From: norm@netcom.com (Norman Hardy)
Date: Wed, 11 Aug 93 23:32:44 PDT
To: cypherpunks@toad.com
Subject: Re: Secure voice software issues
Message-ID: <9308120629.AA17113@netcom4.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


Eric Blossom <eb@srlr14.sr.hp.com> says:
> I have seen estimates that a straight forward implementation requires
> about 13.5 million Mulitply+Accumulates / second.  Most of the time is
> burned up using a brute force search for the best excitation vector to
> use.  There is a fixed 512 entry code book, and a dynamic code book
> with 256 entries (it may be 128).  Each code book entry is an
> excitation vector that is 60 samples long.  Therefore, to evalute each one,
> you have to run a 60 element vector through a 10 pole filter to get
> the predicted output, then compute some measure of error.  This
> requires an additional difference operation that is implemented as
> some kind of "perception weighting filter" (I don't remember the
> details).
 
I have been reading the PowerPC 601 manual (MPC601, The Macs of early
1994). It is dangerous to
believe performance figures. They give you the world in one chapter 
and then take it back here and there in bits and pieces.
Here is what I see however. Simple single precision floating point
operations can issue one per cycle. The book mentions several
floating point ops that take more than one clock in a pipeline stage.
They don't mention floating multiply-add. I think one can issue each
clock. I-unit instructions can issue in the same clock as floating
point ops. If you do the block trick used to multiply matrices then
one load is required per multiply add. All this leads to the optimistic
estimate that the 50MHz machine can sustain nearly 50 fmadd's per
microsecond on a 50MHz chip. Inner products are much like matrix multiply
which is a benchmark where the RS/6000 (The MPC601's father) achieved
nearly one fmadd per clock, and that was double precision!
 
128 excitation vectors each of 60 single precision loats fit in the on
chip cache, but it is tight.
 
There may be enough margin here for it to work with no special DSP.
 
I'll be in Yosemite for a few days so I won't be able to respond
immediately to comments.