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Video Signal Basics

This information applies equally to all modern analogue monochrome television systems, whilst the colour information is valid for PAL and NTSC but not SECAM.

The standard video signal is normally generated and distributed as either a composite or non-composite signal. By composite we mean a 'raw' video signal to which has been added blanking and synchronising (sync) signals; generally this signal has an amplitude of 1.0 volt peak-to-peak (pp) in amateur and broadcast video circles but 1.4 volts pp in some closed-circuit TV applications. Non-composite video has an amplitude of 0.7 volt and is generally confined to studio situations where switching, mixing or other processing is to take place; synchronising signals (syncs) are added afterwards to make the video composite. Non-composite signals are also used in connection with RGB colour.

Composite video is sometimes abbreviated to VBS (video, blanking & syncs) or BAS, its German equivalent. The similar expression for non-composite video is VB or BA. Colour composite video is called CVBS or FBAS.

In traditional broadcast video applications cameras and other picture sources are either driven from a number of separate, centrally generated pulses or arranged to self-synchronise themselves ('genlock') to these pulses. This is to ensure there is no picture break-up or rolling when switching between video sources. Modern equipment is able to lock automatically to a reference source of 'black and burst' or 'colour black', a mixture of mixed syncs, blanking and colour burst. In addition vision mixers and effects generators are nowadays generally provided with synchronisers which delay incoming signals as necessary in order to perform the mixing or cutting cleanly. You may still come across older equipment on the surplus market which uses separate pulses, so here they are.

Line or Horizontal Drive (abbreviated to LD or HD). This is a synchronising signal which initiates the horizontal scan in cameras, monitors, TV receivers, etc. In the case of a 625 line picture its frequency is 15,625Hz. For 405 lines it is 10,125kHz.

Field or Vertical Drive (FD or VD). In a similar this signal starts the vertical scan; its frequency is usually related (although not normally locked) to the mains frequency. In European 625 line systems these pulses are at 50Hz.

Mixed Blanking (MB) pulses suppress the video signal during the retrace period at the end of the horizontal and vertical scans.

Mixed or Combined Syncs. (MS or CS) are the signals used for synchronising monitors and receivers.

These four pulses are essential for all video, monochrome or colour. Colour television also uses Burst Gate (BG) alias Burst Flag (BF). All these pulses are normally distributed at 2 volts peak-to-peak, although some British and most American and Japanese equipment requires 4 volts pp.

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