Square waves are taboo on the phone lines.  The bandwidth is only about 300
to 3000 Hz, and your square waves are making lots of harmonics than can be
confused with a higher transmit frequency.  In addition, the higher
harmonics, sampled at 8 kilohertz will alias down to the 300-3000 Hertz
range and cause additional distortion of the signal you want to detect.
Create sine waves, or low pass filter your square wave, and you will
probably eliminate most of the problems you are having with various phone
lines.

At 09:46 PM 1/7/98 -0500, you wrote:
>> 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.


Regards,
  Ron Fial