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Re: Basic sample code for FFT audio frequency spectrum? Sarcasm Maybe?
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Re: Basic sample code for FFT audio frequency spectrum? Sarcasm Maybe?


  • Subject: Re: Basic sample code for FFT audio frequency spectrum? Sarcasm Maybe?
  • From: Nigel Redmon <email@hidden>
  • Date: Fri, 6 Jul 2007 02:14:53 -0700

Hi Matt,

The answer isn't so clear cut because it's a bit of a loaded question... For instance, if you ask "I need to plot the response my this low-pass filter I made... how do I figure out the response?"-- that's easy. If you talking about doing a spectrum analyzer, then you're asking people to make a ton of implementation decisions for you. Now, maybe you can find some source code that does what you want, but of course you can google for that as well as I can, so that doesn't seem to be the issue you're presenting. (Or maybe it is and you couldn't find exactly what you want, and you're hoping someone else has, but as James noted--"Everything I have looked at is very complex and I am having trouble extracting the basics"--you're not going so find a routine and a few function calls and "ta-duh".)

An FFT converts one cycle of a repeating waveform to its harmonic components. Implicit there is that you have exactly one cycle, and that cycle repeat continuously. Well, that's really a DFT--an FFT is an optimization that makes it more efficient, but stipulates conditions on the number of samples that make up that one cycle-- typically a power of two. And the number of samples dictates the frequency resolution as well, so you have to make a decision there. First you are rarely going to be lucky enough to grab exactly one cycle in the power-of-two you chose, and second, it's unlikely you are trying to measure a static waveform.

From there, you begin your journey of decisions and trade-offs. Basically, you can work around those FFT limitations by repeatedly running FFTs and overlapping them, but that is only to say that the can of worms is now open. There are well-known techniques of doing this sort of thing, and whether it's complex or not pretty much depends on your level of DSP expertise. It's not as complicated as (musically-useful) pitch shifting, but it's a far cry from tossing together a biquad.

I know this doesn't help much, except it might help you understand why you're not getting deluged with source code. I would think you can find something, but then it's probably up to you to search for it, and figure out how you can use it.

Regards,

Nigel



On Jul 5, 2007, at 8:29 PM, IgnisFatuus wrote:
Sorry about this fools response to this forum listed below...

Cocoa has it all (now also 64-bit available)

Google for:

NSLoadAnAudioFile
NSPressTheButton
NSDisplayFrequencySpectrum

Some of us really appreciate honest feedback.

Maybe you should be kicked off this forum since
you don't seem to be of any help!

I wrote software for macs for years but not within
the audio realm! I know your above responses
are crap!

I like to start from the bottom foundation and
work my way up to the details so I can "learn".
If I couldn't find what I was looking for on the
Apple developers site... then I post for
directions or links etc. Not CRAP!

Like say when the audio file is loaded, and how to retrieve
that data for the FFT then the finished result so I can
plot in an NSView.

If all you have is crap to say... then you might as well
resign from this forum! You evidently don't have "any"
programming skills to help out!

Matt
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 >Re: Basic sample code for FFT audio frequency spectrum? Sarcasm Maybe? (From: IgnisFatuus <email@hidden>)

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