Multiplexed Analogue Components (MAC) was an analog television standard where luminance and chrominance components were transmitted separately. This was an evolution from older color TV systems (such as PAL or SECAM) where there was interference between chrominance and luminance. Originally proposed in the 1980s for use on a Europe-wide terrestrial HDTV system, although it was never used terrestrially. However, tests have been done in France with terrestrial transmission but no commercial exploitation.
Various systems were developed, collectively known as the "MAC/packet" family. In 1985 these were recommended for satellite and cable broadcasts by the European Broadcasting Union (EBU). C-MAC/packet was intended for Direct Broadcast Satellite (DBS), D-MAC/packet was intended for wide-band cable, and D2-MAC/packet was intended both for DBS and narrow-band cable.
MAC was originally developed by the Independent Broadcasting Authority (IBA) (dates unknown) in the UK for delivering high quality pictures via direct broadcast satellites that would be independent of European countries' choice of terrestrial colour-coding standard.
In 1982 it was adopted as the transmission format for the UK's forthcoming direct broadcast satellite (DBS) television services (eventually provided by British Satellite Broadcasting). The following year MAC was adopted by the EBU as the standard for all DBS.
By 1986, despite there being two standards, D-MAC and D2-MAC, favoured by different countries in Europe, an EU Directive imposed MAC on the national DBS broadcasters, to provide a stepping stone from analogue PAL and SECAM formats to the eventual high definition and digital television of the future, with european TV manufacturers in a privileged position to provide the equipment required.
However, the Astra satellite system was also starting up at this time (the first satellite, Astra 1A was launched in 1989) and that operated outside of the EU's MAC requirements, due to being a non-DBS satellite. Despite further pressure from the EU (including a further Directive originally intended to make MAC provision compulsory in TV sets, and a subsidy to broadcasters to use the MAC format), most broadcasters outside Scandinavia preferred the lower cost of PAL transmission and receiving equipment.
In the 2000s, the use of D-MAC and D2-MAC ceased when satellite broadcasts changed to DVB-S format.
A number of broadcasting variants exist, according to the way the digital signals are multiplexed with the MAC vision signal.
S-MAC (Studio MAC): Used mostly in North America.
MAC transmits luminance and chrominance data separately in time rather than separately in frequency (as other analog television formats do, such as composite video). This allows for full separation of the components. The signals are also time-compressed (with ratios of 3:2 for luminance and 3:1 for chrominance) and the two color difference signals are transmitted on alternate lines, further increasing compression. The color space was YPbPr, with a luminance component and red blue color difference chrominance components.
In MAC color is encoded using the YPbPr color space. Luma () is derived from red, green, and blue () after gamma-correction (formula similar to PAL):
Color information is computed based on and differences, generating two compressed and weighted color-difference signals know in older MAC references as and or and . To avoid any confusion, and since the signals are analogue and bi-polar, these terms were replaced by and .
and are used to transmit chrominance. On C-MAC, D-MAC and D2-MAC the following formulas apply:
Luminance signal range is -0.5 to 0.5 volts; color difference signals vary between -0.65 to 0.65 volts.
The following table lists the main technical parameters of the various MAC variants:
|B-MAC (525-line)||B-MAC (625-line)||C-MAC||D-MAC||D2-MAC|
|Lines per frame||525||625|
|Aspect Ratio||4:3 / 16:9|
|Primary chromaticities (x y)||Red: 0.67, 0.33
Green: 0.21, 0.71 Blue: 0.14, 0.08
|White point (x y)||D65: 0.313, 0.329|
|Sampling frequency (MHz)||14.318||14.219||13.500|
|Uncompressed bandwidth (MHz)||4.2||5.0||5.6|
|Luminance clock periods||750||696|
|Chrominance clock periods||375||348|
Although the MAC technique is capable of superior video quality, (similar to the improvement of component video over composite in a DVD player), its major drawback was that this quality was only ever realized when the video signals being transmitted remained in component form from source to transmitter. If at any stage the video had to be handled in composite form, the necessary encoding/decoding processes would severely degrade the picture quality.
This is a list of nations that used the MAC standard for television broadcasting:
Since the vast majority of TV stations and similar installations were only wired for composite video, the fitting of a MAC transmitter at the end of the chain had the effect of degrading the transmitted image quality, rather than improving it.
For this and other technical reasons, MAC systems never really caught on with broadcasters. MAC transmission technology was made obsolete by the radically new digital systems (like DVB-T and ATSC) in the late 1990s.
Although MAC transmission systems are still used, the technology is obsolete. It is expected that MAC will cease to be used for TV transmission by 2012.
TV transmission systems: