M-Audio Biport 2x4s [37/48] About frame rates

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