AW: [time-nuts] Re: Accuracy of a sound card

[Ulrich Bangert]

Alberto,

> I don't know the reason, but my suspect is that there is just 
> one ADC, multiplexed between the left and the right channels, 
> hence the time skew between them.

There may be some cards out there, that do it that way. But if you look
at the specs of the better adcs used on sound cards (Burr Brown for
example) you will see, that there are indeed two independand adcs in the
chip for the left and the right channel, clocked by the same source.
Nevertheless, thanks for pointing at that topic. As it turnes out, the
(semi)professional sound cards are better suited for this plan for some
other reasons too.

73's and my best regards
Ulrich Bangert

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Alberto di Bene
> Gesendet: Dienstag, 23. August 2005 18:29
> An: Discussion of precise time and frequency measurement
> Betreff: [time-nuts] Re: Accuracy of a sound card
> 
> 
> Ulrich Bangert wrote:
> 
> >Because it is a differential measurement between the two channels 
> >(which are sampled exactly the same time due to the adc 
> hardware) the 
> >first idea was, that the effects introduced by the not so 
> good sample 
> >clock of a ordinary sound card may cancel out completely.
> >  
> >
> Beware ! The above is true only for high end cards. I learnt 
> it the hard way. I am the author of SDRadio, a software that 
> implements the concepts of a software defined radio. The 
> sound card is fed with the quadrature I and Q components from 
> a QSD (quadrature sampling detector) and the rejection of the 
> unwanted side band is done by the program using the phasing 
> method. This requires a precise phase difference of 90 
> degrees between the two components. On my and many other PCs 
> the program worked ok, but I received a few reports saying 
> that the unwanted band rejection was terrible. After some 
> investigations it turned out that between the left and the 
> right channel of the sound card there was a fixed time 
> difference ( a few tens of microseconds, if I recall 
> correctly). This forced me to implement a fractional unit 
> delay routine, as the usual phase correction techniques 
> didn't work in this case, being of course the phase error 
> depending on the frequency.
> 
> I don't know the reason, but my suspect is that there is just 
> one ADC, multiplexed between the left and the right channels, 
> hence the time skew between them.
> 
> 73  Alberto  I2PHD
> --------------------------------
> 
> 
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