Sequences and live audio live in two very different worlds. Sequences, whether audio or MIDI, are almost always in strict tempo. Every beat has the same duration, as does every bar.
Oddly enough, you don't have to record like this. It's just that it seems to make sense to. And of course if you want to apply quantization, whether the simple kind or groove quantization, then it's a necessity.
And of course if you are working with dance music, then it simply has to be in strict tempo.
But what if you want to work with live instruments such as guitars, drums and keyboards (which of course you don't have to sequence)? Or choose violins and flutes if you like.
If you want to combine these instruments with a sequence, then you have the problem of mixing free tempo with strict tempo. And they just don't want to work together.
But you have options…
- Get the live musicians to play to a click generated by the sequencer.
- Record the sequence first and overdub the live instruments.
Either of these options will get the job done. The problem is that free tempo is the essence of live playing and, if you not going to have that, you have to start to wonder why you're using live musicians in the first place.
But you can do it the other way round too – record the live musicians first, and synchronize the sequence to that.
The problem is that the live musicians will vary in tempo, perhaps slightly, but you won't just be able to set one tempo for the sequence as a whole. The tempo of the sequence will have to adapt as it goes along.
So this is where the 'tempo map' feature of your sequencer comes in. You can set the tempo to vary as often as every beat if you wish. Updating once a bar is often enough.
Some sequencers make this easy – Pro Tools does it well. You can identify a certain point in the audio waveform and then tell the sequencer which bar and beat it should be. Just work your way through the song. It takes a while, but it does the job.
If you like doing things the hard way, or your sequencer doesn't provide easy tools for this, then you can examine the waveform closely and find the position in time of the start of each bar, and then calculate the tempo for the bar. This will take a lot longer, but it's still worth doing.
Remember that just because something is difficult doesn't mean you shouldn't do it. In fact, you should do it, because you'll be doing something that few others will have the patience to attempt.