From: koontzd@lrcs.loral.com (David Koontz )
Date: Mon, 1 Nov 93 11:34:47 PST
To: mg5n+@andrew.cmu.edu
Subject: Re: Secure Phone Progress (fwd)
Message-ID: <9311011627.AA02085@io.lrcs.loral.com>
MIME-Version: 1.0
Content-Type: text/plain


>From: jim@Tadpole.COM (Jim Thompson)

(lets see if I can do this without screwing up the spelling)

>CLEP is a speech encoding algorithm (compressor) that can work well inside
>a 4800bps channel.  It is, however, quite expensive in terms of CPU power.
>A DSP would help here.  :-)  CLEP also tends to diminish the dynamic range of
>its input, with a resulting loss of 'quality'.

Thats CELP - Codebook Excited Linear Predictive coder.

The reason everyone targets 4800 baud is to allow operation over analog
cellular phones, which won't support faster transmissions.  The government
extensively uses secure voice systems over cellular phones.  The market
place for secure voice follows the government.  CELP can be a single chip
solution (QUALCOMM, $70 in 10K, (QCELP)).  

I'm not sure AT&T Surety Communications devices use a single chip solution,
they're guaranteed to use AT&T DSP chips.  A proprietary vocoder (ACELP)
is used, which is supposed to work better with female voices.

A large part of insuring the quality of compressed voice falls under the
heading of signal conditioning.  AGC for dynamic range, pre-emphassis or
otherwise filtering to match the frequency response of the input (mic and
preamps).

AT&T sells 5 plug-in modules to their 3700 unit which are considered 
adequate to interface to all the phones out there.  I recall there was
one for a 500 series phone, and one for NEC phones.