Are high sample rates better for DSP? Options

[Ethan Winer]

I've read many times that audio plug-ins such as equalizers work better at higher sample rates, even if audio generally doesn't benefit. It seems unlikely that there's a practical advantage to up-sampling all your files, or recording at 96 KHz in the first place, but is there a potential theoretical improvement? Or is this just another Internet myth?

--Ethan

[benski]

Yes, many filters work better with higher sampling rates. Particularly resonant filters (of the kind used in EQ) as well as filters feedback paths, such as emulations of analog filters. Many of these kinds of filters are only accurate (desired Fc versus actual Fc) and stable up to perhaps 1/8 Fs. With certain filter topologies, the filter cutoff point and resonance self-oscillation frequency start to drift apart at higher frequencies, also.

For these reasons, it is much better to oversample the input before the filters so that the audio band lies within the "good" range of the filter.

Unfortunately, good resampling is often slow, and more importantly, latent, so it can at times be more advantageous to simply run the whole processing chain at some high sampling frequency, e.g. 384kHz.


Also: non-linear effects, such as distortion, will almost always alias. Certain effects that seem linear are actually not, as fast modulation of parameters such as gain (for a compressor/expander) or filter cutoff (wah pedal) will introduce aliasing as well. Oversampling reduces aliasing in the audio band, as the aliased signal typical falls off 6dB/octave.

This post has been edited by benski: Jun 10 2012, 21:21

[Ethan Winer]

The higher the sample-rate, the better any approximations that assume that the sample-rate is infinite (or that logic propagation speed is infinite) will be.

non-linear effects, such as distortion, will almost always alias. Certain effects that seem linear are actually not, as fast modulation of parameters such as gain (for a compressor/expander) or filter cutoff (wah pedal) will introduce aliasing as well. Oversampling reduces aliasing in the audio band, as the aliased signal typical falls off 6dB/octave.

This is all very interesting, though a bit over my head. Let me ask another way: Considering only "typical" recording studio plug-in effects such as EQ and compression, is there a practical advantage to processing files at, say, 96 KHz rather than at 44.1 KHz? The reason I stress "practical" is because I just tried a simple test in Sound Forge using the Sonitus EQ plug-in bundled with Cakewalk SONAR I use, and I saw no added distortion or other artifacts when applying 10 dB boost at 1 KHz to a 1 KHz sine wave. All that happened is the tone got 10 dB louder.

For completeness, I created two files with the 1 KHz tone at -11 dB. Then I raised the volume of one file by 10 dB, and applied 10 dB of EQ boost with a Q of 24 to the other. The FFT displays of both files are identical, and neither has any added artifacts. Then I did the same test using 10 KHz both with volume boost and EQ at 10 KHz, and again the FFTs were identical showing nothing but the pure tone frequencies.

Edit: I forgot to mention the files were at a sample rate of 44.1 KHz.

--Ethan

This post has been edited by Ethan Winer: Jun 11 2012, 17:22