RE: [linrad] Soundcards and Linrad

[Leif Åsbrink <leif.asbrink@xxxxxxxxxxxxxxxx>]

Hi Ermanno,

Linrad uses Lagranges interpolation formula to fit a third
degree polynomial to four points in the baseband signal.
The output sample is computed at a fractional sampling
point to allow the output signal to have a sampling speed
that has aa non-integral relation to the baseband sampling 
speed.

The baseband is sampled at a low speed, something like 4
times the bandwidth you have selected. For EME it might
be 93.24Hz (96kHz/1024) and when up-sampled to 5 kHz there
are 53.625 output data points for each baseband sample.

The third order polynomial fit is used because a lower order
than that produces too much distortion to the waveform.

The up-sampling routine is not optimised at all, it becomes
extremely inefficient at 96 kHz.

As far as I know, the Delta44 must use the same speed for
output as for input which means that using its output
for Linrad will be very CPU intensive for no reason at all.

If the device driver has a simple up-sampling from 6kHz
(by 16 times) for example by making points equal in groups
of 16 it would work fine. The distortion would be high enough
in frequency to not be a problem.

If you have a modern computer, Delta44 should work fine for
both input and output. In a longer timescale the Delta44 output
would be the ideal hardware for the transmit side and then
you will need a second soundcard for microphone and loudspeaker:-)

Linrad-01.25 which I uploaded a few days ago has a first part
of the transmitter functions but it does not mean that I 
expect to have a complete transmitter running before it is
time to buy a more modern computer. Computers get obsolete
so quickly......

73

Leif / SM5BSZ






 
> Yesterday evening I have finished installing a new PC with 
> Mandrake 10.0 / svgalib-1.9.19 / Delta44/SB-Live.  
> In the old system I had not installed the Delta44 card, and in 
> fact my question is this:  
> There are benefits to use 2 cards sound (  Delta44 in - SB out)?  
> or I can only use Delta 44 (i/o)?  
> Thanks?  
> 
> 
> 73 de Ermanno / ik7ezn
> 

LINRADDARNIL