Video Synchronization 837The standard also specifi es an optional audio control packet. If the control packet is not
transmitted, a receiver defaults to 48 kHz, synchronous operation. For other levels, the
control byte must be transmitted in fi eld interval.
28.10 Time Code
28.10.1 Longitudinal Time Code (LTC)
As we have seen, television (like movie fi lm) gives the impression of continuous
motion pictures by the successive, swift presentation of still images, thereby fooling the
eye into believing it is perceiving motion. It is probably therefore no surprise that time
code (deriving as it does from television technology) operates by “ tagging ” each video
frame with a unique identifying number called a time code address. The address
contains information concerning hours, minutes, seconds, and frames. This information is
formed into a serial digital code, which is recorded as a data signal onto one of the audio
tracks of a videotape recorder. (Some videotape recorders have a dedicated track for this
purpose.)
Each frame’s worth of data is known as a word of time code and this digital word
is formed of 80 bits spaced evenly throughout the frame. Taking the European
Broadcasting Union (EBU) time code^3 as an example, the fi nal data rate therefore turns
out to be 80 bits 25 frames per second 2000 bits per second, which is equivalent
to a fundamental frequency of 1 kHz; easily low enough, therefore, to be treated as a
straightforward audio signal. The time code word data format is illustrated (along with its
temporal relationship to a video fi eld) in Figure 28.16. The precise form of the electrical
code for time code is known as Manchester biphase modulation. When used in a video
environment, time code must be accurately phased to the video signal. As defi ned in the
specifi cation, the leading edge of bit ‘ 0 ’ must begin at the start of line 5 of fi eld 1 ( 1
line). Time address data are encoded within the 80 bits as 8, 4-bit BCD (binary coded
decimal) words (i.e., 1, 4-bit number for tens and 1 for units). Like the clock itself, time
address data are only permitted to go from 00 hours, 00 minutes, 00 seconds, 00 frames
to 23 hours, 59 minutes, 59 seconds, 24 frames.
However, a 4-bit BCD number can represent any number from 0 to 9, so in principle,
time code could be used to represent 99 hours, 99 minutes, and so on. But, as there are no
(^3) The EBU time code is based on a fi eld frequency of 25 frames per second.