At 05:55 PM 1/9/98 GMT, you wrote:
>One doubt:
>
>Still referring to my tones via phone lines problem, will I get any benefit
>by using Manchester encoding in my transmission ?
>
>I understand the advantages of a synchronous approach when you are recording
>the data on a cassette, but how about phone line transmission ?
>

Phone lines are a little different in their frequency spectrum.  They
don't do good with 300 hz tones, so MFM is not a reasonable method.

However, Manchester is still fine, but not optimum.  It would give you
two tones an octave apart.

The biggest concern with phone lines is "Group delay distortion".  This
is basically a inconsistant phase shift throuhout the frequency range.
To minimize this type of distortion, your tones should be closer together.
For example, if you're using 1200 hz with Manchester, the low tone is 600 hz.
Both tones are well within the passband, but the delay of 600 hz will
likely be different than the delay of 1200 hz.

For ease of use and good performance, it's best to use real modem
chips.  Some of them are even good enough to automatically compensate
for group delay distortion.

Only the absolute lowest cost designs should use Manchester, or FSK,
over a phone line. It will work, but it's easier and fairly cheap to use
real modem chips. One popular one was from TI - I think it was only a 1200
baud chip, but it was easy to interface to and cheap.  I've heard that it
may not be available any more - I can't remember the part number.

Does anyone know of some inexpensive modem chips?  Please give us
some part numbers.

Eric Engler