Basic Digital Television

16.9.2.1 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.

7MPEG stands for Motion Picture Experts Group.

• The country receiving the signals via an international circuit uses a color system different from that used in the source country.

• The transmission path is entirely digital, which fits in with the trend toward all-digital systems that is expected to continue.

The advantages of transmitting in the composite form are the following:

• The input to the circuit is provided in the composite form by the signal sources (at the studio).

• The color system used by the receiving country, in the case of an international circuit, is the same as that used by the source country.

• The transmission path consists of mixed analog and digital sections.

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