[linrad] Re: Sound card sample delay

["John Harrison, NI1B" <nei.mv.com; jmh6@xxxxxxxxxxxxxxxx>]

Hi All,

   Regards sound cards and sample switching.

   I NEVER had this problem on my original Linrad machine, an old Pentium
100 using an ISA sound card.

   However, with newer sound cards basically cheap PCI ones, occasionally
Linrad/Knoppix/sound driver would under certain circumstances skip sound
samples. So the whole FFT image would mirror image. A little while later
it would reverse again. It was never stable enough to allow calibrating. 
So, I probably wouldn't have noticed the lack of cancellation effect you
speak of.

   That and the total absence of ANY anti-aliasing filters on these
cheaper sound cards made for some very confusing and noise filled Linrad
displays!!

   Even given all this, you could still use linrad to listen to signals
once you found one, at least until they 'flipped' to the other side :) 

   Lots of fun,
   john



On Sat, 20 Oct 2007, Leif Asbrink wrote:

> 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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