Appendix 3

Time code

All modern audio systems have a time code option. With digital systems, it is effectively built in. At its simplest level, it is easy to understand; it stores time in hours, minutes and seconds. As so often, there are several standards.

The most common audio time display that people meet is on the CD; this gives minutes and seconds. For professional players, this can be resolved down to fraction of second by counting the data blocks. These are conventionally called frames and there are 75 every second. This is potentially confusing as CDs were originally mastered from 3/4 inch U-Matic videotapes (VT) where the data were configured to look like an American television picture running at 30 video frames per second (fps).

The need accurately to edit VT drove the requirement for a standard. VT editing is done by copying from source tapes to the final edited version. With modern microprocessors, tape synchronizers and control gear, this can now be done to frame accuracy (subject to some technical constraints outside the scope of this book).

In 1967, the Society of Motion Picture and Television Engineers (SMPTE) created a standard defining the nature of the recorded signal and the format of the data recorded. Data are separated into 80-bit blocks, each corresponding to a single video frame. The way that the data are recorded (Biphase modulation) allow the data also to be read from analog machines when the machines are spooling, at medium speed, with the tape against the head, in either direction. With digital systems, the recording method is different but the code produced stays at the original standards.

The European Broadcasting Union (EBU) adopted the same standard using the European TV 25 fps frame rate. The core of the format is the actual time code expressed in 24-hour clock mode of HH:MM:SS:FF. It also has eight groups of four user bits that are up to the user to decide how to use. The two have been combined in BS 6865:1987/93, IEC 421:1986/90.

SMPTE/EBU time code can be recorded as audio on a track of a multitrack tape machine. By convention the highest numbered track is used; track 4 on a 4-track; track 16 on a 16- track, etc. It is a nasty screeching noise best kept as far away from other audio as possible.

A3.1 MIDI

It can also be sent to a sequencer, via a converter, as MIDI data allowing the sequencer to track the audio tape. To save too much data overhead MIDI time code (MTC) is sent only every two frames and even then uses up just over 7% of the available MIDI data capacity. The basic time code is sent as eight separate 2-byte MIDI messages. This also includes a code indicating what frame rate is being used. There are four options: 24fps (Cinema film), 25fps (video and film for European TV), 30fps drop-frame (USA/Japan video), 30fps non-drop-frame (used rarely for non-video applications). Sending other information, such as user bits, is optional. It is important to choose the format that is appropriate for the medium for which you are creating the audio. In the UK, the most likely standard is 25fps.

Film has 24 separate pictures per second and these individual pictures are known as frames. Television has adopted a different practice. A European 625 line picture has 25 frames per second but these frames are divided into two 312/2 line fields, which are interlaced together to make the final picture. This doubles the frequency of flicker from 25 Hz to 50 Hz and makes it less visible. Originally, with early technology, TV pictures had to be the same frequency as the local mains to avoid moving ‘hum bars’. This was why 25fps rather than film's 24fps was chosen. Because USA mains is 60 Hz, their picture standard is 30 frames per second (60 fields per second).

The simple relationship between bars, tempo and SMPTE time as shown by sequencers like Cubase is only valid for 120 beats per minute 4/4 time. MIDI time code generators need to be programmed with the music tempo and time signature used by the sequencer, so that they can operate, in a gearbox fashion, so that the sequencer runs at the proper tempo.

A3.2 Drop-frame time code

This used in the USA to deal with a fudge that was made when they introduced color TV. This involves dropping frames so that time code agrees with real time. While American B&W TV ran at 30fps, the color frame rate is nearer 29.97 fps. This means that a programme timed at an hour, using 30 fps time code, will actually run 3.6 seconds longer. This may not seem much, but its worries broadcasters. The SMPTE decided to standardize a way that the 108 ‘extra’ frames could be 'dropped’ every hour. It's similar to the way that the calendar has leap years to keep it in synch with the orbit of the Earth.

The basic rule is that whenever the time code ends a minute, it drops the first two frames on the next minute, for example. The 12:25:59:29 is followed by 12:26:00:02; frames 00 and 01 are dropped. Because this would drop 120 frames rather than 108 frames per hour, there is a further tweak to the rule, so that every 10th minute does not drop any frames, thus 12:29:59:29 is followed by 12:30:00:00. This makes drop-frame time code accurate to 75/1000th of a second in 24 hours, which is good enough for most people.

A3.3 Other systems

Modern digital systems like CD, Digital Audio Tape (DAT) and Minidisc all come with their own internal time codes. Their internal digital audio data is divided into blocks also called frames. Their connection to ‘outside world’ time code systems will translate to the SMPTE standard. DAT, for example store any incoming. SMPTE time code in an internal format that it can then output in any frame rate regardless of the original.

Film has traditionally used a totally different system for dubbing known as 35 mm feet. This increments faster than every second. As you would expect this corresponds the number of feet of 35 mm film but the measure is also used when the film is actually 16 mm!

A3.4 What time?

Time of day recording can be useful when recording of an event. Multiple cameras can stop and start and everything pieced together later. Production assistants can log events using an ordinary accurate clock or watch.

In practice a variation on duration coding is used but it is always unwise to start your recording at zero. There is no negative time so this would give no space for a preroll to allow times for separate equipment to synchronize. Very often the ‘hours' are set to indicate which film roll or VT is being used. Roll 1 will be given a 01:00:00:00 start time, roll 2 gets 02:00:00:00, etc.

The 80 bits of each SMPTE/EBU frame block are allocated as follows: