EQUALIZING PULSES

• To take care of the drawback which occurs on account of the half line discrepancy five narrow pulses are added on either side of the vertical sync pulses.
•  These are known as pre-equalizing and post-equalizing pulses. 
• Each set consists of five narrow pulses occupying 2.5 lines period on either side of the vertical sync pulses.
•  Pre-equalizing and postequalizing pulse details with line numbers occupied by them in each field are given in Fig. 3.8.
•  The effect of these pulses is to shift the half-line discrepancy away both from the beginning and end of vertical sync pulses.
•  Pre-equalizing pulses being of 2.3 µs duration result in the discharge of the capacitor to essentially zero voltage in both the fields, despite the half-line discrepancy before the voltage build-up starts with the arrival of vertical sync pulses.
•  This is illustrated in Fig. 3.9. Post-equalizing pulses are necessary for a fast discharge of the capacitor to ensure triggering of the vertical oscillator at proper time.
•  If the decay of voltage across the capacitor is slow as would happen in the absence of post-equalizing pulses, the oscillator may trigger at the trailing edge which may be far-away from the leading edge and this could lead to an error in triggering.
•  Thus with the insertion of narrow pre and post equalizing pulses, the voltage rise and fall profile is essentially the same for both the field sequences (see Fig. 3.9) and the vertical oscillator is triggered at the proper instants, i.e., exactly at an interval of 1/50th of a second.
•  This problem could possibly also be solved by using an integrating circuit with a much larger time constant, to ensure that the capacitor remains virtually uncharged by the horizontal pulses. 
• However, this would have the effect of significantly reducing the integrator output for vertical pulses so that a vertical sync amplifier would have to be used. 
• In a broadcasting situation, there are thousands of receivers for every transmitter.
• Consequently it is much more efficient and economical to cure this problem in one transmitter than in thousands of receivers. 
• This, as explained above, is achieved by the use of pre and post equalizing pulses. The complete pulse trains for both the fields incorporating equalizing pulses are shown in Fig. 3.10.
•  From the comparison of the horizontal and vertical output pulse forms shown in Figs. 3.7 and 3.9 it appears that the vertical trigger pulse (output of the low-pass filter) is not very sharp but actually it is not so.
• The scale chosen exaggerates the extent of the vertical pulses.
• The voltage build-up period is only 160 µs and so far as the vertical synchronizing oscillator is concerned this pulse occurs rapidly and represents a sudden change in voltage which decays very fast. 
• The polarity of the pulses as obtained at the outputs of their respective fields may not be suitable for direct application in the controlled synchronizing oscillator and might need inversion depending on the type of oscillator used. 
• This aspect will be fully developed in the chapter devoted to vertical and horizontal oscillators.