Low-light cameras convert spatial two-dimensional weak optical images into suitable video signals. This transformation includes:

1) The micro-photographed scene is focused and imaged on the photocathode surface of the camera tube.

2) The photocathode performs photoelectric conversion to convert the optical image into a two-dimensional charge distribution.

3) The camera tube target plate collects the enhanced charge and accumulates continuously within one frame time.

4) The electrons scanned in the subspace can complete the two-dimensional scanning of the entire target surface point by point within a period of time. Since the target charge of the scanning electron beam depends on the amount of charge accumulated on the target surface, the current formed by the scanning electron beam is modulated by the electrical distribution of the target surface, so the video signal of the scene is obtained from the output end.

 

In the reverse process of line scanning, the camera circuit automatically outputs a "line blanking signal" to interrupt the scanning electron beam. There is also a "field blanking signal" that automatically terminates the scanning electron beam during the retrace period after a field scan is completed. The line and field blanking signals become "composite blanking" pulse signals after compounding, which are added to the modulation board of the camera tube to cut off the scanning electron beam.

 

In order to receive and display the receiver, the camera tube will automatically output a narrow pulse signal at the end of the line scanning positive process, so that the electron beam of the camera tube will retrace the line accordingly. This pulse signal is called the line synchronization signal, which is intended to make the transmission and Receive maintains line synchronization. The camera tube also outputs a narrow pulse signal at the end of each field scan, which makes the picture tube perform field retrace accordingly. This pulse signal is called the field synchronization signal. The horizontal and vertical sync signals are combined to form a "composite sync" signal. The sync signal does not need to be displayed, so it always follows the blanking signal.

 

The aforementioned scene video signal is amplified by the preamplifier and mixed with the composite synchronization signal to form a full TV signal output with a peak-to-peak value of 1V.

 

At present, my country's TV adopts the 625-line format, of which 50 lines are in the reverse process of frame scanning, and the actual effective scanning behavior is 575. The frame rate is the same as the mains frequency, 50 frames per second. If it is counted as interlaced scanning, the total number of lines scanned is still 625, the number of effective lines is 575, the field frequency is 50Hz, and the frame frequency is 25Hz. The vertical blanking signal occupies 25 lines of scanning time; the vertical synchronization signal occupies 3 lines of scanning time, it Occurs after the vertical blanking signal is sent. In terms of signal level, the horizontal and vertical sync signals are 100% and the blanking signal is 75%.