[sdiy] IR Reverb

Tim Ressel timr at circuitabbey.com
Fri Feb 16 19:48:15 CET 2018

Ah, okay. But you have to also do inverse FFTs, yes? So the FFT gets the 
input into the frequency domain which gets multiplied by the IR, then an 
iFFT gets you back to time domain. Overlap reduces congruity issues. Am 
I getting that right?


On 2/16/2018 10:11 AM, Eric Brombaugh wrote:
> Yes - a brute force requires 100k MACs / sample.
> An FFT approach reduces the the total number of multiply / add 
> operations required to do a convolution due to the optimization of the 
> "fast" algorithm. FFTs reduce the N^2 operations of a DFT to N*log(N), 
> so the larger the transform you can get by with, the more advantage 
> you'll see. The downside is that FFTs introduce a lot of latency, so 
> to get around this transform-based convolution engines will typically 
> subdivide the process into a small & fast FIR to handle the early 
> results, followed by gradually larger and larger FFTs to handle the 
> later results.
> Eric
> On 02/16/2018 10:53 AM, Tim Ressel wrote:
>> Thanks for all the replies!  So the concept sounds simple enough. I 
>> get the sense the issue is going to be processing time. Am I wrong, 
>> or would a brute-force approach to a 2-second reverb time require 
>> 100K MAC cycles per sample? I found some open-source packages that 
>> use FFTs to do this. I was under the impression FFTs would be less 
>> fast than other methods.
>> Ever get the feeling you're opening Pandora's Box only to find a can 
>> of worms inside?
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--Tim Ressel
Circuit Abbey
timr at circuitabbey.com

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