Timecode

SMPTE ST 262, Binary Groups of Time and Control Codes – Storage and Transmission of Data Control Codes.

SMPTE RP 169, Television, Audio and Film Time and Control Code – Auxiliary Time Address Data in Binary Group – Dialect Specification of Directory Index Locations.

This chapter gives technical details concerning timecode, as used in video, film, audio recording, editing, and sequencing equipment.

Introduction

Timecode systems assign a number to each frame of video to allow each frame to be uniquely identified. Time data is coded in binary-coded decimal (BCD) digits in the form HH:MM:SS:FF, in the range 00:00:00:00 to 23:59:59:29. There are timecode variants for 24, 25, 29.97, and 30 frames per second. Timecode data is digitally recorded with the associated image. Burnt-in timecode (BITC) refers to a recording with timecode numbers keyed over the picture content (that is, embedded in the image data).

In addition to approximately 32 bits required for eight-digit time data, timecode systems accommodate an additional 32 user bits per frame. User bits may convey one of several types of information: a second timecode stream (such as a timecode from an original recording); a stream of ASCII/ISO characters; motion picture production data, as specified in SMPTE ST 262; auxiliary BCD numerical information, such as tape reel number; or nonstandard information. A group of 4 user bits is referred to as a binary group. The information portion of timecode thus totals 64 bits per frame.

A number of synchronization bits are appended to the 64 information bits of timecode in order to convey timecode through a recording channel. Sixteen synchronization bits are appended to form 80-bit linear timecode (LTC). Eighteen sync bits and 8 CRC bits are

See NTSC two-frame sequence, in Chapter 7 of Composite NTSC and PAL: Legacy Video Systems.

See Frame, field, line, and sample rates, on page 389.

In consumer 480i29.97 DV, dropframe timecode is mandatory.

The final field in an hour of 29.97 Hz video has DF code hh:59:59;29 and NDF code hh:59:56:23.

appended to form 90-bit vertical interval timecode

(VITC) that can be inserted into a video signal.

No BCD digit can contain all ones, so the all-ones code is available for other purposes. The highest possible value of certain tens digits is less than 8, so the high-order bits of certain timecode digits are available for use as flags; these flag bits are described on

page 403.

The colourframe flag is asserted when the least significant bit of the timecode frame number is intentionally locked to the colourframe sequence of the associated video – in 480i systems, locked to Colourframes A and B of SMPTE 170M.

Dropframe timecode

In 25 Hz video, such as in 576i video systems, and in 24 Hz film, there is an exact integer number of frames in each second. In these systems, timecode has an exact correspondence with clock time.

During the transition from monochrome to colour television in the United States, certain interference constraints needed to be satisfied among the horizontal scanning, sound, and colour frequencies. These constraints were resolved by reducing the 60.00 Hz

field rate of monochrome television by a factor of exactly 1000⁄1001 to create the colour NTSC field rate of about 59.94 Hz. This leads to a noninteger number of frames per second in 29.97 Hz or 59.94 Hz systems. The dropframe (DF) mechanism can be used to compensate timecode to obtain a very close approximation to clock time. Dropframes are not required or permitted when operating at exact integer numbers of frames per second. Dropframe timecode is optional in 29.97 Hz or 59.94 Hz systems; operation with a straight counting sequence is called nondropframe (NDF).

Counting frames at the NTSC frame rate of 29.97 Hz is slower than realtime by the factor 1000⁄1001, which – in nondropframe code – would result in an apparent cumulative error of about +3.6 seconds in an hour. To make timecode correspond to clock time, approximately once every 1000 frames a frame number is dropped – that is, omitted from the counting sequence. Of course, it is only the number that is dropped, not the video frame! Frame numbers are dropped in pairs in

hh:mm:ss:ff

xx:x0:00:00

xx:x1:06:20

xx:x2:13:10

xx:x3:20:00

xx:x4:26:20

xx:x5:33:10

xx:x6:40:00

xx:x7:46:20

xx:x8:53:10

Figure 33.1

Periodic dropped timecode numbers

Dot present for field 2

Comma for dropframe

Figure 33.2 Timecode as displayed, or represented in ascii, has the final delimiter (separating seconds from frames) selected from colon, semicolon, period, or comma, to indicate dropframe code and field 2.

SMPTE ST 258, Television – Transfer

of Edit Decision Lists.

order to maintain the relationship of timecode (even or odd frame number) to NTSC colourframe (A or B).

Dropping a pair of frames every 662⁄3 seconds – that is, at an interval of 1 minute, 6 seconds, and

20 frames – would result in dropping the codes indicated in Figure 33.1 in the margin. Although this sequence is not easily recognizable, it repeats after exactly ten minutes! This is a consequence of the ratios of the numbers: Two frames in 2000 accumulates 18 frames in 18000, and there are 18000 intervals of

1⁄30 second in 10 minutes (30 frames, times 60 seconds, times 10 minutes). To produce a sequence that is easy to compute and easy to remember, instead of dropping numbers strictly periodically, this rule was adopted:

Drop frame numbers 00:00 and 00:01 at the start of every minute, except the tenth minute. In effect,

a dropped pair that is due is delayed until the beginning of the next minute.

Figure 33.2 depicts the convention that has emerged to represent field identification and of the use of dropframe code in timecode displays.

Dropframe does not achieve a perfect match to clock time, just a very good match: Counting dropframe code at 30⁄1.001 frames per second results in timecode that is about 86.4 ms late (slow) over 24 hours. If the residual error were to accumulate, after 11 or 12 days timecode would fall about one second later than clock time. If

a timecode sequence is to be maintained longer than 24 hours, timecode should be jammed daily to reference clock time at a suitable moment. No standard recommends when this should take place; however, the usual technique is to insert duplicate timecode numbers 00:00:00;00 and 00:00:00;01. Editing equipment treats the duplicate codes as a timecode interruption.

Editing

Timecode is basic to video editing. An edit is denoted by its in point (the timecode of the first field or frame to be recorded) and its out point (the timecode of the first field or frame beyond the recording). An edited sequence can be described by the list of edits used to produce it: Each entry in an edit decision list (EDL) contains the in and out points of the edited material,

SMPTE RP 164, Location of Vertical

Interval Time Code.

the in and out points of the source, and tape reel number or other source and transition identification.

An edited program is ordinarily associated with continuous “nonbroken” timecode. Editing equipment historically treated the boundary between 23:59:59:29 and 00:00:00:00 as a timecode discontinuity, and an edit at that point (such as starting a new program) was problematic. Consequently, it is conventional to start a main program segment with timecode 01:00:00:00. On videotape, it was conventional to include

1.5 minutes of “bars and tone” leader starting at timecode 00:58:30:00. (In Europe, it is common to start a program at 10:00:00:00.)

Linear timecode (LTC)

Timecode was historically recorded on studio videotape and audiotape recorders on longitudinal tracks having characteristics similar or identical to those of audio tracks. This became known as longitudinal timecode (LTC). The word longitudinal became unfashionable, and LTC was renamed linear timecode – thankfully, it retains its acronym. LTC was historically transported in the studio as an audio signal pair interfaced through three-pin XLR connector.s

Vertical interval timecode (VITC)

Due to the limitations of stationary head magnetic recording, longitudinal timecode from a VTR could not be read at very slow speeds or with the tape stopped. Vertical interval timecode (VITC) was coded digitally as a pulse stream onto one or two lines in the vertical interval; the scheme overcomes the disadvantage that LTC cannot be read with videotape stopped or moving slowly.

Timecode structure

Table 33.1A at the top of the facing page illustrates the structure of timecode data. The information bits include a flag bit polarity/field (whose function differs between LTC and VITC), and three binary group flags BGF0, BGF1, and BGF2; they are interpreted in Table 33.1B and Table 33.1C opposite. A clumsy error was made when SMPTE standards were adapted to 25 Hz timecode: The positions of flag bits BGF0, BGF2, and

For details of LTC and VITC encoding, see the first edition of this book.

In LTC, the 64 bits are transmitted serially, followed by 16 LTC sync bits. In VITC, each group of 8 bits is preceded by two VITC sync bits; these ten words are followed by a final pair of VITC sync bits and a CRC.