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Oscillators with time-domain custom wave shape

From: s p <sebpiq@gmail.com>
Date: Mon, 30 Mar 2015 17:10:45 +0300
Message-ID: <CAGKuoCX4X77yuHosgrrwH31kzL_8Thw9-KRbYgy1otV4t_ghNg@mail.gmail.com>
To: "public-audio@w3.org" <public-audio@w3.org>
Hi all!

In the past days I've struggled to get an oscillator that would output an
exact sawtooth. The use case for me is to modulate an AudioParam and the
approximative sawtooth provided by the OscillatorNode is not precise
enough. For example, I wanted to loop through a sound file, using
WaveShaperNode, and drive the read position in that file with a sawtooth,
but since the decrease after the top of the sawtooth is not instant, you
can hear a very loud artifact when you reach the end of the file (peak of
the saw), a bit like fast rewind.

There is 2 solutions I thought about to get this working (I wont mention
the 3rd one, it's too ugly).

First using the OscillatorNode with a custom wave shape, but I didn't even
try because I know I will never get something clean enough, since the wave
shape is specified in the frequency domain.

Second, using BufferSourceNode, and I was sure it would work. I created a
buffer with my wave shape, which I loop, and I control the frequency
through directly modulating playbackRate. Turns out this doesn't work on
Chrome and is glitchy on Firefox. http://jsfiddle.net/gh144L2u/

I think it would be great to have a way to do this, in order to have a
fully predictable wave shape if needed. Is this an intended use-case for
BufferSourceNode? In that case I believe the spec is not precise enough
about how playbackRate should be handled. Paul Adenot told me that in
Firefox the BufferSourceNode is not optimized for fast playbackRate
changes, and also playbackRate is a k-rate param which might be alright ...
Is this rather a use-case for OscillatorNode? This would require a new
param... etc ...

Any thoughts on this?


*S├ębastien Piquemal*

 -----* @sebpiq*
 ----- http://github.com/sebpiq
 ----- http://funktion.fm
Received on Monday, 30 March 2015 14:11:16 UTC

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