Vidicon Tubes

The key component in the vidicon tube is the photo conductive target. The lens focuses the the scene to be viewed on the target, and the target stores the scene in the form of an electrical charge. The electrical conductivity of the target varies with the amount of light absorbed. Where there is minimum light, the material has a high electrical resistance. The points on the target with considerable light have a lower resistance.

The remainder of the tube consists of an electron gun with surrounding magnetic coils. The cathode of the electron gun generates a stream of electron focused into a narrow beam to strike the inside of the target. Other magnetic coils surround the vidicon tube to produce horizontal and vertical deflection of the electron beam. Signals applied to these coils cause the electron beam to scan from left to right and top to bottom across the inside of the target. This causes a current to flow between the target and the transparent signal electrode. This current is proportional to the light amplitude. This is the video output signal.

For scanning the target, many methods are used, but the most widely used method is the standard NTSC pattern of 525 total scanning lines. When the electron beam scans the target, an electrical signal is produced. They varying amplitude in the signal represents brightness variations along one of the scan lines.

As soon as one line is scanned, the vertical deflection signals move the electron beam a small distance farther down, and another horizontal scan line is developed. Special synchronization pulses designate the beginning and end of each horizontally scanned line. The horizontal sync pulse is approximately 5 microseconds wide and occurs at a frequency of 15,750 Hz.

The total number of horizontal scanning lines is 525, but the way in which the target is scanned is unusual. The scanning takes place in two separate fields of 262 1/2 lines, each occurring every 1/60th of a second. The scan lines of the two fields are interlaced to produce a completely scanned image every 1/30th of a second. Of the 525 total lines, only about 480 of them are actually designated to the scene itself. The remaining 22 1/2 lines per field occur but they are not seen. This provides time for the electron beam to re-trace from bottom to top. Special sync pulses included in this interval keep the lines properly synchronized and meshed.

The electrical output signal has an approximate voltage range of .7 volts to 1.5 volts. The analog video signal between the sync pulses, represents the light intensity variations on one scan line. It is this voltage that will be fed to the analog-to-digital converter for translation into binary numbers.