IAN LANG ELECTRONICS
DAB is Digital Audio Broadcasting and it is the favoured choice of HM Government in the UK for the future of terrestrial radio broadcasting. Why is it HM Government's preferred choice? I'd like to say it's because they've done a thorough, competent technical analysis and considered all the practicalities of the issues involved. And indeed a thorough, competent technical analysis considering all the practicalities of the issues involved has been done. By the BBC.
The Government wants it after something along the lines of the following conversation with experts:
Expert : "If we set up a national chain of DAB transmitters, and persuade the public to buy DAB
receivers we can put all the existing national and local stations plus a few more into a much
smaller band of frequencies."
Minister: "Can we really? Oh. Jolly Good."
E: " This means of course that the airwaves currently doing FM and AM transmissions will eventually
M: "Redundancy? Isn't that a bad thing? Don't want that do we?"
E: "Er....well, Minister, actually in this case it's a good thing, because these bands will now be empty."
M: "Not following this at all old boy."
E: "Well, Minister, put simply, the Government can sell them off."
M: "Flog 'em, eh? To whom?"
E: "Anybody that needs extra frequency space Minister. Mobile communications perhaps?"
M: "How much cash will it make?"
E:"I don't know offhand Minister, but last time we sold redundant frequency space it netted billions."
M: (chokes for a while) "Billions??? Do you mean to say you do all this work and the Govt gets a big bag
E: "Yes, Minister."
And so it became.
DAB was actually born out of Eureka Project 147, which was a collaborative effort involving members of the European Broadcasting Union to ETS standards. They didn't tell the Minister that bit.
It works by using a system of multiplexes. A multiplex can take several channels and combine them, broadcasting them on one frequency. At the receiver end the frequency is de-multiplexed and the audio signal is extracted and then amplified and output just like any other system. There are several advantages to this. Because all the signals from anywhere arrive at the same time and in the same phase, you do not get fading as you do when the same analogue signal arrives from two different locations at a different phase. You don't get flat fading either, which is what happens when the signal to noise ratio is too far to noise and you can't get a satisfactory demodulation.
The drawback is it's digital. Either you get a good signal, or you don't get a signal at all. This means that if you can't receive a certain multiplex, you can't receive any channel on that multiplex. For example:
Imagine where you live that you can't receive multiplex 12B. This is the BBC national multiplex. This means you can't get Radio 1. (HUZZZAAHHH!!!).
But- You can't get Radio 2 either. (Meh. It's worth it not to get Radio 1).
Nor can you get Radio 3 or 4. That's a bit more serious. I'm afraid there's no way to get 2, 3 and 4 without also being able to get Radio 1. I'm sorry, but that's how it is. Chin up.
So, before we delve any deeper into the theoretical concerns of the matter, let's have a look at the tramsmission chain. It goes like this:
Now, this is a drawing done by the BBC's Research and Development arm. You'll notice that they consider it to be simplified. That's because they're clever sciency types, and we, the techies, look at this and go "whaaa....?"
Fear not for an English translation is forthcoming. Let's start with the transmission system, at the little circle before the ADC, top left hand corner. That circle represents the audio output from the studio. if it's Radio 3 it'll be Rupert Nancypants blathering on and then playing a bit of some composer you've probably never heard of. If it's Radio 1 it'll be some clicks, whirrs and pops masquerading as music or possibly Cheryl-Bloody-Cole and then some "geezer" will come on and say "innit". And possibly "Bangin' tune!" or "Wicked!" followed by an incomprehensible stream of gobbledegook culminating in the word "innit". (Are you getting the impression yet that I don't like Radio 1 very much? Innit.)
Well, whatever the audio from the studio might be, it goes to the ADC. (Analogue to Digital Convertor). What happens to it there is described by the BBC thus:
Audio programme signals are digitised and multiplexed together with ancillary data
to produce an ‘un-coded’ bit-stream.
For those of us that speak English, what that means is several different outputs are converted to a digital format. The different digital formats are then mixed together into one big stream of data by each ADC in turn (only one is shown in the diagram) passing, at an interval of agreed time which is in fact 1ms, the digital data it currently stores to the multiplexer (MUX). All the data corresponds to audio frequencies set as an agreed code. because the agreed time is 1ms, however many audio inputs to MUX you have is also the cycle time for multiplexing. If you have two, the datastream will contain a bit of information for two channels, and each channel will have a sampling rate of 500 bits per second. A bit of data in this case is not necessarily the same as a computer bit, and it is given the term symbol. The maximum symbol duration is 1ms.
So, in plain English, what happens is this: all the audio inputs are converted to digital codes and then mixed together in an organised time-spread of X milliseconds. As the maximum duration is 1ms, the frequency of hopping about between different outputs is 1kHz.
The next things the BBC says are:
The bit-stream is then encoded for forward error protection by adding redundant bits
with appropriate, calculated values.
During each consecutive 1 ms symbol, the ‘coded’ bits are divided into 1536 pairs,
and each pair is differentially encoded with respect to its counterpart for the previous
Oh. Good. That's nice to know. What this means is that the same bit of data gets done twice, and each one should be the same as the other; if it isn't an error occurs. Why 1536 pairs? Because the chosen bandwidth is 1537 MHz and dividing that by 1 kHz gives 1537. One pair is used for overheads that have nothing to do with the audio input. Some more are actually used to carry data, which allows the station and song title to be displayed on a screen at the receiver, as well as start and stops.
Differential encoding- now here's a fun topic. What you do is to pass the signal into a series of oscillators at very slightly different frequencies. You then send these to an adder. The resulting output looks rather like an AM waveform but consists of digital values spaced regularly apart in time. The inverse Fast Fourrier Transform synthesises this signal as a time domain form. All that's left to do is to reconvert to an analogue form at the broadcast frequency and chuck it out through an amplifier and antenna.
Speaking of which, what are the broadcast frequencies? In Europe they start at Multiplex 5A, which is 174. 928 MHz, and go on to 230MHz. In the United Kingdom, only the top end of this spectrum is used, and one block right at the bottom
So, one frequency carries umpteen channels. On the BBC MUX 12B you get:
Radio 1, Radio2, Radio3, Radio4 (FM version) , Radio 5 Live, BBC 6 Music, Radio4 Extra, Radio 1 Xtra (note the spelling......innit!) BBC Asian Network and BBC World Service. There's two channels that broadcast data too. BBC EPG, the electronic programming guide, and BBC Travel, which is the traffic news. That's ten receivable channels on one frequency.
It follows that the more channels you put on one frequency the less the time each channel has to input to the MUX at the beginning of the transmission chain. The less time they have, the less bits you can put through per second. The less bits per second, the less quality of the output at the receiver end. A clever way round is to have channels inputting bits at different rates. For instance, under normal circumstances, Radio 1 will input 128kbits per second and so will Radio 2. Radio 3, which broadcasts classical music and is thus broadcasting a more dynamic range of audio, does between 160 and 192 kbits/s. Radio 4, which is mainly speech, can go down as low as 80 kbits and as high as 128, Radio 5 does 64 to 80 and the Asian Network and World Service are at 64 bits/s.
In addition, you can lower the bit rates accordingly to accomodate extra broadcasts. The BBC does this for sporting events; It does 5 Live Sports Extra when there's a major sporting event on and some Radio 4 LW transmissions are relayed. It lowers the bit rate on Radio 1, 2, and 3 to compensate (this is called dynamic multiplexing).
It used to lower the bit-rate on Radio 3 to 128 kbit/s. It was awful. Now when grubby oiks like me complain about it, the BBC says "Pah! depart you peasant! We will not be told our business by the likes of you!" but unfortunately for them, not only is Radio 3 listened to by grubby oiks like me but also by people with names like Lady Cecilia-Arabella Montfort-Golightly or Lord Roger de Knightly, and when they complain they do not write letters which are read by the temp in PR. When they complain, they go down the Opera House, grab the Chairman of the BBC and say "You know, old chap, I don't know what you did to Radio 3 this morning but don't do it again or I'll have a word with the PM about the licence fee. Care for a glass of Champers?"
The Chairman of the BBC then scampers back the following morning and hauls the unfortunate engineers over the carpet. Thus proving the old adage: It's never management's fault. They just told you to do it. You actually did it. Ergo the blame lies with you.
64 kbit/s is the absolute minimum you can do and still have a coherent voice transmission. If you try to put music across that, it'll be audible but awful. In fact to achieve the quality of FM you need really to be putting 256 kbit/s in. This would drastically reduce the availability of channels though. For this reason, although DAB is satisfactory, it does not output the quality of a good FM receiver.
Let's look at Digital One. You get: Smooth, Smooth 70s, Absolute, Absolute 80s,90s and Extra,Talksport, Classic FM, Planet Rock, BFBS, Amazing, Premier, UCB and Jazz FM. Fourteen of them. That's because they are a commercial organisation and need to make a profit. If they put any more on there, they'll have to take bit rates down somewhere. So, where once we were constrained by available bandwith in the spectrum, now we're constrained by available bit rates in the multiplex. Nothing's perfect.
You may have gathered that DAB is a very complex technology at the transmission side. It's equally complex at reception. Basically, you have to receive the wave, then work the transmission process backwards.
All of this takes time. The lag can be seconds behind.
What is the coverage? Well, Northern Ireland can't get Digital One at all. UK coverage is thought to be 92 per cent for commercials, and a nominal 100 per cent for the BBC though there are still remote spots where coverage is patchy or non-existent. In my rural location there are many hills and trees; digital radio is achievable upstairs but not down. Nobody knows why. I've put it down to occult practices at work.
Ian Lang April 2012