1
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
4
<meta name="generator" content="HTML Tidy, see www.w3.org">
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>
added
removed
html/driver36.htm
