Re: Problems with a Low Pass Filter process
Re: Problems with a Low Pass Filter process
- Subject: Re: Problems with a Low Pass Filter process
- From: Brian Willoughby <email@hidden>
- Date: Sat, 04 Jun 2011 12:19:20 -0700
Herve,
I suggest that you grab some basic audio DSP texts and study the
background information. But I can quickly point out a few mistakes:
The BitCrunch noise is due to clipping. Your gain weighting factors
are huge. The multiple 2* factors represent significant gain that
you never correct for, and this distorts the audio. The common
solution is to pre-multiply your weighting factors such that they all
add up to < 1.0, and then you won't have distortion. The algorithm
that you're using to calculate alpha, beta, and gamma is convoluted
enough to make this harder that it should normally be, and a simple
low-pass is usually easier than this.
The cutOff problems are probably related to your filter algorithm.
Check your references on this. Some filters are only defined for fs/
4 and below, some are only defined for fs/4 and above. Since digital
audio only carries frequencies between 0 and fs/2, this fs/4
frequency is right in the "middle" as you say. Other filters have
different usable ranges for their cutOff, so if you want to use the
full range from 0 to fs/2 then you'll need a different filter algorithm.
Brian Willoughby
Sound Consulting
On Jun 4, 2011, at 08:34, herve.noury wrote:
float beta = 0.5 * ((1.0 - theSin) / (1.0 + theSin));
float gamma = (0.5 + beta) * cos(theta);
float alpha = (0.5 + beta - gamma)/4.0;
left[frame] += 2*(alpha*(leftB[frame] + 2*leftB[k] + leftB[j])
+ gamma*leftB[k] - beta*leftB[j]);
The result is just noisy : when the cutOf is in the middle, no
effect, but when I want to filter the additive synth, it sounds
like a BitCrunch effect. When Qval is too high, it crashes...
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