[sdiy] BLEP, PolyBLEP, aliasing, etc

Mikael Kopimi Altemark maltemark at gmail.com
Thu Feb 18 20:47:24 CET 2021

I really enjoyed the aliasing-only example; someone should design a synth
around it, really

On Tue, Feb 16, 2021 at 2:40 PM Richie Burnett <rburnett at richieburnett.co.uk>

> I've uploaded some audio examples of what you can expect from these
> different synthesis techniques if anyone is interested:
> http://www.richieburnett.co.uk/temp/blep/
> They all contain a Sawtooth waveform slowly swept from about 47Hz up to
> 12kHz over a duration of 20 seconds...
> File "Saw_Naive" contains what you get if you just output the oscillator's
> phase accumulator directly at 48kHz.  You get a Sawtooth with lots of
> aliasing.  Most noticeable at the higher pitches though.
> File "Saw_PolyBLEP" contains what you get if you apply corrections to one
> sample before and one sample after each discontinuity using the PolyBLEP
> technique I mentioned, with 48kHz sample rate.  This clearly sounds way
> better than just outputting the raw phase accumulator!  But if you view
> the
> spectrum or spectrogram in something like Audacity or Goldwave you will
> see
> there's still plenty of aliasing going on at the top of the audio
> frequency
> range.  You can actually watch harmonics bounce off the Nyquist limit
> (right
> edge of spectrum) and reflect back down.  You can probably hear them too
> if
> you listen carefully.
> File "Saw_PolyBLEPx2" contains what you get if you run the same basic
> PolyBLEP technique with x2 oversampling (i.e. 96kHz sample rate).  You
> still
> see some low-level aliasing right at the top end of the spectrum in
> Goldwave, but not much ventures into the region below the tone's
> fundamental.  You will likely struggle to hear the aliasing now at
> sensible
> volume levels.
> File "Saw_BLEP16" contains a sawtooth synthesised using a full BLEP method
> correcting 16 samples around each discontinuity and running with x2
> oversampling (96kHz.)  Now you will see that the spectrum looks very clean
> everywhere right down to the noise-floor.  (If you can hear aliasing in
> this
> file, then your PC or audio interface is likely performing some sort of
> sample-rate conversion, and doing it badly!)
> File "Saw_Aliases" contains just the corrections applied to the Naive Saw
> waveform around the discontinuities, just for fun.  It essentially
> contains
> all of the aliased rubbish that gets removed by the BLEP algorithm, but
> you
> can hear it more clearly without the main Sawtooth tone masking it!
> As you can see the simple PolyBLEP method does a very good job of
> attenuating aliasing in the low-frequency region of the spectrum where it
> is
> most annoying,  but the full BLEP technique tweaking several samples
> around
> each discontinuity gets close to perfection across the full audio spectrum.
> -Richie,
> --
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Allt gott / Best wishes,
Mikael Altemark
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