812 Chapter 28
the brightness changes that constitute the information of the picture may be conveyed.
Second, it must control the raster scanning so that the beam travels across the tube face
in synchronism with the tube within the transmitting camera. Otherwise information
from the top left-hand side of the televised scene will not appear in the top left-hand
side of the screen and so on! In the analogue television signal this distinction between
picture information and scan synchronizing information (known in the trade as sync-pulse
information) is divided by a voltage level known as black level. All information above
black level relates to picture information, whereas all information below relates to sync
information. By this clever means, all synchronizing information is “ below ” black level.
The electron beam therefore remains cut off—and the screen remains dark—during the
sync information. In digital television the distinction between data relating to picture
modulation and sync is established by a unique codeword preamble, which identifi es the
following byte as a sync byte.
28.4.1 Horizontal and Vertical Sync
The analogy between the eye’s movement across the page during reading and the
movement of the scan spot in scanning a tube face has already been made. Of course
the scan spot doesn’t move onto another page like the eyes do once they have reached
the bottom of the page, but it does have to fl y back to start all over again once it has
completed one whole set of lines from the top to the bottom of the raster. The spot thus
fl ies back in two possible ways: a horizontal retrace, between lines, and a vertical retrace,
once it has completed one whole set of lines and is required to start all over again on
another set. Obviously to stay in synchronism with the transmitting camera the television
receiver must be instructed to perform both horizontal retrace and vertical retrace at the
appropriate times—and furthermore not to confuse one instruction for the other!
It is for this reason that there exist two types of sync information known reasonably
enough as horizontal and vertical. Inside the television monitor these are treated
separately and respectively initiate and terminate the horizontal and vertical scan
generator circuits. These circuits are similar—at least in principle—to the ramp or
sawtooth generator circuits. As the current gradually increases in both horizontal
and vertical scan coils, the spot is made to move from left to right and top to bottom,
the current in the top to bottom circuit growing 312.5 times more slowly than in the
horizontal defl ection coils so that 312.5 lines are drawn in the time it takes the vertical
defl ection circuit to draw the beam across the vertical extent of the tube face.