M-AUDIO Biport 2x4s - Manual (Page 37)

What is MIDI Time Code (MTC)?
MIDI Time Code (MTC) is the MIDI implementation of SMPTE. MTC
was first standardized in 1987 as a method of synchronizing MIDI soft-
ware and hardware sequencers, as well as drum machines. Later it was
also adopted by digital audio recording software and hardware manu-
facturers as a method of synchronizing to audio and video gear.
Interfaces such as the BiPort 2x4s perform the essential task of reading
longitudinal SMPTE from an audio and/or video tape and converting it
to MTC for use by MIDI applications.
MTC implements a special System Common MIDI message that is sent
four times per frame (every quarter-frame). Each of these “quarter-
frame” messages is two bytes long and contains one eighth of a com-
plete time code value. A unit or program receiving MTC must receive
eight of these messages in order to construct the complete time. By the
time 8 quarter-frame messages are received and the complete time is
constructed, that time value is two frames old and it would seem that
any synchronized application would always be two frames behind.
Fortunately, most sequencer programs add two frames to the received
MTC in order to derive the current time.
As long as a longitudinal SMPTE source is running, time code may be
read. However, what happens when it stops? The LTC is no longer
readable and the BiPort 2x4s sends a standard MTC NAK (signal not
acquired) message to the application to indicate the tape has become
idle.
About Frame Rates
All SMPTE time code and MTC carries the same primary information,
Hours:Minutes:Seconds:Frames. However, SMPTE time code can be
written and read at different standard frame rates. These frame rates
designate the number of frames that each second is subdivided into.
Different video, film and audio standard frame rates have been adopted.
These various standards and typical applications are:
Frame Rate Application
24 frames/second Motion Pictures (film)
25 frames/second European Video - both B/W & Color
30 drop frame U.S. Color Video
29.97 frames/sec. U.S. Color Video
30 non-drop U.S. B/W Video and U.S. Audio
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