Mumbai university > Electronics and telecommunication Engineering > Sem 6 > Television Engineering

Marks: 10

Years: May 2016

The main features of the PAL system are:

1. The weighted (B-Y) and (R-Y) signals are modulated without being given a phase shift of 33o as is done in the NTSC system.

2. On modulation, both the colour difference quadrature signals are allowed the same bandwidth of about 1.3MHz. This results in better colour reproduction. However, the chroma signal is of vestigial sideband type. The upper sideband attenuation slope starts at 0.5MHz i.e.(5 – 4.43 = 0.57MHz) but the lower sideband extends to 1.3MHz before attenuation begins.

3. The colour subcarrier frequency is chosen to be 4.43361875MHz. It is an odd multiple of one-quarter of the line frequency instead of the half-line offset as used in the NTSC system. This results in somewhat better cancellation of the dot pattern interference.

4. The weighted (B-Y) and (R-Y) signals are modulated with the subcarrier in the same way as in the NTSC system (QAM) but with the difference that phase of the subcarrier to one of the modulators (V modulator) is reversed from +90o to -90o at the line frequency. In fact the system derives its name, phase alteration by line (i.e. PAL) from this mode of modulation. This technique of modulation cancels hue errors which result from unusual phase shifts in the transmitted signal.

• The problem of cross interference between U and V signals in the coherent detector’s output is solved by using a delay circuit (causing delay of 64µs) before the BM detectors as shown in the circuit shown in the below figure.

• This circuit is called D-PAL decoder circuit and is used in all modern PAL receivers. D stands for delay line, used to delay the signal by 64 µs.

• U and V signals go to an adder stage and a subtractor stage direct as well as through delay line.

• The j operator with V signal indicates that V is at 90o phase difference with U. At adder and subtractor, we get

U + jV (direct signal)

U – jV (through delay line)

(minus sign appears because the delay circuit sends V signal of the previous line)

• Adding the two expressions, we get 2 U signal at the adder’s output. Subtracting the two expressions, we get 2jV at the subtractor’s output.

• If direct signal is U – jV and delayed signal is U + jV, then also we shall get 2U signal at the adder and –jV at the subtractor.

• Thus the first balanced demodulator gets only U signal. V has been automatically eliminated.

• Similarly the second balanced demodulator gets only the V signal as U has been automatically eliminated.

• Although coherent detection is not required, but still it is preferred to allow discount for any fault in the delay line circuit.

• Once the U and V signals have been detected, the rest of the circuit to get R, G and B colours is the same as in simple PAL system.