CCIR System B

CCIR System B was the 625-line analog broadcast television system which at its peak was the system used in the most countries. It is being replaced across Western Europe, part of Asia and Africa by digital broadcasting.

The system was developed for VHF band (part of RF band lower than 300 MHz.) Some of the important specs are listed below.

A frame is the total picture. The frame rate is the number of pictures displayed in one second. But each frame is actually scanned twice interleaving odd and even lines. Each scan is known as a field (odd and even fields.) So field rate is twice the frame rate. In each frame there are 625 lines (or 312.5 lines in a field.) So line rate (line frequency) is 625 times the frame frequency or 625•25=15625 Hz.

The video bandwidth is 5.0 MHz. The video signal modulates the carrier by Amplitude Modulation. But a portion of the lower side band is suppressed. This technique is known as vestigial side band modulation (AC3). The polarity of modulation is negative, meaning that an increase in the instantaneous brightness of the video signal results in a decrease in RF power and vice versa. Specifically, the sync pulses (being "blacker than black") result in maximum power from the transmitter.

The primary audio signal is modulated by Frequency modulation with a preemphasis time constant of τ = 50 μs. The deviation for a 1.0 kHz. AF signal is 50 kHz.

The separation between the primary audio FM subcarrier and the video carrier is 5.5 MHz.

The total RF bandwidth of System B (as originally designed with its single FM audio subcarrier) was 6.5 MHz, allowing System B to be transmitted in the 7.0 MHz wide channels specified for television in the VHF bands with an ample 500 kHz guard zone between channels.

In specs, sometimes, other parameters such as vestigial sideband characteristics and gamma of display device are also given.

System B has variously been used with both the PAL or SECAM colour systems. It could have been used with a 625-line variant of the NTSC color system, but apart from possible technical tests in the 1950s, this has never been done officially.

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