SMPTE timecode

SMPTE time code is a set of cooperating standards to label individual frames of video or film. It has been adapted to synchronize music.

The basic format is an 80-bit code that gives the time of day to the second, and the frame number within the second. Some of the bits also tell whether the timing is for 24 frames/sec (film), 25 frames/sec (European video), 30 frames/sec (black & white NTSC (archaic)), or 29.97 frames/sec (30/sec drop-frame color NTSC).

The data has three standardized formats:

Longitudinal- suitable to be recorded on an audio channel, or by audio wires. This is how it is distributed within a studio to synchronise recorders and cameras.
Vertical Interval Time Code (VITC, pronounced "vitsee") recorded directly on video.
Film label

The longistudinal code is generated by a special electronic clock, and then distributed from a central point. Usually the clock has high precision. Modern clocks usually derive their timing from an atomic clock, either using network time, or GPS. Studios usually maintain two or three clocks, and automatically switch-over if one fails.

A recent development is to mount small GPS-synchronized SMPTE generators on each camera, which eliminates the distribution network for portable set-ups and shooting on location.

Longitudinal SMPTE is widely used to synchronise music. The frame rate used is arbitrary, however the wise frame-rate for most English-speakers to use is 30 FPS drop-frame color NTSC. Most commercial music that ever jumps media goes to video, and NTSC serves the largest English-speaking video market.

Of course, if you are scoring a film or live in Europe, Russia, Asia or South America, use the standard that makes sense!

Drop frame video is a compromise invented when color NTSC was invented. basically, the NTSC redesigners wanted to retain compatibility with black and white TVs. Rather than change the luminance bandwidth to squeeze the color in to 30 frames/sec, they adjusted everything else. including the frame rate.

So, drop frame SMPTE drops frames 0 and 1 of the first second of every minute, and includes them when the number of minutes is divisble by ten.

The bits of the longitudinal SMPTE code:

0..3:Frame units
4..7:user-bits field 1
8,9:Frame tens
10: 1=drop frame format (frame 0 and 1 omitted from first second of each minute, but included when minutes divides by ten; approximates 29.97FPS)
11: 1="color frame", i.e. the time code is intentionally synchronised with a color TV field sequence.
12..15:user-bits field 2
16..19:seconds units
20..23:user bits field 3
24..26:second tens
27: Bi-phase mark-correction bit; set or cleared so every 80-bit word has an even number of zeros. This allows a technician to read the bi-phase coding timing with an oscilloscope, without the signal inverting on every other frame. Modern equipment regenerates the time code to a fixed timing in the video vertical interval, so this is far less needed than before.
28..31:user bits field 4
32..35:minutes unit digit
36..39:user bits field 5
40..42:minutes tens digit
43:binary group flag bit (with bit 59, 43,59 = 00 = no format for user bits, 10 = eight bit format, 01, 11 are unassigned and reserved.
44..47:user bits field 6
48..51:hours units
52..55:user bits field 7
56,57:hours tens
58: unused, reserved, should transmit zero and ignore on receive for compatibility
59:binary group flag bit (see bit 43 for encoding)
60..63:user bits field 8
64..79: sync word, should be 0011 1111 1111 1101

User bits: There are thirty-two bits of user data. These are usually used for a reel number and date. They can be anything at all, as long as bits 43 and 59 are set to zero.

The bits are encoded as "biphase." What this means is that a single transition at the start of the bit period is a zero, while two transitions, at the beginning and middle of the period, are a one. one up & down is a zero, while two up & downs in the same space is a one.

Longitudinal SMPTE should be played back at a middle-level when recorded on an audio track. Too high or too low will both cause distortion.