LPC for speech (fwd)

Forwarded message:
From jp12745@coewl.cen.uiuc.edu Fri Dec 10 14:46:07 1993
From: Jeffrey Wayne Porter <jp12745@coewl.cen.uiuc.edu>
Message-Id: <199312102045.AA16812@eehpx21.cen.uiuc.edu>
Subject: LPC for speech
To: jerry@terminus.dell.com (Jeremy Porter)
Date: Fri, 10 Dec 1993 14:45:49 -0600 (CST)
X-Mailer: ELM [version 2.4 PL22]
Content-Type: text
Content-Length: 948       

Did you know that using LPC (linear predictive coding) on speech can
near-telephone quality at only 8 k BITs/second?  With a signficant
decrease in quality (but still very understandable... probably better
than radio) you can get the rate down to 2kbps.  If you don't mind
sounding like a speak&spell, you can go to 600bps or less.


Using LPC, you could send real-time voice over the
internet.  It would even work (maybe just barely) over a SLIP
connection.  According to my professor, LPC can be implemented
in a simple DSP chip, so I figure a 486 ought to be able to
handle it, too.  Sound like an interesting (granted maybe not
too useful) project?  It would be a way of providing secure
voice communications -- LPC code the speech, encrypt the data
stream, transmit via v.32bis modem, etc.
-- 
--------------------------------------------------------------------
Jeff Porter
jporter@uiuc.edu    TA: ECE 290..."ph Jeff Porter" for office hours


-- 
Jeremy Porter  ------------- Systems Engineering --------
Dell Computer Corp.   ------ jerry@terminus.us.dell.com ----
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Support your Second Amendment rights to encryption technology.

<jp12745@coewl.cen.uiuc.edu>jerry@terminus.us.dell.com> wrote:

> Did you know that using LPC (linear predictive coding) on
> speech can near-telephone quality at only 8 k BITs/second?
> With a signficant decrease in quality (but still very
> understandable... probably better than radio) you can get
> the rate down to 2kbps.  If you don't mind sounding like
> a speak&spell, you can go to 600bps or less.

haha...  Geez oh man I just had to laugh at that speak&spell comment... 
Yeah, those things had funky voices, but it was pretty darn good speech
synthesis for the time. (around 1982-3? I guess)  (It could say alot
more than my Apple IIe :)  But those things had software with little
bugs (or undocumented features) if you pressed combinations of keys at
the same time it would do weird things.  I kinda wish I still had some
of those things, so I could try reprogramming it to do interesting
stuff.  (Imagines scene in US customs office: "Crypto exports?  No, sir,
this isn't a secret cipher machine, just an old speak&spell!  Honest! 
You didn't think I was going to try to phone home with it or something
did you?" ;)  but I digress...

600bps is not realistic.  Most people can READ text faster than 600 bps
(okay it's a little above average, but not if you were skimming; to give
you a rough estimate, it's a little slower than reading one line per
second)  You just can't expect to cram all the intricacies and
inflections of speech into a 600bps channel and be understood.  But why
sacrifice quality when you can use a 14400bps modem?  They aren't all
too expensive these days.

> Using LPC, you could send real-time voice over the
> internet.  It would even work (maybe just barely) over
> a SLIP connection.  According to my professor, LPC can
> be implemented in a simple DSP chip, so I figure a 486
> ought to be able to handle it, too.  Sound like an
> interesting (granted maybe not too useful) project?  It
> would be a way of providing secure voice
> communications -- LPC code the speech, encrypt the data
> stream, transmit via v.32bis modem, etc.

Not too useful???  Sounds very useful to me.  If someone got this
working, it would be all we'd need to kill the clipper chip completely. 
Who would spend several hundred dollars for a "secure" phone when you
can do the same (or better) with a $50 sound card for your computer? 
(Well, some might need to upgrade their modems too, but it would be a
good idea to do that anyway.)  Home (or office) computer cryptophone =
no need for clipper.


loki@cass156.ucsd.edu wrote in response:

> I am working on this very thing. We will be using LPC
> encoding for compression, IDEA for encryption, and DH
> key exchange for key handling.  We plan to use
> something better than DH ASAP (something less
> vulnerable to man in the middle attacks). We plan to use
> 14.4kbps transmission speed.

What kind of hardware will you be using?  486?  DSP?  or something 68xxx-based?
What additional hardware (sound cards) will be supported/required?
Will source be availiable?

> 
> <jp12745@coewl.cen.uiuc.edu>jerry@terminus.us.dell.com> wrote:
> 
> > Did you know that using LPC (linear predictive coding) on
> > speech can near-telephone quality at only 8 k BITs/second?
> > With a signficant decrease in quality (but still very
> > understandable... probably better than radio) you can get
> > the rate down to 2kbps.  If you don't mind sounding like
> > a speak&spell, you can go to 600bps or less.
> 
[...]
> 600bps is not realistic.  Most people can READ text faster than 600 bps
> (okay it's a little above average, but not if you were skimming; to give
> you a rough estimate, it's a little slower than reading one line per
> second)  You just can't expect to cram all the intricacies and
> inflections of speech into a 600bps channel and be understood.  But why
> sacrifice quality when you can use a 14400bps modem?  They aren't all
> too expensive these days.

600 bps is not realistic?  Voice has been gotten down to 400 to 600bps
by doing some coding on the output of a LPC (pitch + gain + filter
parameters) and transmitting that.  The remote decodes to recover
LPC parameters and uses that to synth speech.  The quality is supposed
to be the same as 2400 bps LPC (slightly synthetic sounding,  definitely
not 'toll quality').