[Linrad] Re: SDR-IQ + LINRAD

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

Hi Dom,
 
> DMA rate (10-999) within output sound board configuration ( mine is a 
> realtek built in my Asus laptop)
> 
> I'll be happy to get some informations abt this.

Linrad will compute the delay from antenna to loudspeaker based
on the FFT sizes you have selected. There has to be data in memory
at least corresponding to the longest transform needed.

Based on the computed minimum delay Linrad will allocate buffers
for the soundcards that are small enough to not increase the total
delay significantly. That means that you may end up with very small
IO buffers in case you specified FFTs (filters) that only cause
a delay of say 10 ms. The associated buffers would then only be allowed
to add about 10% additional delay each (sound input and sound output)
which would mean that the hardware would have to issue interrupts at 
a rate of about 1 kHz in its DMA (direct memoty access) and that the 
max latency allowed when other threads hold the CPU has to be in the 
order of a few milliseconds.

Linrad can run under Linux with very high DMA rates on modern computers
(e.g. Pentium IV) but only with svgalib. When running under X11 one has to
limit the dma rate to something like 300 Hz. On slower computers one might
need a lower limit. In case you want to run some other software in parallel
and that other software holds the cpu for half a second occasionally
you might have to set the limit to 10 Hz. (for perhaps 8 buffers holding 
0.1 second of data each) You would also have to set the output delay
margin to perhaps 0.6 seconds in such a case.

In case you have selected a small bin bandwidth in the baseband, the
processing delay will always be long and then Linrad would always
allocate large IO buffers with an associated low DMA rate. Then it
does not matter what limit you have set.

In case the input comes from an SDR-IQ the DMA rate is fixed. The
hardware always sends data in blocks of 8192 bytes.

When you use Linrad with MAP65-IQ you might not use the soundcard
output and then the associated buffer size ddoes not matter at all.


   73

  Leif / SM5BSZ






 

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