Quadrature amplitude modulation
Now that the subcarrier frequency has been set, we can have a look at how it is modulated by the chrominance components. For this, quadrature amplitude modulation, QAM is used for both PAL and NTSC. In QAM, two carriers at right angles (quadrature) to each other are modulated by two separate signals (Figure 2.8): B – Y modulating the in-phase carrier
OU and R – Y modulating the quadrature carrier OV. As with ordinary amplitude modulation, each modulated carrier produces two bands of side frequencies, one on each side of the carrier represented by the pair of phasors shown. Each pair of phasors produces a resultant colour differ- ence phasor: E(B – Y), known as I for in-phase and E(R – Y), known as Q for quadrature, respectively.
As was mentioned in Chapter 1, the information in a modulated carrier is contained in the side frequencies only. The quadrature carriers may thus be suppressed to obtain the two colour difference signals, I and Q illus- trated in Figure 2.9 and by so doing significantly reduce the severity of the dot pattern introduced at the receiver by the presence of the colour signal. These in turn are added to produce a resultant chrominance phasor. The chrominance phasor which has the same frequency as the suppressed car- riers corresponds to the phasor associated with the colour triangle with its length (or amplitude) representing saturation and its angle (or phase), () representing hue. I and Q may have a positive or a negative value which enables all colours and hues to be represented.
The bandwidth of each chrominance component is limited to approxi- mately 1 MHz on each side of the colour carrier (Figure 2.10). The relatively
narrow bandwidth is quite sufficient for an adequate reproduction of a colour image at the receiving end. This is because as was mentioned earlier in this chapter, the eye is not very sensitive to colour and perceives high video frequencies in black and white only.