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I've been thinking a lot about digital aliasing. When running at 44.1/48kHz, many of Logic's distortion plugins seem to alias at least a bit - it's obvious when running a sine sweep through them. It can be heard and seen on the spectrum analyzer.

 

It got me to thinking: some 3rd party plugins sound better, probably because their algorithms have better anti-aliasing. They probably use an upsample->process->downsample structure. But if you use a whole bunch of these plugins for their sound, it seems like a waste of processing power to have every single plugin instance do the up/down sampling.

 

Why not add a mode in Logic where, even though the audio is *recorded* at the original sample rate (44.1/48k; since with good converters, this is all you need for a good unprocessed recording), the *plugins* all run at, say, 4x the recording rate.

 

That would centralize the up/down sampling: upsample at the beginning of each channel strip, and downsample at the output. It would save processing power and disk space, and bring down aliasing artifacts. In my quick experimentation, aliasing artifacts in the 20-20kHz range from a Test Oscillator sine wave into the Clip Distortion plugin are brought down more than 20dB just by doubling the sample rate.

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ok can some one please explain to me how aliasing happens with the distortion plugins when at 44.100kHz aliasing happens at 22.05kHz which our ears cant hear..?.. and if all is EQ'ed right in your mix you wont go past 20kHz.. so i dont understand.. cheers.
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ok can some one please explain to me how aliasing happens with the distortion plugins when at 44.100kHz aliasing happens at 22.05kHz which our ears cant hear..?.. and if all is EQ'ed right in your mix you wont go past 20kHz.. so i dont understand.. cheers.

Aliasing is a bit like when you look at a spinning car wheel (more precisely the rim) in a movie. Sometimes the wheel spins at a rate where it can't be captured correctly. Instead it looks like the wheel is suddenly rotating slowly backwards while it is in fact still moving forward very fast. That happens when the movements are captured at points that still have a meaning to us - only it looks like it's moving backwards now because we happen to capture those exact moments out of order but a specific out of order, that is.

 

So aliasing happens when frequencies above the usable spectrum haven't been removed properly and happen to match two different sets of samples, and thus reproduces two audible tones at the same time.

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ok can some one please explain to me how aliasing happens with the distortion plugins when at 44.100kHz aliasing happens at 22.05kHz which our ears cant hear..?..

It doesn't manifest only at nyquist. It can be audible in the baseband too. You may notice it or not, really depends on the material. You may notice it only after hard comp or limiting, but it will just sound like the usual intermod you get when using dynamics, or it will color it a bit. And, you might like the sound. But compared to what just any dynamics process will do anyway, it's contribution is pretty insignificant.

 

I'm not sure I'd be concerned too much about it with a distortion plug-in.

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It's not only distortion plugins. Try using an EQ on a rich harmonic source like a recorded fuzzy electric guitar. Even applying a low cut filter will "do something" at nyquist and indeed somehow mirror things in the low end.

 

Some Eq's are better than others though, which I guess, links to the same use of the up/down sampling internally, and since things are progressing quite fast in plugin land I wouldn't worry to much nowadays since most Eq's got this feature.

 

As a foot note, there are loads of people using a piece of software from Germany that uses Star Trek terminology to flex audio, and that piece of software is known for not doing something about there aliasing until version 5 or 6. So some people see it as a nasty thing that should be avoided. Other people love the way it somehow makes their mix punchy and crunchy...

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