[sdiy] BLEP, PolyBLEP, aliasing, etc

Amos controlvoltage at gmail.com
Wed Feb 17 16:48:19 CET 2021


A fun test is to run the real-world output of your digitally-generated VCO
through a high-gain analog wave shaper or folder... oh boy does that bring
out the errors!

On Wed, Feb 17, 2021 at 3:57 AM Andrew Simper <andy at cytomic.com> wrote:

> 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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