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Topic: Maths experts - Converting Triplets to 12/8 using stretching

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  1. #1

    Maths experts - Converting Triplets to 12/8 using stretching

    I know this isn't directly related to GPO, but I wonder if anyone can offer any advice ?

    This problem is all the more annoying, in that I know I've solved it in the past by trial and error, but I can never remember how I did it when I come to need it again.

    The feature in most sequencers that I'm looking at (called various things from programme to programme) that allow to to stretch and contract a series of MIDI data by percentage - ie 200% makes it twice as quick (or changes crotchets to quavers in other words) whilst 50% doubles its length.

    On to my specific problem - I'm preparing scores for some of my old tracks, and have realised that whilst I'd composed them in 4/4 using triplets throughout, they are in actual fact more suited to 12/8. What I need to do is alter the MIDI data to make those triplet quavers equal straight quavers. (This needs a tempo change and a bit of tidying up in the score programme before it's complete, but needs to be done to avoid a hell of a nightmare job).

    The main problem is that somewhere along the line you end up with a 33%/66% change which, given the fact that what you actually need is 33.33333 . . . . . the timing cumulatively eventually shifts out of whack.

    I know (because, as I say, I have managed to somehow do it in the past) that there is a way of doing this that avoids the decimal-recurring problem but I can't for the life of me work out what it is. I seem to remember you have to do some sort of combination of 600% followed by a 25% etc etc or something like that !

    Can anyone out there clever with maths come up with a formula that makes a triplet quaver into a quaver ?

    Does this make any sense whatsoever ?!


    Cheers

    Simon

  2. #2
    Senior Member rwayland's Avatar
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    Re: Maths experts - Converting Triplets to 12/8 using stretching

    Quote Originally Posted by sasmith
    I know this isn't directly related to GPO, but I wonder if anyone can offer any advice ?

    This problem is all the more annoying, in that I know I've solved it in the past by trial and error, but I can never remember how I did it when I come to need it again.

    The feature in most sequencers that I'm looking at (called various things from programme to programme) that allow to to stretch and contract a series of MIDI data by percentage - ie 200% makes it twice as quick (or changes crotchets to quavers in other words) whilst 50% doubles its length.

    On to my specific problem - I'm preparing scores for some of my old tracks, and have realised that whilst I'd composed them in 4/4 using triplets throughout, they are in actual fact more suited to 12/8. What I need to do is alter the MIDI data to make those triplet quavers equal straight quavers. (This needs a tempo change and a bit of tidying up in the score programme before it's complete, but needs to be done to avoid a hell of a nightmare job).

    The main problem is that somewhere along the line you end up with a 33%/66% change which, given the fact that what you actually need is 33.33333 . . . . . the timing cumulatively eventually shifts out of whack.

    I know (because, as I say, I have managed to somehow do it in the past) that there is a way of doing this that avoids the decimal-recurring problem but I can't for the life of me work out what it is. I seem to remember you have to do some sort of combination of 600% followed by a 25% etc etc or something like that !

    Can anyone out there clever with maths come up with a formula that makes a triplet quaver into a quaver ?

    Does this make any sense whatsoever ?!


    Cheers

    Simon

    Well, it is a problem I frequently encounter, usually only involving a measure or two. My usual method is to fumble until I remember what will work, or figure it out again. Mostly, I increase to a point where the notes are larger than wanted, say increase value of triplet 8ths to quarters, then change again to value of appropriate per cent, then, the key, quantize to the absolute value, making sure to select change value and start times.

    Or, change note length to smaller than wanted, then quantize to length desired. Make sure to select change note value and start time.

    I think the main thing is to get the note values small, then quantize up to what you need.

    I am not sure if I expressed this clearly, but this is the general idea of what I do. A day or two ago, I changed several strings of triplet 16ths to 32d notes doing something of the sort.

    Richard

  3. #3

    Re: Maths experts - Converting Triplets to 12/8 using stretching

    Simon,
    In SONAR I would:

    1. Change the time sig to 12/8 (this won't affect the following but it will look correct when you've finished)
    2. Increase the length and start times of the notes by 150%
    3. Change the tempo to 2/3 of the original value (because the original tempo would play back the notes more slowly since they are 50% longer and their start times have increased by 50%)

    If you have a complex tempo map, it may not be that simple, as ALL tempos would need to be reduced to 2/3 of their original value.


    The feature in most sequencers that I'm looking at (called various things from programme to programme) that allow to to stretch and contract a series of MIDI data by percentage - ie 200% makes it twice as quick (or changes crotchets to quavers in other words) whilst 50% doubles its length.
    A late note: It's the opposite in SONAR: you can apply the percentages to the note lengths and start times, so 200% would be twice as slow and 50% would be twice as fast at the original tempo...
    Bill

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