[sdiy] BLEP, PolyBLEP, aliasing, etc

Andrew Simper andy at cytomic.com
Wed Feb 17 09:53:19 CET 2021


One thing to note with these listening tests is that oscillators are rarely
used on their own, they normally go through some sort of non-linear
processing eg filter and amp, which will make aliasing more audible.

Cheers,

Andy

On Wed, 17 Feb 2021 at 15:48, Roman Sowa <modular at go2.pl> wrote:

> I can hear difference between BLEP and BLEPx2, hesitate to say "clearly
> hear" but it's aparent with no special conditions, lousy acoustic
> environment and radio playing near by.
> I must be youg then :)
> To hear the difference in x16 version I would probably need $28k silver
> speaker cables laid down in circle with marble cable supporters.
>
> Those audio examples are the best what has happened in this thread and
> the one it came out from.
>
> Roman
>
> W dniu 2021-02-16 o 17:35, Antonio Tuzzi pisze:
> > Thank you
> >
> > in a blind test using my monitors before and headphones after I can't
> hear any difference between Saw_PolyBLEP Saw_PolyBLEPx2 and Saw_BLEP16
> >
> > but I can see clearly the aliasing in the spectrum plot for the
> Saw_PolyBLEP
> >
> > I must be old ! :D
> >
> > Antonio
> >
> >> On 16 Feb 2021, at 14:39, Richie Burnett <rburnett at richieburnett.co.uk>
> wrote:
> >>
> >> 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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> >> https://www.avg.com
> >>
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