« Previous | Main | Next »

Digital Radio Mondiale - our medium-wave experiences

Post categories:

James Cridland James Cridland | 13:50 UK time, Thursday, 21 May 2009

My colleague Tom writes...

Hello. My name is Tom and I work for the part of the BBC which looks after our television and radio transmissions in the UK. For a bit more than two years, I was also looking after a project to do with digital broadcasting on medium-wave and as we've just published the final report for that, I've been asked to write a few words here to explain what we did and what we found out.

It might surprise you to know that we're still interested in technologies like medium-wave given everything else that's going on but there are good reasons for that. Whichever way you cut it, the BBC is still one of the world's largest medium-wave broadcasters and a considerable chunk of our audience still makes use of our transmissions. From an engineer's point of view, medium-wave is great because it's relatively simple and goes much further than, say, FM: it's not as blocked by trees and buildings and terrain as technologies which use higher frequencies. But it has its problems as well: it's mono, it's not very high quality, it can suffer from interference from domestic appliances. So, a decade or so ago, research engineers from around the world - including the BBC's team at Kingswood Warren - began development of a new transmission standard which would turn medium-wave digital: something which ended up being called digital radio mondiale, or DRM.

Our colleagues at the BBC World Service have been using DRM for a while now on some of their European medium-wave and short-wave transmissions but before April 2007, we in the UK-bit of the BBC (the "home" or "domestic" service) had never really had a chance to experiment with it. We'd certainly never tried running a DRM transmission as a service for an extended period of time to see what the listeners made of it.

In April 2007, we did just that. We took a frequency used by BBC Radio Devon in Plymouth, closed down the medium-wave service, and re-engineered the transmitter so that it could carry DRM. We then ran Radio Devon using this system - and over the course of the next year, we worked with a panel of listeners in Plymouth and west Devon (to whom we had given special DRM-capable radios) to understand how the technology worked for them and what their experience of it was.

The results were interesting. For the most part, the panel's reaction to DRM was positive. They enjoyed the generally improved audio quality and they took easily to their new radios. And for most of them, the reception was good; one or two glitches but normally ok.

This was borne out by our own measurements of the signal. We found that the area we were able to cover during the day was very much bigger than the area we could cover with the old AM signal that had been there previously.

But, on the downside, some of our panel experienced problems at night - and we saw these effects in our measurements as well. The problem will be familiar to many listeners to medium-wave: at night, changes in the atmosphere mean that signals from distant transmitters reach our shores more easily. On medium-wave, you might hear this as cross-talk: a foreign voice underneath what you're trying to listen to, or the occasional 'splat' of another transmission. The issue we came across with DRM is that this interference causes the radio to stop decoding the signal: sometimes only momentarily, sometimes for a while longer. So rather than listening through the interference, it's like all digital systems: you either get it (and so get it at a consistently high quality) or you don't get it at all.

So while most of the panel continued to listen without experiencing any problems, some of them found they were only served during the day and had patchy coverage at night. And of course this problem became worse as the hours of daylight shrunk during the autumn; so by winter, some were beginning to experience poor reception in the late afternoon.

This problem can be solved - but it would require us to re-plan our transmission network: either by moving the frequencies around so that we use ones that aren't quite so damaged by interference, or by building higher power transmitters, or by simply building more of them. (One of the nice things about DRM - which we also proved in the trial - is that you can run two transmitters on the same frequency without causing interference between them, so building more transmitters doesn't necessarily mean using more frequencies.)

DRM still has potential: indeed, considerable potential for international broadcasting where it remains of great interest to our colleagues at the BBC World Service and others, and it might have an application at home as well. But our trial has shown that the migration from analogue to digital at many of the frequencies which are currently allocated to the UK has its own set of challenges that would need to be addressed.

Anyway, there's much more in our final reports - because we worked so closely with the people at BBC Radio Devon, they're available over on their website. Have a read and see what you think.

Tom Everest works within BBC Distribution

Comments

  • Comment number 1.

    "it's like all digital systems: you either get it (and so get it at a consistently high quality) or you don't get it at all."

    Please tell me that you're not trying to claim that DAB or DRM deliver "high quality". DAB provides dire audio quality, and DRM is even worse!

    Anyway, what I'd like to know from James Cridland is why the BBC's live Internet radio streams are still "testing" three months after "tests" began, when performance testing should only take a couple of weeks?

    What I'd also like to know from James Cridland is why the BBC has kept the live Internet radio streams at low-quality for an additional YEAR compared to the on-demand Internet radio streams, especially considering that James Cridland said last year that he wanted to provide the live Internet radio streams at lower quality than the on-demand Internet radio streams - which he later retracted, but which has been the case for the last year anyway!

    Coincidence? I think not.

  • Comment number 2.

    Yes DRM has great potential but will we ever get it in the UK ?! If so when ? The years go by and nothing happens apart from a ickle test in the west country which did't amount to much. We need atleast a dozen DRM tests in MW scattered around the country using existing BBC local radio frequencies then maybe the media and joe bloggs will take notice

    Unfortunatly for DRM internet radio is taking over as now most of the country have wireless routers and the low cost of these small radios which give you 14,000 stations at your fingertips as apposed to half a dozen if your luckly on a DRM radio and even these are not available 24/7

    DRM and espcially DAB can sound very good. I think some just make a career of complaining and moaning

    So let's have a lot more DRM MW tests BBC and ILR, bring it on

  • Comment number 3.

    To quote from the summary report

    Without specific guidance and repeated listening, panellists rated the audio quality of DRM as comparable to FM, although when pushed through detailed exercises most rated DAB as better than DRM

    Very odd. How on earth can DRM be comparable to FM. Every study I have ever seen shows DAB to to far worst that FM and DRM at the extreamly low bit rates is very poor.

    The trial used 23 kbit/s AAC + SBR which is roughly the same quality as 46kbit/s with mpeg2 which is used by dab. The lowest bit rate that DAB uses is 64kbit/s and so the trial is of a lower sound quality than DAB. The maximum bit rate of DAB is 192kbit/s but the BBC's own research showed that it would need to be 256kbit/sec to equal FM quality. So why did the tial rate the sound quality so high. The report is also very suspicious of the results. I think that the reason the trial sound quality was rated so high is the fact the trials were not blind. To make an accurate comparison the listeners should not know which medium they are listening to when grading the sound quality. It is a pity the BBC did not do a blind trial using the same sound equipment in making the comparison.

    The BBC used a 9khz channel bandwidth which restricted the bit rate used. If DRM was to be adopted I would hope that the bandwidth would be increased to 18 or 20Khz so the 55kbit/s bit rate could be used which would give much better quality. We should also bear in mind the trade off between error correction and bit rate. One of the big failing of DAB is insufficient error correction and the BBC should not make the same mistake twice.

    I was not in the reception area so I only got to listen to recordings posted on the net. In comparison to AM the wider bandwidth of DRM was to me the most noticable benefit but the compression artifacts were very obvious especially on music. The artifacts are often described as irritations which make listening for long periods of time very irritating. For me I could only put up with listening to 23kbit/s for short periods of time.

    As drmradiofan said the internet seems to be the future of radio. I understand that car radios with internet will be available soon.

  • Comment number 4.

    The BBC is keeping its Internet stream at low quality, in order to force interest in over-the-air digital radio. Digital radio is nothing but hype, and listeners aren't falling for it. Looks like DRM is dead:

    "Death of Digital Radio Mondiale in 2008 as well?"

    "From both formal and informal discussions among participants at the HFCC, it is now clear that the proposed DRM (Digital Radio Mondiale) system, that would have converted analogue Shortwave to digital, FM like quality reception would hardly be implemented if ever on a large scale, beyond the current experimental stage."

    http://tinyurl.com/b6z5lr

    The same farce is trying to be forced on the US (HD Radio), but like all digital radio, it jams and reception is poor with dropouts and silence:

    http://hdradiofarce.blogspot.com

  • Comment number 5.

    This sounds great! But are there any DRM portable receivers on sale in the UK? Can I borrow one from the BBC to try it out?

 

Copyright © 2015 BBC. The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.