ugh
...
lads if thats not it dont slate me...i couldn't give a shit
Why the fed-up, world-weary sounding start if you don't care? Especially as your knowledge of the whole thing seems pretty slim.
Impulses and convolution are actually quite an old techniques in general - it's just the cab sim thing that's a new(er) use for it. Essentially it's a way to record the sound of a certain space/room.
Traditionally, you put a bunch of microphones around your room; say one close, one 5ft back, one 10ft back. Then you record a wide-band, easily reproducible sound (usually a starting pistol being fired or a balloon being popped) through your mics. You end up with the sound of a gunshot in your room - technically called a
convolution as it's the combination of two functions (the source sound and the room).
You then take a sample of your gunshot recorded in a room with no reverb sound (an anechoic chamber ideally).
You then essentially take the room recording (the big reverby one) and subtract the source (the dead, no reverb version) - that process is called
deconvolution. What you're left with is an
impulse - the way it reacts to different frequencies over time at a specific point in space. Each of your three mics will sound different, as the sound takes different times to reach them, and there will be different reflections affecting them etc.
That's how reverbs used to be modelled on some early rack gear, before computers were used to just work out the maths instead. They're really good for big, echo-ey rooms like theatres and churches, and can make drum tracks sound enormous. Oddly enough, people now create impulses of the reverb units because they sounded good...
Then some bright spark had the idea of doing the same thing with guitar cabs. Instead of a gunshot/balloon/whatever, you normally use either a sine wave continuously increasing in pitch (although pink noise would probably work as it's equal energy per octave) and play it through the cab via a poweramp. Then when you deconvolve it, you can use
exactly the same source (so it's really accurate), and you end up with the sound of the cab. The only problem with the results is that they're very static and one-dimensional - there's no dynamics as your cab is being pushed at the same level the whole time.
And that's where Nebula comes in - Nebula 3 programs use several source sounds (a sine wave at different volumes) to create impulses which are more dynamic. The theory is that it can react differently depending on the volume of the input, so if you play gently it mostly uses an impulse created at low volume so the speakers are hardly moving; if you play heavy it uses one of the louder impulses where the speakers are really moving some air.
Steve
