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Re: [softrock40] Re: Audio Card Sampling Rate Errors

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  • pe0fko
    ... beloved iMic which showed close to 3 Hz variance across 22 KHz on a Win 7 machine, when measured on a slower Win XP laptop, only showed about 210
    Message 1 of 8 , Jul 20 7:54 AM
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      >> One interesting (to me anyway) observation made since I posted this: My beloved iMic which showed close to 3 Hz variance across 22 KHz on a Win 7 machine, when measured on a slower Win XP laptop, only showed about 210 milliHertz change across the same range. It seems that the drivers and perhaps even the PC itself come into play and Win 7 seems to make the problem worse in the case of the iMic.

      The USB connection need a stable (with in 500ppm I think) frequency from the device, so many devices used a crystal for it. When the USB chip is working on a RC (internal) oscillator most of the time the oscillator is locked (software pll) to the USB Start Of Frame signal from the PC site (1ms). Maybe that is the reason why the USB sound-card freq changed with the change of PC?

                        
      73, Fred
      PE0FKO



      2013/7/20 warrenallgyer <allgyer@...>
       

      Yes Alan, you are absolutely correct. In fact, measuring the Doppler shift on HF signals is one of the applications that I am building this mess for! :-)

      For the purposes of measuring sound card sampling errors it would be better to use a stable, local frequency source. We are only interested in how much it changes when measured at the center of the SDR spectrum as compared to the edge. The absolute frequency of the source is not particularly important in this case.

      One interesting (to me anyway) observation made since I posted this: My beloved iMic which showed close to 3 Hz variance across 22 KHz on a Win 7 machine, when measured on a slower Win XP laptop, only showed about 210 milliHertz change across the same range. It seems that the drivers and perhaps even the PC itself come into play and Win 7 seems to make the problem worse in the case of the iMic.

      One additional thought: This concept does impact those who transmit from a different sound card than the one they use on receive. Calibration of the receiver would not necessarily extend to the transmit side in such cases.

      But before everyone starts throwing their sound cards in the river remember: for the most part here the variance we are talking about is considerably less than one Hertz. This is not a practical issue for anyone but we nerds who are trying to make cheap equipment jump through extraordinary hoops.

      Warren Allgyer - 9V1TD



      --- In softrock40@yahoogroups.com, "Alan" <alan4alan@...> wrote:
      > Yes Warren, because you can!
      >
      > But how far was WWV away from it's exact frequency?
      > Many times signals can be seen to vary and be a few Hz from their actual transmitted frequency after only one refraction from the


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