Horizontal Sync Details

HORIZONTAL SYNC DETAILS

The horizontal blanking period and sync pulse details are illustrated in Fig. The interval between horizontal scanning lines is indicated by H. As explained earlier, out of a total line period of 64 s, the line blanking period is 12 s. During this interval a line synchronizing pulse is inserted. The pulses corresponding to the differentiated leading edges of the sync pulses are actually used to synchronize the horizontal scanning oscillator. This is the reason why in Figand

other figures to follow, all time intervals are shown between sync pulse leading edges.

The line blanking period is divided into three sections. These are the ‘front porch’, t he ‘line sync’ pulse and the ‘back porch’. The time intervals allowed to each part are summarized below and their location and effect on the raster is illustrated in Fig.

Details of Horizontal Scanning

Period Time         (µ s)

Total line (H)        64

Horz blanking      12 ± .3

Horz sync pulse   4.7 ± 0.2

Front porch          1.5 ± .3

Back porch 5.8 ± .3

Visible line time   52

Front porch.

This is a brief cushioning period of 1.5 µ s inserted between the end of the picture detail for that line and the leading edge of the line sync pulse. This interval allows the receiver video circuit to settle down from whatever picture voltage level exists at the end of the picture line to the blanking level before the sync pulse occurs.

Thus sync circuits at the receiver are isolated from the influence of end of the line picture details. The most stringent demand is made on the video circuits when peak white detail occurs at the end of a line.

Despite the existence of the front porch when the line ends in an extreme white detail, and the signal amplitude touches almost zero level, the video voltage level fails to decay to the blanking level before the leading-edge of the line sync pulse occurs.

This results in late triggering of the time base circuit thus upsetting the ‘horz’ line sync circuit. As a result the spot (beam) is late in arriving at the left of the screen and picture information on the next line is displaced to the left.

This effect is known as ‘pulling-on-whites’.

Line sync pulse.

After the front proch of blanking, horizontal retrace is produced when the sync pulse starts. The flyback is definitely blanked out because the sync level is blacker than black.

Line sync pulses are separated at the receiver and utilized to keep the receiver line time base in precise synchronism with the distant transmitter.

The nominal time duration for the line sync pulses is 4.7 s. During this period the beam on the raster almost completes its back stroke (retrace) and arrives at the extreme left end of the raster.

Back porch.

This period of 5.8 µ s at the blanking level allows plenty of time for line flyback to be completed. It also permits time for the horizontal time-base circuit to reverse direction of current for the initiation of the scanning of next line.

In fact, the relative timings are so set that small black bars (see Fig) are formed at both the ends of the raster in the horizontal plane.

These blanked bars at the sides have no effect on the picture details reproduced during the active line period.

The back porch also provides the necessary amplitude equal to the blanking level (reference level) and enables to preserve the dc content of the picture information at the transmitter.

At the receiver this level which is independent of the picture details is utilized in the AGC (automatic gain control) circuits to develop true AGC voltage proportional to the signal strength picked up at the antenna