Low/High Pass Filter (Audio)

ciatog
ciatog used Ask the Experts™
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I was wondering if someone has a software implementation of a low and high pass filter for audio processing. My search results brought up a basic low pass but no high pass implementations.

Although any implementation would be helpful I would be particularly happy with an implementation in Objective C using the AVAudioRecorder class

Note: current low pass implementation I've found: http://mobileorchard.com/tutorial-detecting-when-a-user-blows-into-the-mic/

Thanks for you help
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mccarlIT Business Systems Analyst / Software Developer
Top Expert 2015
Commented:
Generally when thinking about this sort of problem, I think about using convolution to do the filtering. I have zero experience with Obj-C, etc so I can't help you with your code, but this may at least be a start. Take a look at http://www.dspguide.com/ch6/1.htm which describes the process and some pseudocode and gives hints as to what a low-pass and high-pass filter kernel might looks like.

I realise that this response is rather vague but I hope that at least it may give you a direction start looking in.
If you have the code for a low-pass, you can convert add a step to right-shift the frequency response to either a band-pass filter, or a high-pass filter. (If you shift it far enough to the right so that the max low-pass frequency now becomes the max high pass frequency (i.e., PI), then the band-pass is actually a high-pass filter.)
     http://stackoverflow.com/questions/820944/how-to-convert-a-low-pass-filter-to-a-band-pass-filter
     http://en.wikipedia.org/wiki/Prototype_filter#Lowpass_to_highpass
     http://cnx.org/content/m16913/latest/
     http://academicearth.org/lectures/discrete-filters-lowpass-and-highpass

Commented:
what about

1-lowpass = highpass
Commented:
if you know how filters work (like FIR, IIR, ...) then you know how to implement a simple HP or LP Filter.
if you need code examples on this topic the soft archive of http://www.musicdsp.org will be a good help.

check: http://www.musicdsp.org/archive.php?classid=3

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Commented:
Neither comment fully answered my question but each was helpful in giving me ideas and links to other resources so I'm splitting the points amongst everyone for their assistance.

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