System B
Encyclopedia
System B is the analog broadcast television
system used in the most countries. There are several systems in use and letter B is assigned for the European system which is also used in the majority of Asian and African countries. (However some countries in Europe use different systems.)
Frame is the total picture. 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 named a field (odd and even fields.) So field rate is two times 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 MHz. It modulates the carrier by AM
technique. But a portion of lower side band is suppressed. This technique is known as vestigial side band modulation (AC3). The polarity of modulation is negative, meaning that the increase in video signal results in decrease in RF power and vice versa.
The audio signal
with a preemphasis
time constant of τ = 50 μs. is modulated by FM
technique , maximum deviation for 1 kHz. AF signal being 50 kHz.
The total RF bandwidth is 7 MHz. The separation between the aural and visual carriers is 5.5 MHz.
In specs, sometimes, other parameters such as vestigial sideband characteristics and gamma of display device are also given.
In most system B countries, UHF band is used with system B specifications, the only difference being that there is a guard band of 1 MHz. between the channels. (Of course this difference is only meaningful in RF.) This system for UHF band is known as G and all specifications given above also apply to system G.
A few countries (Belgium, Luxembourg and Netherlands) use the other version of system B which is known as system H . System H is similar to system G . But lower side band (vestigal sideband characteristics) is wider.
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
system used in the most countries. There are several systems in use and letter B is assigned for the European system which is also used in the majority of Asian and African countries. (However some countries in Europe use different systems.)
Specifications
Some of the important specs are listed below.| Frame rate | Interlace | Field rate | Line/frame | Line rate | Visual b/w | Vision mod. | Preemphasis | Sound mod. | Sound offset | Channel b/w | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 25 | 2/1 | 50 | 625 | 15625 | 5 MHz. | AC3 neg. | 50 μs | F3 | 5.5 MHz. | 7 MHz. |
Frame is the total picture. 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 named a field (odd and even fields.) So field rate is two times 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 MHz. It modulates the carrier by AM
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
technique. But a portion of lower side band is suppressed. This technique is known as vestigial side band modulation (AC3). The polarity of modulation is negative, meaning that the increase in video signal results in decrease in RF power and vice versa.
The audio signal
Audio signal
An audio signal is an analog representation of sound, typically as an electrical voltage. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical...
with a preemphasis
Preemphasis
In processing electronic audio signals, pre-emphasis refers to a system process designed to increase the magnitude of some frequencies with respect to the magnitude of other frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as...
time constant of τ = 50 μs. is modulated by FM
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
technique , maximum deviation for 1 kHz. AF signal being 50 kHz.
The total RF bandwidth is 7 MHz. The separation between the aural and visual carriers is 5.5 MHz.
In specs, sometimes, other parameters such as vestigial sideband characteristics and gamma of display device are also given.
System G and H
System B is an old system . It was proposed for VHF 1 and VHF 3 (Channels 2-4 and 5-12) But when UHF bands came into use, two versions of System B began to be used.In most system B countries, UHF band is used with system B specifications, the only difference being that there is a guard band of 1 MHz. between the channels. (Of course this difference is only meaningful in RF.) This system for UHF band is known as G and all specifications given above also apply to system G.
A few countries (Belgium, Luxembourg and Netherlands) use the other version of system B which is known as system H . System H is similar to system G . But lower side band (vestigal sideband characteristics) is wider.