OK, about time to post this here. I just realised there's no explanation around what others in a jam are hearing when you're playing and it's one of the really, really not obvious parts of the whole thing.
tl/dr: People don't hear you when you play. They hear you later. You play on "1", they hear you when that gets to them, delayed until the next time they get to "1". Different people may hear different people's "1" mixed together with theirs...
That's the short version.
Let's start the long version with a picture. Yeah, it's not pretty...
Code:
A: |0: a0/b-/c- :|1: a1/b-/c- :|2: a2/b1/c1 :|3: a3/b2/c2 :|
B: |0: a-/b-/c- :|1: a0/b1/c- :|2: a1/b2/c- :|3: a2/b3/c1 :|
C: |0: a-/b-/c- :|1: a0/b-/c1 :|2: a1/b1/c2 :|3: a2/b2/c3 :|
"A", "B" and "C" are three people who have joined a single NINJAM session. The "x: ../../.. :|" bits are what each of them hears and plays for each interval after they join.
A starts playing, so that's "a0", playing to nothing from B or C.
B and C wait until they hear "a0", A's first interval, and start playing when they hear it. Both happen to get it at the start of the next interval.
A, of course, does not hear B or C until after they finish playing, so plays "a1" to nothing, too.
B's "b1" gets to A and C ready for the next interval.
C's "c1", however - due to the timing of A, B and C joining the server - gets to A for the next interval but to B for the one after.
I'll let that lot sink in - I go on about it more in the next message.
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So how do you make coherent music with this apparent "mess"? You understand musical form (and you follow the metronome).
Like I said, "x: ../../.. :|" (or, for example B's, "3: a2/b3/c1 :|") is an "interval" in NINJAM terms.
Each interval is a number of beats.
Each time an interval starts, each player starts hearing an interval from the other players, delayed to synchronise each other player's "1" to their "1", like I said at the start. (That's why following the metronome is important.)
Examine most simple songs and their form becomes apparent. There are even names like "12 bar blues" to describe that form.
Taking that example, what does "12 bar blues" mean?
Aside for those not familiar with the terms:
It is important to read "bars" as "bars" and "beats" as "beats" and understand that they are not the same thing.
In "12 bar blues", each "bar" has four "beats". That means you could call it "48 beat blues".
That's the important thing: the interval should be set to 48 beats. You multiply the number of bars by the number of beats in a bar, which you can take as 4. (Most people struggle if the beats per bar isn't 4, to be honest.)
Then each player will hear the blues chords from one 12 bar phrase at the start of the next.
Most rock and pop uses 8 bar phrases, two very similar 8 bar phrases or a 16 bar phrase. (Variations on that, such as four or eight 2 bar phrases, etc, are fairly common, too.)
Even rap might repeat one 4 bar phrase three times and then use a new 4 bar phrase, before repeating the form. In that case you could consider either 8 (if the last 4 bars isn't much different from the one 8 bars before) or 16 bars as the form. 4 bars would not work -- especially in NINJAM, because of the clash between the various different stages each player in the jam might be at.
Note that "form" can also be thought of as the chord sequence. Each chord will cover one or more beats but usually there will be one or two per bar, though sometimes more in passing. What you don't want to hear is two dissonant chords played against each other because the interval doesn't match the form.
So...
32BPI (8 bars) is good for most things - but not 12 bar blues.
48BPI (12 bars) is good for blues and straightforward jazz without too much cleverness.
64BPI (16 bars) is pushing most people's patience, though it's not too bad above 140BPM...
But you can go beyond this. I've had a few vaguely successful attempts at Brubeck's Take Five, with willing participants.