Inter-range instrumentation group time codes
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Inter-range instrumentation group time codes, commonly known as "IRIG" timecodes, were created by the TeleCommunications Working Group of the Inter-Range Instrumentation Group, a US Navy Standards organization. The work on these standards was started in October of 1956, and the original standards where accepted in 1960. The original formats were described in IRIG Document 104-60, which was later revised and reissued in August 1970 as IRIG Document 104-70, leading to an upgrade later that year of the IRIG Document to the status of a Standard, IRIG Standard 200-70. The latest version of the Standard is IRIG Standard 200-95. The different timecodes defined in the Standard are given alphabetic designations. A, B, D, E, G, and H are the standards currently defined by 200-95. C was in the original specification, but was replaced by H. The main difference between the codes is their rate, which varies between 1 pulse per minute and ten thousand pulses per second.
- A: 1000 PPS
- B: 100 PPS
- D: 1 PPM
- E: 10 PPS
- G: 10000 PPS
- H: 1 PPS
Other differences are:
- Form Designation:
- 0) (DCLS) Direct Current Level Shift (width coded)
- 1) Sine wave carrier (amplitude modulated),
- Carrier Resolution:
- 0) No carrier (DCLS)
- 1) 100 Hz (10 ms resolution)
- 2) 1 kHz (1 ms resolution)
- 3) 10 kHz (100 µs resolution)
- 4) 100 kHz (10 µs resolution)
- 5) 1 MHz (1 µs resolution)
- Coded expressions:
- 0) BCD (binary-coded decimal), CF (control functions), SBS (straight binary seconds)
- 1) BCD, CF
- 2) BCD
- 3) BCD, SBS
The recognized combinations according to the standard are:
- A: A000, A003, A130, A132, A133
- B: B000, B003, B120, B120, B122, B123, B150, B152, B153
- D: D001, D002, D111, D112, D121, D122
- E: E001, E002, E111, E112, E121, E122
- G: G001, G002, G141, G142
- H: H001, H002, H111, H112, H121, H122
The most commonly used of the standards is IRIG B, then IRIG A, then probably IRIG G. Time code formats directly derived from IRIG H are used by NIST radio stations WWV, WWVH and WWVB.
As an example, one of the most common formats, IRIG B122: IRIG B122 transmits one hundred pulses per second on an amplitude modulated 1 kHz sine wave carrier, encoding information in BCD. This means that 100 bits of information are transmitted every second. The time frame for the IRIG B standard is 1 second, meaning that one data frame of time information is transmitted every second. This data frame contains information about the day of the year (1-366), hours, minutes, and seconds. Information about which year it is is not transmitted. It should be noted that although information is transmitted only once per second, by using a phase locked loop, a device can sychronize its time very accurately with the device transmitting. Typical commercial devices will synchronize to within 1 microsecond using IRIG B timecodes.