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Hello,

I’ve been playing around with FMOD_DSP_TYPE_PARAMEQ on the iPhone and I see a few issues with the FMOD_DSP_PARAMEQ_BANDWIDTH parameter, which is supposed to be the octave range around the center of the filter. I take this to mean that if I set up a bandwidth of 4 octaves, that would be 2 octaves on either side of the center.

I took a white noise sample and looped it while applying various filter settings. As I got to a center of 3500hz, things got interesting. I piped this through the sonogram feature from aurioTouch to illustrate.

Here is the white noise, sans filter:
[img:3eahirhs]http://www.kreation.com/George/fmod/No-filter.png[/img:3eahirhs]

Here it is with a low range. Nothing too interesting here, except for it seems a little narrow for a 4 octave range:
[list:3eahirhs]dspParamEQ>setParameter(FMOD_DSP_PARAMEQ_CENTER, 100.0f);
dspParamEQ->setParameter(FMOD_DSP_PARAMEQ_BANDWIDTH, 4.0f);
dspParamEQ->setParameter(FMOD_DSP_PARAMEQ_GAIN, 0.5f);
[/list:u:3eahirhs]
[img:3eahirhs]http://www.kreation.com/George/fmod/100hz-4.0octaves.png[/img:3eahirhs]

Below is a mid range filter. This is interesting, notice that it is wider, even though it is only 1.5 octaves, [b:3eahirhs]AND[/b:3eahirhs] it also cuts out a range [b:3eahirhs]ABOVE[/b:3eahirhs].
[list:3eahirhs]dspParamEQ->setParameter(FMOD_DSP_PARAMEQ_CENTER, 3500.0f);
dspParamEQ->setParameter(FMOD_DSP_PARAMEQ_BANDWIDTH, 1.5f);
dspParamEQ->setParameter(FMOD_DSP_PARAMEQ_GAIN, 0.5f);
[/list:u:3eahirhs]
[img:3eahirhs]http://www.kreation.com/George/fmod/3500hz-1.5octaves.png[/img:3eahirhs]

Finally, below is a higher range. Seems normal.
[list:3eahirhs]dspParamEQ->setParameter(FMOD_DSP_PARAMEQ_CENTER, 12000.0f);
dspParamEQ->setParameter(FMOD_DSP_PARAMEQ_BANDWIDTH, 4.0f);
dspParamEQ->setParameter(FMOD_DSP_PARAMEQ_GAIN, 0.5f);
[/list:u:3eahirhs]
[img:3eahirhs]http://www.kreation.com/George/fmod/12000hz-4.0octaves.png[/img:3eahirhs]

Any idea on what is going on here? Why is the mid range filter so screwy? I thought implementing the octave range accommodated for the logarhythmic progression of HZ to octaves?

I appreciate any and all help. Thanks!

  • George

Edit: forgot to mention that I am quieting the bands by setting the gain to the minimum which is 0.5.

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