MOTU MIDI Express XT [85/94] How does synchronization work

APPENDIX B: TIME CODE BASICS

Now, if we play a color version of the same program, it

actually runs slower at 29.97 frames per second so that the

actual elapsed time is 60 minutes and 3.6 seconds! Here’s

where the discrepancy arises: the time code that counts the

frames shows that one hour’s worth of frames has gone by,

which is 01:00:00:00 on the ﬁnal frame. But this does not

match the actual elapsed time, which is 01:00:03:18! In

broadcast situations, where edits are calculated down to

fractions of a second, 3.6 seconds is a long, long time —

too large a degree of inaccuracy.

Drop Frame time code ﬁxes this problem by skipping

ahead every once in a while as it counts color video frames

to catch up with actual elapsed time. The result is that over

the period of several minutes, the time code matches the

actual elapsed time.

It is important to note that since frames are dropped only

once every minute, Drop Frame time code does not always

reﬂect the exact actual elapsed time: it may be up to a 10th

of a second faster or slower than actual elapsed time,

depending on how recently the last frame number was

dropped.

SHOULD I USE DROP FRAME?

Use Drop Frame time code only when it is absolutely

necessary. Drop Frame is required only with color video

projects in which the SMPTE time code numbers must

precisely match the actual elapsed time, such as when

preparing a television broadcast. Otherwise, we suggest

that you use 30 Non-drop time code because of the slight

inaccuracy mentioned above, as well as the confusion that

drop frame can cause.

HOW DOES SYNCHRONIZATION WORK?

The syncing process is straightforward. It involves one

device following another. As you play back a tape with

SMPTE time code on it, the time code feeds into a

hardware device called a SMPTE-to-MIDI converter. The

converter translates the SMPTE audio signal into MIDI

Time Code and sends the MIDI Time Code to a MIDI

device such as a sequencer. The MIDI device receives the

time code and adjusts its playback position to match the

time code. All of this happens very quickly, around 30

times per second, which is fast enough so that the MIDI

device follows the tape smoothly.

!USB Interfaces Manual Book Page 85 Tuesday, August 16, 2011 11:21 AM