Remember that echo is not unknown on phone lines. This won't help you,
either. Also various equalisation is going to play havoc with your
square waves.

> ----------
> From:         Pedro Drummond[SMTP:pdrummond@IBM.NET]
> Reply To:     pic microcontroller discussion list
> Sent:         Thursday, January 08, 1998 6:01 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: tones via phone line
>
> True, I am sending square waves. But at the receiver I have an
> opamp-based
> filter/amplifier, so I got kind of confident about the square wave
> problem.
>
> One extra information: it seems that on the great majority of times
> the
> decoder misses the 2300 Hz separator tone...
>
> Thanks again.
>
>
> Pedro.
>
>
>
>
>
>
> At 20:41 07/01/98 -0800, you wrote:
> >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.
>