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. It belongs to the MAC - Multiplexed Analogue Components standard family. It is an early attempt by the EEC to provide high-definition television (HDTV) in Europe. It is a complex mix of analogue signal (based on the Multiplexed Analogue Components standard), multiplexed with digital sound, and assistance data for decoding (DATV). The video signal (1250 lines/50 fields per second in 16:9 aspect ratio, with 1152 visible lines) was encoded with a modified D2-MAC encoder.
HD-MAC could be decoded by normal D2-MAC standard definition receivers, but no extra resolution was obtained and certain artifacts were visible. To decode the signal in full resolution a specific HD-MAC tuner was required .
Naming convention
The European Broadcasting Union video format description is as follows: width x height [scan type: i or p] / number of full frames per second
European standard definition digital broadcasts use 720×576i/25, meaning 25 interlaced frames measuring 720 pixels wide and 576 pixels high per second: 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... Thus, there are two fields in a frame, resulting in a field frequency of 25 × 2 = 50 Hz.
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 amount of information is multiplied by 4, considering the encoder started its operations from a 1440x1152i/25 sampling grid.
Standard history
Work on HD-MAC specification started officially in May 1986. 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). Truth be said, the precisely 60 Hz of the Japanese proposal was also worrying the US, as their NTSC M-based standard definition infrastructure used a practical frequency of 59.94 Hz, potentially leading to incompatibility problems.
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. This show included the first progressive scan HD video camera prototypes (Thomson/LER).High Definition Television; the Creation, Development and Implementation of HDTV Technology, Philip J. Cianci
Golden SCART was developed as a transmission interface for consumer devices,Jim Slater: Modern Television Systems to HDTV and beyond. Pitman Publishing, 1991, ISBN 0-203-16851-8, S. 180/181 a special and backward-compatible implementation of the normal SCART connection. Some television sets from Philips and Telefunken are said to have been equipped with it.
For the Albertville 1992 Winter Olympics and Barcelona 1992 Summer Olympics, a public demonstration of HD-MAC broadcasting took place. 60 HD-MAC receivers for the Albertville games and 700 for the Barcelona games were set up in "Eurosites" to show the capabilities of the standard. 1250 lines (1152 visible) CRT projectors were used to create an image a few meters wide in public spaces in Barcelona for the Olympics. There were some Thomson "Space system" 16:9 CRT TV sets as well. The project sometimes used rear-projection televisions. In addition, some 80,000 viewers of D2-MAC receivers were also able to watch the channel (though not in HD). It is estimated that 350,000 people across Europe were able to see this demonstration of European HDTV. 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 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. Production equipment was visible to the public through windows.
Because UHF spare bandwidth was very scarce, HD-MAC was usable "de facto" only to cable and satellite providers, where their bandwidth was less constricted, similarly to Hi-Vision that was only broadcast by the NHK through a dedicated satellite channel called BShi. However, the standard never became popular among broadcasters. For all this, analogue 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 use MAC from 1986. However, the launch of middle-powered satellites by SES and the use of PAL allowed broadcasters to bypass HD-MAC, reducing their transmission costs. HD-MAC was left for transcontinental satellite links, however.
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.
This article about IFA 1993 provides a view of the project's status close to its end. It mentions "a special BBC compilation encoded in HD-MAC and replayed from a D1 Video Tape Recorder".
HD-MAC development was stopped alongside the EUREKA project in 1996, because picture quality was not deemed to be good enough, receiving TVs didn't have enough resolution, the 16:9 aspect ratio that would later become standard was seen as exotic, and receiving TVs weren't large enough to exhibit the image quality of the standard, and those that were, were CRT TVs which made them extremely heavy.
Technical details
Transmission
PAL/SECAM analogue SDTV broadcasts use 6-, 7- (VHF), or 8 MHz (UHF). The 819-line (System E) used 14 MHz wide VHF channels. For HD-MAC, the transmission medium must guarantee a baseband bandwidth of at least 11.14 MHz.ETSI specification of the D2-HDMAC/Packet system (ETS 300 352), section 4.1 This translates to a 12 MHz channel spacing in cable networks. 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. For satellite broadcasting, due to FM modulation spectrum expansion, an entire satellite transponder would be used, resulting in 27 to 36 MHz of bandwidth.ETSI specification of the D2-HDMAC/Packet system (ETS 300 352), section 10.3 The situation is pretty much the same in analogue standard definition : a given transponder can only support one analogue channel. So from this point of view, going to HD does not represent an inconvenience.
Bandwidth reduction
BRE (Bandwidth Reduction Encoding) operation started with analogue HD video (even when the source was a digital recorder, it was reconverted to analogue to feed the encoder). It was specified to have a 50 Hz field frequency. It could be interlaced, with 25 frames a second (called 1250/50/2 in the recommendation), or it could be progressively scanned with 50 full frames a second (called 1250/50/1). The interlaced version was the one used in practice. In any case, the number of visible lines was 1152, twice the standard 576 lines vertical definition. The full number of lines in a frame period, included those that cannot be displayed, was 1250. This made for a 32 μs line period. According to ITU recommendation for HDTV standards parameters the active part of the line was 26.67 μs long (see also the LDK 9000 camera document ).
Had the modern trend for square pixels applied, this would have yielded a 2048x1152 sampling grid. There was no such requirement in the standard, though, since CRT monitors don't need any extra scaling to be able to show non-square pixels. According to the specification, the sampling rate for the interlaced input to use was 72 MHz, resulting in 72 x 26.67 = 1920 horizontal samples. It was then reconverted to 1440 from within the sampled domain. The input signal often originated from sources previously sampled at only 54 MHz, for economical reasons, and therefore already containing no more than the analogue equivalent of 1440 samples per line. Ultimately, the starting point for BRE was a 1440x1152 sampling grid (twice the horizontal and vertical resolutions of digital SD), interlaced, at 25 fps.ETSI specification of the D2-HDMAC/Packet system (ETS 300 352), section 5.2.1
To improve 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 vertical bandwidth was more complex, as the line frequency had to stay at 15.625 kHz to be compatible with D2-MAC. This offered three choices:
50 frames per second with only 288 lines for fast moving scenes (20 ms mode) 25 frames per second with 576 lines for normally moving scenes (40 ms mode) 12.5 frames per second with all 1152 lines for slow motion (80 ms mode)
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 16×16 pixels. The signal then contained hints (the DATV digital stream) that controlled which de-interlacing method the decoder should use.
The 20 ms mode offered an improved temporal resolution, but the 80 ms was the only one that provided High spatial definition in the usual sense. The 40 ms mode threw away one the HD fields and reconstructed it in the receiver with the assistance of motion compensation data. Some indications were also provided in case of a whole frame movement (camera panning,..) to improve the quality of the reconstruction.
The encoder could work in "Camera" operating mode, using the three coding modes, but also in "film" mode where the 20 ms coding mode was not used.
The 80 ms mode took advantage of its reduced 12.5 fps frame rate to spread the contents of an HD frame over two SD frames, meaning four 20 ms fields = 80 ms, hence the name.
But that was not enough, as a single HD frame contains the equivalent of 4 SD frames. This could have been "solved" by doubling the bandwidth of the D2-MAC signal, thus increasing the allowed horizontal resolution by the same factor. Instead, the standard D2-MAC channel bandwidth was preserved, and one pixel out of two was dropped from each line. This sub-sampling was done in a quincux pattern. Assuming pixels on a line independently numbered from 1 to 1440, only pixels 1, 3, 5... were retained from the first line, pixels 2, 4, 6... from the second, 1, 3, 5...again from the third, and so on. That way, information from all the columns of the HD frame were conveyed to the receiver. Each missing pixel was surrounded by 4 transmitted ones (except on the sides) and could be interpolated from them. The resulting 720 horizontal resolution was further truncated to the 697 samples per line limit of the D2-HDMAC video multiplex.ETSI specification of the D2-HDMAC/Packet system (ETS 300 352), 5.3.6 Baseband format