VIT signals
In television broadcasting, VIT signals are a group of test signals inserted in the composite video signal. These signals are used to weight the transmission characteristics of the system between the test generator and the output of the demodulator, where the system includes the microwave links, or TVROs as well as the TV transmitters and the transposers. There are both ATSC and EBU standards for VIT.
Blanking in CVS
In a composite video signal there are two types of blanking: horizontal and vertical. Horizontal blanking is between lines and vertical blanking is between fields. In a poorly tuned TV receiver the horizontal blanking can be seen at the right or left of the image and the vertical blanking can be seen at the top or bottom of the image. VIT signals are inserted in the vertical blanking.Vertical blanking
In each field vertical blanking is about 1612 μs in System B and 1333 μs in System M. This duration is equal to 25 lines in system B and 21 lines in system M. Although 7.5 lines are used for synchronization of the image, the remaining lines can be used for other purposes. Two of these lines in each field are reserved for test signals. Since there are two fields in each frame, the number of lines reserved for test signals is four per frame.Test signals
In both systems, line numbers 17 and 18 are assigned for VIT signals in each field.Usually the following test signals are used:
- Luminance bar
- 2T signal
- 20T signal
- Staircase
- Group of sine waves with different frequencies
- Color carrier superimposed on staircase
- Group of color carriers with different amplitudes