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anonymous 14 April 2003

Hi,

Does anybody know the maths ( or where I can find it ) behind layering audio files on top of each other?

Cos I have this idea:
In cutting up drum loops or beats extracted from a song, say to take out a snare drum or kick etc but most of the time it will have a hi-hat or something on top of it.

I guess I'd be simplifying it by saying that:
x=hi hat
y=kick
x*y=kick and hi hat?

But if there exists an inverse to y then you could remove the hi hat if you had the hi hat seperately which is easy to get. (can be extracted from other part of loop pretty easy).
I know that in acoustic drum loop the hi hat hits won't be identical but similar enough to use in that equation?

So thats my theory.

I know the maths will be a lot harder than the simple algebra above but can it be done? Are there any simpler existing methodologies for cutting up drum beats for the single shot samples?

Also I'd like to introduce myself, my names Adam and Im doing a BE in Software at melbourne uni australia and I'm heavy into PC music ( mostly funk, trip hop, breaks and stuff ) and I'm looking for a BBS filled with some Pro Audio/PC Heavyweights to talk about something a bit more stimulating than what MIDI means.
Hopefully I can bring something to the board.

Peace.

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Comments

anonymous Sun, 04/27/2003 - 17:29

hi...
the math of what you described for kick and hi-hat would be x + y. x * y would be multiplying the two signals together, resulting in amplitude (aka ring) modulation. if you had a perfectly clean solo track of the hi-hats, you could invert the phase and mix it back in...this would remove some of the hi-hat sound from the mixed track. in terms of your equation, the phase-inverted hihat would be -y, so (x + y) - y would in theory give you x. in practice, it's probably impossible to completely remove an entire hi-hat track just with phase cancellation, but you could probably make it less prominent.
chris

falkon2 Sun, 04/27/2003 - 20:43

If it was sequenced drums and no non-linear effects or algorithms were applied to it after the addition, you SHOULD be able to get the original kick or snare down to the last bit.

That's in theory, of course. In practice, the signal for the hi hat superimposed on the kick and snare isn't just a simple A + B because of subsequent effects and artefacts like compression, dithering, etc. Basically, anything non-linear will screw up the algebra equation.

If not, you STILL could get the hi hats to almost inaudible states by phase-inverting. However, you probably can't do this with live drums because each hit will cause a slightly different sample.

Just be sure that the two samples line up EXACTLY horizontally, or you'd just be comb-filtering the hi hat instead of taking it out.

anonymous Sun, 04/27/2003 - 21:10

Hi Atom,

I'm going to take a stab at this one and assume you're talking about removing specific elements of drum loops such as maybe removing the hi-hat or snare drum to later replace them with a different hi-hat or snare.

Unless you just like crunching numbers, there's need to rack your brain using formulas and such.

There are a few apps out there that will accomplish this task for you just fine. One in particular that I'm familiar with is "Cyclone". A very handy DXi that comes with Cakewalks' "SONAR XL". Cyclone will allow you to remove any element of an ACID formatted loop that you want. SONAR will also allow you to convert a normal .wav file into an ACID format loop file. What Cyclone does is slice up the ACID formatted wave file into all the hits that make up that loop. At that point you can remove any part of the loop that you want and Cyclone will then replace what you removed with silence thus allowing you to play that loop minus what you removed. You can then replace what you removed with anything you want.

Hope I'm on the right track with this one.

JHC

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