On Tue, 02 Dec 2008 13:58:37 +0000, Dave wrote:

JosephKK wrote:
On Mon, 01 Dec 2008 14:13:23 +0000, Dave wrote:

JosephKK wrote:
On Sun, 30 Nov 2008 22:52:16 +0000, Dave wrote:

JosephKK wrote:
Which make and model / series? There are dozens of variations.

These are the standard CTCSS freqs:

http://www.batnet.com/~mfwright/ht220plcodes.html

http://en.wikipedia.org/wiki/CTCSS
Have you read your own references?

Bottom line, most any tunable tone decoder.

If a vibrating reed can be considered a "decoder" then you are correct.

However, there are not "dozens of variations".

Sure can. Collins Radio used mechanical resonators in an HF radio ISB
diversity 2240 baud modems. Then they used a switched capacitor meta
resonator. Then they used DDS synchronous comparison. And of course
there are various narrow band filter methods. And of course whatever
internal method the 8870 DTMF decoder uses. Who knows what methods
can be implemented in fast UC or small DSP or even an FPGA, except it
will not be a mechanical resonator. I suppose a mathematical model of
a mechanical resonator though.

Damn, you overthink things. The subject is PL, as generally used in
2-way mobile and hand-held radios and base stations. There are a few
variations, as a handful of tones have been added to the original group.

The receiver looks for a tone in the audio of an incoming signal. If
that tone is heard, that part of the squelch system unmutes the audio.

I have no idea how you got from that to Collins HF Modems, but I'm sure
it was a lovely trip for someone.

You wanted a tone detector. Then you asked if a vibrating reed
(mechanical resonator) can be one. Then you said that there was not
many methods, so i named some that even did more complex things. You
asked for the discussion and received it.
Moreover, the tone squelch system does some other complex things as
well. Particularly when multiple users (groups) are on a single
channel in a single locale.