[linrad] Re: Sound card sample delay

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

Hi Alberto,

> I am using a notebook with Audigy2 ZS PCMCIA audio card running at 96000 
> sample 16 bit and an SDR-X I/Q receiver.
> With other software (Winrad, Rocky) I must enable 1 sample delay on the 
> right channel to get proper image cancellation.
> If I don't delay the right channel it's impossible to get more than 15 / 20 
> dB of cancellation, while with delay ON I can get easily 70 dB on any 
> specific frequency and about 50 dB on the 90k band.
> I know this problem is typical of most low cost audio card and laptop 
> internal card, while is absent on the Delta 44.
> I see no way of enabling this delay in Linrad (I am using it currently under 
> Windows). Any help / advice?
I do not have any such soundcard so I have no experience 
with this behaviour.

In Linrad the image balancing is done in the frequency 
domain in order to allow steep anti-alias filters that
are not well balanced between the channels. To invoke
the balancing you have to run the calibration procedure 
"Calibrate I/Q phase and amplitude". This could give
you about 70 dB rejection over the entire 90 k band - 
but I am not quite sure because a one sample delay
is a really big phase error near the Nyquist frequency....

Please report to the list whether the Linrad calibration
will give a satisfactory image suppression on your
system.

You say you "know this problem is typical of most low cost 
audio card and laptop internal card". This seems odd to me,
it looks like a simple bug in the drive routine. It would
be interesting to know whether the problem exists under Linux
or whether this is a bug present under Windows only.

Is there anyone on this list who knows?

The correct data from e.g. Delta44 would be like this.
Left[n], Right[n],Left[n+1],Right[n+1],Left[n+2],Right[n+2]

Is this how the data your soundcard gives is arranged?
Left[n], Right[n+1],Left[n+1],Right[n+2],Left[n+2],Right[n+3]

If that is correct, one would just have to throw away one 
sample to get the correct data (but with the channels in 
reversed order)

Maybe updating the drive routine would help?

73

Leif



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