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I'm downsampling a given .wav from 44100 to 22050 samples/second. When I play it back I'm looking at the signal with an EQ, and I would expect that I wouldn't see any content above ~11k. However, I see lots of high frequency content all the way up to ~40k.

How is this possible? Is this a sign that I'm downsampling incorrectly?

Edit: The issue seems to be that I'm upsampling after downsampling to return the sample to it's original length. If I don't do that then the observed frequency content is as expected.

Valevalorin
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  • Can you share your downsampling code? You're correct, you shouldn't see content above ~11k (at least not significant content), and this might be indication of a bad down-sampling routine. – Jdip Jan 05 '23 at 20:50
  • There's very little code that isn't just from a library. However, I think the issue is that I'm upsampling after downsampling to return the sample to it's original length. If I play back the sample without upsampling it, the EQ shows the frequency content stopping where expected.

    Funny though, because I was comparing against another downsampling effects plugin and it was producing the sample results, so it must also be doing the same thing.

     – Valevalorin Jan 05 '23 at 21:37
  • proper interpolation is the combination of upsampling and filtering, perhaps the issue is in the interpolation filter. – Dan Boschen Jan 05 '23 at 22:03
  • How are you up-sampling then? – Jdip Jan 06 '23 at 11:05
  • Just resampling with sinc interpolation. – Valevalorin Jan 06 '23 at 14:51

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