[PD] Combat aliasing!

[Pierre Massat]

That sounds like a sensible reason why a karplus-strong resonator could not
alias. That's good news, although i suppose it isn't new at all to most
people on the list...
By the way i had read the FLOSS pages before...
Thanks Derek!

Pierre

2010/3/31 Derek Holzer <derek at umatic.nl>

> Correct, nothing played back at original sampling rate will alias. If you
> speed that sample up, then some of the recorded harmonics will go over the
> Nyquist number and alias.
>
> Please read the page I sent you on aliasing:
>
>
> http://en.flossmanuals.net/PureData/Antialiasing
>
> and also
>
> http://en.flossmanuals.net/PureData/WhatIsDigitalAudio
>
> Aliasing happens when you try to synthesize or play back a frequency higher
> than 1/2 the sampling rate (this is called the Nyquist number). In non-sine
> wave oscillators, it often comes from the highest harmonics. In samples, it
> comes from playing them back faster than the original sampling rate. It
> happens at the moment those frequencies are synthesized, and cannot be
> removed later. Thus the oversampling approach documented in the FLOSS Manual
> (and taken directly from Miller's Pd manual patches).
>
> A Karplus-Strong resonator is a delay line and as I understand it, so long
> as no pitch shifting is going on then it can't alias. You would not be able
> to create a Karplus-Strong resonator at 30KHz unless you have a sampling
> rate of 60KHz, because the shortest delay time you can get is still one
> sample (1/44100 of one second at normal sampling rate). Again, math gurus
> are welcome to correct my calculations.
>
> D.
>
>
>
> On 3/31/10 6:39 PM, Pierre Massat wrote:
>
>> I m not sure i understand aliasing well... So anything that's sampled
>> and played back without altering the pitch would not suffer from
>> aliasing? When exactly does aliasing occur? during the DAC conversion,
>> or before that? Let's say i set a karplus-strong resonator to a
>> frequency of 30 KHz (assuming i'm a dog and i can hear a pitch that
>> high), at a 44.1 KHz sampling rate, than what happens? No aliasing at all?
>>
>>
>>
>> 2010/3/31 Derek Holzer <derek at umatic.nl <mailto:derek at umatic.nl>>
>>
>>
>>    I was thinking about this the other day.... is it possible to have
>>    aliasing with Karplus-Strong? Because it's a delay line, nothing is
>>    being played back at any higher rate than it was sampled at, so no
>>    aliasing should be possible. Right? Math-gurus correct me if I'm wrong.
>>
>>    Otherwise, any signal generator needs to be bandlimited or oversampled:
>>
>>    http://en.flossmanuals.net/PureData/Antialiasing
>>    http://en.flossmanuals.net/PureData/GeneratingWaveforms
>>
>>    Frank Barknecht has some spliced-transition trick he uses as well,
>>    I'm sure it will come up in a reply or two on this thread as well...
>>
>>    D.
>>
>>
>>    On 3/31/10 6:27 PM, Pierre Massat wrote:
>>
>>        Hi!
>>
>>        I ve been reading the on-going debate about interpolation for a few
>>        days, and it just occured to me that i don't how go about avoiding
>>        aliasing more generally than with band-limited wavetables. If i
>>        wanted
>>        to play a sample at a pitch higher than the original, or if i
>>        wanted to
>>        use a karplus-strong resonator to generate notes, what would be the
>>        proper way of ensuring that no aliasing occurs? Do people
>>        generally use
>>        low-pass filters with a cut-off somewhere below the Nyquist
>>        frequency?
>>        Or is there a trick that one can use earlier on in the signal
>>        path of a
>>        patch?
>>
>>
>>    --
>>    ::: derek holzer ::: http://macumbista.net :::
>>    ---Oblique Strategy # 139:
>>    "Revaluation (a warm feeling)"
>>
>>
>>
> --
> ::: derek holzer ::: http://macumbista.net :::
> ---Oblique Strategy # 151:
> "Take away the important parts"
>
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