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Re: kAudioUnitID_BandPassFilter
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Re: kAudioUnitID_BandPassFilter


  • Subject: Re: kAudioUnitID_BandPassFilter
  • From: Chris Rogers <email@hidden>
  • Date: Mon, 14 Jan 2002 23:21:43 -0800

An equalizer (say 31 band) and bass & treble shelving filters would both be
good for Apple to ship as part of its standard suite of AudioUnits.
This is something we'll have to consider for a future release...

Chris
CoreAudio Engineer
Apple Computer





> > the reason I'd like to know is I would like to implement something like
> the equalizer in iTunes, so I'd take some Band Pass filters each for a
> different frequency interval, pass the audio data through them, and then
> mixing the output back together adjusting the gain. Is this the right
> way
> to do it, or are you all laughing at me right now? :-)

This will work--sort of. The problem is that all audio filters cause
phase shift, to differing degrees depending on their design and the
parameters they're using. Where your filters overlap, you will be mixing
together sound which has been phase-shifted by different amounts, and
this will probably sound pretty terrible. You can get around this by
being careful about the particular filter design you use; unfortunately
we have no details about how exactly the filters in the Apple audio
units work, as far as I know.

can anyone at apple tell us more about it?

I guess I'll drop the equalizer plan if the phaze shifting differs too
much for different frequency intervals, because I don't have the time nor
the courage to start digging deep into digital signal processing. I was
really glad to see all those effects CoreAudio is capable of without /me
becoming a dsp professional :-)
Maybe an idea for those dsp professionals at apple: an equalizer
AudioUnit... wouldn't that be cool?

My other option is bass & treble gain in stead of a complete equalizer. Is
this hard to implement using the AudioUnits?

thanks,

--Lieven


References: 
 >Re: kAudioUnitID_BandPassFilter (From: Lieven Dekeyser <email@hidden>)

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