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[linrad] Re: PCM2900 sound chip in Linrad



Leif,
The PCM2900 has a design defect so that one channel is a clock tick later on the usb line to the computer. Sorry I didn't make it clear. I have played with small corrections in amplitude and phase. Actually in Winrad, I did the 1 clock shift and the fine adjustments, and never got the image at negative frequency to be less than 15 db lower than the main signal.
David Underwood

Leif Asbrink wrote:
Hi David,

I have not yet found how to set up Linrad to compensate for one channel of a sound card having a delay. The PCM2900 chip seems to have this problem.

A delay difference between I and Q will cause a phase error that grows linearly with frequency. In Linrad there is a calibration procedure that will compensate for an arbitrary variation with frequency of both phase and amplitude errors. The reason is that anti-alias filters (such as those used in the WSE units) would be extremely difficult to match between the channels. The calibration should take care of the delay
so there is no feature in Linrad to treat it as a special case.

73

Leif / SM5BSZ

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