Monday, January 14, 2013

Breaking Squelch


Beginning in the 1620s, squelch meant "to fall, drop, or stomp on something (soft) with crushing force." The sense we use in radio meaning to  "suppress completely" is first recorded 1864. The word squelch is actually a corruption of the old English verb "cwelan" which meant "to kill."  It also survives as the verb "quell" meaning to subdue. It may originate in the early Germanic "qvale" meaning to choke.  the word has almost come full circle.

In telecom, squelch is a circuit function used to suppress the audio (or video) output of a receiver in the absence of a sufficiently strong desired input signal. It's more formally known as carrier squelch. It sets a threshold below which it will silence your receiver suppressing noisy channels.  For that reason it's also sometimes called noise squelch. More here.


Next we have tone squelch. This is a form of selective calling. It's just a technique to select individual receivers when there is a set of receivers on one frequency. With tone squelch  the sender also transmits a sub-audible audio frequency tone beside the voice message.  The receiver detects the specific frequency and will only operate if the signal tone is present. This is often used with repeaters. In FM radio this is build-in using CTCSS (Continuous Tone-Coded Squelch System.) The original tone set was 10, then 32 tones, and has been expanded even further over the years to 50 tones between 67 Hz and 250.3 Hz. Some manufactures have their own implementations of CTSS. GE uses a variant called "Channel guard" and RCA had one called "quiet channel." Motorola calls theirs "private line."There were others as well. various military entities had their own tones and frequencies also.

Later, Motorola developed what they called "Digital Private Line", or DPL. This was the first Digital-Coded Squelch (DCS). Here instead of a constant low frequency tone, DPL sends a continuous stream of 23-bit square wave data at 134.4 bps repeating. There are anywhere from 83 to 104 codes depending on the manufacturer of your gear. Now you should also know that 9 bits of data could hypothetically provide 512 codes. But because patterns repeat, shuffling that shift can create false positives. Lost you there?  Let me show you by shifting blocks of sequenced characters. See below:

ABCDABCDABCDABCD
BCDABCDABCDABCDA
CDBACDBACDBACDBA
DABCDABCDABCDABC

Starting with four unique repeating sets (ABCD, BCDA, CDAB, DABC) I eventually cycle through a series that matches another code. It's called a false positive.