Advantages of Digital Radio
Ben - If you're not currently listening online, then you could be using one of three different ways to receive your radio. We've already heard about AM and FM, but now, there's also DAB - that's Digital Audio Broadcast. The BBC first started broadcasting digital radio back in 1995 and now, most of their output can be found on a DAB radio. But, if FM was working, then why go digital? Still with us is John Adamson, BBC Production Editor for London and the Southeast, and as we said earlier, 'the Digital Doctor'. So John, why did we want to go digital?
John - I suppose it was a combination of quality and quantity. So, it enhances listeners choice and offers better quality, particularly in marginal signal conditions.
If we go back to AM just very briefly - medium wave and long wave, there's another radio station every 9 kilohertz apart. So, the audio bandwidth is only 4.5 kilohertz, barely better than telephone quality, not very good at all. Also, a finite number of frequencies that cannot be re-used very often because their transmissions spill over a very wide area, they can't be very well targeted. In the case of long wave, there's only 15 channels for the whole of Europe, so you can imagine it's quite difficult to manage that frequency of the spectrum effectively. Medium wave has rather more, but it's still four and a half kilohertz audio bandwidth to avoid interference to adjacent stations. Aerials are rather large. Not a lot of choice there really. Also, medium wave has a problem of after dark interference; Strange noises, the background continental interference, whistles, whines, and buzzes in the background. FM made things a lot better of course because we got stereo as we said earlier and more immunity to electrical interference. Also, because it was on the VHF band, around about 3 metres wavelength, aerials were a sensible size, so you could target your audience quite effectively. So, in the case of BBC Radio Cambridgeshire, they've got a transmitter at Madingley that's targeted just for the environment around there.
Why do we go to DAB? Well, FM still had a problem in marginal areas where there's something called multipath interference. I don't know if you've noticed this as you're driving along or using a portable radio, but sometimes you get all sorts hisses and burbles, and squeaks and things. That's multipath interference and that's a function of analogue radio - that if you get a wanted signal and a reflected signal which inherently must be delayed with respect to the original, it will interfere.
If you think of analogue television, which was shut down just last year, you may remember seeing ghosting on the picture. That is the visual equivalent of the interference you would get on audio.
So, the reflected signal always causes a problem to the main. DAB offered something rather new and that was, that because of the modulation system used and the coding systems used, you could have a number of transmitters, all working on the same frequency.
So, for example, the BBC national digital multiplex operates on one frequency right around the country. All the transmitters work together and although some of them will be delayed with respect to others, if there are in a certain window of timing some delay, they will add, and the listener won't get any interference.
High quality audio is possible as well. Thanks to digital audio encoding, mpeg compression, that allowed us to squeeze more audio into a given bit of spectrum. If you were using linear digital audio, you would need around about 1.5 megabits or 1.5 megahertz worth of space for one stereo audio channel. That's not very efficient. But with mpeg, this allowed us to reduce that to say, 256 kilobits, and that was alleged to give around about CD quality. So, in the early days, what we were thinking of with DAB was, we'd have more channels, but not many more, but high quality, and a more rugged, reliable, transmission system
Ben - Lets come back to the audio compression in a minute, because I think we need to point out that that's different from the type of compression we were talking about before. When we were talking about amplitude modulation and frequency modulation, that's essentially building a model of the sound of that pressure wave in the modulation. But with digital radio, you're not doing that. It's not a direct model of the sound. It's differently encoded.
John - Yes, it's all digitally sampled with a lot of ones and zeros effectively, and all gets assembled back in the decoder again. So, if you were to listen to DAB radio spectrum on an analogue set, all you'd hear is a lot of random noise.
Ben - And how does the signal change with distance? I have a digital radio at home, and it seems to be, ironically, digital. It's either perfect and sounds beautiful or the signal is dreadful. Whereas with an FM radio, I can tune it in, and it sort of seems to fade out with distance, the quality fades out. Is digital always as good as it's going to be, or not present?
John - The theory is that you either get a perfect signal or you're getting no signal at all, but in reality, there's that grey area where the signal isn't quite strong enough for your set decoder to make sense of. So, what you get is this characteristic sound which has been described as "bubbling mud". The difficulty with that is that it's a really irritating sound. With FM, you just get more and more noise. The signal gets weaker or you might get these burbles that I mentioned to you earlier, and squeak sounds with multipath distortion. But with the digital burbling mud sound, it's really quite difficult to follow the programme. So, it's not perfect, that is true. Because of the modulation and coding system that is used, it's generally better for mobile and portable reception. So if you're driving along in your car, it should be more rugged than an FM equivalent. And of course, you're on the same frequency, so you're not having to retune every few miles as you find a different transmitter.