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.
Automatic squelch systems are a little different: a receiver stays quiet until it detects a coded signal from a transmitter, at which point it opens the squelch and allows the transmission to be heard. Probably the place that most people are familiar with it is with those blister-pack two-way Family Radio Service (FRS) radios you buy at Walmart for a couple of bucks. All those little UHF walkie-talkie radios have some sort of “privacy code” feature, which is actually a coded squelch system.
Automatic squelch systems came along very early in the days of two-way public service and business radio. With limited bandwidth and lots of users on the air, radio manufacturers came up with ways to share frequencies without necessarily hearing all the transmissions on the channel. A florist could share the same frequency as a tow truck operator, for instance, but each company would only hear their own traffic. That was the theory, at least; such features are rarely as simple as the marketing team makes them seem.
Early squelch systems were based on “subaudible” tones that were transmitted every time the transmitter was keyed. The tones, ranging from 60 Hz to 250 Hz or so, were clearly in the range of human hearing, but given the limitations of audio reproduction at the receiver, the tones were effectively inaudible. But they still got modulated onto the carrier, and once decoded the receiver would break the squelch and allow all the radios set to the same tone to hear the transmission.
This system, known formally as continuous tone-coded squelch system, or CTCSS, went by different names for different manufacturers. Motorola’s system was called “Private Line” or “PL”, General Electric has “Channel Guard,” and RCA went by “Quiet Channel.” Tone frequencies were standardized under EIA RS-220 for interoperability between brands, and the whole system eventually became known generically as “PL tones.”
Early radio used vibrating electromechanical reeds, essentially tiny tuning forks attached to a small coil of wire. Audio from the receiver was fed through the coil, and if the correct PL tone was present, the reed, resonant for that frequency, would vibrate and make close contacts to open the audio channel. Tones were encoded on the transmit side with similar reeds.
Digital Squelch and Beyond
Resonant reed PL systems were the state of the art well into the 1980s, and more than a few amateur radio repeaters in service today are based on old Motorola UHF and VHF base stations that have been retuned and still have reeds in them. As sketchy as reeds may seem from a modern perspective, especially given the solid state phase-locked loop decoders that replaced them, the reeds were robust and reliable under often harsh mobile and remote conditions, and were marvels of miniaturized electromechanical engineering.
But technology marches on, and eventually CTCSS systems were joined by digital coded squelch (DCS) systems. Implementations vary, but Motorola’s “Digital Private Line” or DPL became the standard. It mixes a continuous 134-bps square wave into the audio signal and provides 83 separate codes, far more than the 38 tones PL specified.
Two-way land mobile radio technology has come a long way since the early days of vibrating reeds in transceivers stuffed with vacuum tubes that took up most of a vehicle’s trunk space. Today’s entirely digital trunked systems resemble packet-switching networks and rely heavily on out-of-band signaling for control. But in-band signaling for squelch control isn’t going anywhere anytime soon.