Re: Advice on the convolver node

On Thu, Mar 22, 2012 at 3:18 AM, Chris Lowis <chris.lowis@bbc.co.uk> wrote:

> On 20/03/2012 17:18, Raymond Toy wrote:
>
>>    My question - is it possible to set the convolver buffer from a
>>    regular javascript array? For example to generate the impulse
>>    response using Math.sin()?
>>
>>
>> Yes, you can do this.  Something like the following should work:
>>
>
> Hi Raymond,
>
> Thank you for your advice! I've been trying to get to grips with the
> convolver node by reading the source in Webkit, but I'm having a little
> trouble understanding whether I can do what I want with it.
>
> I'm attempting to create a "ring modulation" effect. In the time domain
> I'd like to multiply the input signal (an audio buffer node) with a 30Hz
> sine wave. I thought I could achieve the same in the frequency domain using
> a convolution.
>

If your impulse response is a 30Hz sine wave, consider what happens in the
frequency domain.  You transform the input signal and transform your 30Hz
sine.  The transform of that is a narrow spike at 30 Hz.  Multiply the
transforms together and you get a narrow spike at 30Hz.  Inverse transform
to get the desired signal.  All you get out would be a 30Hz sine wave, at
most.

One possible way to do your time domain multiplication is to use an audio
gain node with a setValueCurveAtTime.  Use your 30Hz sine wave as the curve
and feed your input into the gain node.  This should multiply your signal
by your 30Hz sine.


>
> Am I missing something here? The documentation of the Convolution node
> suggests that it is operating as a FIR filter, I'm not quite sure whether
> it is possible to do a straight convolution with this node.
>

This should work.  But note that there is an internal normalization going
on.  If you want just straight convolution, you should set the normalize
attribute to false.

Ray