IRIG Timecode - Time Codes

Time Codes

The different timecodes defined in the Standard have alphabetic designations. A, B, D, E, G, and H are the standards currently defined by 200-04. C was in the original specification, but was replaced by H. The main difference between codes is their rate, which varies between one pulse per second and 10,000 pulses per second.

The bits are modulated on a carrier. A three-digit suffix specifies the type and frequency of the carrier, and which optional information is included:

Modulation type

0. (DCLS) Direct Current Level Shift (width coded)
1. Sine wave carrier (amplitude modulated)
2. Manchester modulated

Carrier frequency

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

Binary-coded decimal day of year, hours, minutes, and (for some formats) seconds and fractions) are always included. Optional components are:

• Year number (00–99; century is not coded)
• User-defined "control functions" occupying bits not defined by IRIG
• "Straight binary seconds", a 17-bit binary counter that counts from 0 to 86399.

0. BCD, CF, SBS
1. BCD, CF
2. BCD
3. BCD, SBS
4. BCD, BCD_Year, CF, SBS
5. BCD, BCD_Year, CF
6. BCD, BCD_Year
7. BCD, BCD_Year, SBS

The recognized signal identification numbers for each format according to the standard 200-04 consist of:

Thus the complete signal identification number consists of one letter and three digits. E.g. the signal designated as B122 is deciphered as follows: Format B, Sine wave (amplitude modulated), 1 kHz carrier, and Coded expressions BCDTOY.

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.

For 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. Year numbers are not included, so the time code repeats annually. Leap second announcements are not provided. Although information is transmitted only once per second, a device can synchronize its time very accurately with the transmitting device by using a phase locked loop to synchronize to the carrier. Typical commercial devices will synchronize to within 1 microsecond using IRIG B timecodes.