HORIZONTAL SYNC DETAILS
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
figures to follow, all time intervals are shown between sync pulse leading
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.
of Horizontal Scanning
Time (µ s)
Total line (H) 64
Horz blanking 12
Horz sync pulse 4.7
Front porch 1.5
Back porch 5.8
Visible line time 52
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.
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.
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
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
effect is known as ‘pulling-on-whites’.
Line sync pulse.
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
pulses are separated at the receiver and utilized to keep the receiver line
time base in precise synchronism with the distant transmitter.
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.
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.
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.
blanked bars at the sides have no effect on the picture details reproduced
during the active line period.
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.
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