIR
80 filters, a 600-Hz bandpass filter centered on 1100 Hz and a 150-Hz
81 lowpass filter. The minute sync pulse is extracted using a 800-ms
82 synchronous matched filter and pulse grooming logic which
83 discriminates between WWV and WWVH signals and noise. The second
84 sync pulse is extracted using a 5-ms FIR matched filter and
85 8000-stage comb filter.</p>
87 <p>The phase of the 100-Hz subcarrier relative to the second sync
88 pulse is fixed at the transmitter; however, the audio highpass
89 filter in most radios affects the phase response at 100 Hz in
90 unpredictable ways. The driver adjusts for each radio using two
91 170-ms synchronous matched filters. The I (in-phase) filter is used
92 to demodulate the subcarrier envelope, while the Q
93 (quadrature-phase) filter is used in a tracking loop to discipline
94 the codec sample clock and thus the demodulator phase.</p>
96 <p>The data bit probabilities are determined from the subcarrier
97 envelope using a threshold-corrected slicer. The averaged envelope
98 amplitude 30 ms from the beginning of the second establishes the
99 minimum (noise floor) value, while the amplitude 200 ms from the
100 beginning establishes the maximum (signal peak) value. The slice
101 level is midway between these two values. The negative-going
102 envelope transition at the slice level establishes the length of
103 the data pulse, which in turn establish probabilities for binary
104 zero (P0) or binary one (P1). The values are established by linear
105 interpolation between the pulse lengths for P0 (300 ms) and P1 (500
106 ms) so that the sum is equal to one. If the driver has not
107 synchronized to the minute pulse, or if the data bit amplitude,
108 signal/noise ratio (SNR) or length are below thresholds, the bit is
109 considered invalid and all three probabilities are set to zero.</p>
111 <p>The difference between the P1 and P0 probabilities, or
112 likelihood, for each data bit is exponentially averaged in a set of
113 60 accumulators, one for each second, to determine the semi-static
114 miscellaneous bits, such as DST indicator, leap second warning and
115 DUT1 correction. In this design, an average value larger than a
116 positive threshold is interpreted as a hit on one and a value
117 smaller than a negative threshold as a hit on zero. Values between
118 the two thresholds, which can occur due to signal fades or loss of
119 signal, are interpreted as a miss, and result in no change of
122 <p>The BCD digit in each digit position of the timecode is
123 represented as four data bits, all of which must be valid for the
124 digit itself to be considered valid. If so, the bits are correlated
125 with the bits corresponding to each of the valid decimal digits in
126 this position. If the digit is invalid, the correlated value for
127 all digits in this position is assumed zero. In either case, the
128 values for all digits are exponentially averaged in a likelihood
129 vector associated with this position. The digit associated with the
130 maximum over all of the averaged values then becomes the maximum
131 likelihood selection for this position and the ratio of the maximum
132 over the next lower value becomes the likelihood ratio.</p>
134 <p>The decoding matrix contains nine row vectors, one for each
135 digit position. Each row vector includes the maximum likelihood
136 digit, likelihood vector and other related data. The maximum
137 likelihood digit for each of the nine digit positions becomes the
138 maximum likelihood time of the century. A built-in transition
139 function implements a conventional clock with decimal digits that
140 count the minutes, hours, days and years, as corrected for leap
141 seconds and leap years. The counting operation also rotates the
142 likelihood vector corresponding to each digit as it advances. Thus,
143 once the clock is set, each clock digit should correspond to the
144 maximum likelihood digit as transmitted.</p>
146 <p>Each row of the decoding matrix also includes a compare counter
147 and the difference (modulo the radix) between the current clock
148 digit and most recently determined maximum likelihood digit. If a
149 digit likelihood exceeds the decision level and the difference is
150 constant for a number of successive minutes in any row, the maximum
151 likelihood digit replaces the clock digit in that row. When this
152 condition is true for all rows and the second epoch has been
153 reliably determined, the clock is set (or verified if it has
154 already been set) and delivers correct time to the integral second.
155 The fraction within the second is derived from the logical master
156 clock, which runs at 8000 Hz and drives all system timing
159 <p>The logical master clock is derived from the audio codec clock.
160 Its frequency is disciplined by a frequency-lock loop (FLL) which
161 operates independently of the data recovery functions. At averaging
162 intervals determined by the measured jitter, the frequency error is
163 calculated as the difference between the most recent and the
164 current second epoch divided by the interval. The sample clock
165 frequency is then corrected by this amount using an exponential
166 average. When first started, the frequency averaging interval is
167 eight seconds, in order to compensate for intrinsic codec clock
168 frequency offsets up to 125 PPM. Under most conditions, the
169 averaging interval doubles in stages from the initial value to over
170 1000 seconds, which results in an ultimate frequency precision of
171 0.125 PPM, or about 11 ms/day.</p>
173 <p>It is important that the logical clock frequency is stable and
174 accurately determined, since in most applications the shortwave
175 radio will be tuned to a fixed frequency where WWV or WWVH signals
176 are not available throughout the day. In addition, in some parts of
177 the US, especially on the west coast, signals from either or both
178 WWV and WWVH may be available at different times or even at the
179 same time. Since the propagation times from either station are
180 almost always different, each station must be reliably identified
181 before attempting to set the clock.</p>
183 <p>Station identification uses the 800-ms minute pulse transmitted
184 by each station. In the acquisition phase the entire minute is
185 searched using both the WWV and WWVH using matched filters and a
186 pulse gate discriminator similar to that found in radar acquisition
187 and tracking receivers. The peak amplitude found determines a range
188 gate and window where the next pulse is expected to be found. The
189 minute is scanned again to verify the peak is indeed in the window
190 and with acceptable amplitude, SNR and jitter. At this point the
191 receiver begins to track the second sync pulse and operate as above
192 until the clock is set.</p>
194 <p>Once the minute is synchronized, the range gate is fixed and
195 only energy within the window is considered for the minute sync
196 pulse. A compare counter increments by one if the minute pulse has
197 acceptable amplitude, SNR and jitter and decrements otherwise. This
198 is used as a quality indicator and reported in the timecode and
199 also for the autotune function described below.</p>
203 <p>It is the intent of the design that the accuracy and stability
204 of the indicated time be limited only by the characteristics of the
205 propagation medium. Conventional wisdom is that synchronization via
206 the HF medium is good only to a millisecond under the best
207 propagation conditions. The performance of the NTP daemon
208 disciplined by the driver is clearly better than this, even under
209 marginal conditions. Ordinarily, with marginal to good signals and
210 a frequency averaging interval of 1024 s, the frequency is
211 stabilized within 0.1 PPM and the time within 125 <font face=
212 "Symbol">m</font>s. The frequency stability characteristic is
213 highly important, since the clock may have to free-run for several
214 hours before reacquiring the WWV/H signal.</p>
216 <p>The expected accuracy over a typical day was determined using
217 the DSP93 and an oscilloscope and cesium oscillator calibrated with
218 a GPS receiver. With marginal signals and allowing 15 minutes for
219 initial synchronization and frequency compensation, the time
220 accuracy determined from the WWV/H second sync pulse was reliably
221 within 125 <font face="Symbol">m</font>s. In the particular DSP-93
222 used for program development, the uncorrected CPU clock frequency
223 offset was 45.8±0.1 PPM. Over the first hour after initial
224 synchronization, the clock frequency drifted about 1 PPM as the
225 frequency averaging interval increased to the maximum 1024 s. Once
226 reaching the maximum, the frequency wandered over the day up to 1
227 PPM, but it is not clear whether this is due to the stability of
228 the DSP-93 clock oscillator or the changing height of the
229 ionosphere. Once the frequency had stabilized and after loss of the
230 WWV/H signal, the frequency drift was less than 0.5 PPM, which is
231 equivalent to 1.8 ms/h or 43 ms/d. This resulted in a step phase
232 correction up to several milliseconds when the signal returned.</p>
234 <p>The measured propagation delay from the WWV transmitter at
235 Boulder, CO, to the receiver at Newark, DE, is 23.5±0.1 ms.
236 This is measured to the peak of the pulse after the second sync
237 comb filter and includes components due to the ionospheric
238 propagation delay, nominally 8.9 ms, communications receiver delay
239 and program delay. The propagation delay can be expected to change
240 about 0.2 ms over the day, as the result of changing ionosphere
241 height. The DSP93 program delay was measured at 5.5 ms, most of
242 which is due to the 400-Hz bandpass filter and 5-ms matched filter.
243 Similar delays can be expected of this driver.</p>
245 <h4>Program Operation</h4>
247 The driver begins operation immediately upon startup. It first
248 searches for one or both of the stations WWV and WWVH and attempts
249 to acquire minute sync. This may take some fits and starts, as the
250 driver expects to see three consecutive minutes with good signals
251 and low jitter. If the autotune function is active, the driver will
252 rotate over all five frequencies and both WWV and WWVH stations
253 until three good minutes are found.
255 <p>The driver then acquires second sync, which can take up to
256 several minutes, depending on signal quality. At the same time the
257 driver accumulates likelihood values for each of the nine digits of
258 the clock, plus the seven miscellaneous bits included in the WWV/H
259 transmission format. The minute units digit is decoded first and,
260 when five repetitions have compared correctly, the remaining eight
261 digits are decoded. When five repetitions of all nine digits have
262 decoded correctly, which normally takes 15 minutes with good
263 signals and up to an hour when buried in noise, and the second sync
264 alarm has not been raised for two minutes, the clock is set (or
265 verified) and is selectable to discipline the system clock.</p>
267 <p>As long as the clock is set or verified, the system clock
268 offsets are provided once each second to the reference clock
269 interface, where they are saved in a buffer. At the end of each
270 minute, the buffer samples are groomed by the median filter and
271 trimmed-mean averaging functions. Using these functions, the system
272 clock can in principle be disciplined to a much finer resolution
273 than the 125-<font face="Symbol">m</font>s sample interval would
274 suggest, although the ultimate accuracy is probably limited by
275 propagation delay variations as the ionspheric height varies
276 throughout the day and night.</p>
278 <p>As long as signals are available, the clock frequency is
279 disciplined for use during times when the signals are unavailable.
280 The algorithm refines the frequency offset using increasingly
281 longer averaging intervals to 1024 s, where the precision is about
282 0.1 PPM. With good signals, it takes well over two hours to reach
283 this degree of precision; however, it can take many more hours than
284 this in case of marginal signals. Once reaching the limit, the
285 algorithm will follow frequency variations due to temperature
286 fluctuations and ionospheric height variations.</p>
288 <p>It may happen as the hours progress around the clock that WWV
289 and WWVH signals may appear alone, together or not at all. When the
290 driver is first started, the NTP reference identifier appears as
291 <tt>NONE</tt>. When the driver has acquired one or both stations
292 and mitigated which one is best, it sets the station identifier in
293 the timecode as described below. In addition, the NTP reference
294 identifier is set to the station callsign. If the propagation
295 delays has been properly set with the <tt>fudge time1</tt> (WWV)
296 and <tt>fudge time2</tt> (WWVH) commands in the configuration file,
297 handover from one station to the other will be seamless.</p>
299 <p>Once the clock has been set for the first time, it will appear
300 reachable and selectable to discipline the system clock, even if
301 the broadcast signal fades to obscurity. A consequence of this
302 design is that, once the clock is set, the time and frequency are
303 disciplined only by the second sync pulse and the clock digits
304 themselves are driven by the clock state machine and ordinarily
305 never changed. However, as long as the clock is set correctly, it
306 will continue to read correctly after a period of signal loss, as
307 long as it does not drift more than 500 ms from the correct time.
308 Assuming the clock frequency can be disciplined within 1 PPM, the
309 clock could coast without signals for some 5.8 days without
310 exceeding that limit. If for some reason this did happen, the clock
311 would be in the wrong second and would never resynchronize. To
312 protect against this most unlikely situation, if after four days
313 with no signals, the clock is considered unset and resumes the
314 synchronization procedure from the beginning.</p>
316 <p>To work well, the driver needs a communications receiver with
317 good audio response at 100 Hz. Most shortwave and communications
318 receivers roll off the audio response below 250 Hz, so this can be
319 a problem, especially with receivers using DSP technology, since
320 DSP filters can have very fast rolloff outside the passband. Some
321 DSP transceivers, in particular the ICOM 775, have a programmable
322 low frequency cutoff which can be set as low as 80 Hz. However,
323 this particular radio has a strong low frequency buzz at about 10
324 Hz which appears in the audio output and can affect data recovery
325 under marginal conditions. Although not tested, it would seem very
326 likely that a cheap shortwave receiver could function just as well
327 as an expensive communications receiver.</p>
331 <p>The driver includes provisions to automatically tune the radio
332 in response to changing radio propagation conditions throughout the
333 day and night. The radio interface is compatible with the ICOM CI-V
334 standard, which is a bidirectional serial bus operating at TTL
335 levels. The bus can be connected to a serial port using a level
336 converter such as the CT-17. The serial port speed is presently
337 compiled in the program, but can be changed in the driver source
340 <p>Each ICOM radio is assigned a unique 8-bit ID select code,
341 usually expressed in hex format. To activate the CI-V interface,
342 the <tt>mode</tt> keyword of the <tt>server</tt> configuration
343 command specifies a nonzero select code in decimal format. A table
344 of ID select codes for the known ICOM radios is given below. Since
345 all ICOM select codes are less than 128, the high order bit of the
346 code is used by the driver to specify the baud rate. If this bit is
347 not set, the rate is 9600 bps for the newer radios; if set, the
348 rate is 1200 bps for the older radios. A missing <tt>mode</tt>
349 keyword or a zero argument leaves the interface disabled.</p>
351 <p>If specified, the driver will attempt to open the device <tt>
352 /dev/icom</tt> and, if successful will activate the autotune
353 function and tune the radio to each operating frequency in turn
354 while attempting to acquire minute sync from either WWV or WWVH.
355 However, the driver is liberal in what it assumes of the
356 configuration. If the <tt>/dev/icom</tt> link is not present or the
357 open fails or the CI-V bus or radio is inoperative, the driver
358 quietly gives up with no harm done.</p>
360 <p>Once acquiring minute sync, the driver operates as described
361 above to set the clock. However, during seconds 59, 0 and 1 of each
362 minute it tunes the radio to one of the five broadcast frequencies
363 to measure the sync pulse and data pulse amplitudes and SNR and
364 update the compare counter. Each of the five frequencies are probed
365 in a five-minute rotation to build a database of current
366 propagation conditions for all signals that can be heard at the
367 time. At the end of each rotation, a mitigation procedure scans the
368 database and retunes the radio to the best frequency and station
369 found. For this to work well, the radio should be set for a fast
370 AGC recovery time. This is most important while tracking a strong
371 signal, which is normally the case, and then probing another
372 frequency, which may have much weaker signals.</p>
374 <p>Reception conditions for each frequency and station are
375 evaluated according to a metric which considers the minute sync
376 pulse amplitude, SNR and jitter, as well as, the data pulse
377 amplitude and SNR. The minute pulse is evaluated at second 0, while
378 the data pulses are evaluated at seconds 59 and 1. The results are
379 summarized in a scoreboard of three bits</p>
382 <dt><tt>0x0001</tt></dt>
384 <dd>Jitter exceeded. The difference in epoches between the last
385 minute sync pulse and the current one exceeds 50 ms (400
388 <dt><tt>0x0002</tt></dt>
390 <dd>Minute pulse error. For the minute sync pulse in second 0,
391 either the amplitude or SNR is below threshold (2000 and 20 dB,
394 <dt><tt>0x0004</tt></dt>
396 <dd>Minute pulse error. For both of the data pulses in seocnds 59
397 and 1, either the amplitude or SNR is below threshold (1000 and 10
398 dB, respectively).</dd>
401 <p>If none of the scoreboard bits are set, the compare counter is
402 increased by one to a maximum of six. If any bits are set, the
403 counter is decreased by one to a minimum of zero. At the end of
404 each minute, the frequency and station with the maximum compare
405 count is chosen, with ties going to the highest frequency.</p>
409 <p>The autotune process produces diagnostic information along with
410 the timecode. This is very useful for evaluating the performance of
411 the algorithm, as well as radio propagation conditions in general.
412 The message is produced once each minute for each frequency in turn
413 after minute sync has been acquired.</p>
415 <p><tt>wwv5 port agc wwv wwvh</tt></p>
417 <p>where <tt>port</tt> and <tt>agc</tt> are the audio port and
418 gain, respectively, for this frequency and <tt>wwv</tt> and <tt>
419 wwvh</tt> are two sets of fields, one each for WWV and WWVH. Each
420 of the two fields has the format</p>
422 <p><tt>ident score comp sync/snr/jitr</tt></p>
424 <p>where <tt>ident</tt>encodes the station (<tt>C</tt> for WWV,
425 <tt>H</tt> for WWVH) and frequency (2, 5, 10, 15 and 20), <tt>
426 score</tt> is the scoreboard described above, <tt>comp</tt> is the
427 compare counter, <tt>sync</tt> is the minute sync pulse amplitude,
428 <tt>snr</tt> the SNR of the pulse and <tt>jitr</tt> is the sample
429 difference between the current epoch and the last epoch. An example
432 <p><tt>wwv5 2 111 C20 0100 6 8348/30.0/-3 H20 0203 0
433 22/-12.4/8846</tt></p>
435 <p>Here the radio is tuned to 20 MHz and the line-in port AGC is
436 currently 111 at that frequency. The message contains a report for
437 WWV (<tt>C20</tt>) and WWVH (<tt>H20</tt>). The WWV report
438 scoreboard is 0100 and the compare count is 6, which suggests very
439 good reception conditions, and the minute sync amplitude and SNR
440 are well above thresholds (2000 and 20 dB, respectively). Probably
441 the most sensitive indicator of reception quality is the jitter, -3
442 samples, which is well below threshold (50 ms or 400 samples).
443 While the message shows solid reception conditions from WWV, this
444 is not the case for WWVH. Both the minute sync amplitude and SNR
445 are below thresholds and the jitter is above threshold.</p>
447 <p>A sequence of five messages, one for each minute, might appear
451 wwv5 2 95 C2 0107 0 164/7.2/8100 H2 0207 0 80/-5.5/7754
452 wwv5 2 99 C5 0104 0 3995/21.8/395 H5 0207 0 27/-9.3/18826
453 wwv5 2 239 C10 0105 0 9994/30.0/2663 H10 0207 0 54/-16.1/-529
454 wwv5 2 155 C15 0103 3 3300/17.8/-1962 H15 0203 0 236/17.0/4873
455 wwv5 2 111 C20 0100 6 8348/30.0/-3 H20 0203 0 22/-12.4/8846
458 <p>Clearly, the only frequencies that are available are 15 MHz and
459 20 MHz and propagation may be failing for 15 MHz. However, minute
460 sync pulses are being heard on 5 and 10 MHz, even though the data
461 pulses are not. This is typical of late afternoon when the maximum
462 usable frequency (MUF) is falling and the ionospheric loss at the
463 lower frequencies is beginning to decrease.</p>
465 <h4>Debugging Aids</h4>
467 <p>The most convenient way to track the driver status is using the
468 <tt>ntpq</tt> program and the <tt>clockvar</tt> command. This
469 displays the last determined timecode and related status and error
470 counters, even when the driver is not discipline the system clock.
471 If the debugging trace feature (<tt>-d</tt> on the <tt>ntpd</tt>
472 command line)is enabled, the driver produces detailed status
473 messages as it operates. If the <tt>fudge flag 4</tt> is set, these
474 messages are written to the <tt>clockstats</tt> file. All messages
475 produced by this driver have the prefix <tt>chu</tt> for convenient
476 filtering with the Unix <tt>grep</tt> command.</p>
478 <p>In the following descriptions the units of amplitude, phase,
479 probability and likelihood are normalized to the range 0-6000 for
480 convenience. In addition, the signal/noise ratio (SNR) and
481 likelihood ratio are measured in decibels and the words with bit
482 fields are in hex. Most messages begin with a leader in the
483 following format:</p>
485 <p><tt>wwvn ss stat sigl</tt></p>
487 <p>where <tt>wwvn</tt> is the message code, <tt>ss</tt> the second
488 of minute, <tt>stat</tt> the driver status word and <tt>sigl</tt>
489 the second sync pulse amplitude. A full explanation of the status
490 bits is contained in the driver source listing; however, the
491 following are the most useful for debugging.</p>
494 <dt><tt>0x0001</tt></dt>
496 <dd>Minute sync. Set when the decoder has identified a station and
497 acquired the minute sync pulse.</dd>
499 <dt><tt>0x0002</tt></dt>
501 <dd>Second sync. Set when the decoder has acquired the second sync
502 pulse and within 125 <font face="Symbol">m</font>s of the correct
505 <dt><tt>0x0004</tt></dt>
507 <dd>Minute unit sync. Set when the decoder has reliably determined
508 the unit digit of the minute.</dd>
510 <dt><tt>0x0008</tt></dt>
512 <dd>Clock set. Set when the decoder has reliably determined all
513 nine digits of the timecode and is selectable to discipline the
517 <p>With debugging enabled the driver produces messages in the
518 following formats:</p>
520 <p>Format <tt>wwv8</tt> messages are produced once per minute by
521 the WWV and WWVH station processes before minute sync has been
522 acquired. They show the progress of identifying and tracking the
523 minute pulse of each station.</p>
525 <p><tt>wwv8 port agc ident comp ampl snr epoch jitr offs</tt></p>
527 <p>where <tt>port</tt> and <tt>agc</tt> are the audio port and
528 gain, respectively. The <tt>ident</tt>encodes the station
529 (<tt>C</tt> for WWV, <tt>H</tt> for WWVH) and frequency (2, 5, 10,
530 15 and 20). For the encoded frequency, <tt>comp</tt> is the compare
531 counter, <tt>ampl</tt> the pulse amplitude, <tt>snr</tt> the SNR,
532 <tt>epoch</tt> the sample number of the minute pulse in the minute,
533 <tt>jitr</tt> the change since the last <tt>epoch</tt> and <tt>
534 offs</tt> the minute pulse offset relative to the second pulse. An
537 <p><tt>wwv8 2 127 C15 2 9247 30.0 18843 -1 1</tt><br>
538 <tt>wwv8 2 127 H15 0 134 -2.9 19016 193 174</tt></p>
540 <p>Here the radio is tuned to 15 MHz and the line-in port AGC is
541 currently 127 at that frequency. The driver has not yet acquired
542 minute sync, WWV has been heard for at least two minutes, and WWVH
543 is in the noise. The WWV minute pulse amplitude and SNR are well
544 above the threshold (2000 and 6 dB, respectively) and the minute
545 epoch has been determined -1 sample relative to the last one and 1
546 sample relative to the second sync pulse. The compare counter has
547 incrmented to two; when it gets to three, minute sync has been
550 <p>Format <tt>wwv3</tt> messages are produced after minute sync has
551 been acquired and until the seconds unit digit is determined. They
552 show the results of decoding each bit of the transmitted
555 <p><tt>wwv3 ss stat sigl ampl phas snr prob like</tt></p>
557 <p>where <tt>ss</tt>, <tt>stat</tt> and <tt>sigl</tt> are as above,
558 <tt>ampl</tt> is the subcarrier amplitude, <tt>phas</tt> the
559 subcarrier phase, <tt>snr</tt> the subcarrier SNR, <tt>prob</tt>
560 the bit probability and <tt>like</tt> the bit likelihood. An
563 <p><tt>wwv3 28 0123 4122 4286 0 24.8 -5545 -1735</tt></p>
565 <p>Here the driver has acquired minute and second sync, but has not
566 yet determined the seconds unit digit. However, it has just decoded
567 bit 28 of the minute. The results show the second sync pulse
568 amplitude well over the threshold (500), subcarrier amplitude well
569 above the threshold (1000), good subcarrier tracking phase and SNR
570 well above the threshold (10 dB). The bit is almost certainly a
571 zero and the likelihood of a zero in this second is very high.</p>
573 <p>Format <tt>wwv4</tt> messages are produced for each of the nine
574 BCD timecode digits until the clock has been set or verified. They
575 show the results of decoding each digit of the transmitted
578 <p><tt>wwv4 ss stat sigl radx ckdig mldig diff cnt like
581 <p>where <tt>ss</tt>, <tt>stat</tt> and <tt>sigl</tt> are as above,
582 <tt>radx</tt> is the digit radix (3, 4, 6, 10), <tt>ckdig</tt> the
583 current clock digit, <tt>mldig</tt> the maximum likelihood digit,
584 <tt>diff</tt> the difference between these two digits modulo the
585 radix, <tt>cnt</tt> the compare counter, <tt>like</tt> the digit
586 likelihood and <tt>snr</tt> the likelihood ratio. An example
589 <p><tt>wwv4 8 010f 5772 10 9 9 0 6 4615 6.1</tt></p>
591 <p>Here the driver has previousl set or verified the clock. It has
592 just decoded the digit preceding second 8 of the minute. The digit
593 radix is 10, the current clock and maximum likelihood digits are
594 both 9, the likelihood is well above the threshold (1000) and the
595 likelihood function well above threshold (3.0 dB). Short of a
596 hugely unlikely probability conspiracy, the clock digit is most
599 <p>Format <tt>wwv2</tt> messages are produced at each master
600 oscillator frequency update, which starts at 8 s, but eventually
601 climbs to 1024 s. They show the progress of the algorithm as it
602 refines the frequency measurement to a precision of 0.1 PPM.</p>
604 <p><tt>wwv2 ss stat sigl avint avcnt avinc jitr delt freq</tt></p>
606 <p>where <tt>ss</tt>, <tt>stat</tt> and <tt>sigl</tt> are as above,
607 <tt>avint</tt> is the averaging interval, <tt>avcnt</tt> the
608 averaging interval counter, <tt>avinc</tt> the interval increment,
609 <tt>jitr</tt> the sample change between the beginning and end of
610 the interval, <tt>delt</tt> the computed frequency change and <tt>
611 freq</tt> the current frequency (PPM). An example is:</p>
613 <p><tt>wwv2 22 030f 5795 256 256 4 0 0.0 66.7</tt></p>
615 <p>Here the driver has acquired minute and second sync and set the
616 clock. The averaging interval has increased to 256 s on the way to
617 1024 s, has stayed at that interval for 4 averaging intervals, has
618 measured no change in frequency and the current frequency is 66.7
621 <p>If the CI-V interface for ICOM radios is active, a debug level
622 greater than 1 will produce a trace of the CI-V command and
623 response messages. Interpretation of these messages requires
624 knowledge of the CI-V protocol, which is beyond the scope of this
627 <h4>Monitor Data</h4>
629 When enabled by the <tt>filegen</tt> facility, every received
630 timecode is written to the <tt>clockstats</tt> file in the
634 sq yy ddd hh:mm:ss.fff ld du lset agc stn rfrq errs freq cons
642 fff millisecond of second
643 l leap second warning
645 dut DUT sign and magnitude
646 lset minutes since last set
648 ident station identifier and frequency
649 comp minute sync compare counter
650 errs bit error counter
651 freq frequency offset
655 The fields beginning with <tt>year</tt> and extending through <tt>
656 dut</tt> are decoded from the received data and are in fixed-length
657 format. The <tt>agc</tt> and <tt>lset</tt> fields, as well as the
658 following driver-dependent fields, are in variable-length format.
663 <dd>The sync indicator is initially <tt>?</tt> before the clock is
664 set, but turns to space when all nine digits of the timecode are
669 <dd>The quality character is a four-bit hexadecimal code showing
670 which alarms have been raised. Each bit is associated with a
671 specific alarm condition according to the following:
674 <dt><tt>0x8</tt></dt>
676 <dd>Sync alarm. The decoder may not be in correct second or minute
677 phase relative to the transmitter.</dd>
679 <dt><tt>0x4</tt></dt>
681 <dd>Error alarm. More than 30 data bit errors occurred in the last
684 <dt><tt>0x2</tt></dt>
686 <dd>Symbol alarm. The probability of correct decoding for a digit
687 or miscellaneous bit has fallen below the threshold.</dd>
689 <dt><tt>0x1</tt></dt>
691 <dd>Decoding alarm. A maximum likelihood digit fails to agree with
692 the current associated clock digit.</dd>
695 It is important to note that one or more of the above alarms does
696 not necessarily indicate a clock error, but only that the decoder
697 has detected a condition that may in future result in an
700 <dt><tt>yyyy ddd hh:mm:ss.fff</tt></dt>
702 <dd>The timecode format itself is self explanatory. Since the
703 driver latches the on-time epoch directly from the second sync
704 pulse, the fraction <tt>fff</tt>is always zero. Although the
705 transmitted timecode includes only the year of century, the
706 Gregorian year is augmented 2000 if the indicated year is less than
707 72 and 1900 otherwise.</dd>
711 <dd>The leap second warning is normally space, but changes to <tt>
712 L</tt> if a leap second is to occur at the end of the month of June
717 <dd>The DST state is <tt>S</tt> or <tt>D</tt> when standard time or
718 daylight time is in effect, respectively. The state is <tt>I</tt>
719 or <tt>O</tt> when daylight time is about to go into effect or out
720 of effect, respectively.</dd>
722 <dt><tt>dut</tt></dt>
724 <dd>The DUT sign and magnitude shows the current UT1 offset
725 relative to the displayed UTC time, in deciseconds.</dd>
727 <dt><tt>lset</tt></dt>
729 <dd>Before the clock is set, the interval since last set is the
730 number of minutes since the driver was started; after the clock is
731 set, this is number of minutes since the time was last verified
732 relative to the broadcast signal.</dd>
734 <dt><tt>agc</tt></dt>
736 <dd>The audio gain shows the current codec gain setting in the
737 range 0 to 255. Ordinarily, the receiver audio gain control or IRIG
738 level control should be set for a value midway in this range.</dd>
740 <dt><tt>ident</tt></dt>
742 <dd>The station identifier shows the station, <tt>C</tt> for WWV or
743 <tt>H</tt> for WWVH, and frequency being tracked. If neither
744 station is heard on any frequency, the station identifier shows
747 <dt><tt>comp</tt></dt>
749 <dd>The minute sync compare counter is useful to determine the
750 quality of the minute sync signal and can range from 0 (no signal)
753 <dt><tt>errs</tt></dt>
755 <dd>The bit error counter is useful to determine the quality of the
756 data signal received in the most recent minute. It is normal to
757 drop a couple of data bits under good signal conditions and
758 increasing numbers as conditions worsen. While the decoder performs
759 moderately well even with half the bits are in error in any minute,
760 usually by that point the sync signals are lost and the decoder
761 reverts to free-run anyway.</dd>
763 <dt><tt>freq</tt></dt>
765 <dd>The frequency offset is the current estimate of the codec
766 frequency offset to within 0.1 PPM. This may wander a bit over the
767 day due to local temperature fluctuations and propagation
770 <dt><tt>avgt</tt></dt>
772 <dd>The averaging time is the interval between frequency updates in
773 powers of two to a maximum of 1024 s. Attainment of the maximum
774 indicates the driver is operating at the best possible resolution
775 in time and frequency.</dd>
778 <p>An example timecode is:</p>
780 <p><tt>0 2000 006 22:36:00.000 S +3 1 115 C20 6 5 66.4
783 <p>Here the clock has been set and no alarms are raised. The year,
784 day and time are displayed along with no leap warning, standard
785 time and DUT +0.3 s. The clock was set on the last minute, the AGC
786 is safely in the middle ot the range 0-255, and the receiver is
787 tracking WWV on 20 MHz. Excellent reeiving conditions prevail, as
788 indicated by the compare count 6 and 5 bit errors during the last
789 minute. The current frequency is 66.4 PPM and the averaging
790 interval is 1024 s, indicating the maximum precision available.</p>
794 <p>The <tt>mode</tt> keyword of the <tt>server</tt> configuration
795 command specifies the ICOM ID select code. A missing or zero
796 argument disables the CI-V interface. Following are the ID select
797 codes for the known radios.</p>
799 <table cols="6" width="100%">
873 <h4>Fudge Factors</h4>
876 <dt><tt>time1 <i>time</i></tt></dt>
878 <dd>Specifies the propagation delay for WWV (40:40:49.0N
879 105:02:27.0W), in seconds and fraction, with default 0.0.</dd>
881 <dt><tt>time2 <i>time</i></tt></dt>
883 <dd>Specifies the propagation delay for WWVH (21:59:26.0N
884 159:46:00.0W), in seconds and fraction, with default 0.0.</dd>
886 <dt><tt>stratum <i>number</i></tt></dt>
888 <dd>Specifies the driver stratum, in decimal from 0 to 15, with
891 <dt><tt>refid <i>string</i></tt></dt>
893 <dd>Ordinarily, this field specifies the driver reference
894 identifier; however, the driver sets the reference identifier
895 automatically as described above.</dd>
897 <dt><tt>flag1 0 | 1</tt></dt>
899 <dd>Not used by this driver.</dd>
901 <dt><tt>flag2 0 | 1</tt></dt>
903 <dd>Specifies the microphone port if set to zero or the line-in
904 port if set to one. It does not seem useful to specify the compact
905 disc player port.</dd>
907 <dt><tt>flag3 0 | 1</tt></dt>
909 <dd>Enables audio monitoring of the input signal. For this purpose,
910 the speaker volume must be set before the driver is started.</dd>
912 <dt><tt>flag4 0 | 1</tt></dt>
914 <dd>Enable verbose <tt>clockstats</tt> recording if set.</dd>
917 <h4>Additional Information</h4>
919 <a href="refclock.htm">Reference Clock Drivers</a> <br>
920 <a href="audio.htm">Reference Clock Audio Drivers</a>
923 <a href="index.htm"><img align="left" src="pic/home.gif" alt=
926 <address><a href="mailto:mills@udel.edu">David L. Mills
927 <mills@udel.edu></a></address>
projects / dragonfly.git / blob