[sdiy] A new shade of pink (noise)

[Brian Willoughby]

I'll add another voice of agreement that "reconstruction filter" is the best name.

In my estimation, it's a great word to use because it's a subtle reminder that digital audio is *not* a stair-stepped approximation of continuous analog audio. As everyone here already knows, proper use of sampling requires a signal to be filtered (band-limited) before it's digitized, and then reconstructed through a filter after the DAC. Calling that final stage a "reconstruction filter" emphasizes that it's returning the same, continuous, analog signal that went into the process - without the jagged edges that you *might* see on your computer screen.

That said, I am guilty of saying "aliasing" when "imaging" would be more appropriate.

Would it be correct to say that half of the images are mirror images? In terms of high frequencies being output lower than low input frequencies?

Between Nyquist and the sample rate, would you call those frequencies "images" or "aliases"?

I assume we can all agree that frequencies above the sample rate are aliased down below the sample rate.

Sounds like I'm going to have to review some terminology. That's always a good idea, though, anyway.

Brian


On Nov 24, 2020, at 13:31, Eric Brombaugh <ebrombaugh at gmail.com> wrote:
> Yes - the secret sauce of how Stenzel got 128 voices of anti-aliased oscillators out of a lowly STM32F373 with the Streichfet was an eye-opener. Lots of optimizations that are only obvious *after* someone tells you how they work.
> 
> I agree 100% on the use of "anti-image" or "reconstruction" for the filtering that happens after a DAC - I prefer reconstruction. As I became more familiar with the subtleties of discrete-time processing I also became more conscious of the difference between imaging and aliasing and began to strive to use the terms correctly.
> 
> Eric
> 
> On 11/24/20 1:14 PM, Richie Burnett wrote:
>> Thanks for that link Eric.  Very interesting, I always wondered how they achieved 128 voice poly on the Streichfet.
>> The section towards the end about using BLEP techniques to emulate "variable sample rate playback with Zero-Order-Hold" on a modern DSP with fixed sample rate is very interesting.  To me this is the secret to getting that gritty PPG type sound when playing back wavetables at low pitches.  I would call the additional spectral content "Imaging" rather than "Aliasing" though, because it is produced by repeated images of the spectrum due to the sampling process that aren't being supressed properly during playback.  Not aliasing that I think of happened during recording of the wavetable.  A lot of people do seem to call the filter that follows a DAC an "Anti-aliasing filter" though, when it is really an "anti-imaging filter" in my opinion, or even "reconstruction filter" is a better description, I think.
>> -Richie,