C-MAC
C-MAC is the variant approved by the European Broadcasting Union for satellite transmissions. The digital information is modulated using 2-4PSK, a variation of quadrature PSK where only two of the phaser angles are used.
- The data capacity for C-MAC is 3Mbit/s.
- C-MAC data has to be sent to the transmitter separately from the vision.
- The transmitter switches between FM and PSK modulation during each television line period.
C-MAC variants : E-MAC
E-MAC is 16:9 version of C-MAC. 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 wide-screen data into a seamless 16:9 picture.
MAC transmits luminance and chrominance data separately in time rather than separately in frequency.
Audio and Scrambling
- Audio, in a format similar to NICAM was transmitted digitally rather than as an FM sub-carrier.
- The MAC standard included a standard scrambling system, Euro-Crypt, a precursor to the standard DVB-CSA encryption system.
