Serial time codes have their beginnings in the early days of the missile and space program, when a need to correlate data gathered from widely separated geographical locations arose during the testing of long-range missiles. The development of these time codes yielded: 1) A means of synchronizing the time at remote locations via a telemetry link or land lines; and 2) A means of recording time on the same media (typically magnetic tape) as the test data. In addition, an important benefit of the co-recorded time code is the ability to perform automatic data location by searching the magnetic tape using the stored time code information.
The basic time information is pulse width coded in either a BCD or binary format. Originally, the Inter-Range Instrumentation Group (IRIG) proposed a series of time code formats which later became the NASA codes. Soon after these, another series called the IRIG Standard Time Code Formats was approved and has become the industry standard.
The codes, which have names like IRIG-A, IRIG-B, and so on through IRIG-H, differ in the frequency of the amplitude modulated carrier, which ranges from 100 Hz to 100 KHz, and the resolution of the time which may be encoded. Varying numbers of control bits may also be encoded in these time code streams. Time codes may be used either as DC-level shifts suitable for direct connection into digital systems or as amplitude-modulated audio tones suitable for recording onto magnetic tape or for transmission over some other medium.
All our Time & Frequency Standards provide IRIG-B AM as a standard output with IRIG-B DC-level shift as an option.