Matrix network
The purpose of the matrix is to reproduce the original RGB signals to be fed to the tube electrodes. The first stage is to obtain the green colour difference (Y’- G’) which will be used to produce the RGB signals.
The proportions of R’- Y’ and B’- Y’ components which are necessary to obtain the third colour difference component G’- Y’ may be mathe- matically determined as G’- Y’= 0.51(R’- Y’) – 0.186(B’- Y’) However, if G’ – Y’ matrixing takes place before deweighting of the U and V components is carried out, different proportions are necessary G’- Y’= -0.29(R’- Y’) – 0.186(B’- Y’) Since the required R’- Y’ and B’- Y’ levels are both less than unity, they may be derived by using a simple resistor network.
Now to the RGB network itself at the c.r.t., a picture is produced by modulating the three c.r.t. electron beams using the gamma-corrected R’, G’ and B’ signals. These are derived by the addition of the colour dif- ference signals to the gamma-corrected luminance signals Y’
During addition, the difference in the bandwidth of the luminance sig- nal and the colour difference signals (5.5 MHz for the luminance and 1 MHz for the colour difference) introduces glitches or notches at points of fast colour transition. The narrow bandwidth of the colour difference sig- nals restricts the rise time of rapidly changing signals. When they are added to the rapidly changing luminance signal, a ‘glitch’ is introduced. On a colour-bar display, the effect of the glitch appears as a narrow dark band between the colour bars.
The addition may be carried out by a dedicated RGB matrix before the colour signals are applied to the cathode of the c.r.t., a technique known as direct drive. Alternatively, matrixing may be carried out by the tube itself, a technique known a colour difference drive. Colour difference drive invol- ves feeding the colour difference signals to the control grids of the c.r.t. while applying a negative luminance of -Y’ to the cathodes. Applying a negative luminance to the cathode is equivalent to applying a positive luminance to the grid; it results in the mathematical addition of the two signals.
In the colour difference drive, separate colour difference and luminance signals are fed into different electrodes of the c.r.t. This creates problems with timing, accentuated because the two signals have different band- widths. Further problems arise from the relative sensitivities of the elec- trodes, which have to be the same for all levels of the input signals. Direct RGB drive overcomes these problems by performing the necessary matrix- ing close to the point of demodulation. For these reasons, direct RGB drive is used in modern TV receivers.