Calculate values for spectrum analyser
How would I go about implementing a spectrum analyser like the ones in WinAmp below?
Just by looking at it, I think that these bars are rendered to display the 'volume level' of a specific frequency band of the incoming audio data; however, I'm not sure how to actually calculate this data needed for the rather easy task of drawing the bars.
From what I've been told and understand, calculating these values can be done by using an FFT — however, I'm not exactly sure how to calculate those, given a buffer of input data — am I on the right track about FFTs? How would I apply an FFT on the input data and get, say, an integer out of the FFT that represents the 'volume' of a specific frequency band?
The drawing part isn't a problem, since I can just draw directly to my framebuffer and render that out. I'm doing this as a project on an FPGA, using a Nios II soft-CPU, in case anyone's wondering about potential hardware limitations. Audio data comes in as 24-bit data at 96kHz.
You're probably looking for FFTw.
edit:
To elaborate on your question:
calculating these values can be done by using an FFT — however, I'm not exactly sure how to calculate those, given a buffer of input data: yes, you're right; that's exactly how it's done. You take a (necssarily small, due to the time-frequency uncertainty principle) sample segment out of the currently playing audio data, and feed it to a (typically) discrete, real-only FFT (one of the best known, most widely used and fastest being the DCT family of DFTs - in fact there are highly optimized versions of most DCTs in FFTw). Then you take out the next sample segment and repeat the process. The output of the FFT will be the freqeuncy decomposition of the audio signal that has been fed in - you then need to decide how to display it (i.e. which function to use on the outputs of the FFT, common candidates being f(x) = x; f(x) = sqrt(x); f(x) = log(x)) and also how to present/animate the following readings (e.g. you could average each band in the temporal direction or you could have the maximums "fall off" slowly).
rage-edit: Additional links since it appears somebody knows how to downvote but not how to use google: