658 lines
25 KiB
HTML
658 lines
25 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
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<head>
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<meta name="generator" content="HTML Tidy, see www.w3.org">
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<title>Radio CHU Audio Demodulator/Decoder</title>
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</head>
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<body>
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<h3>Radio CHU Audio Demodulator/Decoder</h3>
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<hr>
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<h4>Synopsis</h4>
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Address: 127.127.7.<i>u</i> <br>
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Reference ID: <tt>CHU</tt> <br>
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Driver ID: <tt>CHU</tt> <br>
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Modem Port: <tt>/dev/chu<i>u</i></tt>; 300 baud, 8-bits, no parity
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<br>
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Autotune Port: <tt>/dev/icom</tt>; 1200/9600 baud, 8-bits, no
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parity <br>
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Audio Device: <tt>/dev/chu_audio</tt> and <tt>/dev/audioctl</tt>
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<h4>Description</h4>
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<p>This driver synchronizes the computer time using data encoded in
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radio transmissions from Canadian time/frequency station CHU in
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Ottawa, Ontario. It replaces an earlier one, built by Dennis
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Ferguson in 1988, which required a special line discipline to
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preprocessed the signal. The new driver includes more powerful
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algorithms implemented directly in the driver and requires no
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preprocessing.</p>
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<p>CHU transmissions are made continuously on 3330 kHz, 7335 kHz
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and 14670 kHz in upper sideband, compatible AM mode. An ordinary
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shortwave receiver can be tuned manually to one of these
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frequencies or, in the case of ICOM receivers, the receiver can be
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tuned automatically as propagation conditions change throughout the
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day and night. The performance of this driver when tracking the
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station is ordinarily better than 1 ms in time with frequency drift
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less than 0.5 PPM when not tracking the station.</p>
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<p>While there are currently no known commercial CHU receivers, a
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simple but effective receiver/demodulator can be constructed from
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an ordinary shortwave receiver and Bell 103 compatible, 300-b/s
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modem or modem chip, as described in the <a href="gadget.htm">
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Gadget Box PPS Level Converter and CHU Modem</a> page. The driver
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can use the modem to receive the radio signal and demodulate the
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data or, if available, the driver can use the audio codec of the
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Sun workstation or another with compatible audio interface. In the
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latter case, the driver implements the modem using DSP routines, so
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the radio can be connected directly to either the microphone on
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line input port.</p>
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<p>This driver incorporates several features in common with other
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audio drivers such as described in the <a href="driver36.htm">Radio
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WWV/H Audio Demodulator/Decoder</a> and the <a href="driver6.htm">
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IRIG Audio Decoder</a> pages. They include automatic gain control
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(AGC), selectable audio codec port and signal monitoring
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capabilities. For a discussion of these common features, as well as
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a guide to hookup, debugging and monitoring, see the <a href=
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"audio.htm">Reference Clock Audio Drivers</a> page.</p>
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<p>Ordinarily, the driver poll interval is set to 14 (about 4.5 h),
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although this can be changed with configuration commands. As long
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as the clock is set or verified at least once during this interval,
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the NTP algorithms will consider the source reachable and
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selectable to discipline the system clock. However, if this does
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not happen for eight poll intervals, the algorithms will consider
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the source unreachable and some other source will be chosen (if
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available) to discipline the system clock.</p>
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<p>The decoding algorithms process the data using
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maximum-likelihood techniques which exploit the considerable degree
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of redundancy available in each broadcast message or burst. As
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described below, every character is sent twice and, in the case of
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format A bursts, the burst is sent eight times every minute. In the
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case of format B bursts, which are sent once each minute, the burst
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is considered correct only if every character matches its
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repetition in the burst. In the case of format A messages, a
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majority decoder requires at least six repetitions for each digit
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in the timecode and more than half of the repetitions decode to the
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same digit. Every character in every burst provides an independent
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timestamp upon arrival with a potential total of over 60 timestamps
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for each minute.</p>
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<p>A timecode in the format described below is assembled when all
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bursts have been received in the minute. The timecode is considered
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valid and the clock set when at least one valid format B burst has
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been decoded and the above requirements are met. The <tt>yyyy</tt>
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year field in the timecode indicates whether a valid format B burst
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has been received. Upon startup, this field is initialized at zero;
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when a valid format B burst is received, it is set to the current
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Gregorian year. The <tt>q</tt> quality character field in the
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timecode indicates whether a valid timecode has been determined. If
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any of the high order three bits of this character are set, the
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timecode is invalid.</p>
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<p>Once the clock has been set for the first time, it will appear
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reachable and selectable to discipline the system clock, even if
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the broadcast signal is lost. Since the signals are almost always
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available during some period of the day and the NTP clock
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discipline algorithms are designed to work well even in this case,
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it is unlikely that the system clock could drift more than a few
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tens of milliseconds during periods of signal loss. To protect
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against this most unlikely situation, if after four days with no
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signals, the clock is considered unset and resumes the
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synchronization procedure from the beginning.</p>
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<p>The last three fields in the timecode are useful in assessing
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the quality of the radio channel during the most recent minute
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bursts were received. The <tt>bcnt</tt> field shows the number of
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format A bursts in the range 1-8. The <tt>dist</tt> field shows the
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majority decoder distance, or the minimum number of sample
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repetitions for each digit of the timecode in the range 0-16. The
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<tt>tsmp</tt> field shows the number of timestamps determined in
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the range 0-60. For a valid timecode, <tt>bcnt</tt> must be at
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least 3, <tt>dist</tt> must be greater than <tt>bcnt</tt> and <tt>
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tsmp</tt> must be at least 20.</p>
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<h4>Program Operation</h4>
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<p>The program consists of four major parts: the DSP modem, maximum
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likelihood UART, burst assembler and majority decoder. The DSP
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modem demodulates Bell 103 modem answer-frequency signals; that is,
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frequency-shift keyed (FSK) tones of 2225 Hz (mark) and 2025 Hz
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(space). This is done using a 4th-order IIR filter and
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limiter/discriminator with 500-Hz bandpass centered on 2125 Hz and
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followed by a FIR raised-cosine lowpass filter optimized for the
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300-b/s data rate. Alternately, the driver can be compiled to
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delete the modem and input 300 b/s data directly from an external
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modem via a serial port.</p>
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<p>The maximum likelihood UART is implemented using a set of eight
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11-stage shift registers, one for each of eight phases of the
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300-b/s bit clock. At each phase a new baseband signal value from
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the DSP modem is shifted into the corresponding register and the
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maximum and minimum over all 11 samples computed. This establishes
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a slice level midway between the maximum and minimum over all
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stages. For each stage, a signal level above this level is a mark
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(1) and below is a space (0). A quality metric is calculated for
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each register with respect to the slice level and the a-priori
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signal consisting of a mark bit (previous stop bit), space (start)
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bit, eight arbitrary information bits and the first of the two mark
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(stop) bits.</p>
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<p>The shift registers are processed in round-robin order as each
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modem value arrives until one of them shows a valid framing pattern
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consisting of a mark bit, space bit, eight arbitrary data bits and
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a mark bit. When found, the data bits from the register with the
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best metric is chosen as the maximum likelihood character and the
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UART begins to process the next character.</p>
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<p>The burst assembler processes characters either from the maximum
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likelihood UART or directly from the serial port as configured. A
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burst begins when a character is received and is processed after a
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timeout interval when no characters are received. If the interval
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between characters is greater than two characters, but less than
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the timeout interval, the burst is rejected as a runt and a new
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burst begun. As each character is received, a timestamp is captured
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and saved for later processing.</p>
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<p>A valid burst consists of ten characters in two replicated
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five-character blocks. A format B block contains the year and other
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information in ten hexadecimal digits. A format A block contains
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the timecode in ten decimal digits, the first of which is a framing
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code (6). The burst assembler must deal with cases where the first
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character of a format A burst is lost or is noise. This is done
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using the framing code to correct the phase, either one character
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early or one character late.</p>
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<p>The burst distance is incremented by one for each bit in the
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first block that matches the corresponding bit in the second block
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and decremented by one otherwise. In a format B burst the second
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block is bit-inverted relative to the first, so a perfect burst of
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five 8-bit characters has distance -40. In a format A block the two
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blocks are identical, so a perfect burst has distance +40. Format B
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bursts must be perfect to be acceptable; however, format A bursts,
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which are further processed by the majority decoder, are acceptable
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if the distance is at least 28.</p>
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<p>Each minute of transmission includes eight format A bursts
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containing two timecodes for each second from 31 through 39. The
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majority decoder uses a decoding matrix of ten rows, one for each
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digit position in the timecode, and 16 columns, one for each 4-bit
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code combination that might be decoded at that position. In order
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to use the character timestamps, it is necessary to reliably
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determine the second number of each burst. In a valid burst, the
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last digit of the two timecodes in the block must match and the
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value must be in the range 2-9 and greater than in the previous
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burst.</p>
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<p>As each hex digit of a valid burst is processed, the value at
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the row corresponding to the digit position in the timecode and
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column corresponding to the code found at that position is
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incremented. At the end of each minute of transmission, each row of
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the decoding matrix encodes the number of occurrences of each code
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found at the corresponding position of the timecode. However, the
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first digit (framing code) is always 6, the ninth (second tens) is
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always 3 and the last (second units) changes for each burst, so are
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not used.</p>
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<p>The maximum over all occurrences at each timecode digit position
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is the distance for that position and the corresponding code is the
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maximum likelihood candidate. If the distance is zero, the decoder
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assumes a miss; if the distance is not more than half the total
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number of occurrences, the decoder assumes a soft error; if two
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different codes with the same distance are found, the decoder
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assumes a hard error. In all these cases the decoder encodes a
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non-decimal character which will later cause a format error when
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the timecode is reformatted. The decoding distance is defined as
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the minimum distance over the first nine digits; the tenth digit
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varies over the seconds and is uncounted.</p>
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<p>The result of the majority decoder is a nine-digit timecode
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representing the maximum likelihood candidate for the transmitted
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timecode in that minute. Note that the second and fraction within
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the minute are always zero and that the actual reference point to
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calculate timestamp offsets is backdated to the first second of the
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minute. At this point the timecode block is reformatted and the
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year, days, hours and minutes extracted along with other
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information from the format B burst, including DST state, DUT1
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correction and leap warning. The reformatting operation checks the
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timecode for invalid code combinations that might have been left by
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the majority decoder and rejects the entire timecode if found.</p>
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<p>If the timecode is valid, it is passed to the reference clock
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interface along with the backdated timestamp offsets accumulated
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over the minute. A perfect set of nine bursts could generate as
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many as 90 timestamps, but the maximum the interface can handle is
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60. These are processed by the interface using a median filter and
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trimmed-mean average, so the resulting system clock correction is
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usually much better than would otherwise be the case with radio
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noise, UART jitter and occasional burst errors.</p>
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<h4>Autotune</h4>
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<p>The driver includes provisions to automatically tune the radio
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in response to changing radio propagation conditions throughout the
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day and night. The radio interface is compatible with the ICOM CI-V
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standard, which is a bidirectional serial bus operating at TTL
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levels. The bus can be connected to a standard serial port using a
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level converter such as the CT-17. The serial port speed is
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presently compiled in the program, but can be changed in the <tt>
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icom.h</tt> header file.</p>
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<p>Each ICOM radio is assigned a unique 8-bit ID select code,
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usually expressed in hex format. To activate the CI-V interface,
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the <tt>mode</tt> keyword of the <tt>server</tt> configuration
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command specifies a nonzero select code in decimal format. A table
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of ID select codes for the known ICOM radios is given below. Since
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all ICOM select codes are less than 128, the high order bit of the
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code is used by the driver to specify the baud rate. If this bit is
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not set, the rate is 9600 bps for the newer radios; if set, the
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rate is 1200 bps for the older radios. A missing <tt>mode</tt>
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keyword or a zero argument leaves the interface disabled.</p>
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<p>If specified, the driver will attempt to open the device <tt>
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/dev/icom</tt> and, if successful will tune the radio to 3.330 MHz.
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If after five minutes at this frequency not more than two format A
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bursts have been received for any minute, the driver will tune to
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7.335 MHz, then to 14.670 MHz, then return to 3.330 MHz and
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continue in this cycle. However, the driver is liberal in what it
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assumes of the configuration. If the <tt>/dev/icom</tt> link is not
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present or the open fails or the CI-V bus or radio is inoperative,
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the driver quietly gives up with no harm done.</p>
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<h4>Radio Broadcast Format</h4>
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<p>The CHU time broadcast includes an audio signal compatible with
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the Bell 103 modem standard (mark = 2225 Hz, space = 2025 Hz). It
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consist of nine, ten-character bursts transmitted at 300 b/s and
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beginning each second from second 31 to second 39 of the minute.
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Each character consists of eight data bits plus one start bit and
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two stop bits to encode two hex digits. The burst data consist of
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five characters (ten hex digits) followed by a repeat of these
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characters. In format A, the characters are repeated in the same
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polarity; in format B, the characters are repeated in the opposite
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polarity.</p>
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<p>Format A bursts are sent at seconds 32 through 39 of the minute
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in hex digits</p>
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<p><tt>6dddhhmmss6dddhhmmss</tt></p>
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<p>The first ten digits encode a frame marker (<tt>6</tt>) followed
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by the day (<tt>ddd</tt>), hour (<tt>hh</tt>), minute (<tt>mm</tt>)
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and second (<tt>ss</tt>). Since format A bursts are sent during the
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third decade of seconds the tens digit of <tt>ss</tt> is always 3.
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The driver uses this to determine correct burst synchronization.
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These digits are then repeated with the same polarity.</p>
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<p>Format B bursts are sent at second 31 of the minute in hex
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digits</p>
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<p><tt>xdyyyyttaaxdyyyyttaa</tt></p>
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<p>The first ten digits encode a code (<tt>x</tt> described below)
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followed by the DUT1 (<tt>d</tt> in deciseconds), Gregorian year
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(<tt>yyyy</tt>), difference TAI - UTC (<tt>tt</tt>) and daylight
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time indicator (<tt>aa</tt>) peculiar to Canada. These digits are
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then repeated with inverted polarity.</p>
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<p>The <tt>x</tt> is coded</p>
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<dl>
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<dt><tt>1</tt></dt>
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<dd>Sign of DUT (0 = +)/dd></dd>
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<dt><tt>2</tt></dt>
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<dd>Leap second warning. One second will be added.</dd>
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<dt><tt>4</tt></dt>
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<dd>Leap second warning. One second will be subtracted. This is not
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likely to happen in our universe.</dd>
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<dt><tt>8</tt></dt>
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<dd>Even parity bit for this nibble.</dd>
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</dl>
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<p>By design, the last stop bit of the last character in the burst
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coincides with 0.5 second. Since characters have 11 bits and are
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transmitted at 300 b/s, the last stop bit of the first character
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coincides with 0.5 - 10 * 11/300 = 0.133 second. Depending on the
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UART, character interrupts can vary somewhere between the beginning
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of bit 9 and end of bit 11. These eccentricities can be corrected
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along with the radio propagation delay using the <tt>fudge
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time1</tt> variable.</p>
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<h4>Debugging Aids</h4>
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<p>The most convenient way to track the program status is using the
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<tt>ntpq</tt> program and the <tt>clockvar</tt> command. This
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displays the last determined timecode and related status and error
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counters, even when the program is not discipline the system clock.
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If the debugging trace feature (<tt>-d</tt> on the <tt>ntpd</tt>
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command line)is enabled, the program produces detailed status
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messages as it operates. If the <tt>fudge flag 4</tt> is set, these
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messages are written to the <tt>clockstats</tt> file. All messages
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produced by this driver have the prefix <tt>chu</tt> for convenient
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filtering with the Unix <tt>grep</tt> command.</p>
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<p>With debugging enabled the driver produces messages in the
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following formats:</p>
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<p>A format <tt>chuA</tt> message is produced for each format A
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burst received in seconds 32 through 39 of the minute:</p>
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<p><tt>chuA n b s code</tt></p>
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<p>where <tt>n</tt> is the number of characters in the burst
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(0-11), <tt>b</tt> the burst distance (0-40), <tt>s</tt> the
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synchronization distance (0-40) and <tt>code</tt> the burst
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characters as received. Note that the hex digits in each character
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are reversed and the last ten digits inverted, so the burst</p>
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<p><tt>11 40 1091891300ef6e76ecff</tt></p>
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<p>is interpreted as containing 11 characters with burst distance
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40. The nibble-swapped timecode shows DUT1 +0.1 second, year 1998
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and TAI -UTC 31 seconds.</p>
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<p>A format <tt>chuB</tt> message is produced for each format B
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burst received in second 31 of the minute:</p>
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<p><tt>chuB n b f s m code</tt></p>
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<p>where <tt>n</tt> is the number of characters in the burst
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(0-11), <tt>b</tt> the burst distance (0-40), <tt>f</tt> the field
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alignment (-1, 0, 1), <tt>s</tt>the synchronization distance
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(0-16), <tt>m</tt>the burst number (2-9) and <tt>code</tt> the
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burst characters as received. Note that the hex digits in each
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character are reversed, so the burst</p>
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<p><tt>10 38 0 16 9 06851292930685129293</tt></p>
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<p>is interpreted as containing 11 characters with burst distance
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38, field alignment 0, synchronization distance 16 and burst number
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9. The nibble-swapped timecode shows day 58, hour 21, minute 29 and
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second 39.</p>
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<p>If the CI-V interface for ICOM radios is active, a debug level
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greater than 1 will produce a trace of the CI-V command and
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response messages. Interpretation of these messages requires
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knowledge of the CI-V protocol, which is beyond the scope of this
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document.</p>
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<h4>Monitor Data</h4>
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When enabled by the <tt>filegen</tt> facility, every received
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timecode is written to the <tt>clockstats</tt> file in the
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following format:
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<pre>
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sq yy ddd hh:mm:ss.fff ld dut lset agc rfrq bcnt dist tsmp
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s sync indicator
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q quality character
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yyyy Gregorian year
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ddd day of year
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hh hour of day
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mm minute of hour
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ss second of minute
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fff millisecond of second
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l leap second warning
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d DST state
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dut DUT sign and magnitude in deciseconds
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lset minutes since last set
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agc audio gain (0-255)
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rfrq radio frequency
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bcnt burst count
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dist decoding distance
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tsmp timestamps captured
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</pre>
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The fields beginning with <tt>year</tt> and extending through <tt>
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dut</tt> are decoded from the received data and are in fixed-length
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format. The <tt>agc</tt> and <tt>lset</tt> fields, as well as the
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following driver-dependent fields, are in variable-length format.
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<dl>
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<dt><tt>s</tt></dt>
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<dd>The sync indicator is initially <tt>?</tt> before the clock is
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set, but turns to space when the clock is correctly set.</dd>
|
|
|
|
<dt><tt>q</tt></dt>
|
|
|
|
<dd>The quality character is a four-bit hexadecimal code showing
|
|
which alarms have been raised during the most recent minute. Each
|
|
bit is associated with a specific alarm condition according to the
|
|
following:
|
|
|
|
<dl>
|
|
<dt><tt>8</tt></dt>
|
|
|
|
<dd>Decoder alarm. A majority of repetitions for at least one digit
|
|
of the timecode fails to agree.</dd>
|
|
|
|
<dt><tt>4</tt></dt>
|
|
|
|
<dd>Timestamp alarm. Fewer than 20 timestamps have been
|
|
determined.</dd>
|
|
|
|
<dt><tt>2</tt></dt>
|
|
|
|
<dd>Format alarm. The majority timecode contains invalid bit
|
|
combinations.</dd>
|
|
|
|
<dt><tt>1</tt></dt>
|
|
|
|
<dd>Frame alarm. A framing or format error occurred on at least one
|
|
burst during the minute.</dd>
|
|
</dl>
|
|
|
|
It is important to note that one or more of the above alarms does
|
|
not necessarily indicate a clock error, but only that the decoder
|
|
has detected a condition that may in future result in an
|
|
error.</dd>
|
|
|
|
<dt><tt>yyyy ddd hh:mm:ss.fff</tt></dt>
|
|
|
|
<dd>The timecode format itself is self explanatory. Note that the
|
|
Gregorian year is decoded directly from the transmitted
|
|
timecode.</dd>
|
|
|
|
<dt><tt>l</tt></dt>
|
|
|
|
<dd>The leap second warning is normally space, but changes to <tt>
|
|
L</tt> if a leap second is to occur at the end of the month of June
|
|
or December.</dd>
|
|
|
|
<dt><tt>d</tt></dt>
|
|
|
|
<dd>The DST code for Canada encodes the state for all
|
|
provinces.</dd>
|
|
|
|
<dt><tt>dut</tt></dt>
|
|
|
|
<dd>The DUT sign and magnitude shows the current UT1 offset
|
|
relative to the displayed UTC time, in deciseconds.</dd>
|
|
|
|
<dt><tt>lset</tt></dt>
|
|
|
|
<dd>Before the clock is set, the interval since last set is the
|
|
number of minutes since the program was started; after the clock is
|
|
set, this is number of minutes since the time was last verified
|
|
relative to the broadcast signal.</dd>
|
|
|
|
<dt><tt>agc</tt></dt>
|
|
|
|
<dd>The audio gain shows the current codec gain setting in the
|
|
range 0 to 255. Ordinarily, the receiver audio gain control or IRIG
|
|
level control should be set for a value midway in this range.</dd>
|
|
|
|
<dt><tt>rfrq</tt></dt>
|
|
|
|
<dd>The current radio frequency, if the CI-V interface is active,
|
|
or 'X' if not.</dd>
|
|
|
|
<dt><tt>bcnt</tt></dt>
|
|
|
|
<dd>The number of format A bursts received during the most recent
|
|
minute bursts were received.</dd>
|
|
|
|
<dt><tt>dist</tt></dt>
|
|
|
|
<dd>The minimum decoding distance determined during the most recent
|
|
minute bursts were received.</dd>
|
|
|
|
<dt><tt>tsmp</tt></dt>
|
|
|
|
<dd>The number of timestamps determined during the most recent
|
|
minute bursts were received.</dd>
|
|
</dl>
|
|
|
|
<h4>Modes</h4>
|
|
|
|
<p>The <tt>mode</tt> keyword of the <tt>server</tt> configuration
|
|
command specifies the ICOM ID select code. A missing or zero
|
|
argument disables the CI-V interface. Following are the ID select
|
|
codes for the known radios.</p>
|
|
|
|
<table cols="6" width="100%">
|
|
<tr>
|
|
<td>Radio</td>
|
|
<td>Hex</td>
|
|
<td>Decimal</td>
|
|
<td>Radio</td>
|
|
<td>Hex</td>
|
|
<td>Decimal</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td>IC725</td>
|
|
<td>0x28</td>
|
|
<td>40</td>
|
|
<td>IC781</td>
|
|
<td>0x26</td>
|
|
<td>38</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td>IC726</td>
|
|
<td>0x30</td>
|
|
<td>48</td>
|
|
<td>R7000</td>
|
|
<td>0x08</td>
|
|
<td>8</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td>IC735</td>
|
|
<td>0x04</td>
|
|
<td>4</td>
|
|
<td>R71</td>
|
|
<td>0x1A</td>
|
|
<td>26</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td>IC751</td>
|
|
<td>0x1c</td>
|
|
<td>28</td>
|
|
<td>R7100</td>
|
|
<td>0x34</td>
|
|
<td>52</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td>IC761</td>
|
|
<td>0x1e</td>
|
|
<td>30</td>
|
|
<td>R72</td>
|
|
<td>0x32</td>
|
|
<td>50</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td>IC765</td>
|
|
<td>0x2c</td>
|
|
<td>44</td>
|
|
<td>R8500</td>
|
|
<td>0x4a</td>
|
|
<td>74</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td>IC775</td>
|
|
<td>0x46</td>
|
|
<td>68</td>
|
|
<td>R9000</td>
|
|
<td>0x2a</td>
|
|
<td>42</td>
|
|
</tr>
|
|
</table>
|
|
|
|
<h4>Fudge Factors</h4>
|
|
|
|
<dl>
|
|
<dt><tt>time1 <i>time</i></tt></dt>
|
|
|
|
<dd>Specifies the propagation delay for CHU (45:18N 75:45N), in
|
|
seconds and fraction, with default 0.0.</dd>
|
|
|
|
<dt><tt>time2 <i>time</i></tt></dt>
|
|
|
|
<dd>Not used by this driver.</dd>
|
|
|
|
<dt><tt>stratum <i>number</i></tt></dt>
|
|
|
|
<dd>Specifies the driver stratum, in decimal from 0 to 15, with
|
|
default 0.</dd>
|
|
|
|
<dt><tt>refid <i>string</i></tt></dt>
|
|
|
|
<dd>Specifies the driver reference identifier, an ASCII string from
|
|
one to four characters, with default <tt>CHU</tt>.</dd>
|
|
|
|
<dt><tt>flag1 0 | 1</tt></dt>
|
|
|
|
<dd>Not used by this driver.</dd>
|
|
|
|
<dt><tt>flag2 0 | 1</tt></dt>
|
|
|
|
<dd>When the audio driver is compiled, this flag selects the audio
|
|
input port, where 0 is the mike port (default) and 1 is the line-in
|
|
port. It does not seem useful to select the compact disc player
|
|
port.</dd>
|
|
|
|
<dt><tt>flag3 0 | 1</tt></dt>
|
|
|
|
<dd>When the audio driver is compiled, this flag enables audio
|
|
monitoring of the input signal. For this purpose, the speaker
|
|
volume must be set before the driver is started.</dd>
|
|
|
|
<dt><tt>flag4 0 | 1</tt></dt>
|
|
|
|
<dd>Enable verbose <tt>clockstats</tt> recording if set.</dd>
|
|
</dl>
|
|
|
|
<h4>Additional Information</h4>
|
|
|
|
<a href="refclock.htm">Reference Clock Drivers</a> <br>
|
|
<a href="audio.htm">Reference Clock Audio Drivers</a>
|
|
|
|
<hr>
|
|
<a href="index.htm"><img align="left" src="pic/home.gif" alt=
|
|
"gif"></a>
|
|
|
|
<address><a href="mailto:mills@udel.edu">David L. Mills
|
|
<mills@udel.edu></a></address>
|
|
</body>
|
|
</html>
|
|
|