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 -------------------------------- _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
