PAL Encoder/Coder:

Figure given below is the functional diagram of a PAL coder. The gamma corrected R, G and B signals are matrixed to form the Y and the weighted colour difference signals. The bandwidthsof both (B – Y) and (R – Y) video signals are restricted to about 1.3 MHz by appropriate low pass filters. In this process these signals suffer a small delay relative to the Y signal. In order to compensate for this delay, a delay line is inserted in the path of Y signal.

The double sideband suppressed carrier signals from the modulators are added to yield the quadrature amplitude modulated (Q.A.M.) chrominance (C) signal. This passes through a filter which removes harmonics of the sub-carrier frequency and restricts the upper and lower sidebands to appropriate values. The output of the filter feeds into an adder circuit where it is combined with the luminance and sync signals to form a composite colour video signal. The bandwidth and location of the composite colour signals (U and V) is shown along with the Y signal in the figure.

Notice that the colour burst signal is also fed to the modulators along with the U and V signals through the adders. The burst signals are obtained from the circuits that feed the colour subcarrier signal to the two modulators. However, before feeding the burst signals to the U and V adders these are passed through separate burst gates. Each burst gate is controlled by delayed pulses at fH rate obtained from the frequency dividing circuit. The gating pulses appear during the back porch period. Thus, during these intervals the (B – Y) i.e., U modulator yields a subcarrier burst along –U while the (R – Y) i.e., V modulator gives a burst of the same amplitude but having a phase of $\pm 90^°$ on alternate lines relative to the – U phasor. At the outputs of the two modulators, the two burst components combine in the adder to yield an output which is the vector sum of the two burst inputs. This is a subcarrier sinewave ( 10cycles) at $45^°$ on one line and $- 45^°$ on the next line with reference to U phasor.

PAL Decoder:

Various designs of PAL decoder have been developed. The one shown in the colour receiver block diagram of Fig is a commonly used arrangement. It will be noticed that the general pattern of signal flow is very close to that of the NTSC receiver.

Tuner

It is necessary to maintain local oscillator frequency at the correct value to obtain exact colour burst frequency for proper reproduction of different colours in the picture. Therefore, colour receiver tuners employ an additional circuit known as automatic frequency tuning (AFT). This circuit actually controls the local oscillator frequency to obtain a picture IF of exactly 38.9MHz at the converter output. The discriminator in the AFT circuit measure the intermediate frequency and develops a dc control voltage proportional to the frequency deviations if any.

Sound Strip

The frequency modulated sound IF signal is processed in the usual way to obtain audio output.The volume and tone controls are associated with the audio amplifier, the output of which feeds into the loudspeaker. Thus the sound strip of a colour receiver is exactly the same as ina black and white receiver.

AGC, Sync-separator and Deflection Circuits

The AGC and sync-separator circuits function in the same way as in a monochrome receiver.However, the deflection circuits, besides developing normal horizontal and field scanning currents also provide necessary wave-forms for dynamic convergence and pincushion correction.In addition, pulses from the horizontal output transformer are fed to several circuits in the colour section of the receiver.

Merits and Demerits of the PAL System

The problem of differential phase errors has been successfully overcome in the PAL system.This is its main merit. In addition the use of PAL-D technique in receivers for electrically accumulating adjacent line colour signals considerably circumvents hue errors. Thus a manual hue control becomes unnecessary. However, the delay line technique of reception also involves a reduction in the vertical resolution of the chrominance signal but the effect is less pronounced because the two chrominance signals are radiated continuously and the receiver interpolates between the signals of two consecutive lines.

The use of phase alternation by line technique and associated control circuitry together with the need of a delay line in the receiver makes the PAL system more complicated and expensive. The receiver cost is higher for the PAL colour system. In addition, the PAL system presents problems in magnetic recording since a complete colour coding sequence requires eight fields instead of four necessary in the NTSC system.