SITOR-B (FEC)

SITOR, SImplex Telex Over Radio, is seen here in Mode B, a continuous, broadcast mode that uses Forward Error Correction redundancy instead of the short, ARQ bursts of SITOR-A. While its FSK signal shows a 170-Hz shift and a superficial resemblance to the older Baudot, visible differences include a constant pulse length at 100 baud, and 6 data bits instead of 5.

This plot rather dramatically shows SITOR's spectrum efficiency in the crowded maritime bands. We see how the blistering signal in the middle fits neatly, grunge and all, into its 500-Hz channel. The next channel up shows a very strong ARQ sync burst from the old site of the Power House, KPH, in California, which definitely generated some serious RF in its time. The channel right below our broadcast looks like NMC, US Coast Guard CAMSPAC Point Reyes, which occupies an adjacent antenna farm to KPH and is not exactly a peanut whistle either. We can even read parts of the Morse code identifiers in both adjacent signals. The square things are sync bursts in their channel markers, so ARQ calling vessels can synchronize to them.

The old KPH is now part of the Pt. Reyes National Seashore, and several of the vintage transmitters have been restored for special events. ARQ is no longer used by either them or the Coast Guard. NMC is now unstaffed, and controlled from USCG Commcom in Virginia.