Zero reference pulse: Difference between revisions

Revision as of 05:33, 21 June 2010

Revision as of 10:27, 29 June 2010

Line 1:

'''Zero reference pulse''' or '''Zero reference pulse''' is an artificially produced pulse in a professional television receiver imitating no [[RF|radio frequency]] case for [[modulation]] index measurements in analogue [[TV transmitter]]s.

== Video signal ==

== Video signal ==

{{Main|Composite video signal}}

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

== Modulation Measurement==

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 (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.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.

Line 23 ⟶ 15:

== Reference ==

<references/>

{{Analogue TV transmitter topics}}

r

[[Category:Broadcast transmitters]]

[[Category:Television technology]]

[[Category:Broadcast engineering]]

[[Category:Broadcast transmitters]]