Ideas About The Future Of The Medium Wave

some arguments for wide-band-AM, some thoughts about DRM

by Peer-Axel Kroeske (DL2LBP).

Sound Quality must be improved

Because of the poor sound quality, the medium wave has become very much out-of-fashion in many parts of Europe. In some countries, it is still alive though, e.g. in the UK and Spain or America. In these countries MW programmes exist, which cannot be heard on FM. This is the key to MW attraction I think. However, MW stations often have less success because of the poor sound.

Is DRM the future?

The digital standard DRM has tried to push forward for a couple of years now. The break-through still seems far away. And I remain sceptical. At least I'd rather see DRM on short wave with a flexible RDS-like frequency concept. The main problem is probably the lack of cheap receivers. As long as there aren't enough potential listeners, stations won't invest into DRM (and DAB). The competition is high: FM usually offers about 20 stations, internet radio via flatrate cell phone networks will become an alternative. Listeners get the ultimate variety there. There must be a few 24h high quality music and news programmes in DRM before listeners accept it. Foreign services must provide worldwide 24h coverage on short wave. When many people buy receivers, DRM could become highly attractive for commercial stations, at least on shortwave, because you have large coverages with only one transmitter. On medium wave it's probably difficult to set up complex SFN-DRM networks. Crowded frequencies at night don't leave much space for adequate DRM-signal-to-noise-ratios. There would be an option for local stations which couldn't get FM frequencies. But they should rather use VHF frequencies to avoid interference problems (see below). DRM on the former TV band I frequencies has already been discussed. And DAB still offers capacity in most places.

One advantage could be the sound quality. The DRM-site claims that FM-sound quality (mono) will be possible on medium wave. Now you don't need to be an expert to reveal that 24kbps AAC is far from FM-Hifi sound. The DRM signal is heavily compressed resulting in a very artificial sound impression. Cosmetics by SBR technique help only a little (and could theoretically also be implemented to analogue AM). 48kbps is possible with double channels (20 KHz). I'd recommend that but it doesn't look like any station would be using this in the near future. As far as sound is concerned DRM is still an improvement to nowaday-AM. However, the main potential for DRM can be found on short wave. Once it has become popular, single transmitters of commercial programms could cover large areas in a sufficient sound quality. But it hasn't happened yet.
A main problem of DRM is interference: I've experienced that DRM needs absolutely free frequencies. If there's only one weak carrier (could be from an AM station using the same channel) the DRM signal-to-noise-ratio drops down and the signal disappears. By the way this makes it very easy for countries with censorship to prevent reception. Indoor environments are full of interfering signals produced by computers, power supplies and various electronics. While AM can still be received here more-or-less, DRM reception becomes impossible.

DRM introduction would take time

New receivers would be needed for DRM. In poorer countries only few people could afford them immediately. If you've ever had a look at radios in 3rd world countries this becomes clear: People still use receivers from the 60ies and 70ies, they repair them again and again. It would take much time until all receivers are exchanged.

In the rich world people could afford new receivers. But only a few would buy them. The big majority is not so interested in radio and satisfied with FM. International radio does not attract people so much. This is why people wouldn't spend too much extra money on DRM. The only chance is that DRM technique must becomes as cheap as normal AM. Then the industry would start building FM/DRM radios as they now produce FM/AM combinations. In the best case, the DRM community would grow slowly. So it would again take many years.

Is a co-existence of DRM and AM possible?

Yes. But not on the same frequency. As mentioned earlier, DRM causes hard intereference on AM and vice versa. Both modes should get their own frequency segments.

Wide-band AM could help immediately

The existing AM standard could be boosted up and provide half-hifi-sound. Wideband-AM would be up- and downward compatible with all existing receivers. Right now, the official audio bandwidth is 4.5 kHz. This is not much better than telephone sound. You can't really enjoy music like that. A few stations already use more than the allowed - some values can be found in the list. It's interestin: in those countries with a vivid AM scene, many stations use 6 KHz (UK/Netherlands) or even up to 10 KHz (many US stations). This is better than DRM, and it's not compressed. Still not FM like but close to it. About 8k is the maximum possible in Europe because AM carriers are covering the band in 9k-steps. Wideband can be easily be introduced in the daytime when the AM band (medium wave) is relatively empty, only occupied by local stations. Most receivers would play the 8 khz sound already now, because they only use cheap filters.
Maybe the bandwidth should be increased in the night, too. In the night there is only the problem that interefering neighbouring channels could interfere with each other. Strong stations would push down the weaker and distant ones, but most people are more interested in their local station with a clear signal anyway.

Wide-band AM doesn't require more frequency resources

If stations suppress one sideband, wide-band AM wouldn't even occupy more frequency bandwidth and would not produce additional interference in the night. There is one disadvantage: Sideband suppression could lead to a little distrtion. The solution could be in suppressing only parts of the sideband: reduced power or a limited frequency range. This would have a similar effect as if you tune a receiver one or two KHz from the center frequency to catch up more treble. Anyway, synchronous AM-detectors would solve the problem.

Synchronous AM (S-AM)

The distortion problem of AM could be minimized with a cheap technique: At the moment signals get distorted as soon as the carrier is phased out for a while. But it's easily possible to amplify this carrier in the receiver. This technique is called synchronous AM and should be implemented in more AM receivers.

Intelligent DSP for AM

With digital sound processing you can improve sound quality of AM signals much further. Chip power can not only demodulate DRM but also boost up AM signals. It could adjust frequency phasing, notch out beeping carriers, simulate stereo and treble and so on. And as long as it's AM it would still be downward compatible. New stations could immediately reach many potential listeners.

The character of the medium wave

But which stations should be on medium wave at all? Coveage completely changes from day to night. During light hours, it's a local band. In the night signals reach out a few 1000 km. Unfortunately, the frequency coordination in Europe seems to ignore this unconvenient fact. In most cases transmitters are allowed to broadcast 24 hours with power only a little reduced at night. The result: in the day MW is relatively empty, at night an interference chaos rules. So what can be improved? It would be a bad idea to create a MW scenario with different stations at day and night I think. No comercial station could afford parttime presence. Think of the northern regions: it's dark during 6-8 am prime time in the winter time, in the summer the sun is already high in the sky. The better concept would be 24h with one nationwide program on one frequency and a flexible usage of transmitters. With some intelligence the unconvienient propagation characteristics could be turned into an advantage.

Build up more single frequency networks (SFN)

There are networks which use up to 20 frequencies for the same programme. This has historic reasons but today it leads into chaos. In the old days it was difficult to modulate transmitters simultaneously. As soon as you received two transmitters of one program, there would be a terrible echo. Today with satellite feeds, SFNs are no problem anymore. You might argue that SFNs sound bad as soon as you receive more than one transmitter at once. This results in frequency shaping and stronger distortion (without S-AM). But it's still better than the existing mix of interfering signals. 80% of the channels can't be used at the moment when it's dark. Listeners appreciate to know their station in one and the same place. Transmitters should be launched close to every big cities. Single or double-frequency networks with small-sized transmitters close to the cities would reach many people. A small number of big transmitters would back up the signal in the countryside.

International coordination only for strong transmitters

Unfortunately it is almost impossible in many countries to find new MW frequencies for new stations. International coordination is a big hurdle to take. But is international coordination really neccessary for local daytime stations on medium wave? Transmitters of let's say 50 kw(day) and 5kW(night) don't reach much further than 100 km in the day (with simple radios). With a simplified coordination procedure, many wide-band-AM stations could be launced around the metropolitan areas.

Foreign services should transmit from the target country

The principle that foreign services use transmitters in their own country seems to be old-fashioned nowadays. This is a more or less a relict of the cold war when stations weren't allowed to transmit their propaganda from the other block. Today with freedom of speech this should be posible. Something like World Radio Network on satellite could work on medium wave, too. Here you hear different international services every hour like Bulgaria at 20UTC, China at 21UTC, Radio France at 22UTC (just an example, real times are different). You could even build up national networks for such a service.

Conclusion

So what we need is a "Zerobase" (re coordination from scratch) for the medium wave with wide-band AM. The potential of good old amplitude modulation is big and has not been discussed enough yet. Clear channels without interference and a better sound quality are the keys. DRM could also be worth a try but only on exclusive channels, best with 20 KHz bandwidth. But AM is just easier to handle.

Comments

Jacques (ON5MJ) wrote:
You suggest on your web site to enlarge the bandwith of AM stations. This is not that simple to do as increasing the audio bandwidth because at the same time the power must be reduced in order to respect the maximum power dissipation in the final amplifier of the station. It is well known that SSB permits 12 times more power efficiency than AM due to one side band and carrier suppression. Nevertheless one can imagine a tradeoff in using a single side band modulation WITH the carrier remaining (because usual AM receiver don't have a BFO in AM). This would be compatible with the present AM usual receivers and be more powerfull than simply increasing the audio bandwidth.

Questions? Corrections? Ideas? Comments?
Mail me: PeerAxel@aol.com

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