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Measuring sample rates on a sound card

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  • Measuring sample rates on a sound card

    > I have a question regarding your soundcheck software.
    > It is my experience that not all sound cards sample at
    > the exact rates they are programmed to sample at.
    > Does your software provide a facility to verify sample rates?
    > ie, if the sound card is programmed to sample at
    > 8KHz, what frequency it is actually sampling at?
    > Jim

    Yes. When you are recording sound you should be able see any variation from the requested sample rate. The real sample rate is displayed in the "Average In" field.

    The "Average In" field displays the current sample rate from start of the recording session. Because sound arrives from the sound card in blocks (and not as a continuous flow). The Average In value will initially jump around a fair amount until a large number of samples have arrived. This is because it is not possible to get a good average value until a lot of samples have arrived. After 30sec or so the value should start to exactly match the requested sample rate and have a 0.00% variation (at least that is what you hope for).

    The size of each incoming data block (full of sampled sound) in bytes is
    512 * ( Resolution / 8 ) * Number of Channels

    So as an example, requesting a rate of 11025Hz with 8bit resolution and mono (1 channel), should result in 11025 bytes arriving per second. But because data arrives in blocks of 512 bytes you never get exactly 11025 bytes in any one second. So in this example, in a typical second, you get either 21 or 22 data blocks. Corresponding to 10752 or 11264 bytes.

    This is all from the SoundCheck applications point of view. Obviously the sound card does not change its sample rate from one second to the next. The card runs at a stable speed and buffers the sample on the card until the requested buffer size is reached.

    So as a consequence is it possible to measure the sample rate variation over a long period, as a average, but it is not possible to notice short fluctuations.