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It is technically possible to synchronise a system to a jittering external
clock source. To be prepared for jitter, some products make use of
complex signal processing.
With a PLL for example, it is possible to manage difficult clock signals.
That implies that the slave calculates an average value for the tempo
to compensate the jitter. This calculated moving average is then used
for the playback. This average tempo is continuously refreshed by the
incoming clock ticks but the playback is freewheeling on the calculated
BPM. Sometimes you can observe that the song position is not based
on the counted beats, but on the calculated tempo and the elapsed
time.
Moreover, most devices for electronic music production are equipped
with a buffer for the sound output.
These buffers cause constant latencies that have to be merged with
the playback time. It depends on the implementation what happens
when a 'stop' command is received. Sometimes you can recognize
some sort of a run-out time. In this run-out time the data in the audio
buffer is played till the end and the playback does not stop immedia-
tely. This causes that some sequencers do not stop instantly with the
arrival of the 'stop' command, but with delay of the buffer. If now the
master does not define a new song position, the slaves are shifted to
each other on a 'continue' command. In this case all slaves run with the
same tempo, but not in sync with the beat.
Please consider the following hints to prevent those problems:
First, use a sound card with a small buffer size if you use DAWs and
software-sequencers.
Furthermore, a professional MIDI interface is essential for good
results.
Avoid merging midiclock signals with MIDI commands from other
sources, since you cannot ensure beat accuracy in this case..
It is possible that you still notice shifted tracks after pausing. In this
case you can get around this by sending a 'MIDI song position pointer'.
The midiclock sends this pointer at the beginning of playback and
re-sync, but not on 'MIDI continue'.
That's the only way to ensure that all slaves are situated at the same
song position and start in sync.
