Date: Mon, 28 Sep 92 01:03 PDT
From: john@zygot.ati.com (John Higdon)
Organization: Green Hills and Cows
Subject: Re: LD Transmission Quality Comparison

Jack Decker <Jack@myamiga.mixcom.com> writes:

> I also wish there was a simple add-on circuit for phones that would
> increase the receiver volume level WITHOUT increasing sidetone.  Most
> "amplified" handsets can raise the volume of a long distance call, but
> when you talk you blast yourself in the ear!  I'd LOVE to see a phone
> with both adjustable receiver volume AND adjustable sidetone ... would
> that be too much to ask?

The simple passive hybrid used in most telephones would be incapable
of providing sufficient transhybrid loss to be acceptable for use with
an amplified earpiece. Sidetone is simply the leakage from an
imperfect hybrid and is not something that can be adjusted. To do
that, one would have to use an adaptive hybrid and then intentionally
and adjustably feed transmitter audio into the receiver. The best
adaptive hybrids are executed in the digital domain, something that is
out of the question for use in a simple telephone.

The usual workaround for an amplified handset is to employ a noise
cancelling transmitter. This cuts the ambient noise pickup and the
resultant sidetone distraction.

> [Moderator's Note: I've always wondered about this also. I have
> various amplified handsets around and if I get a call with very poor
> transmission I turn the volume up ... all I wind up getting is lots
> more sidetone and very little increase (if any) in the voice on the
> other end.  PAT]

An effective trick is to put your hand over the transmitter. This
blocks the ambient noise pickup. It is surprising to see people on the
telephone in a noisy environment putting a finger in the opposite ear
in order to hear the caller on the line more clearly. It is a much
more effective practice to cover the mouthpiece. The brain can
differentiate signals that come in through separate ears with little
difficulty, so what you want to do is to reduce as much as possible
the distractive noise coming into the same ear as the distant caller.


John Higdon  |   P. O. Box 7648   |   +1 408 264 4115     |       FAX:
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