[Linrad] Some succes on an embedded ALIX system

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hello Xtof,

> I did get a question I forgot with a range 10-999Hz to answer at
> hardware setup,
> but I could not find what this about so started with 100.
That was "max DMA rate"

Linrad will set the buffer size for input and output
devices and through that it forces the hardware to
its direct memory access (DMA) at a high rate in case
the user enters bandwidths and other parameters that
could produce a short dealy from antenna to loudspeaker.

On a slow computer one might want to limit the DMA rate
to avoid the extra overhead.

> On normal CW, this runs fine, with about 60% cpu. No choppy audio
> unless one starts other stuff.
> On weak signal CW, audio is choppy and CPU at 130%. The rest functions
> fine.
> 
> I ll see if I can optimize to squeeze out a little more. Any hints
> appreciated, I do need X since
> the system also runs my HPIB controlled band switching GUI.
There are many things you can do to make Linrad run more
efficiently. Start with X11 itself.


1) Make sure you do not have "visual effects" of any 
   kind enabled. (Ubuntu, at least recent versions have them
   by default and they destroy the performance of X11)

2) Try to set a smaller colour depth. Linrad is happy with
   a colour depth of 256 only so if your other softwares
   can accept that it would save some resources.

Besides setting the SDR-IQ decimation factors high enough
to make the input data stream slow enough you could 
compromise with the window functions. In case you do
not need a high dynamic range you might go for a sine
(to power 1) window. Check the different fft versions.
on some CPUs they might differ significantly.

I guess the problem with weak signal CW is the second
fft. In case you need the noise blanker, it should run
fine in case the CPU has MMX instructions so you can
select those fft2 variants.

A large "first mixer bandwidth reduction" as well as a low
sampling rate for the loudspeaker output should also
save some resources.

Please report further progress:-)

73

Leif / SM5BSZ

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