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Squelch Techniques

The detector circuits in wireless microphone receivers generate high-level audio noise when no RF input signal is present. If allowed into the audio system, this noise would be intolerable to listeners, and it could pose a serious risk to amplifiers and speakers. For a wireless microphone system to be useful, the receiver must include some form of squelch circuitry to mute the audio output when no RF signal is present.

The simplest form of squelch circuit monitors the RF input signal to the receiver and mutes the audio output if the signal level drops below that necessary to obtain satisfactory audio. If the squelch circuit is set correctly and everything works as intended, the receiver will only output audio that is of acceptable quality. At other times, the receiver output will be muted. This type of squelch works extremely well unless interference is present.

Interference complicates the situation considerably. Interfering signals can trick the receiver into unmuting the audio output, especially when the wireless transmitter is turned off. One approach to this problem is to make the squelch point adjustable. Often a squelch setting can be found that will unmute the receiver when the strong transmitter signal is received, but not when only the weaker interfering signal is present. Unfortunately, a high or "tight" squelch setting usually will also reduce the maximum range of the wireless system. This is because the transmitter must be relatively close in order to deliver the necessary strong signal to the receiver.

To help overcome this problem, other more complicated squelch techniques have been developed. One is to add a high-frequency pilot tone to the transmitter signal. The receiver checks incoming signals to see if the pilot tone is present. If not, the receiver assumes that the signal is not from its own transmitter and keeps the receiver muted even if the interfering signal is very strong. This form of squelch is generally referred to as "pilot tone" or "tone coded" squelch.

A third approach is to monitor the level of high-frequency noise at the output of the receiver detector. A high noise level indicates that the signal being received is being interfered with, or that it is not at the correct frequency and is probably the wrong signal, or that it has other characteristics that indicate that it is not the desired signal. When combined with the signal-level-monitoring type of squelch, this technique is sometimes referred to as "dual mode" squelch.

Other combinations of squelch circuits have been used, as well as even more complicated techniques. Unfortunately, none of these approaches is completely foolproof. Pilot tone squelch systems can be tricked by intermodulation. An intermodulation product caused by a transmitter with a pilot tone and another signal will have the pilot tone present. Similarly, a strong, clean interfering signal on the correct frequency will trick a receiver with dual mode squelch.

There is also a fundamental problem with all squelch techniques. While sophisticated squelch circuits can usually keep the receiver muted when interference is present, they can't make the wireless system usable. That is, the real choice is between no audio and bad audio. Obtaining good audio requires the elimination of the underlying interference problem. In the absence of interference, very simple squelch circuits work quite well; but when serious interference is present, even the most sophisticated squelch technique can only break the audio path and silence the system.

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