HD-MAC

HD-MAC (High Definition Multiplexed Analogue Components) was a broadcast television standard proposed by the European Commission in 1986, as part of Eureka 95 project.

The European Broadcasting Union video format description is as follows: width x height [scan type: i or p] / number of full frames per second[2] European standard definition digital broadcasts use 720×576i/25, meaning 25 720 pixels wide and 576 pixels high interlaced frames: odd lines (1, 3, 5 ...) are grouped to build the odd field, which is transmitted first, then it is followed by the even field containing lines 2, 4, 6...

The visible part of the video signal provided by an HD-MAC receiver was 1152i/25, which exactly doubles the vertical resolution of standard definition.

The purpose was to react against a Japanese proposal, supported by the US, which aimed to establish the NHK-designed Hi-Vision (also known as MUSE) system as a world standard.

Besides preservation of the European electronic industry, there was also a need to produce a standard that would be compliant with the 50 Hz field frequency systems (used by a large majority of countries in the world).

In September, 1988, the Japanese performed the first High Definition broadcasts of the Olympic games, using their Hi-Vision system (NHK produced material using this format since 1982).

In that same month of September, Europe showed for the first time a credible alternative, namely a complete HD-MAC broadcasting chain, at IBC 88 in Brighton.

[6] 1250 lines (1152 visible) CRT projectors were used to create an image a few meters wide in public spaces in Barcelona for the Olympics.

The HD-MAC standard was also demonstrated at Seville Expo '92, exclusively using equipment designed to work with the standard such as Plumbicon and CCD cameras, direct view and rear projection CRT TVs, BCH 1000 Type B VTRs, single mode fiber optic cables, and Laserdisc players with their respective discs.

For all this, analogue HDTV could not replace conventional SDTV (terrestrial) PAL/SECAM, making HD-MAC sets unattractive to potential consumers.

However, the launch of middle-powered satellites by SES and the use of PAL allowed broadcasters to bypass HD-MAC, reducing their transmission costs.

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.

The specification allows for 8 MHz channels, but in this case assistance data can no longer be correctly decoded, and it is only possible to extract a standard definition signal, using a D2-MAC receiver.

According to ITU recommendation for HDTV standards parameters[15] the active part of the line was 26.67 μs long (see also the LDK 9000 camera document [16]).

Ultimately, the starting point for BRE was a 1440x1152 sampling grid (twice the horizontal and vertical resolutions of digital SD), interlaced, at 25 fps.

[18] As a consequence of those operations, a 4:1 reduction factor was achieved, allowing the high definition video signal to be transported in a standard D2-MAC channel.

The samples retained by the BRE were assembled into a valid standard definition D2-MAC vision signal and finally converted to analogue for transmission.

The BRD (Bandwidth Restoration Decoder) in the receiver would then reconstruct a 1394x1152 sampling grid from it, under the control of the DATV stream, to be fed into its DAC.

The final output was a 1250 (1152 visible) lines, 25 fps, interlaced, analogue HD video signal, with a 50 Hz field frequency.

[23] This requirement meant pushing the technology boundaries of the time, and would have added to the notorious lack of sensitivity of some Eu 95 cameras (particularly CRT ones).

The Eu95 system would have provided better compatibility with cinema technology than its competitor, first because of progressive scanning, and second because of the convenience and quality of transfer between 50 Hz standards and film (no motion artifacts, one just needs to invert the usual "PAL speed-up" process by slowing down the frame rate in a 25/24 ratio).

So in theory, it would have been possible to use an SD digital video recorder, assuming it provides enough room for the DATV assistance stream, which requires less than 1.1 Mbit/s.

But there is no real reason for the studio equipment to be constrained by HD-MAC, as the latter is only a transmission standard, used to convey the HD material from the transmitter to the viewers.

At the start of the Eureka95 project the only means of recording the HD signal from a camera was on a massive 1-inch reel-to-reel tape machine, the BTS BCH 1000, which was based on the Type B videotape format but with 8 video heads instead of the two normally used, together with a higher linear tape speed of 66 cm/s, thus accommodating the higher bandwidth requirements of HD Video.

This gave the system a full bandwidth performance in the diagonal direction, but halved horizontally or vertically depending on the exact image temporal-spatial characteristics.

Excepting a slightly longer processing delay, it otherwise worked similarly to the BBC approach, and both versions of the Quadriga equipment were made to be interoperable, switchable between interleaved and quadrant modes.

For a historical perspective on HD-originated movies, one can mention early attempts such as 'Harlow', shot in 1965 using a near-HD analogue 819 lines process that later evolved to higher resolutions (see Electronovision).

In France, a company called VTHR (Video Transmission Haute Resolution) used the Eu95 hardware for some time to retransmit cultural events to small villages (later, they switched to upscaled 15 Mbit/s MPEG2 SD).

Some legacy of this development may be seen in "2K" and "4K" digital movie projectors using TI DLP chips, which run a slightly wider than usual 2048x1080 or 4096x2160 resolution, giving 1.896:1 aspect ratio without anamorphic stretching (vs the 1.778:1 of regular 16:9, with 1920 or 3840 horizontal pixels), give a little (6.7%) more horizontal resolution with anamorphic lenses when showing 2.21:1 (or wider) movies specifically prepared for them, and further enhancement (~13.78%) through reduced letterboxing if used without such lenses.

In this way, it can be considered a form of convergent specification evolution - although there's little chance the two standards are directly related, their particulars will have been landed on by broadly similar methods.