Creating a Time Code Track for 29.97 FPS Video

QuickTime Movie File Format Specification, May 1996

Creating a Time Code Track for 29.97 FPS Video

A time code track specifies time code information for other tracks. The time code keeps track of the time codes of the original source of the video and audio. After a movie has been edited, the time code can be extracted to determine the source tape and the time codes of the frames.

It is important that the time code track has the same time scale as the video track. Otherwise, the time code will not tick at the exact same time as the video track.

For each contiguous source tape segment, there is a single time code sample that specifies the time code value corresponding to the start of the segment. From this sample, the time code value can be determined for any point in the segment.

The sample description for a time code track specifies the time code system being used (for example, 30 fps drop-frame) and the source information. Each sample is a time code value.

Since the time code media handler is a derived from the base media handler, the media information atom starts with a generic media header atom. The time code atoms would contain the following data values:

Atom size	77
Atom type	`'gmhd'`
	Atom size	69
	Atom type	`'gmin'`
	Version/Flags	0
	Graphics mode	0x0040
	Opcolor (red)	0x8000
	Opcolor (green)	0x8000
	Opcolor (blue)	0x8000
	Balance	0
	Reserved	0
	Atom size	45
	Atom type	`'tmcd'`
		Atom size	37
		Atom type	`'tcmi'`
		Version/Flags	0
		Text font	0 (system font)
		Text face	0 (plain)
		Text size	12
		Text color (red)	0
		Text color (green)	0
		Text color (blue)	0
		Background color (red)	0
		Background color (green)	0
		Background color (blue)	0
		Font name	`"\pChicago"` (Pascal string)

The sample table atom contains all the standard sample atoms and has the following data values:

Atom size	174
Atom type	`'stbl'` (sample table)
	Atom size	74
	Atom type	`'stsd'` (sample description)
	Version/Flags	0
	Number of entries	1
	Sample description size [1]	58
	Data format [1]	`'tmcd'`
	Reserved [1]	0
	Data reference index [1]	1
	Flags [1]	0
	Flags (time code) [1]	7 (drop frame + 24 hour + negative times ok)
	Time scale[1]	2997
	Frame duration[1]	100
	Number of frames[1]	20
		Atom size	24
		Atom type	`'name'`
		String length	12
		Language code	0 (English)
		Name	"my tape name"
	Atom size	24
	Atom type	`'stts'` (time-to-sample)
	Version/Flags	0
	Number of entries	1
	Sample count[1]	1
	Sample duration[1]	1
	Atom size	28
	Atom type	`'stsc'` (sample-to- chunk)
	Version/Flags	0
	Number of entries	1
	First chunk[1]	1
	Samples per chunk[1]	1
	Sample description ID[1]	1
	Atom size	20
	Atom type	`'stsz'` (sample size)
	Version/Flags	0
	Sample size	4
	Number of entries	1
	Atom size	20
	Atom type	`'stco'` (chunk offset)
	Version/Flags	0
	Number of entries	1
	Offset[1]	(offset into file of chunk 1)

In the example, let's assume that the segment's beginning time code is 1:15:32.4 (1 hour, 15 minutes, 32 seconds, and 4 frames). This time would be expressed in the data file as 0x010F2004 (0x01 = 1 hour; 0x0F = 15 minutes; 0x20 = 32 seconds; 0x04 = 4 frames).

The video and audio tracks must contain a track reference atom to indicate that they reference this time code track. The track reference is the same for both and is contained in the track atom (at the same level as the track header and media atoms).

This track reference would contain the following data values:

Atom size	12
Atom type	`'tref'`
Reference type	`'tmcd'`
Track ID of referenced track (time code track)	3

In this example, the video and sound tracks are tracks 1 and 2. The time code track is track 3.