Beats 1 and beats 2 are actually the same thing. It only becomes an issue if you only have one BPM value for the entire track and switch between time signatures with different denominators, in which the notes used to count off the beats change (which essentially leads to a doubling/halving of the bpm 99% of the time).tony tony chopper wrote:So this is what the denominator of the timesig is for:
-we have the "beats" in BPM, aka "beats 1"
-the "beats on a grid/in a metronome", aka "beats 2" relate to "beats 1" according to the denumerator of the timesig.
And this is why a metronome, supposed to play on the beat, should change its speed while the tempo does not, when the denumerator of the timesig changes. Because the metronome plays on "beat 2" while the BPM define "beat 1".
What you're calling beat 1 is actually just the corresponding bpm in 4/4, because you're only counting quarter notes as beats. But that's not actually how it works. All a metronome does is count out the beats for you as they come. There's no difference between these beats and any other beats. There is no "computer bpm" that is in some way removed from bpm for the rest of music. The whole point of a metronome is that each "tock" indicates a beat.
As an example, look at the following 'BPM indicators' for traditional sheet music:
The note indicated here is a quarter note, so we have 120 quarter notes per minute. In other words, bpm is 120, and each beat corresponds to a quarter note. This will be for a time signature with 4 in the denominator.
Here we have Mozart's Sonata K. 331, and the bpm is indicated as 120 as well, however each beat corresponds to an eighth note (that squiggle on the note indicates an eighth note), which makes sense because we're in 6/8 time, and each eighth note corresponds to a beat.
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You need to understand that the bpm doesn't have to be in quarter notes. As I posted before, the denominator in the time signature determines the type of note that represents a beat. How frequently these notes are played determines the bpm. Have 120 eighth notes a minute in 6/8? Then you have a bpm of 120. Simples.