HD-MAC

HD-MAC was a proposed television standard by the European Commission in 1986 (MAC standard) . It was an early attempt by the EEC to provide HDTV in Europe. It was a complex mix of analogue signal (Multiplexed Analogue Components), multiplexed with digital sound. The video signal ( 1250 (1152 visible) lines/50 frames in 16:9 aspect ratio) was encoded with a modified D2-MAC encoder.

HD-MAC could be decoded by a standard D2-MAC receivers(SDTV), but in that mode only 625 (576) lines and certain artifacts were visible. In order to decode the signal in full resolution, a specific HD-MAC tuner was needed.

36 MHz were required to broadcast in HD-MAC (PAL SDTV uses 6-, 7- (VHF), or 8 MHz (UHF)). Because UHF spare bandwidth was very scarce, HD-MAC was usable "de facto" only to cable and satellite providers, where there bandwidth was less constricted. However, the standard never became popular among broadcasters. For all this, analog HDTV could not replace conventional SDTV(terrestrial) PAL/SECAM, making HD-MAC sets unattractive to potential consumers.

It was required that all high-powered satellite broadcasters to use MAC from 1986. However, the launch of middle-powered satellites by SES Astra and the use of PAL allowed broadcasters to bypass HD-MAC, reducing their transmission costs. HD-MAC (the high-definition variant of MAC) was left for transcontinental satellite links, however.

In the 1992 Summer Olympics, an experimental HD-MAC broadcasting took place. 100 HD-MAC receivers (in that time, retro-projectors) in Europe were used to test the capabilities of the standard. This project was financed by the EEC. The PAL-converted signal was used by mainstream broadcasters such as SWR, BR and 3Sat.

The HD-MAC standard was abandoned in 1993, and since then all EU and EBU efforts have focused on the DVB system ("Digital Video Broadcasting"), which allows both SDTV and HDTV.

Technical details

The signal was captured with 50 full frames per second, each at a resolution of 2048×1152. In order to increase the horizontal resolution of the D2-MAC norm, only its bandwidth had to be increased. This was easily done as unlike PAL, the sound is not sent on a sub-carrier, but multiplexed with the picture. However, to increase the vertical bandwidth was a bit more complicated, as the line frequency had to stay at 15'625 Hz, in order not to break compatibility with D2-MAC. This offered three choices:

* 50 frames per second with only 288 lines for fast moving scenes,
* 25 frames per second with 576 lines for normally moving scenes, or
* 12.5 frames per second with all 1152 lines for slow motion.

As none of the three modes would have been sufficient, the choice during encoding was not made for the whole picture, but for little blocks of 8×8 pixels. The signal then contained hints which deinterlacing method the decoder should use.

In order to fully decode the picture, the signal has to be digitized and then read from the memory several times.

An interesting situation occurs with 50 Hz HDTV systems. Human peripheral vision is much more sensitive to contrast and movement than foveal vision. As a consequence, the 50 Hz field rate (25 Hz frame rate) has been found to be too slow to avoid perception of flicker. The edges of a 50 Hz HDTV image will noticeably flicker under most lighting conditions and screen brightness and contrast settings. Most European HDTV systems implementers advocate 100 Hz refresh rate.

See also

TV transmission systems
* Analog high-definition television systems
* PAL, what MAC technology tried to replace
* SECAM, what MAC technology tried to replace
* A-MAC
* B-MAC
* C-MAC
* D-MAC
* E-MAC
* S-MAC
* D2-MAC
* HD-MAC, an early high-definition television standard allowing for 2048x1152 resolution.
* DVB-S, MAC technology was replaced by this standard
* DVB-T, MAC technology was replaced by this standard

Related standards:
* NICAM-like audio coding is used in the HD-MAC system.
* Chroma subsampling in TV indicated as 4:2:2, 4:1:1 etc...

External links

* [http://www.stjarnhimlen.se/tv/tv.html#MAC Multiplexed Analogue Components] in "Analog TV Broadcast Systems" by Paul Schlyter