EBU • UER - Technical Review

EBU Technical Review : No. 280 (Summer 1999)

AM broadcasting - is the end in sight?

The introduction in the 1950s and 1960s of FM broadcasting sounded the death-knell for AM broadcasting. As FM was so much better than AM, it seemed obvious that AM would disappear within 10 years – or 20 years at the most. In fact, AM is still with us. Why has it refused to lay down and die?

From a technical perspective, there should be no contest between AM and FM. The 9 kHz RF channels in the LF and MF bands set the maximum audio bandwidth for AM at 4.5 kHz. In practice, the frequency response of most AM radios is typically -3 dB at 3.5 kHz, whereas FM offers 15 kHz bandwidth as well as stereo.

Although ground-wave reception of LF and MF stations can be reasonably good during the day, reception at night is often ruined by interference from sky-wave signals, either from the wanted transmitter or from distant co-channel transmitters.

Despite the clear technical benefits of FM, AM did not quickly replace FM. In countries (such as the UK) where the FM transmissions simply duplicated the existing AM services, admittedly in higher quality, there was little incentive for listeners to buy relatively expensive new FM radios. Conversely, in countries (such as Germany) where the LF/MF assignments were insufficient to meet the programming requirements, many services were available only on the FM band – and hence FM listening became popular more quickly.

Until the late 1960s or early 1970s, most portable radios and car radios could only receive AM services. By the 1980s, most new radios included FM, but AM radios were not discarded in favour of the new FM radios. Some of the AM radios remained in use – with the consequence that many listeners continued to listen for part of the time to AM services, because their old radios could not receive FM services.

However, this was only part of the problem. Not everybody accepted the technical arguments in favour of FM. Perversely, some people preferred the "mellow" sound of AM, rather than the "tinny" sound of FM. There is little doubt that such complaints were justified: some cheap portable radios sounded terrible. Furthermore, even within the last few years, some FM radios are so insensitive that they are essentially "deaf", since they cannot receive any weak FM signals.

More substantially, some listeners acknowledged the superiority of FM reception on a hi-fi system with a good antenna but, rightly, observed that FM reception on portable or car radios was often unsatisfactory. Whereas AM reception is relatively stable, the use of VHF transmissions means that moving an FM radio by, say, one metre can cause dramatic variations in reception quality. In a car, such variations can cause temporary "drop-outs" which might be acceptable if they are infrequent. However, the variability of reception with location is judged by many to be completely unacceptable on a portable radio at home. Unfortunately, adequate reception of a specific FM transmission can require a portable radio to be physically moved from its "natural" position on a table or kitchen shelf to a less satisfactory location – only to be moved back to receive a different FM transmission. AM reception cannot match the best quality obtainable on FM, but AM might be more reliable - at least during the day!

At long last, at the end of the 1990s, AM listening is showing signs of decline. FM seems to have won the long battle. AM remains viable for news and sports services, but is less likely to be successful for music formats. New broadcasters are reluctant to build AM transmitters because FM transmitters are cheaper to build and operate. Furthermore, some AM services have already been closed down. This trend is likely to continue as broadcasters examine their high-power AM services in terms of cost per listener.

Sky-wave propagation is generally regarded as a disadvantage in the MF bands. However, it also offers the possibility of covering large areas with a single transmitter, especially in the MF and HF bands. Where there are low levels of both co-channel interference and man-made noise, sky-wave coverage is very attractive for international broadcasting. A major problem for sky-wave services is that multipath propagation through the ionosphere causes time-varying selective fading.

As with DAB and digital terrestrial TV, the combination of advanced digital modulation schemes with new algorithms for the digital compression of audio signals offers tremendous potential – even within 9 kHz or 10 kHz RF channels. Digital systems can offer enhanced performance – probably giving performance equivalent to monophonic FM services – whilst being much less fragile than AM in terms of immunity to interference and selective fading.

Digital Radio Mondiale (DRM) is investigating such systems, with the objective of agreeing a single standard for digital radio in the AM bands. This could be used as the long-term replacement of AM broadcasting in the HF bands, as well as in the LF and MF bands. Ideally, the DRM solution will be applicable to existing AM transmitters with only minor modifications. Broadcasters are naturally attracted by the potential re-use of expensive hardware, such as high power transmitters and transmitting antennas. Unfortunately, the real cost of switching from AM to digital services is in the purchase of millions of new radios – this cost will have to be paid directly by the general public.

The timetable for the introduction of digital services in the AM bands will be set by broadcasters, but the speed of the transition to digital will be set by consumers. Judging by past experience, the transition could be slow – but DRM deserves substantial support from the broadcasting community.

Philip Laven
Director
EBU Technical Department

techreview@ebu.ch