Timecode

SMPTE timecode, containing information about the time of day of a recording, allows a device to be synchronised with professional recording machines or video equipment. The timing is accurate to one frame, the duration of which is set by your television standard, such as NTSC or PAL.

Timecode exists in several forms, including longitudinal timecode (LTC), recorded on analogue audio tracks, vertical interval timecode (VITC), conveyed within a video signal, and as MIDI timecode (MTC), sent over a Musical Instrument Digital Interface (MIDI) connection.

Timecode on Analogue Audio Tape

When used with an analogue tape machine, SMPTE timecode is recorded as an audio signal in the form of longitudinal timecode (LTC). This is encoded using a technique known as bi-phase modulation, employing the following frequencies:-

Frame RateLogic ‘0’ (Hz)Logic ‘1’ (Hz)
249601920
2510002000
3012002400

An example signal waveform is shown below:-

In practice, the signal is ‘rounded off’ to make it suitable for transferring directly onto an audio recording device. For timecode at the PAL frame rate of 25 frm/s, the average frequency is between 1 and 2 kHz, whilst the period length is around 500 milliseconds, with rise and fall times of about 50 microseconds. Overshoot, undershoot and tilt should be less than 5% of the peak-to-peak amplitude.

LTC can be recorded on a tape machine in several ways. If you only need a mono sound track, you can record the audio on the left-hand channel and timecode, in the form of LTC, on the right-hand channel. Unfortunately, there’s some risk of audible crosstalk reaching the track used for audio.

However, if stereo sound is required you’ll need a machine with a separate timecode track. In a quarter-inch tape recorder the two audio tracks, each 2 mm wide, are separated by a wide guard track, also 2 mm wide but with the 0.38 mm timecode track running down the middle.

In some machines, the timecode track doesn’t use pre-emphasis or de-emphasis, making it unsuitable for other purposes, such as an audio cue track. Recorders sometimes have separate heads for timecode and audio, requiring a fixed delay circuit to keep the two signals in step. Others have combination heads, such as a combined head for erasing audio and playing timecode, with another for erasing timecode and recording audio. Once again, delay circuitry is required.

Timecode on DAT

Timecode can also be used to synchronise the sound output of a Digital Audio Tape (DAT) machine with other recording machines and video equipment. The timecode is recorded onto DAT by recording on its subcode area, an area not used by DAT recorders that don’t support timecode.

To ensure precise timing accuracy, you should record timecode, as generated by the DAT recorder itself, at the same time as recording the audio. However, if the recipient of your DAT material doesn’t use the digital output of their DAT machine, reasonable timing ‘lock’ can be maintained for up to 30 minutes without worrying about timecode. In this situation your recipient can use their own source of timecode or you can stripe your DAT tape with timecode after making the audio recording.

A modern 4-head machine with off-tape monitoring can work in three modes:-

In Insert mode the machine can read timecode or audio from the DAT and then replace it.

VITC (Vertical Interval Timecode)

This form of SMPTE timecode is contained within a normal video signal, as recorded onto a video recorder or VCR, allowing timecode to be extracted later. The information is stored in a pair of ‘spare’ lines, typically lines 19 and 21, which are in the blanking period at the top of each picture frame.

VITC data can be extracted at up to 10 times play speed and when a recorder is in Pause. Some types of VITC to LTC converter continue to generate timecode even when a machine in Pause, sometimes also when in Stop, producing a signal at a rate of 110 to 1 times play speed. Unfortunately, such data can confuse other devices, requiring you to disconnect the LTC signal when not in use.

Synchronisation using Timecode

Timecode can be used to synchronise several video or audio recording devices, one of which acts a master whilst the others work as slaves. A synchroniser, consisting of at least two timecode readers connected to the master and slave machines, compares the timecode from each machine and uses this information to control the function and speed of the slave machines. The synchroniser’s control unit sometimes lets you to apply a fixed timecode offset between chosen machines.

More advanced studio systems often employ a separate controller for each machine. These controllers are then linked together by a control bus such as ES-Bus. Connections for video, audio, cue track, timecode and CTL pulse circuits must be provided to each machine as required.

There are three traditional video editing techniques:-

SMPTE Coding

Irrespective of how SMPTE data is transferred, it always contains the same information about the time of day. The time is described using eight digits, in the form of hh:mm:ss:ff, where hh are hours, mm are minutes, ss are seconds and ff are elapsed picture frames. A timecode reading at around 2:36 in the afternoon would therefore look something like 14:36:09:15.

Each digit is represented using binary-coded decimal (BCD), where bit 3 is the most significant bit (MSB) and bit 0 is the least significant bit (LSB), as shown below:-

DecimalBit 3 (MSB)Bit 2Bit 1Bit 0 (LSB)
00000
10001
20010
30011
40100
50101
60110
70111
81000
91001

The number of bits needed for each digit are given by the following table:-

DigitsBits
Frames, seconds, minutes and hours (units)4
Seconds and minutes (tens)3
Frames and hours (tens)2

These bits, together with other data and a synchronisation word, are kept in a frame whose length is 80 bits. For precise synchronisation with video material this length is made exactly equal to one television frame. For this reason most types of SMPTE/EBU timecode generator can be locked to the vertical sync pulse used in a video signal. Television studios invariably have a sync pulse generator (SPG) which provides such a signal, which then acts as the studio’s timing reference.

SMPTE timecode also includes User Bits that can be interpreted as four ASCII (ISO-7) characters per frame, although they usually contain eight hexadecimal numbers. The EBU standard assigns bits 27 and 43 to indicate what the User Bits convey. Typically, they can carry a cue name or date, although the information itself doesn’t usually change within a run of timecode.

The full 80 frames are detailed below:-

BitsData
79-0Start clock on edge between bits 79 and 0
0-3Frames Units: Bits 0 - 3
4-7User Bits Group 1
8-9Frames Tens: Bits 0 - 1
10Drop Frame Flag (’on’ for drop frame)
11Colour Frame Flag (’on’ for colour info locked to TC)
12-15User Bits Group 2
16-19Seconds Units: Bits 0 - 3
20-23User Bits Group 3
24-26Seconds Tens: Bits 0 - 3
27Bi-phase Mark: Phase Correction Bit
28-31User Bits Group 4
32-35Minutes Units: Bits 0 - 3
36-39User Bits Group 5
40-42Minutes Tens: Bits 0 - 2
43Binary Group Flag Bit
44-47User Bits Group 6
48-51Hours Units: Bits 0 - 3
52-55User Bits Group 7
56-57Hours Tens: Bits 0 -1
58Unassigned (’off’)
59Binary Group (User Bits) Flag
60-63User Bits Group 8
64-79Sync Word: bits sent in following order:-
 0011111111111101 

The number of bits assigned to each function are as follows:-

FunctionBits
Timecode and Flags31
Unassigned
User Bits31
Word Sync16

©Ray White 2004.