I did think of that too, and should be pretty easy to build with a .wav
editor on any PC. My only concern is about channel separation in the
pocket player, and whether MP3 compression would throw away the tone or
not.

It depends upon the compressor and the type of compression chosen. There
are three options in most compressors: Mono, Joint Stereo and Stereo.

Keep in mind that MP3 files are really MPG movie streams with no video.

Mono is obvious.

Stereo is two seperate mono channels. This is best for programs with
multi language sound tracks.

Joint stereo is the same system used in FM stereo. FM uses a SUM track
(mono) and a DIFFERNCE track. I think MP3's use one track and a difference
track. Since it is decoded by a computer, this saves processor time
over the sum and difference which requires two tracks to be calculated
after decompression instead of one.

Because of MP3 compression I think the tone should not be chosen either
too high or too low.

Anything in the higher audible range, say 300 Hz on up, but 2600Hz is
sort of a standard in the telephone industry and 1000Hz in the audio visual,
so your choice may be influenced by what you can find easily.

Analog phone cells identified themselves with tones in the 5kHz range.
They were used to determine which cell your phone was actually receiving.
Obvioulsy the phone filtered out the tone.

A simple set of capacitor audio filters such as the ones used in SCAF
units would do the trick and you could use a mono signal. You just make
a 3kHz low pass filter for the audio and a 4khz high pass filter
for the control signals.


I'm not sure if it would be any value, but you could also use different
tones to key different things; standard DTMF would be one way to do it,

DTMF would be simple to use, encoder and decoder hardware is very easy
to get and well documented. The tones are audible so you would need
to use two channels, or introduce a delay to prevent them from being
transmitted.

The old motorola pagers used reeds at very specific filters. Subaudible
for signal identification and audible tones to unlock the recevier.
You may be able to find lots of them in junk bins.


Well, walking uphill with antenna, RTX and battery (one each) is tiring
enough - that's about 3kg -, having even more radios is not for me. :-)

You could just get a bunch of audible greeting cards (or the chips from
inside them) or use a pocket pc or late model Palm.

Geoff.

--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
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