From: andrey@cco.caltech.edu (Andre T. Yew)
Newsgroups: comp.sys.amiga.misc,comp.sys.amiga.advocacy
Subject: Re: DSP not as good as we thought?
Date: 21 Nov 1992 16:43:15 GMT
Organization: California Institute of Technology, Pasadena
NNTP-Posting-Host: punisher.caltech.edu

ins739l@lindblat.cc.monash.edu.au (William Fang) writes:

>Anybody out there has a true idea of what can be done with a DSP chip?

>Some questions I would like answered would be:

>o	Who makes them

	Motorola, TI, Analog Devices, and AT&T are some of the
more popular ones.

>o	Various prices and specs, is xxx better than yyy?

	Very cheap to very expensive (less than $10 to hundreds of
dollars for really fancy ones like the Motorola 96002).  There
are too many to list.  As for one being better than the other,
there are lots of factors affecting this: cost, efficiency
at what you want it to do, ease at building it into your
system, availability of programming tools, etc.

>o	What can be done with a DSP

	Lots of things.  They're designed to deal with signal-
processing algorithms best like fast Fourier transforms,
convolutions and filter networks.  Motorola doesn't hesitate
to tell you how close to the theoretical limit its DSPs come
when performing the FFT, for example.

	Some distinguishing features of DSPs are:  on chip register
stack (good for really fast do-while loops), modulo anything
memory addressing (good for cycling over a buffer of samples),
multiple address spaces, parallel operation of (usually) fetch
and arithmetic operations.  For example, the Motorola 56001 can
dereference a register, increment it, and move it to another
register for two different sets of registers while clearing
another.  Multiply and accumulate (multiple by something and
add it back to itself, x = a*x+b) is standard.  And most instructions
execute in 1 or 2 cycles.

>o	How this is done (not just.. can emulate modem, make sound, etc..)

	Well, you just calculate the samples you need, and output that
to a digital-to-analog convertor.  A lot of people out there make
glueless logic convertors for DSPs.  It doesn't hurt that a lot of
DSPs have their own serial ports, too.  Note that any non-DSP chip can
calculate those samples for you -- it's all a matter of speed and
convenience.

>o	Why it is better than using the main processor

	It's another processor working if you can use it well.
It's faster for for certain specialized operations like
Fourier transforms and convolutions.  Although, I've seen
a realtime raytracer done with 64 AT&T DSPs.  It's great
for a sound synthesizer, too.

	DSPs can't replace main processors because of their
restricted instruction set and naive memory handling (no
MMU).  I think the only notable exception to this is
the Intel i860.  Is it a DSP or is it a micro?  I don't
know, you tell me.

	In fact, for a lot of things, fast RISC chips like the
MIPS R3000 or 4000, or HP's PA chipset tend to outperform
cheap DSPs on many traditional DSP tasks.  PA and IBM's
RS/6000 chipset both come with a multiply and accumulate
instruction, can execute multiple instructions simultaneously,
although only the IBM chipset can do really insane stuff, and
have 1-2 cycle instructions.

>Or is there a better place such as a newsgroup that I should be reading?

	Try comp.dsp.  There's usually a FAQ list with the technical
specs on lots of different chips.  People there also talk about
DSP algorithms, so you can get an idea of what people do with
DSPs.

>Thanks 

	No problem.

--Andre

-- 
             Andre Yew andrey@cco.caltech.edu (131.215.139.2)
Newsgroups: comp.sys.amiga.misc