When you have 2 or more devices synced using MIDI Clock, it's normal to think that they would all display the same BPM value and that, if they don't, then there must be some kind of problem. But devices will display different, even fluctuating, BPM though there is no problem with sync at all.
This post is made to help understand why.
What's in MIDI Clock?
MIDI Clock is only a series of regularly spaced messages that are sent out at a speed that's dependent on BPM. MIDI Clock contains no other data points, such as reporting what the BPM actually is.
Devices that follow MIDI Clock can only display a BPM value by calculating it for themselves using the pulses that come in. That, of course, means that each device must rely on its own internal timing in order to make the calculation. The catch is that the internal timing of each device is slightly different. It's the reason why MIDI Clock (or any master clock protocol) exists in the first place: to bring multiple pieces of hardware together in sync because, if left to their own internal timing, they'd each drift apart.
So, it's entirely normal that they'll come up with slightly different answers. The "severity" of this will vary.
First, a higher BPM tends to have the largest variance, so expect that you'll see the BPM display fluctuate a bit more when using a higher BPM.
Also, each device will decide how frequently it will refresh its calculation of BPM, how much precision they will use for the number, and whether to round fractions of a whole number up or down. We can easily have the Master Clock device set at 120 BPM, another device following that reports 121 BPM that has rounded up to the nearest whole number from its internal calculation of 120.5 BPM, and still another device that's reporting fluctuations between 119.6 and 120.4 BPM.
Even though this will seem like a problem on the surface, it doesn't mean that sync itself is misbehaving. As long as each device is using the clock messages to override their relevant timing functions, the BPM display is not what's important.
What about Jitter?
If you've been using MIDI Clock for any length of time then you've probably heard about Jitter. Jitter is irregular spacing between the pulses themselves. In an ideal system, every pulse would be sent and would arrive perfectly spaced from one another. There is no ideal though, jitter is always present, and certain factors, such as MERGING data through devices, will even increase jitter in the signal.
But that doesn't mean that a BPM display that's fluctuating is evidence of a problem with jitter in the clock. Generally, any fluctuation that is within 1 BPM is more likely caused by the calculation of BPM as described above.
However, there are times when the BPM display could be telling us something that does indicate more of a problem. Values that swing wildly outside of 1 BPM could be an indication of the presence of high amounts of "jitter" somewhere. In that case, perhaps better results can be obtained by distributing the signal through MIDI Splitters and bypassing any detrimental merges of data that may be unnecessary.