On Thu, 21 Jun 2007 17:44:13 -0700, john wrote:

I'm working on a SDR design using the AD9874 to
digitize the IF producing 280 Ksamples/sec and
am trying to size the DSP. I'm leaning towards
the ADSP-BF532 which is a fixed point DSP rated
at 400 MIPS / 800 MMACS and is available in a LQFP
package. I'd like to be able to handle everything
from decoding standard shortwave SSB signals to
broadcast FM stereo signals (including RDS).

1) Any pointers to information which can be used
to size the DSP in terms of MIPS, program RAM,
and data RAM? Something like a minimum of X MIPS
are needed to do a reasonable job handling
broadcast FM stereo signals (which I'm assuming
require more MIPS than SSB) would be useful
to know.

2) Anyone have a specific DSP they favored for
this type of application? Keep in mind I
need the DSP to be in a package I can handle
using home equipment. It would be nice if the
DSP was flexible enough so that it can be used
for general purpose things such as controlling
the keyboard and LCD instead of adding a
microcontroller.

I'm not looking to use a FPGA at this time.

1. I generally do this by prototyping the algorithms using Scilab, or
designing them on paper, then figuring out the number of operations (for
MIPS) and data storage requirements. You'll probably find that the FM
demodulation takes the most processor time, unless you're decoding
multiple SSB channels for some odd reason.

2. I make a conscious effort to not fall in love with any particular
processor. Look at a few different choices in your target word
size and data type, and try to get a feel for how much time it'll take to
run your algorithms.

1 & 2. _Don't_ forget the 'regular processor' stuff when you're sizing
memory and processor bandwidth! Wouldn't you feel silly if you had 20
times the RAM you needed for your DSP, but didn't have the buffer space
necessary for your TCP/IP stack, or if your algorithm took up 500 words of
memory but you ran out of space for menu lines in ROM? Generally with
these mixed-target DSP projects you can expect 1% of the code to take 95%
of the consumed processing power; you need to make sure you have the code
space for the other stuff.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html