Promax PROMAX-4, Руководство пользователя

Страница: 69 / 108

USER’S MANUAL. PROMAX-4
Novembre 1999 Page 27
E
n
g
lis
h
APPENDIXES
APPENDIX A.- MEASUREMENT OF THE VIDEO CARRIER LEVEL (C
L
)
A) Negative Video Modulation (PAL/NTSC)
The measurement of the video carrier level is carried out taking the modulation
peak as the measurement value, this being the maximum value of the signal during the
line synchronism. The system requires a minimum length of time in order to make this
measurement, since it has to detect the peak of the modulated signal.
Figure 10.- Measurement of the video carrier level.
The typical values between which the video carrier level fluctuates are:
- In the transmission line: From 70 to 100 dBµV (From 10 to 40 dBmV)
- In the user’s terminal: From 60 to 80 dBµV (From 0 to 20 dBmV)
B) Positive Video Modulation (SECAM)
On this type of modulation, the line synchronism is determinated by a minimum
carrier level. The maximum signal level (measurement point) is variable in time, and it is
a function of the picture that is being transmitted. It could vary from 10 dB among white
and black image; nevertheless white signals, Video Insertion Test (VIT), are transmitted
in the sweep pulses, which reduce this margin to 4 dB approximately.
Due to this fact, and the small duration of the VIT, when we measure levels of
SECAM signals, it is advisable to add 2 dB to the quantity showed on the display, in
order to obtain a measure, its average value, more precise.