C-MAC
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C-MAC/packet is a variant of the family of MAC (Multiplexed Analogue Components) systems for television broadcasting, developed by IBA and approved by the European Broadcasting Union (EBU) for satellite TV transmissions.[1][2]
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It was an analog television standard where luminance and chrominance components are transmitted separately in time rather than separately in frequency.[3][4] This was an evolution from older color TV systems (such as PAL or SECAM) where there was interference between chrominance and luminance.
Technical details
C-MAC required a bandwidth of about 22 MHz, making it unsuitable for direct-to-home or cable broadcast.[5]
The transmitter switches between FM (vision) and PSK (sound/data) modulation during each television line period. The digital information is modulated using 2-4PSK (phase-shift keying), a variation of quadrature PSK where only two of the phaser angles (±90°) are used, with a data rate of 20.25 Mbit/s.[6][7] This allows for the transmission of up to eight high quality (15 kHz bandwidth) sound channels.[6]
C-MAC data was sent to the transmitter separately from the vision. The C-MAC standard included a standard scrambling system, EuroCrypt, a precursor to the standard DVB-CSA encryption system.
E-MAC
E-MAC (Extended MAC) is 16:9 version of C-MAC.[8] Originally E-MAC was designed for 15:9 pictures, it later adopted the 16:9 aspect ratio.
- In E-MAC all the 4:3 information is transmitted exactly as in C-MAC so that C-MAC receivers are still compatible.
- E-MAC hides extra luminance and chrominance information in the field blanking interval and parts of the line blanking interval.
- E-MAC has a lower data capacity because luminance is hidden where data would usually be located.
- A 'steering' signal is transmitted to indicate to the 16:9 receiver whereabouts the 4:3 picture information.
- E-MAC receivers stitch the 4:3 and helper widescreen data into a seamless 16:9 picture.
See also
TV transmission systems
References
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