Service manual

STP 11-25R13-SM-TG
S - 43
b. In the deletion mode, the 147A and 1430 delete the entire active portion of the line and insert a
noise-free pedestal. This pedestal establishes a zero noise reference at that point in the video chain.
Another 147A or 1430 inserted in the video chain at a later point, used in the insertion mode, can
determine the amount of noise generated between the two points.
S-33. Vertical Interval Reference Signal (VIRS).
VIRS consists of a 7.5-IRE pedestal, a 50-IRE pedestal, and a 70-IRE pedestal with 40 IRE of
chrominance modulation in phase with the burst. The 70-IRE luminance level with modulation
approximates the average program level of typical skin tones. The 50 IRE luminance pedestal represents
average scene brightness and corresponds to the bottom of the chrominance reference to allow easy
comparison to the two levels. The 7.5-IRE luminance pedestal provides a setup reference. See Figures
S-43, S-44, and S-45.
Figure S-43. Vertical Interval Reference Signal (VIRS)