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Originally Posted by chb
Thank you for this extremely interesting and comprehensive paper! It never occurred to me that the mismatch between the sampling frequency and the PWM-period introduces noise.
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In the end, (Re)Sampling is amplitude modulation, just with an impulse train instead of the customary complex exponential (or cosine). Since in the case of digital resampling the originating signal is discrete already, it will have a periodic spectrum that gets convoluted with a periodic spectrum of the (Re)sampler. If just point sampling is done (i.e. somewhat near ideal sampling), then the original spectrum is kept and maps to the new "usable" spectrum. Anyway, that's outside the audible range.
In case of digital resampling, especially the approximation of point sampling (or nearest neighbor), is the more important other side of the coin. Nearest neighbor resampling can be decomposed into two fundamental operations: convolution of the input signal with a rectangle (=implicit interpolation), followed by sampling at the new rate. The frequency domain correspondence to the rectangle is part of the new spectrum and introduces the relevant noise.
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Do you have an estimate what fraction of the total noise is due to that internal resampling? What SNR would be achievable at 55.4 kHz? It's rather pointless for practical purposes because it'S such an uncommon sampling frequency, and good quality resampling probably beyond the capabilities of an Amiga CPU.
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I don't have actual figures at hand, simply because I tend to avoid nearest neighbor as often as possible.
55 kHz: I could not get any of my Amigas to play sound at 55.4 kHz properly. The maximum I could obtain was 54.5 kHz and I didn't do any measurements at that rate.