M-AUDIO MIDISPORT 8x8/s - Manual (Page 41)

to audio and video gear. Interfaces such as the MIDISPORT 8x8/s 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 complete 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
MIDISPORT 8x8/s 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.30
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
Black and white video frame rates were originally derived from the A/C line
frequency of the indigenous country (e.g. 50 Hz in Europe, 60 Hz in the U.S.). In
the U.S. when color video was introduced, part of the black and white frame
information was used to encode the color information. The result was that the
color frame rate ran at an effectively slower rate of 29.97 frames per second. This
slower rate can be approximated by running at 30 frames per second and
skipping 108 frames per hour in the numbering scheme. This method of
“dropping” 108 frames/hour to slow the effective frame rate was termed “drop
frame.” Hence, the standard frame rate used when doing U.S. color video is
called “30 drop frame.” When running at a t rue 30 frames per second, this is
known as “30” or sometimes “30 non-drop.”
As listed in the above table, in the U.S. and other “NTSC” countries, black and
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