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Time Code

From Chace Audio, Wikipedia

Time code is a linear audio program of a sequence of numbers generated by a timing system that specifies location by hours:minutes:seconds:frames (i.e. 01:58:30:00). Time code is used to synchronize different audio and video machines. In fact, the invention of time code made modern video/audiotape editing possible and eventually led to the creation of non-linear editing systems.

There are many types of time code, including linear or longitudinal time code (LTC), vertical interval time code (VITC), and control track longitudinal (CTL). Visible time code is a representation of time code burned onto the video image and visible as hours:minutes:seconds:frames on a video monitor.

In our industry, time code and speed are easily confused. While there are standard relationships, they are not always adhered to, and one cannot assume these relationships have been maintained.

There are three standard speeds used in audio post production: film, NTSC, and PAL.

Film travels at a speed of 24 frames per second (fps) – 90 feet of film per minute. NTSC video runs at a speed of 29.97fps, which is equivalent to 23.976 (23.98) fps film speed and is exactly 0.1% slower than standard film speed (24 film frames = 30 NTSC video frames). PAL video runs at 25fps, 4% faster than standard film speed (25 film frames = 25 PAL video frames).

It’s important to note that NTSC videotape always travels at the same speed, 29.97fps. NTSC videotape cannot travel at 30fps. That said, there are literally 30 frames of NTSC video on the cassette, but they cannot be played back at 30fps due to technological restrictions in the NTSC format.

Time code is a positional reference that we use to synchronize elements together. While there are standard time code rates that are used with NTSC, PAL and film speed elements, they can sometimes be incorrectly matched, which can lead to a lot of confusion. The following time code types (or “flavors” as they are so euphemistically referred) are standard industry time codes. They are listed with their standard corresponding speed:

24 = film speed
30 NDFTC = film speed
23.976 (23.98) = NTSC speed
29.97 NDFTC = NTSC speed
29.97 DFTC = NTSC speed
25 (EBUTC) = PAL speed

Notice that NDFTC and DFTC are the same speed. These two time code flavors both run at the same speed but count frames differently from one another. DFTC (drop frame time code) was invented specifically to address the fact that NTSC video cannot playback 30 frames per second. Over time, NDFTC (non-drop frame time code) will begin to differ from real time, while DFTC will constantly adjust itself to compensate.

HD video can become more confusing, but with comprehension of the three speeds discussed above, it can be more easily understood. To understand HD video you must first understand the various video resolutions and time codes that are standard across HD formats.

There are three basic picture resolutions: 720PsF (Progressive Scan Frames), 1080i (interleaved frames), and 1080PsF. Each number refers to the lines of vertical resolution in the picture. Standard NTSC is capable of 480PsF (progressive scan DVD), but is typically displayed as 240 interleaved fields (two fields per frame totaling 480 total fields).

HD also supports all six standard time code formats: 23.976, 24, 25 (EBUTC), 29.97 NDFTC, 29.97 DFTC, and 30 NDFTC. (Please note: 23.98 is really 23.976 rounded up; you may see it referred to either way depending on the machine or platform).

Tapes are labeled with their video resolution and corresponding frequency (which equates to video fields and time code flavor).

These are the various types of HD cassettes for 720p and 1080p resolutions:
720 23.98PsF
720 24PsF
720 25PsF
720 29.97PsF (NDF and DF)
720 30PsF
1080 23.98PsF
1080 24PsF
1080 25PsF
1080 29.97PsF (NDF and DF)
1080 30PsF

These are the various types of cassettes for 1080i resolutions. (To figure out the time code flavor, simply divide the frequency value):
1080 50i (25 EBUTC)
1080 59.94i (29.97 NDF or DF)
1080 60i (30 NDFTC)

This definition/image is licensed under the GNU Free Documentation License found at http://www.gnu.org/copyleft/fdl.html. It uses material from the Wikipedia articles at http://en.wikipedia.org/wiki/Time_code.

The most irritating sound in audio post production is also one of its most important - the sound of time code.