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Black and Burst also known as bi-level sync and black burst is an analogue signal used in broadcasting. It consists of a video signal with a blacked-out picture. It is used to synchronise video signals together to put them on the same timing plane. It allows vertical timing that allows synchronous switching between two video signals. Black/Burst also locks the colour phase between signals. The burst section of the waveform allows the phase relationship of the two signals to be timed together by sending pulses to the internal oscillators of the equipment.
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History
PAL is a broadcast transmission standard for Europe, and from this standard the Black/Burst analogue reference was created. Black and Burst became common when the PAL video format became standard in Europe, then became the most commonly used reference signal. It is commonly used today, it is sometimes referred to as a SD reference but this has no relevance to SD-SDI.
Black and Burst is now a predecessor to Tri Level Sync also known as HD reference. Please see Genlock
Waveform
Its natural waveform is a negative pulse of -40 IRE followed by 10 cycles of the colour sub carrier of video. For PAL video, the frequency of the sub carrier is 4.43361875MHZ.