This feature (sometimes called РІР‚Сљtone burstРІР‚Сњ) is common in European systems. Selcall (Selective Calling) transmits a burst of five inband audio tones at the beginning of each transmission. Old and new radios with CTCSS and radios across manufacturers are compatible. Any CTCSS system that has compatible tones is interchangeable. There are many other company-specific names used by radio vendors to describe compatible options. RCA Corporation used the name Quiet ChannelР’В or QC. General ElectricРІР‚в„ўs implementation of CTCSS is called Channel GuardР’В (or CG). The original tone set was 10, then 32 tonnes, and has been expanded even further over the years.Р”В¬TCSS is often called PL toneР’В (for Private Line, a trademark of Motorola), or simply tone squelch. CTCSSР”В¬TCSS (Continuous Tone-Coded Squelch System) continuously superimposes any one of about 50 low-pitch audio tones on the transmitted signal, ranging from 67 to 254 Hz. Selective calling can be regarded as a form of in-band signaling. It will still degrade system performance but by using selective calling the user will not have to hear the noises produced by receiving the interference.Р”Р‡our different techniques are commonly used. Receivers with poor specifications-such as scanners or low-cost mobile ham radios-can not reject the strong signals present in urban environments. In non-critical uses, selective calling can also be used to hide the presence of interfering signals such as receiver-produced intermodulation. Selective calling locks out all signals except ones with the correct code. A carrier squelch is unlocked and will let any signal in. This is akin to the use of a lock on a door. Instead of turning on the receive audio for any signal, the audio turns on only in the presence of the correct selective calling code. Where more than one user is on the same channel, selective calling addresses a subset of all receivers. Tone squelch, or other forms of selective calling, is sometimes used to solve interference problems. Some applications have the receiver tied to other equipment that uses the audio muting control voltage as a РІР‚Сљsignal presentРІР‚Сњ indication. When a signal with little or no noise is received, the noise-derived voltage goes away and the receiver audio is unmuted. The presence of continuous noise on an idle channel creates a DC voltage that turns the receiver audio off. The audio output of the filter and amplifier is rectified and produces a DC voltage when noise is present. The squelch control adjusts the gainР’В of an amplifier which varies the level of noise coming out of the filter. A typical filter might pass frequencies over 4 kHz. It takes out the voice components of the receive audio bypassing the detected audio through a high-pass filter.

However, if a weak signal is annoying, the operator can adjust the squelch to open only when stronger signals are received.Р”Р„ typical FM two-way radio carrier squelch circuit is noise operated. The usual operation is to adjust the control until the channel just shuts off РІР‚вЂњ then only a small threshold signal is needed to turn on the speaker. Backing off the control will turn on the audio, and the operator will hear white noise (also called РІР‚СљstaticРІР‚Сњ or squelch noise) if there is no signal present. This setting adjusts the threshold at which signals will open (un-mute) the audio channel. Some have push buttons or a sequence of button presses. In devices such as two-way radios, the squelch can be adjusted with a knob. Receivers in base stations at remote mountain top sites are usually not adjustable remotely from the control point. For example, television squelch settings are usually preset. In some designs, the squelch threshold is preset. It operates strictly on the signal strength, such as when a television mutes the audio or blanks the video on РІР‚СљemptyРІР‚Сњ channels, or when aР’В transceiverР’В mutes the audio when no signal is present. A carrier squelchР’В or noise squelch is the most simple variant of all.

Astro saber squelch 1 or 9