Hello, friends.

I need some help.

In this project I am involved with, I have to send certain numbers through
the phone line. Since it is just a few numbers each time, I decided not to
use a modem chip, and just code my digits in 3 frequencies (440 Hz, 1400 Hz,
2300 Hz) right out of the processor.

Basically, what I do to send the code "32" is send 3 pulses (100 ms each) of
1400 Hz, one pulse of 2300 Hz (same duration) as a separator, and then 2
more pulses of 1400 Hz. Pulses are separated by a 100ms silence. Roughly,
that's what I am doing. Could not be simpler.

Concerning the hardware, I have a strong clear signal going out and a
beautiful square wave at the receiver's opamp output.

I am decoding these frequencies by measuring the pulse width. If the result
is the same for, say, 5 consecutive measurements, then the corresponding
frequency is validated.

Well, it works great... in some phone lines. And not so great in others,
specially if both phones are separated by a greater distance. I don't know
why, but the decoder tends to miss some pulses in this situation.

Why this is happening ? Someone told me that it would be better to fill the
100 ms silences with some frequency, just to keep feeding signal to my
receiver's amplifier.
I don't think it makes much sense, though...

Any help will be much appreciated.


Pedro.