When you use ACW in BIAB, you can generate a tempo map to mostly match each bars tempo to the underlying song. BIAB only handles whole note tempos and only supports tempo changes at each bar, but as long as you start each bar on beat 1 of the underlying song, then you would only drift within a bar - the whole song won't drift as you move through it.

Once you have finished with establishing the chord progression, etc. and decide you don't need the original recording anymore, you can remove the tempo map from the song and set whatever consistent tempo you wish to use.

If you use RealBand instead of BIAB, it supports fractional tempos, so you can get much closer to the underlying song. ACW will still only make changes at each bar, but instead of setting the tempo to say, 121, it will set it to 10.83. After sending the song back to RealBand from ACW, you can even insert additional tempo changes anywhere along the time line, if you wish.

John

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