In the first part of our series on in-band signaling, we discussed one of the most common and easily recognizable forms of audio control, familiar to anyone who has dialed a phone in the last fifty years – dual-tone multifrequency (DTMF) dialing. Our second installment will look at an in-band signaling method that far fewer people have heard, precisely because it was designed to be sub-audible — coded squelch systems for public service and other radio services.
To review, an in-band signaling system is loosely defined as any system that sends control information along with the main content of a transmission. In the case of a telephone call, the main content would be the conversation you have with a friend on the other end, or shouting at the voice response system that’s currently screwing up your electric bill. The control signals would be the DTMF tones sent by your phone to the telco central office to connect your phone to your friends, or the angry mashing of keys to try to get connected to a human being. In both cases, the DTMF tones travel on the same channel, or band, as the voice conversation, one-sided though it may be.
In-band signaling works in radio transmissions, too, as a way for one radio to control another. The most common form of remote control is the squelch system. Ham radio operators and other two-way radio users will no doubt be familiar with the squelch control on a receiver, which is used to manually set a threshold which an incoming signal needs to be stronger than to be heard. This quiets the constant white noise and blanks out distant, weak transmissions.