Zero reference pulse or Zero pulse is an artificially produced pulse in a professional television receiver imitating no

radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the ...

case for modulation index measurements in analogue

TV transmitter A television transmitter is a transmitter that is used for terrestrial (over-the-air) television broadcasting. It is an electronic device that radiates radio waves that carry a video signal representing moving images, along with a synchronized au ...

Video signal

The composite

monochrome A monochrome or monochromatic image, object or palette is composed of one color (or values of one color). Images using only shades of grey are called grayscale (typically digital) or black-and-white (typically analog). In physics, monochrom ...

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

In analogue broadcasting the composite video signal modulates the carrier by a type of amplitude modulation named

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. 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 The modulation index (or modulation depth) of a modulation In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the '' carrier signal'', with a separate signal called ...

is so arranged that, the maximum level of CVS yields a RF level of % 10 (sometimes % 12.5).Bernard Grob - Charles E.Herndon:''Basic Television and Video systems'', Glencoe-McGraw Hill,, p.411-412 This value is known as the level of the

residual carrier In analogue TV technology, residual carrier is the ratio of carrier level which is modulated by the maximum video signal to the unmodulated carrier level. Video signal Video signal (VF) or more formally composite video signal (CVS) is the sig ...

. 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

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

{{Analogue TV transmitter topics Broadcast transmitters Television technology