Zero reference pulse: Difference between revisions

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{{One source|date=June 2024}}{{short description|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 transmitter]]s.

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{{Main|Composite video signal}}

The composite [[monochrome]] video signal (CVS) is composed of a video signal superimposed on an auxiliary signal of 300 mV. The levels between 0 toand 300 mV are assigned for the auxiliary signal and the levels between 300 toand 1000 mV are assigned to video information.

==Modulation==

In analogue broadcasting the composite video signal modulates the carrier by a type of [[amplitude modulation]] named [[Single-sideband modulation#vestigal sideband (VSB)|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 [[amplitude modulation#Modulation index|modulation index]] is so arranged that, the maximum level of CVS yields a RF level of % 10 (sometimes % 12.5).<ref>Bernard Grob - Charles E.Herndon:''Basic Television and Video systems'', Glencoe-McGraw Hill,{{ISBN |0-02-800437-X}}, p.411-412</ref> 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==

[[File:0 pulse.svg|thumb|525px400px|Left: Input CVS (1000 mV., sawtooth) &ndash; Right: Demodulated output with zero ref pulse]]

To adjust the modulation index, an input of maximum level CVS (1000 mV) is applied to the modulator. The modulated RF signal is thanthen 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 [[Analogue television synchronization|sycnsync]] 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==