M-Audio MIDISPORT 8x8/s [41/52] About frame rates

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