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Signal transmitted for test purposes in TV transmitters
Zero reference pulse
or
Zero pulse
is an artificially produced pulse in a professional television receiver imitating no
radio frequency
case for
modulation
index measurements in analogue
TV transmitters
.
Video signal
[
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The composite
monochrome
video signal (CVS) is composed of a video signal superimposed on an auxiliary signal of 300 mV. The levels between 0 and 300 mV are assigned for the auxiliary signal and the levels between 300 and 1000 mV are assigned to video information.
Modulation
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In analogue broadcasting the composite video signal modulates the carrier by a type of
amplitude modulation
named
VSB
. The polarity of the modulation is negative, i.e., higher the level of the CVS, lower the level of the RF signal. If the level of CVS is 0 volt the level of the RF signal is % 100. The
modulation index
is so arranged that, the maximum level of CVS yields a RF level of % 10 (sometimes % 12.5).
[1]
This value is known as the level of the
residual carrier
. If the modulation index yields more than % 10 for maximum level input (high residual carrier), the efficiency of the transmission drops, i.e., low contrast. On the other hand, if the RF level is below % 10 (low residual carrier), aural and visual signals begin to interfere each other. So it is important to keep %10 for 1000 mV input.
Measurement
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To adjust the modulation index, an input of maximum level CVS (1000 mV) is applied to the modulator. The modulated RF signal is then applied to a professional TV receiver . The receiver has a facility to switch off RF for a short interval in each consecutive line. So during this interval, modulation ratio is effectively 0%. The interruption on all lines in a frame is observed as a vertical white bar on a visual monitor. This bar is named as 0 reference pulse (or simply 0 pulse).
The oscillogram of the 0 pulse is a pulse with a level more than the maximum level of the CVS. Taking the level difference between the
sync
tip and the 0 pulse as % 100, the maximum CVS should be 10% or 12.5%. The adjustment of the modulation index is simply the level adjustment of the modulating signal (CVS) at the input of the modulator.
References
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- ^
Bernard Grob - Charles E.Herndon:
Basic Television and Video systems
, Glencoe-McGraw Hill,
ISBN
0-02-800437-X
, p.411-412