Generation of Colour Rasters-Delta-gun colour picture tube
The following section explains the Generation of Colour Raster for Delta gun Colour picture tube.
Generation of Colour Rasters:
- The overall colour seen is determined both by the intensity of each beam and the phosphors which are being bombarded.
- If only one beam is ‘on’ and the remaining two are cut-off, dots of only one colour phosphor get excited. Thus the raster will be seen to have only one of the primary colours.
- If one beam is cut-off and the remaining two are kept on, the rasters produced by excitation of the phosphors of two colours will combine to create the impression of a complementary colour.
- The exact hue will be determined by the relative strengths of the two beams.
- When all the three guns are active simultaneously, lighter shades are produced on the screen.
- The is so because red, green and blue combine in some measure to form white, and this combines with whatever colours are present to desaturate them.
- Black in a picture is just the absence of excitation when all the three beams are cut-off.
Primary Colour Signals:
- The demodulators in the receiver recover (B – Y) and (R – Y) video signals.
- The (G – Y) colour video signal is obtained from these two through a suitable matrix.
Figure 1.1 Delta-gun colour picture tube (c) schematic diagram showing application of ‘Y’ and colour difference signals between the cathodes and control grids
All the three colour difference signals are then fed to the three grids of colour picture tube (see Fig. 1.11).
- The inverted luminance signal (– Y) is applied at the junction of the three cathodes.
- The signal voltages subtract from each other to develop control voltages for the three guns, i.e.,
V′G1 – Vk = (VR – VY) – (– VY) = VR
V″G1 – Vk = (VG – VY) – (– VY) = VG
and V′″G1 – Vk = (VB – VY) – (– VY) = VB
- In some receiver designs the Y signal is subtracted in the matrix and resulting colour voltages are directly applied to the corresponding control grids. The cathode is then returned to a fixed negative voltage.