NTSC –Phasor Diagram
The NTSC colour system is compatible with the American 525 line monochrome system. In order to save bandwidth, advantage is taken of the fact that eye’s resolution of colours along the reddish blue-yellowish green axis on the colour circle is much less than those colours which lie around the yellowish red-greenish blue axis.
Overview of NTSC Phasor Diagram:
- The two new colour video signals, which correspond to these colour regions, are generated.
- These are designated as I and Q signals.
- The I signal lies in a region 33° counter clockwise to (R – Y) where the eye has maximum colour resolution.
- It is derived from the (R – Y) and (B – Y) signals and is equal to 0.60R – 0.28G – 0.32B.
- As shown in Fig. 1.1 it is located at an angle of 57° with respect to the colour burst in the balanced modulator circuits.
- Similarly the Q signal is derived from colour difference signals by suitable matrix and equals 0.21R –0.52G 0.31B.
- It is located 33° counter
Figure 1.1 Phasor diagrams of the I and Q signals in the NTSC system.
Clockwise to the (B – Y) signal and is thus in quadrature with the I signal.
- In Fig. 1.1 the Q signal covers the regions around magenta (reddish-blue) and yellow-green shades.
- Similarly orange hues correspond to phase angles centred around I and the complementary blue-green (cyan) hues are located around the diametrically opposite – I signal.
- Since the eye is capable of resolving fine details in these regions, I signal is allowed to possess frequencies up to 1.5 MHz.
- However, the eye is least sensitive to colours that lie around the ±Q signals, and therefore it is allowed a bandwidth of only ±0.5 MHz with respect to the colour subcarrier.
- It may be noted that both I and Q signals are active up to 0.5 MHz and being at right angles to each other, combine to produce all the colours contained in the chrominance signal.
- However, the Q signal drops out after 0.5 MHz and only I signal remains between 0.5 and 1.5 MHz to produce colours, the finer details of which the eyes can easily perceive.
- To help understand this fact it may be recalled that only one colour difference signal is needed for producing colours which are a mixture of only two colours.
- Thus the Q signal is not necessary for producing colours lying in the region of orange (red green) and cyan (green blue) hues.
- Hence at any instant when Q = 0 and only I signal is active the colours produced on the screen will run the gamut from reddish orange to bluish green.