Squelch Circuit Technology in Wireless Audio Systems

The Technical FAQ section on shure.com contains 4,162 questions and answers. That’s everything from “Help! I forgot to take my Shure earphones out of my jeans pocket, and they were washed. Have I destroyed them?” to “How can I test phantom power voltage and current?”

We keep track of which questions interest you the most. For the past several months, one of the most frequently visited FAQs addresses the squelch circuitry in wireless systems. So, we thought we’d expand upon that here. Below is what you need to know to be in the know about squelch.

What does a squelch circuit do?

A squelch or muting circuit is critical to proper receiver behavior in wireless systems. The function of this circuit is to mute or silence the audio output of the receiver in the absence of the desired radio signal. When the desired signal is lost (due to multi-path dropout, excessive distance, loss of power to the transmitter, etc.), the open receiver may pick up another signal or background radio noise. Typically, this is heard as white noise and is often much louder than the audio signal from the desired source.

What’s the history of squelch circuit technology?

Motorola pioneered the technology as Private Line for radio in the early 1950s, but as competitors developed their own variations, the generic version was known as Continuous Tone Coded Squelch System or Continuous Sub Audible Tone Coded Squelch System. Back then, tones were generated and decoded by vibrating reeds encased in copper. These materials were chosen for their ability to withstand the rigors of weather and vibration in two- or three-way radio systems. These days, tones are generated electronically, and an entire wireless transmitter is a fraction of the size of Motorola’s vintage Vibrasponder.

Motorola Vibrasponder

Motorola TU333 Vibrasponder
Image: Courtesy of repeater-builder.com

Squelch technology in wireless audio systems

All wireless transmitters have some form of squelch circuitry. The real difference is in the levels of refinement offered by various manufacturers and models.

Basic: The traditional squelch circuit is an audio switch controlled by the radio signal level using a fixed or manually adjustable threshold (level). When the received signal strength falls below this level, the output of the receiver is muted. Ideally, the squelch level should be set just above the background radio noise level or at the point where the desired signal is becoming too noisy to be acceptable. Higher settings of squelch level require higher received signal strength to unmute the receiver. Since received signal strength decreases as transmission distance increases, higher squelch settings will decrease the operating range of the system.

Squelch Diagrams

Noise Squelch: One refinement of the standard squelch circuit is referred to as noise squelch. This technique relies on the fact that the audio from undesirable radio noise has a great deal of high frequency energy compared to a typical audio signal. The noise squelch circuit compares the high-frequency energy of the received signal to a reference voltage set by the squelch adjustment. In this system, the squelch control essentially determines the “quality” of signal (signal-to-noise ratio) required to unmute the receiver. This allows operation at lower squelch settings with no likelihood of noise if the desired signal is lost.

Tone-key or Pilot-tone Squelch: A further refinement is a dual-squelch method that involves the transmission of a tone-key, tonecode or pilot-tone circuit. It enables the receiver to identify the desired radio signal by adding a supra- or sub-audible tone in the transmitter (32 kHz in the Shure ULX® wireless system) that is sent along with the normal audio signal. The receiver will unmute only when it picks up a radio signal of adequate strength and detects the presence of the tone-key. This prevents noise from the receiver when the desired transmitter signal is lost, even in the presence of a (non-tone-key) interfering signal at the same frequency.

Turn-on and turn-off delays are incorporated in the transmitter tone-key circuits so that the transmitter power switch operates silently. When the transmitter is switched on, the radio signal is activated immediately, but the tone-key is briefly delayed, keeping the receiver muted until the signal is stable. This masks any turn-on noise (often a pop). When the transmitter is switched off, the tone-key is deactivated instantly, muting the receiver, but actual turn-off of the transmitted signal is delayed slightly. This masks any turn-off noise and eliminates the need for a separate mute switch. Finally, the tone-key signal often is used to transmit additional information to the receiver. This may include battery voltage, transmitter audio gain settings, transmitter type, and transmitter power level.

Troubleshooting Tips

There are situations when you may want to disable the tone-key squelch, especially when doing RF channel testing or product diagnostics. Even so, disabling tone-key isn’t recommended for general wireless microphone operation. Disabling tone-key is only a temporary setting; it will be restored to its normal On state upon power cycling the receiver.

And there’s this: squelch circuitry is designed to eliminate background noise without unmuting the receiver. If you’re consistently experiencing wireless interference, it may be a good idea to check the stewardship of your wireless practices.  Make sure that you’re:

  • Setting the proper input gain
  • Using the right antennas and spacing them properly
  • Maintaining line-of-sight between the transmitter and the receiver
  • Coordinating frequencies
  • Using fresh batteries
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Davida Rochman

Davida Rochman

A Shure associate since 1979, Davida Rochman graduated with a degree in Speech Communications and never imagined that her first post-college job would result in a lifelong career that had her marketing microphones rather than speaking into them. Today, Davida is a Communications Manager, lending her skills to a wide spectrum of activities, from public relations and social media to content development and sponsorships.

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