Duplex Stereo over the phone? (littlle OT)

Theo t.hogers at home.nl
Tue Aug 1 09:05:16 CEST 2000

We used to do this at a (not exactly leagal) radio station I worked for.

The problem is the phone lines bandwith.
If you have a unfiltered line there is no problem.
However in some cases the band is limmedted to about 8kHz.

We tried serveral methods.
The winner was to sent two AM modulated signals (L&R) over the line.
I don't remember the modulation frequenties that we used,
think they were around 500kHz or somthing.

The sine carrier waves where generated using a digital counter
and a 4051 that steped through a voltage divider repesenting a quater sine
The audio input was the top of the voltage divider.
The resulting sine wave has virtually no side bands, real squiky clean.
(If I remember well the first side band was the 32th hamonic at somehting
like -80dB)

On the other side of the line where two ppl based demodulators.
This way we could send 20kHz sterio over one unfiltered line.
The results where really good.

An other method was to shift one channel 20kHz up the band,
add both channels and send them over the line.
On the other side a crossover splitted the channels
and the shifted channel was shifted back to the original pitch.
This was more complex to do and had bad effects on the audio quallity.

Method number three was to use one sine carrier and use AM modulation for
one channel and FM for the other.
Worked ok, however since the signals where not treated in the same way
the sterio immaging was not optimal.

I've looked for schematics, but can only find the anti alliasing filter that
we used. :(

Hope this helps,


----- Original Message -----
From: Barry L Klein <Barry.L.Klein at wdc.com>
To: <synth-diy at node12b53.a2000.nl>
Sent: Monday, July 31, 2000 6:40 PM
Subject: Duplex Stereo over the phone? (littlle OT)

> Wonder if anyone here has any experience with this....
> What would be the least expensive method to achieve stereo duplex audio
> over a phone line?
> I want to be able to have a typical interactive conversation in stereo at
> both ends using a single line.  Would you go with a laptop at each end and
> some software program, or is there some other way?
> Barry
> barry.l.klein at wdc.com

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