ANALOG BASICS

COMPONENT COLOUR

The colour picture can be distributed in two forms, whether in 625 or 525 line standards:

This is the basic signal produced by a camera etc and fed to a colour c.r.t. It consists of three primary signals, Red, Green and Blue. By convention, black level is at 0mV and peak brightness is at + 700mV.

As the human eye can see less resolution with colours, the video can be modified to take advantage of this to reduce the amount of information needed. The picture is separated into monochrome and colour components. The monochrome Y signal is formed from:

Y = (0.3 x Red) + (0.59 x Green) + (0.11 x Blue) approximately.

This signal has black level at 0mV and maximum white level at + 700mV.

The colour components are two colour difference signals:

Cr = (R-Y) and Cb = (B-Y)

These are weighted to give maximum values of +/- 350mV and are bandwidth restricted to half that of the Y component.

shows an encoded 100% colour bar signal. The two colour components of Cr and Cb are used to amplitude modulate a 4.43361875Mhz carrier signal. The two carriers are arranged to be 90 degrees apart before they are combined with the Y luminance signal, so that they can be decoded separately. The PAL system is designed to minimise hue errors by phase reversing the Cr axis on alternate lines (Phase Alternate Line). This reversal is copied by the decoder, so that the hue error will now alternate in phase. By combining the chrominance from two adjacent lines, the error is thus cancelled out.

NTSC