CCIR System B was the 625-line analog broadcast standard for the VHF band television system used in most countries of the world. It is being replaced by digital broadcasting across Western Europe, part of Asia, and Africa.
The system was developed for VHF band (part of RF band lower than 300 MHz.) Some of the most important specs are listed below.
A frame is a 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 the 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 sideband is suppressed. This technique is known as vestigial sideband 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 the display device are also given.
System B has variously been used with both the PAL or SECAM color 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.
When used with PAL, the color subcarrier is 4.43361875 MHz and the sidebands of the PAL signal have to be truncated on the high-frequency side at +570 kHz (matching the roll off of the luminance signal at +5.0 MHz). On the low-frequency side, the full 1.3 MHz sidebands are radiated. ( This behavior would cause massive U/V crosstalk in the NTSC system, but delay-line PAL hides such artifacts. )
When used with SECAM, the 'R' lines' carrier is at 4.40625 MHz deviating from +350±18 kHz to -506±25 kHz. The 'B' lines' carrier is at 4.250 MHz deviating +506±25 kHz to -350±18 kHz.
Neither color encoding system has any effect on the bandwidth of system B as a whole.
Enhancements have been made to the specification of System B's audio capabilities over the years. The introduction of Zweiton in the 1970s allowed for stereo sound or twin monophonic audio tracks (possibly in different languages for instance). This was implemented by adding a second FM audio subcarrier at +5.74 MHz. Alternatively, starting in the late 1980s and early 1990s it became possible to replace the second audio FM subcarrier with a digital signal carrying NICAM sound. Either of these extensions to audio capability has eaten into the guard band between channels. Zweiton uses an extra 150 kHz. The alternative NICAM system uses an extra 500 kHz and needs to be spaced further from the primary audio subcarrier, thus System B with NICAM has only 150 kHz guard zones between channels.
System B was the first internationally accepted 625-line broadcasting standard in the world. The European 41-68 MHz Band I television allocation was agreed at the 1947 ITU (International Telecommunications Union) conference in 1947, and the first European channel plan (i.e. the use of channels E2 - E4) was agreed in 1952 at the ITU conference in Stockholm. The extension to VHF Band III (i.e. Channels E5 - E12) was also agreed in the 1950s.
Since then, the System B specification has been used with slightly different broadcast frequencies in many other countries.