What are convolution impulses?

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I've seen convolution impulses for thing like the POD, or Vox tonelab, and Tech 21 Sansamp, etc.

What are these, what do they do, and how do they work??

Thanks.
 
Convolution is a digital signal processing (DSP) technique for transforming an input signal. Basically it simulates the transformation of the signal that takes place within a physical signal path -- which could be an acoustic space, or an electronic device like an amp.

The convolution technique is based on the idea that the physical transformation can be characterized by putting an impulse (a very short, broadband signal, like a handclap or a rapid oscillator sweep) into the space or device, and recording the results. That recording is the impulse response. The IR becomes an input to the convolution algorithm.

Some of the most realistic digital reverbs I've ever heard used this technique.
 
I understand how this could work in a reverb plugin or something, but what are impulse responses for like the POD, or a Neve preamp used for??
 
Theoretically, any signal transormation can be modeled and simulated using the convolution technique. You just put the impulse into the unit (amp, preamp, washtub, Grand Canyon, or whatever) and record the output.
 
So are you saying you could take impulse of say a vintage preamp and apply that sound to your audio? Thanks for your help. This is new to me.
 
That is exactly right. The downside is that convolution requires a lot of calculations, which means it takes a lot of cpu power. It also involves significant latency (delay), so it's difficult to do in real time. (You can get close if the latency is kept low enough, but then the cpu power required increases.)

Here's a quick link to more background information:
http://digital-signal-processing.brainsip.com
 
Ok, it's becoming somewhat clearer. Now, I understand how I could say, load a reverb impulse into SIR, and use SIR to manipulate that impulse to an extent (I think that's how it works?)

But, How would I take (not actually make one, but use an existing one) maybe an impulse of a Neumann mic, or a guitar cabinet, or compressor though, and use it to manipulate my audio track? how do you apply it to it, or "run" your audio through it?

Thanks, you're really are a big help, and this is quite fascinating.
 
The process is exactly the same. The impulse is what determines how the signal is transformed. As you might imagine, it's critical that the impulse is captured accurately. Keep in mind, with your examples of the Neumann mic and the guitar cab, that the sound you put into the convolution filter is already colored by your preamp, mic, etc. So the resulting sound won't be exactly the same as if you'd used the Neumann, or whatever. It may not even be close.
 
So, can you use the SIR for impulses other than reverbs?
 
cactuseskimo said:
So, can you use the SIR for impulses other than reverbs?
Absolutely! I have even used a recording of me speaking as an IR to pass a synth through which then sounds like a wacky vocoder :eek:

You can use ANY audio file as an IR and it will impart its sonic character to whatever is passing through it. It opens up the door to so many possibilities.

Sometimes I have wanted to use a snare sound of which I only had one sample, but wanted to create some other snare sounds that are related to it, yet somewhat different. So, I loaded another snare .wav file as an IR and sent the main snare through it. Do this 3-4 times with different sources for IR and you've got yourself 4-5 snares that are related yet not exactly the same. Helps when you don't want to have the machine gun effect or eliminate the "sameness" from your drum patterns when programming drums electronically.

Don't stop there. You could use IRs of specific EQs and such. Although they're not replacement for the actual EQ units as the "controlls" are frozen, but if the original setting works, then great!

An extension of this is the LiquidChannel which uses IRs of different compressors, although this requires a different method of convolution.
 
Thanks, I 'm excited to give this all a go. Thanks for your help guys . Rep points for all!!!! :D
 
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