[hpsdr] Odyssey dsPIC33 RAM usage

[Frank Brickle]

On Sun, Jun 1, 2008 at 10:07 AM, Murray Lang <murray.lang at bbnet.com.au>
wrote:


> Is the prototype code available? I haven't found any SVN links, but can
> understand why it would be under wraps given that it's probably quite fluid.
> Is it based on DttSP? Are the free versions of the Microchip tools likely to
> be sufficient? Are you using an operating kernel or is it just basically
> interrupt driven?


The SDX code is moving forward fast now.

The free Microchip tools are OK for learning, but you really need the
optimizing compiler which is not free.

There are a few fundamental problems using the dsPIC33 for SDR. The biggest
one is that it's not fast enough to sustain 256-point complex FFTs at 48kHz.
For Suitsat 2 we're using a by-hook-or-by-crook approach in place of the
general algorithm exemplified by DttSP. Basically the idea is to stipulate
that incoming data arrive at the dsPIC at baseband, so the I and Q arms can
be filtered separately with simpler low-pass filters. The processed signal
is then mixed to one of a select set of channel bandcenters. The bandcenters
are the ones which correspond to "simple" values of the (cos, sin) mixing
LO.

Another big issue impacting generality is filter scaling. Since internal
data is fixed point, you have to juggle the coefficients to make best use of
the 16 bits available for the filter coefficients and gain. There is no
direct path from a filter spec to a design. You have to experiment using a
combination of science, art, and cunning.

Along the same lines, the other big snag is AGC. We're using very elementary
power computations on the input signals to determine binary shifts on the
output.

None of this means that the dsPIC33 is unsuitable for SDR applications. It's
not a general-purpose engine, though. You need to look carefully at your
application and figure out where you can cut corners.

73
Frank
AB2KT

>
>
> Cheers,
> Murray VK6HL
> _______________________________________________
> HPSDR Discussion List
> To post msg: hpsdr at hpsdr.org
> Subscription help: http://lists.hpsdr.org/listinfo.cgi/hpsdr-hpsdr.org
> HPSDR web page: http://hpsdr.org
> Archives: http://lists.hpsdr.org/pipermail/hpsdr-hpsdr.org/
>



-- 
The only thing we have to fear is whatever comes along next. -- Austin Cline
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/attachments/20080601/df877d62/attachment-0003.htm>