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 that's often not the case, devices will display different BPM even though there is no problem at all with sync. Read on to find out why.
What's in MIDI Clock?
MIDI Clock is only a series of regularly spaced messages that are sent out at a speed dependent on BPM. MIDI Clock contains no other data points.
Devices can only display a BPM value by calculating it for themselves from these pulses. To calculate BPM, each device relies on its own internal timing. The catch is that the internal timing of each device is slightly different, which is the exact reason why MIDI Clock even exists in the first place.
It's entirely normal that they'll come up with slightly different answers.
Each device will decide how frequently it will calculate the BPM to refresh their own displays, 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 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 may seem bad on the surface, as long as each device is using the clock messages to override their relevant timing functions, the display is not what's important, so it's best not to dwell on it.
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 increases the more devices the pulses have to merge through.
But that doesn't mean that a BPM display that's fluctuating is the result of severe jitter in the clock. Generally, I'll disregard any fluctuation that is happening within 1 BPM for the reasons I explained 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 would 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, bypassing any detrimental merges of data.