| Microsoft DirectX 8.1 (C++) |
SMPTE timecode is the glue that holds the post-production process together. It identifies video and audio sources, makes automatic track synchronization possible, and provides a container for ancillary data related to the production. You will need to understand this data stream and its application to media production, tool development, or system design.
SMPTE timecode, more properly known as SMPTE time and control code, is a series of digital frame address values, flags and additional data applied to a video or audio stream, and is defined in ANSI/SMPTE 12-1986. Its purpose is to provide a machine-readable address for video and audio.
The most common form of an SMPTE timecode data structure is an 80-bit frame that contains the following information:
The DirectShow TIMECODE_SAMPLE structure is an example of a timecode data structure that contains timecode information for video or audio data.
SMPTE timecode comes in one of two types. Timecode recorded on an analog audio track as a bi-phase mark encoded signal is known as LTC, or Linear TimeCode (formerly known as Longitudinal TimeCode). Each timecode frame is one video frame time in duration. The other common type of timecode is known as VITC, or Vertical TimeCode. VITC is usually stored on two lines of a video signal's vertical blanking interval, somewhere between lines 10 and 20.
LTC timecode is easy to add to a prerecorded tape, because it is encoded in a separate audio signal. However, it cannot be read when the tape is paused, moving very slowly, or moving very quickly. In addition, it consumes one audio channel on nonprofessional VCRs.
VITC timecode, on the other hand, can be read from speeds of zero to 15 times normal speed. It can contain field-dependent data and can be read from video capture cards. However, it is not easily added to a prerecorded tape and often requires expensive hardware.
SMPTE timecode also comes in one of two modes, nondrop frame and drop frame. Nondrop frame is timecode that is consistently increasing and sequential. It can act as a real-time clock and works fine for monochrome video that runs at a frame rate of exactly 30 frames per second.
However, NTSC color video actually runs at a frame rate of 29.97 Hz (frames per second) because of some compatibility issues with monochrome television. This causes a problem with nondrop frame timecode because it gets out of step with real time at the rate of 108 frames (or 3.6 seconds) per hour. This means that after 1 hour of playback, the timecode would read 00:59:56:12, assuming a start point of 00:00:00:00. This causes problems when trying to calculate show duration or using "time-of-day" referencing.
A solution to this problem is to skip some frames in the count every so often so the error is reduced to something tolerable. This compensation method is called "drop frame" and is implemented by skipping the first two frames from the count at the start of each minute except minutes 00, 10, 20, 30, 40 and 50. The net result is an error of less than 1 frame per hour, or about 3 frames per 24 hour period.
Drop frame is used more commonly in today's productions, although any implementation should support mixing both modes.