The system involves the use of a sub-audible tone injected into the voice-modulation line of the transmitter and a matching tone detector in the receiver after the FM discriminator. This mechanism is used by both mobile handheld radios and base stations/repeaters to maximize the use of a single shared frequency among many users. It's also known under the brand names PL (Private Line) to Motorola users or Channel Guard to GE/Ericsson users. Here is a relatively simple technique that uses the modified form of the Goertzel algorithm to detect squelch tones that can be adapted to many applications where the detection or analysis of a limited set of frequencies is desired.Ĭontinuous Tone Coded Squelch System (CTCSS) is a generic term used by the mobile radio industry the technology itself accepts desired signals and rejects undesired ones on a voice radio channel. The lesser-known Goertzel Discrete Fourier Transform (DFT) may not be as familiar to many. Most engineers involved in the design of Software Defined Radio (SDR) applications are well acquainted with the Fast Fourier Transform (FFT) and related algorithms.
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