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From: gronk!johnl@uunet.uu.net (John Limpert)
Newsgroups: comp.dcom.telecom
Subject: Re: IRIDIUM: Motorola's New Cellular Phone System
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Date: 10 Aug 90 19:45:47 GMT
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mk59200@metso.tut.fi (Kolkka Markku Olavi) writes:

>Does anybody know what kind of compression they plan to use to squash
>8kHz bandwith sound through a 4800bps channel?

The original article said that the system used vocoders, not telco
style A/D converters.  A vocoder (voice encoder/decoder) can operate
at very low data rates.  The Texas Instruments Speak and Spell toy
used vocoder technology (linear predictive coding) to fit digitized
voice into the toy's ROM chip.  A vocoder uses a model of the human
vocal tract to transmit speech.  It continually adjusts the model to
approximate the speech input and periodically transmits the parameters
to the decoder on the other end.  

The receiving vocoder uses the parameters to synthesize speech.
Vocoders do not transmit a waveform, they transmit a description of
their vocal input.  This can include energy in various frequency
bands, whether the speech is voiced (ah sound) or unvoiced (sh sound),
dominant pitch etc.  Although vocoders can be efficient, they are not
without problems.  At low data rates the output can sound like Donald
Duck and you can confuse the vocoder by talking too fast.  You may not
be able to verify the identity of the caller by the sound of their
voice.


John Limpert		johnl@gronk.UUCP	uunet!n3dmc!gronk!johnl