Sunday, 4 May 2014

Blanking, Retrace, Synchronizing and Equalizing Pulses

Blanking, Retrace, Synchronizing and Equalizing Pulses:

In TV, ‘blanking’ means ‘going to black’ as part of the video signal, the blanking voltage is at the black level. Video voltage at the black level cuts off the beam currents in the picture tube to black out the light from screen. The purpose of providing the blanking pulses is to make invisible the retraces of the scanning process. The horizontal blanking pulse at the frequency of 15625 Hz, blanks out the retrace from right to left for each line. The vertical blanking pulses at 50 Hz blank out the retrace from bottom to top for each field.

The time period of blanking pulses is 16% of the each horizontal line, i.e., 16 % of 64 µs = 10.2 µs. In other words, retrace from left to right must be completed in 10.2 µs.

The time period of vertical blank pulses is 8% of each vertical field. It comes equal to 8% of 1/50 s = 0.0016 s. In other words, the vertical retrace must be completed within 1.6 ms.

 A blanking pulse comes first to put the video signal at black level, then a sync pulse comes to start the retrace. This sequence applies to blanking, horizontal and vertical retraces.

To produce a true and undistorted picture, it is necessary that the scanning process at the transmitter camera tube should be quite in step with that at the receiver picture tube. Thus the timing pulses generated by the synchronizing generator to trigger the saw tooth generator for vertical and horizontal plates are not only applied to the transmitter camera tube system but also transmitted to the receiver along with the image signals. At the receiver, these triggering pulses are separated from the signal components, which are then differentiated (horizontal synchronizing pulses) to trigger saw-tooth wave generators for the application of saw-tooth voltage to horizontal and vertical deflection plates of picture tube respectively.

Frequency of scanning, synchronizing and blanking pulses:

The sync pulses and blanking pulses have the same frequency as that of scanning. The values are shown in the table given below.

No:
Particles
Frequency in Hz
1
Horizontal Scanning Pulses
15625
2
Horizontal Sync Pulses
15625
3
Horizontal blanking Pulses
15625
4
Vertical Scanning Pulses
50
5
Vertical Sync Pulses
50
6
Vertical blanking Pulses
50
7
Equalizing Pulses
31250


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