From: wcs@anchor.ho.att.com (bill.stewart@pleasantonca.ncr.com +1-510-484-6204)
Date: Tue, 19 Apr 94 21:42:03 PDT
To: pgf@srl01.cacs.usl.edu
Subject: Re: cryptophone ideas
Message-ID: <9404200440.AA02524@anchor.ho.att.com>
MIME-Version: 1.0
Content-Type: text/plain


Phil Fraering writes: 
> Lady Ada writes:
> >The ideal phone might be based on CPU's, RAM, and DSP's, with no
> >DES chips or anything like that.
...
> Anyway, why do you need a DSP? I have read in several places that
> DSP's are going to be "replaced" by the CPU as time goes on and the
> CPUs just get more and more powerful.

For full-scale general-purpose computers with post-Pentium CPUs,
it's probably more cost-effective to do any number-crunching in the CPU,
though you can get a lot of crunchons for $25-50 of DSP these days
(if you're willing to add the cost of the I/O interfaces for it.)
Any extra price-performance you gain by skipping it makes the whole 
system faster, and if you need real-time audio-hacking you can
handle the extra OS overhead if your OS is well-designed.

However, for a cheaper single-purpose device like a fancy-processing phone 
(whether crypto or high-quality speakerphone or whatever),
the main activities are modeming, A/D conversion, bit-crunching 
(mostly signal-processing flavors), and some call-setup signalling.
A/D converters live on chips, DSPs are real good at digital signal processing,
modems chips are cheap and software on DSPs is another approach,
and the call-setup logic can fit on almost anything as long as 
you've got some spare ROM space; your design sophistication and
cost analysis will tell you whether you want to do it on an 8086
(or similar flavor of cheap microcontroller), which has enough horsepower
to do 10 kb/s of crypto in its spare time, or whether to add some 
program complexity to the DSP instead (popular if you're building ASICs).

Several of AT&T's DSPs have a miminal operating system built in which lets
you switch between different programs easily during processing
(I think it's non-preemptive, so you have to plan a bit in your code,
but it only burns about 5% of CPU for typical applications.)

# Bill Stewart  AT&T Global Information Solutions, aka NCR Corp
# 6870 Koll Center Parkway, Pleasanton CA, 94566 Phone 1-510-484-6204 fax-6399
# email bill.stewart@pleasantonca.ncr.com billstewart@attmail.com
# ViaCrypt PGP Key IDs 384/C2AFCD 1024/9D6465