This driver replaces an earlier one built by Dennis Ferguson in 1988. The earlier driver required a special line discipline which preprocessed the signal in order to improve accuracy and avoid errors. The new driver includes more powerful algorithms implemented directly in the driver and requires no line discipline. It decodes the data using a maximum-likelihood technique which exploits the considerable degree of redundancy available to maximize accuracy and minimize errors.
While there are currently no known commercial CHU receivers, a simple but effective receiver/demodulator can be constructed from an ordinary shortwave receiver and Bell 103 compatible, 300-bps modem or modem chip, as described in the Pulse-per-second (PPS) Signal Interfacing page. The driver can be compiled to use this modem to receive the radio signal and demodulate the data. Alternatively, the driver can be compiled to use the audio codec of the Sun workstation or another with compatible audio drivers. In the latter case, the driver implements the modem using DSP routines, so the radio can be connected directly to either the microphone on line input port.
The CHU time broadcast includes an audio signal compatible with the Bell 103 modem standard (mark = 2225 Hz, space = 2025 Hz). It consist of nine, ten-character bursts transmitted at 300 bps and beginning each second from second 31 to second 39 of the minute. Each character consists of eight data bits plus one start bit and two stop bits to encode two hex digits. The burst data consist of five characters (ten hex digits) followed by a repeat of these characters. In format A, the characters are repeated in the same polarity; in format B, the characters are repeated in the opposite polarity.
Format A bursts are sent at seconds 32 through 39 of the minute in hex digits
6dddhhmmss6dddhhmmss
The first ten digits encode a frame marker (6) followed by the day (ddd), hour (hh in UTC), minute (mm) and the second (ss). Since format A bursts are sent during the third decade of seconds the tens digit of ss is always 3. The driver uses this to determine correct burst synchronization. These digits are then repeated with the same polarity.
Format B bursts are sent at second 31 of the minute in hex digits
xdyyyyttaaxdyyyyttaa
The first ten digits encode a code (x described below) followed by the DUT1 (d in deciseconds), Gregorian year (yyyy), difference TAI - UTC (tt) and daylight time indicator (aa) peculiar to Canada. These digits are then repeated with inverted polarity.
The x is coded
1 Sign of DUT (0 = +)
2 Leap second warning. One second will be added.
4 Leap second warning. One second will be subtracted.
This is not likely to happen in our universe.
8 Even parity bit for this nibble.
By design, the last stop bit of the last character in the burst coincides with 0.5 second. Since characters have 11 bits and are transmitted at 300 bps, the last stop bit of the first character coincides with 0.5 - 10 * 11/300 = 0.133 second. Depending on the UART, character interrupts can vary somewhere between the beginning of bit 9 and end of bit 11. These eccentricities can be corrected along with the radio propagation delay using fudge time1.
n b f s m code
where n is the number of characters in the burst (0-11), b the burst distance (0-40), f the field alignment (-1, 0, 1), s the synchronization distance (0-16), m the burst number (2-9) and code the burst characters as received. Note that the hex digits in each character are reversed, so the burst
10 38 0 16 9 06851292930685129293
is interpreted as containing 11 characters with burst distance 38, field alignment 0, synchronization distance 16 and burst number 9. The nibble-swapped timecode shows day 58, hour 21, minute 29 and second 39.
When the audio driver is compiled, format A is preceded by the gain (0-255) and relative signal level (0-9999). The receiver volume control should be set so that the gain is near the middle of the range 0-255, which results in a signal level near 1000.
Following is format B:
n b s code
where n is the number of characters in the burst (0-11), b the burst distance (0-40), s the synchronization distance (0-40) and code the burst characters as received. Note that the hex digits in each character are reversed and the last ten digits inverted, so the burst
11 40 1091891300ef6e76ecff
is interpreted as containing 11 characters with burst distance 40. The nibble-swapped timecode shows DUT1 +0.1 second, year 1998 and TAI - UTC 31 seconds.
In addition to the above, the reference timecode is updated and written to the clockstats file and debug score after the last burst received in the minute. The format is
qq yyyy ddd hh:mm:ss nn dd tt
where qq are the error flags, as described below, yyyy is the year, ddd the day, hh:mm:ss the time of day, nn the number of format A bursts received during the previous minute, dd the decoding distance and tt the number of timestamps. The error flags are cleared after every update.
For accuracy better than the low milliseconds, the fudge time1 variable can be used to set the propagation delay and compensate for inherent latencies in the serial port hardware and operating system. This can be done conveniently using the minimuf program.