MIDI time code

The time code developed by the Society of Motion Picture and Television Engineers (SMPTE) is currently the most accurate system available for frame identification of video tape. It serves as the central mechanism by which different processes involving video and audio can be synchronized on a very accurate basis. MIDI time code is compatible with SMPTE. It synchronizes MIDI applications to video and audio applications that use SMPTE. The SMPTE format is expressed as Hours:Minutes:Seconds:Frames, where:

    0 <= hours<= 23*
    0 <= minutes<= 59
    0 <= seconds<= 59
    0 <= frames<= 29**
    *     TSMPTETimeCode allows hours to be greater than 23.
    **The maximum frame value depends on the frame rate.
The MIDI system provides a set of TTime derivatives to make SMPTE time code easy to use. The classes derived from TSMPTETimeCode support conversion between the different frame rates and other TTime derivatives. TSMPTETimeCode uses a floating-point value to extend the SMPTE format to provide fractional frames.


MIDI ticks

A MIDI tick is a dimensionless time unit derived from TTime. You can use MIDI ticks for time stamps in MIDI sequences when expressing metrical or SMPTE time formats.


Although MIDI data can be specified in real time, for musical flexibility MIDI time formats are often expressed in metrical or SMPTE contexts. To do this, you use a tick combined with information for translating ticks to actual time values. Then you can characterize MIDI data by a tempo (units per second) and tick rate (ticks per unit).

For example, a MIDI file might specify a tempo of 1 quarter note per second and 2 ticks per quarter note. To express MIDI data in a SMPTE context, you specify the number of frames per second and the number of ticks per frame.

Tempo maps

You can model a MIDI tempo map by changing the rate of a MIDI player at different points in time--that is, changing the playback rate of its sequence. You change the playback rate by changing the rate of the player's internal clock, or the rate of the clock to which it is synchronized. It is important that you use the tempo map to back out of a sequence when the player is reversed, rewound, or restarted.


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