Analogue enhanced or high- definition television systems tend, by nature, to be amenable to evolutionary development and implementation. HD-MAC, the newest member of the MAC/packet family of standards whose development has been fostered by the EBU for a number of years, is the fruit of just such an evolutionary approach. Having gained the support of the European Commission as the only non-digital high-definition satellite emission system for Europe, HD-MAC is set to establish an early foothold in the high-definition market. It therefore has an important rôle in promoting public demand for better pictures, better sound and, above all, for the 16:9 aspect ratio.

PALplus, a 16:9 compatible development of the PAL system which has been the backbone of European 4:3 colour services for a quarter of a century, is set to sustain the interest of millions of viewers in the services of broadcasters who must continue their terrestrial services in the period of transition to all-digital HDTV – a transition which is expected to last well into the next century.

If broadcasters can find commercial or other justification for breaking away from the constraints imposed by compatibility with the existing analogue receiver base, digital technologies offer many interesting possibilities for new services which may – or may not – include HDTV.

Research is in progress in two projects, Spectre and HD-Divine, which both draw on digital emission techniques developed initially for Digital Audio Broadcasting. They will offer rugged emission in the presence of multipath propagation, adaptability allowing control over interference to and from conventional television services in the same frequency band, and flexibility in terms of the programme signals conveyed.

Past experience – especially in relation to the analogue enhanced systems – highlights the importance of the timely establishment of a standard for digital terrestrial television, preferably on a world-wide basis.

While the European broadcast industry offers its wares to public scrutiny and invites viewers to define demand, the United States of America has preferred to play out its ATV/EDTV/HDTV battles in the privacy of the laboratories, under the arbitration of the Federal Communications Commission, and away from public gaze.

The FCC's aim is a radical one – the rapid introduction of all-digital terrestrial HDTV services, first in parallel with existing NTSC services (simulcast) and followed, within a relatively short time, by the withdrawal of NTSC, leaving the chosen digital HDTV system to stand alone.

Starting from over 20 candidate systems in 1989, the number of contenders has been whittled down to just five. Those five have much in common – but sufficient differences to ensure that there will be winners – and loosers – when the final choice is made in 1993.