for world-wide coverage. However, care must be taken with the satellite delay problem
if programming is interactive when two satellites in tandem relay a TV signal.
Table 16.3 summarizes TV performance through satellite relay.
Up to this point in the chapter we have covered the baseband TV signal and radio
relaying of that signal via LOS microwave and satellite, where both are analog systems
using frequency modulation. A "raw" TV video signal is naturally analog.
Now convert a TV signal to a digital format. Let's assume it is PCM (Chapter 6) with
those three steps required to develop a digital PCM signal from an analog counterpart.
We will remember that those steps are sampling, quantization, and coding. The bit rate of
such a digital signal will be on the order of 80160 Mbps. If we were to assume one bit
per hertz of bandwidth, the signal would require in the range of 80 to 160 MHz for just
one TV channel! This is a viable alternative on extremely wideband fiber-optic systems.
However, for narrower bandwidth systems, such as radio and coaxial cable, compression
techniques are necessary.
Television, by its very nature, is highly redundant. What we mean here is that we
are repeating much of the same information over and over again. Thus a TV signal is a
natural candidate for digital signal processing to remove much of the redundancy, with
the potential to greatly reduce the required bit rate after compression.
In this section we will discuss several digitizing techniques for TV video and pro-
vide an overview of some bit rate reduction methods including a brief introduction to a
derivative of MPEG-2.
Section 16.10 reviews several approaches to the development of
a conference television signal.
Basic Digital Television
Two Coding Schemes
. There are two distinct digital coding methods for
color television: component and composite coding. For our discussion here, there are four
components that make up a color video signal. These are R for red, G for green, B for
blue, and Y for luminance. The output signals of a TV camera are converted by a linear
matrix into luminance (Y) and two color difference signals R-Y and B-Y.
With the component method of transmission, these signals are individually digitized by
an analog-to-digital (A/D) converter. The resulting digital bit streams are then combined
with overhead and timing by means of a multiplexer for transmission over a single medium
such as specially conditioned wire-pair or coaxial cable.
Composite coding, as the term implies, directly codes the entire video baseband. The
derived bit stream has a notably lower bit rate than that for component coding.
CCIR Rep. 646-4 (Ref. 14) compares the two coding techniques. The advantages of
separate-component coding are the following:
The input to the circuit is provided in separate component form by the signal sources
(in the studio).
The component coding is adopted generally for studios, and the inherent advantages
of component signals for studios must be preserved over the transmission link in
order to allow downstream processing at a receiving studio.
MPEG stands for Motion Picture Experts Group.