Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!uakari.primate.wisc.edu!uflorida!gatech!bloom-beacon!mcgill-vision!mcgill-vision!jean
From: jean@mcgill-vision.uucp (Pierre Racz)
Newsgroups: comp.dsp
Subject: Re: US Gov. sends real-time audio at 2400 baud!
Keywords: Low bitrate speech coding
Message-ID: <1990Nov9.183212.1441@larry.mcrcim.mcgill.edu>
Date: 9 Nov 90 18:32:12 GMT
Sender: news@larry.mcrcim.mcgill.edu (Usenet File Owner)
Distribution: na
Organization: McGill University, McRCIM
Lines: 20

It is possible to send intelligible voice at bit rates as low as 1k However
the quality suffers.  Speech quality can fall in one of the following
classes:

1 - Broadcast
2 - Toll
3 - Communication
4 - Synthetic

Currently, broadcast quality requires 64kbps and is your typical AM radio
station quality.  Toll quality is the standard telephone fare.  By complex
coding the bit rate can be reduced to about 10k bps.  Communication quality
is highly intelligible but has noticeable distortions, your would expect this
from short wave radio.  This level can be achieved with 4.8kbps.  Synthetic
quality, while 80% to 90% of the speech is intelligible, sounds "machine
like", "buzzy" and the identity of the speaker cannot be recognized any more.
Code rates of less than 4.8kbps (and as low as 1kbps) can be achieved at
this level of quality.

As time goes on, these bit rates will be reduced further.