Re: oscillators and the Web Audio API

On Mon, Feb 6, 2012 at 6:19 PM, Phil Burk <philburk@mobileer.com> wrote:

> I have begun doing some experiments with the Web Audio API described here:
>
> <https://dvcs.w3.org/hg/audio/**raw-file/tip/webaudio/**specification.html<https://dvcs.w3.org/hg/audio/raw-file/tip/webaudio/specification.html>
> >
>
> In general, I really like the API. It will be great to have a flexible
> set of synthesis modules available from JavaScript.  I was impressed
> with the inclusion of FFT support, convolution, occlusion, etc. I was
> also very happy to see that timestamps are supported. That is so crucial.
>

Phil, thanks for having a look at the specification.  Sample-accurate
timing was one of the primary design goals of the system, and we're doing
our best to test these APIs with automated tests (called "layout tests"):
http://svn.webkit.org/repository/webkit/trunk/LayoutTests/webaudio/

One specific test we have for "note" scheduling is here:
http://svn.webkit.org/repository/webkit/trunk/LayoutTests/webaudio/sample-accurate-scheduling.html

We're currently working on layout tests for the AudioParam automation APIs
to make sure that the engine renders precisely.


>
> I am now trying to do some "Hello World" type examples and have hit a
> few snags. Perhaps someone could help me.
>
> My first test was to generate a tone using an oscillator.
>
>   <http://www.softsynth.com/**webaudio/tone.php<http://www.softsynth.com/webaudio/tone.php>
> >
>
> But I could not find the oscillator class. I could use a looping buffer
> but that would be difficult to use for FM or multi-octave sweeps. It is
> also difficult to control aliasing with a looped wavetable, even with
> multi-point interpolation.
>
> Luckily I was able to use a JavaScriptNode to implement a simple sine
> oscillator. It would be nice to have an AudioOscillator class with sine,
> sawtooth, square, pulse and impulse. The oscillator should accept
> frequency values that can go negative for extreme modulation.
>
> An AudioNoise class would also be very useful with white noise, pink
> noise and red noise generation. Stochastic synthesis is very handy for
> wind, explosions, percussive sounds, etc.
>

Up to this point we've relied on AudioBufferSourceNode for general sample
playback, and also periodic waveform playback.  I completely agree that
there are some challenges and limitations to this approach.  Nevertheless,
I've worked on a demo using this approach in my "wavetable-synth" and have
made some initial (although incomplete and not fully developed)
anti-aliased wavetables:
http://chromium.googlecode.com/svn/trunk/samples/audio/lib/wavetable.js

But, back to your proposal.  Having the pure and extremely high-quality
anti-aliased oscillators would be great to have, especially if you have
code to contribute.  Sound quality does matter, so I really appreciate
this.  One thing I'm curious about is how these oscillators can avoid
aliasing when used in FM applications.  It's certainly a goal of mine to
have audio-rate signals modulating AudioParams like .playbackRate or
.frequency

The sine, sawtooth, square, pulse and impulse oscillators are considered
common in computer-music and so would fit into the Web Audio API's
philosophy of implementing common audio building blocks.

I would suggest that the AudioOscillator could have an attribute called
.oscillatorType which could be one of several values.  I would also be
interested in your take on how to implement anti-aliased oscillators with
arbitrary waveforms.  Optimally, the AudioOscillator would be able to
handle more than just the simple waveform types.  I have some ideas of my
own, but we can leave that for later discussion.

Cheers,
Chris







>
> Thanks,
> Phil Burk
>
>