文件第7信号分析在本章中我们将重点.pdf

Chapter 7 Signal ysis In this chapter we will focus on: • The composite signal ludes picture information, sync pulses and blanking pulses. • The front and back porches refer to the blanking time before and after the H sync pulse. • That before and after the V sync pulses is a group of equalizing pulses. • Why the g is a number indication how contrast is expanded or compressed. • How advances in digital-signal processing made possible line double circuits. The three parts of the composite signal are: • The camera signal corresponding to the variations of light in the scene. • The sync pulses to synchronize the scanning • The blanking pulses to make the retraces invisible. For color TV, the 3.58MHz chrominance signal and color sync burst are needed. 7.1 Construction of the composite signal As time reases at the right direction, the amplitudes vary for shades of white, gray, or black in the picture. Starting at the left, the signal is at a whi evel and the scanning beam is at the left side of the image. After H trace produces the desired camera signal for one line, the scanning beam is at the right side of the image. Then the blanking pulse is inserted to bring the signal amplitude up to the black level so that the retrace can be blanked out. Af retrace, the blanking voltage is removed. Then the scanning beam is at the left side, ready to scan the next line. SYNC POLARITY IN THE COMPOSITE SIGNAL The two possibilities are • Apositive sync polarity , with the sync pulses in the “up” position. • A negative sync polarity , with the sync pulses in the “down” position. The two kinds of signals contain the same picture information, but 0 have the difference that they are 180 out-of-phase.