Squelch

Lore & Background

Squelch was invented for use in two-way radio and remains widely employed. The simplest form, carrier squelch, functions strictly on signal strength, using the receiver's automatic gain control (AGC) to determine the squelch threshold. It is used in television to mute audio or blank video on empty channels, and in walkie-talkies to mute audio when no signal is present. Single-sideband modulation typically uses carrier squelch.

Noise squelch is more reliable than carrier squelch. It is noise-operated and can be used in AM or FM receivers, relying on receiver quieting in the presence of a carrier. The audio from the detector is passed through a high-pass filter (typically 4 kHz and above) to minimize the effects of voice audio. The remaining high-frequency noise is rectified to produce a DC voltage that turns the receiver audio off when noise is present; when a signal with little noise is received, the voltage drops and the audio unmutes. Noise squelch can be defeated by intermodulation in the high-pass band, so many receivers also use a carrier squelch set at a higher threshold.

Tone squelch and selective calling techniques address co-channel interference. CTCSS (Continuous Tone-Coded Squelch System) continuously superimposes one of about 50 low-pitch audio tones (67 to 254 Hz) on the transmitted signal. DCS (Digital-Coded Squelch) adds a 134.4 bit/s sub-audible bitstream using a 23-bit Golay code, with 83 standardized codes. Selcall transmits a burst of up to five in-band audio tones at the start of each transmission. XTCSS is the newest technique, providing 99 codes with silent operation, implemented as a combination of CTCSS and in-band signalling.

Reader's Guide

Squelch is fundamental to modern radio communications, enabling receivers to remain silent until a valid signal appears. Its significance lies in its ability to suppress unwanted noise, making two-way radio practical for everyday use. Carrier squelch, the simplest variant, is still widely used in consumer devices like walkie-talkies and televisions. Noise squelch offers improved reliability in AM and FM receivers, though it can be vulnerable to intermodulation interference. The development of tone squelch and selective calling systems—such as CTCSS, DCS, Selcall, and XTCSS—addressed the need for privacy and interference rejection on congested channels. These techniques allow groups of users to share a frequency without hearing each other's transmissions unless they share the same code. In professional wireless microphones, squelch prevents reproduction of noise when the signal is weak. In repeater systems, squelch provides a signal-present indication that keys the transmitter, ensuring repeaters do not transmit continuously. The legacy of squelch is its enduring role as a simple yet essential function in both consumer and professional radio equipment, with its various implementations continuing to evolve to meet the demands of crowded radio spectrum.

Did You Know?

• Noise squelch passes audio through a high-pass filter at 4 kHz and above to isolate noise.
• CTCSS is often called PL tone (Private Line, a trademark of Motorola) or simply tone squelch.
• DCS uses a 23-bit Golay (23,12) code that can detect and correct errors of 3 or fewer bits.