Zero reference pulse: Difference between revisions

Revision as of 11:25, 17 June 2010

Revision as of 11:43, 17 June 2010

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== 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,ISBN0-02-800437-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 (higherhigh 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.

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[[File:0 pulse.jpg|thumb|525px|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 than 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 bar on a visual monitor. This bar is named as 0 reference pulse.

The osilogramoscillogram 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|sycn]] tip and the 0 pulse as % 100, the maximum CVS should be 10 % . The adjustment of the modulation index is simply the level adjustment of the modulating signal at the input of the modulator.