Black and burst, also known as bi-level sync and black burst, is an

analogue signal An analog signal (American English) or analogue signal (British and Commonwealth English) is any continuous-time signal representing some other quantity, i.e., ''analogous'' to another quantity. For example, in an analog audio signal, the inst ...

used in broadcasting. It is a

composite video Composite video, also known as CVBS (composite video baseband signal or color, video, blanking and sync), is an analog video format that combines image information—such as brightness (luminance), color (chrominance), and synchronization, int ...

signal with a black picture. It is a reference signal used to synchronise video equipment, in order to have them output video signals with the same timing. This allows seamless switching between two video signals. Black and burst can also be used to synchronise colour phase and provides timing accuracy in the order of tens of nanoseconds which is necessary to perform e.g. analogue video mixing. Black and burst exists for various colour TV standards, such as

PAL Phase Alternating Line (PAL) is a color encoding system for analog television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...

NTSC NTSC (from National Television System Committee) is the first American standard for analog television, published and adopted in 1941. In 1961, it was assigned the designation System M. It is also known as EIA standard 170. In 1953, a second ...

and

SECAM SECAM, also written SÉCAM (, ''Séquentiel de couleur à mémoire'', French for ''sequential colour memory''), is an analog color television system that was used in France, Russia and some other countries or territories of Europe and Africa. ...

. Because the black and burst signal is a normal video signal, it is transportable via normal video cables and through video distribution equipment.

History

Before colour TV existed, the reference signal was also a black video signal. Inaccuracies meant the video picture would be shifted. With the introduction of colour, the reference had to be much more accurate. In every composite video signal a reference burst is present in the horizontal sync portion, so all equipment in the chain will be synchronised roughly 16000 times per second. This regular synchronisation is necessary because the colour information is transmitted via

quadrature amplitude modulation Quadrature amplitude modulation (QAM) is the name of a family of digital modulation methods and a related family of analog modulation methods widely used in modern telecommunications to transmit information. It conveys two analog message signa ...

on the high-frequency colour signal. Incorrect synchronisation means the phase will be off, and consequently the colour will be incorrect. Creating broadcast television usually involves mixing video signals. When doing this in an analogue way, it is essential that all signals have the same colour phase, which was achieved by synchronising all cameras with a black and burst signal. Because of cable length differences, every camera required a (often only slightly) different timing. This could be tuned at the reference source, and/or at the camera. Black and burst is being replaced by

tri-level sync Tri-level sync is an analogue video synchronization pulse primarily used for the locking of high-definition video signals ( genlock). It is preferred in HD environments over black and burst, as timing jitter is reduced due to the nature of it ...

, but as of 2020 it is still quite common. Because the signal chains are now digital, which allows buffering, the timing requirements are not as strict anymore.

Waveform

Its natural waveform is a negative pulse with a level of -40 IRE followed by 10 cycles of the colour sub carrier of video. For most variants of PAL video, the frequency of the sub carrier is 4.43361875 MHz.

References

{{reflist Synchronization Film and video technology Broadcast engineering Television terminology

History

Waveform

See also

References