Some users find it necessary to monitor the presence of an audio signal using the RFC-1. For example, the absence of audio at the output of a modulation monitor can be made to trigger the RFC-1 telephone alarm system.

The schematic for a very simple, easy to build audio detector is shown below. Any audio level of -6 dBv or greater will maintain at least 0.5 volts DC at the output. This covers most "line level" audio sources. If greater audio sensitivity is needed, germanium diodes (1N34) or Schottky diodes (1N5819) can be used for D1 and D2.

The easiest way to set this up as a "loss of audio" alarm with the RFC-1/B is to turn the telemetry fully counter-clockwise. Then set the upper limit for this channel to 2040 and the lower limit to around 0150. With audio present, the reading will be "Status on" almost all the time meaning that the telemetry is pegged against the upper end of the scale--2040. During long pauses the reading will change to numerical values. With a long period of silence, the value with drop very low. This will trigger an alarm in the RFC-1/B when the value drops below 0150 during the RFC-1/B alarm scan.