John Adamson, BBC
Ben - If you're not currently listening online, then you could be using one of three different ways to receive your radio. Weíve already heard about AM and FM, but now, there's also DAB Ė 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, Ďthe Digital Doctorí. 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 Ė 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 canít be very well targeted. In the case of long wave, there's only 15 channels for the whole of Europe, so you can imagine itís 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, theyíve 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 donít 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 wonít 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, weíd 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. Itís not a direct model of the sound. Itís differently encoded.
John - Yes, itís 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 youíd 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. Itís 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 itís 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 isnít 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, itís really quite difficult to follow the programme. So, itís not perfect, that is true. Because of the modulation and coding system that is used, itís 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.
The core reason for digital radio and digital TV broadcasting is not really for the consumers benefit, it is to save an incredible amount of money in transmission costs, it probably costs over 100 times less in watts power to transmit in digital compared to frequency modulation and vhf. Have you noticed the picture quality?, has it taken your breath away?, I think not, and if you get a poor signal your immage is de-pixelated :) confusious says, Tue, 7th May 2013
Frankly I couldn't disagree more with Confucius/Confusious - digital hdtv channels are strikingly better, and going back to normal channels is such a disappointment. On the radio front I am unsure if I could tell the difference as I have no direct comparisons (ie all the radios in my house went from analogue to digital as soon as I found out about BBC radio 6 music). But the fact that the beeb were willing to push out a host of new stations on digital only is enough for me to be a great fan. imatfaal, Wed, 15th May 2013
Digital is neither inherently better or worse. But it allows extensive compression, and that's mostly a good thing, you can get better resolution with lower bandwidth, and the artifacts from the compression are largely not noticeable. Reduced bandwidth means many more channels from the same radio spectrum.
The PAL system produced rather poor results when it was first introduced in 1964 but by 2010 when digital processing of the decoding had taken over the results were almost indistinguishable from 720p which surprised me when I purchased a modern TV. I would add I have a clear view of a transmitter 10 miles away so noise or multi path is not problem a problem.
I agree with Imatfaal; digital TV provides a much better service - more channels (of course that does not always translate to better programs), better quality, TVs which can accept broadcast signal inputs or WiFi so you can see high quality downloaded or streamed video. The broadcast cost in the power used is not a significant cost compared with everything else. The advantage is in better usage of available bandwidth, of which there is much demand now.
With FM you usually have to hold the aerial all the time to stop the thing farting, and MW now has a terrible buzz on it from those weird things that have replaced light bulbs. Digital radios cure both those problems (though they can still go a bit garbled if the signal's weak or being blocked by too much of the building you're in). The worst problem until recently was the battery consumption, but you can now get tiny digital radios that run for 8 hours on a single rechargeable AAA battery for £25 (I've been using one to try it out), so we've got to the point where it's well worth dipping a toe in the water, and once you've done that, you'll probably want to jump right in.
Digital is good, except for poor signal areas and in areas of multipath reception, where it will work and then stop. Analogue goes noisy then eventually drops to noise. The push for digital is only driven by money, the transmitter costs a lot to run, and the power bill is not a small one, you have power outputs for each station of upwards of 10kW for a tiny local station to 500kW for a large broadcaster. That power is at the antenna, typically at the meter the input is doubled, so a transmitter site with multiple stations broadcast from it might use 1MW of power or more, 27/7/365, with the need to have generation capacity in case of supply failure of that amount of power, and a fuel tank that will keep it running for a week or more. Then consider that the broadcasters pay for each channel to government, and that is not a small cost, it is a massive bill, and if you want an extra channel you pay again.
I've noticed with digital TV coverage of formula 1 and cycle racing that the road can be very blocky due to the compression and it's extremely offputting if you look at it, though it also depends on how compressed the channel is - the compression on ITV4 is particularly bad. The same applies to ripples on water, smoke and dust in the air where it all turns into ugly flickering squares if the compression is too great, but on some channels which aren't too compressed you only see these defects if footage has been recorded on cameras that do too much compression.
I'm a broadcasting engineer and back in the early 90's when DVB was first being developed the 38Mbit MUX that fits into the old 5.5Mhz TV channel space was going to carry "..no more than two TV stations and maybe four radios" - but, the accountants got involved and now it's not uncommon for transport streams within each multiplex to be down around 2 Mbit/s. The BBC keep the quality higher than most - BBC1 is often averaging 4Mbits/sec on Freeview (the SD version) but some of the infomercial stations have such a low data rate that the interactive application ("press the red button" has a higher data rate than the pictures. Remember that uncompressed SD video is 270Mbit/sec - so at 1% of the original data load we still have watchable pictures. MPEG2 compression seems to have doubled it's performance in the last twenty years; PSNR figures show that what looks OK at 4Mbits/sec today would have needed 8Mbit/sec with a mid-90s vintage compressor.
Yes, I've never heard anyone claim that NTSC was better than PAL before either. "Never the same colour" was the acronym I use to remember it also.
PAL stands for "Phase Alternate Line" - the phase of the colour subcarrier signal varies by 180 degrees every line so that in a simple PAL decoder any phase errors (which would give a colour shift in NTSC) are visually "averaged" so that you don't notice the colour shift so much. In the case of a delay-line decoder the averaging actually de-saturates the chrominance portion of the signal which is visually more pleasing than a colour shift.