[sdiy] IR Reverb

Tim Ressel timr at circuitabbey.com
Fri Feb 16 18:53:55 CET 2018

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?


On 2/15/2018 1:43 AM, Tom Wiltshire wrote:
>> On 15 Feb 2018, at 07:42, Tim Ressel <timr at circuitabbey.com> wrote:
>> (note: laughter is understandable and probably mandatory)
>> So I just did a cannonball into the murky waters of impulse response reverb. My tenuous grasp of DSP coupled with my sketchy math skills is making this, well, interesting. So far I have acquainted myself with linear convolution, which looks suspiciously like an FIR. Since it says "impulse response" on the box, that seemed to make sense. But then I stepped back and tried to imagine a reverb system as I understand it and got confused.
>> Reverbs have two things going on: time delay and filtering. The time component gives the reverb time and overall thickness of the reverb, while the filtering can make the effect warmer or colder (yes, oversimplified). So I am guessing the impulse response is a room characterization used to color a reverb. However that seems incomplete. Unless the impulse response is really long or is sets of impulse responses over time.
> You’re dead right about this looking suspiciously like a FIR, 'cos it is a FIR.
> But it isn’t only a filter - when the IR gets long enough, you’re literally mixing signals that can cover many seconds, so there is a very clear time delay element too. Mostly when we’re thinking of FIR filters that’s not so apparent because the FIR kernel is short when compared to the sample rate, but it’s still there. It does both things.
> In fact, the two aspects can’t really be separated. If you start adding time-delayed signals, you’re going to get some frequency filtering effects however you do it - think of the comb filtering effect of a typical BBD flanger, for example.
>> I suspect that gurgling sound is me in over my head.
> Or the neighbour in the apartment upstairs is emptying the bath?!
> Tom
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--Tim Ressel
Circuit Abbey
timr at circuitabbey.com

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