Delusional DAB arguments page
Brian Butterworth published on UK Free TV
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Saturday, 26 February 2011
The DAB situation that we are lumbered with in the UK and Norway (Who I believe also use the same DAB standard as the UK)is nothing short of a joke! To be told by polititians that it's all part of a Digital Ecconomy Act, is just laughable indeed for many reasons.
The first point I'd like to make is that DAB uses band 3; a terrible band to use for domestic broadcasting for many reasons mainly due to it's poor propagation and lack of field stregnth per metre; i.e: the ammount of voltage developed accross a 1 metre length of antenna rod in free space. This is measured in microVolts per metre, so for a frequency in band 3 of 200 MHz with a received field strength devloped across such an antenna of say, 50 microVolts per metre, the same antenna in band 2(FM)at 100 MHZ, would read 100 microVots, a 6 dB increase in level. Therefore to acheive the same punch as an FM band 1 transmission a DAB transmitter needs to put out a carrier wave of 4 times the effective radiated power to acheive the same level of building penetration. This is why DAB does not work at a moderate distance from a transmitter.
This is why we have people moaning that their brand new shiny DAB set only works if the set has been placed in a special spot on a window ledge, or with the antenna hangining out of a window of a tall building.
So to counteract this lack of field strength what do the idiots at OFCOM in charge of spectrum management do?... Yep!... they sanction the building of lots and lots of new DAB transmitter sites with even more power demand on the National Grid than the entire FM network of transmitters in the UK.
So, Digital Ecconomy Act, me arse! Oh, and then there's the power hungry DAB radios and the chargers that are needed to charge them up for portable use... Ever tried to operate a DAB radio on a battery of cells for more than 4 hours without them going almost flat anyone??! As I said before, digital radio, fine, but let's develop a worldwide compatible system that works and works well. The American IBOC digital system is on the right lines as it's backwards compatible with FM transmitter sites, using a digital sub carrier alongside of the FM carrier. Although IBOC is not perfect, it could be developed more and then the problem of poor propagation is then negated. After all, AM radio is a good univerally adopted broadcast medium and before anyone shoots me down in flames here, the sound quality on AM is restricted to a 4.5 KHz audio bandwidth in region 1 Europe by the broadcast regulators, not the broadcasters. A good quality AM signal with a 15 KHz bandwidth and using a properly designed wideband AM receiver sounds awsome! (As good as FM) Remember just how good the audio on Radio Caroline sounded with it's 50 KW transmitter and 10 KHz plus wideband transmissions using an Optimod AM processor.
If the RF spectrum was set up by broadcast engineers only and not politicians with their advizors then we would have more room to spread out and operate stations without all of the technical restrictions of reduced bandwith, too much digital compression and a money grabbing corporate industry who are only hell bent on making lots of cash, selling advertizing and pleasing their shareholders at the expense of higher technical standards for the listeners.
One final poit that also worries me about DAB is that it's a totalitarian medium... Think about it, right, if you don't get on to a multiplex, then your station can't broadcast; but with FM the ball game is in your court. In other words DAB is an exclusive club, right... in other words: "You pay us whatever we say or else we take you of the air"... Now that's hardly demorcratic is it?
It's not easy to broadcast as an independent operator on DAB it's either you join a multiplex or you have no future on the airwaves if they close down FM and AM as well. Maybe that's the Government's plan for even more ways in which to dumb down an already poor broadcasting system in the UK.
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Actually Brianist I think you are wrong about MP3. Freeview uses MP2 audio and satellite uses MP2 or AC3 (stereo or 5.1). Freeview HD uses AAC (stereo or 5.1).
Actuall modern FM recievers handle mutipath and doppler rather well as is demostrated by many in car radios. As for premphasis this is nothing compared to the distortion added by the BBC using optimod signal processing. This processing is designed to disguise the artifacts produced by low bit rate MP2.
As I am sure you know DAB only has one level of error correction and so is much more vunerable to interference than FM.
Actually Steve I would favour DRM+ at the moment. This can be fitted on Band 2 between the existing FM stations. You are correct in about multiplex owners . They were the ones that lobbied Ofcom for lower bit rates. They could make more money from a larger number of low bit rate stations.
The BBC has been doing research into DRM+ but has been very quiet about it. Their papers always seen to sujest that it will be several years for DRM+ to be viable. This is just not true.
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Brian, hate to dissagree with you about FM, but all RF signals have multipath reception, analog broadcasts show this up as, phasing due to skywave propagation on the long, medium and shortwave bands using AM and is I agree a very annoying disturbance to long range AM broadcasts. However, with FM there is the capture effect, which mostly cancells out any form of multipath reception reaching the receiver, why?... Well a multipath signal can never be stronger than the original carrier wave, it would have to be a very very weak distant FM signal to produce any noticeable audio phasing effects on a receiver, even then this would be very hard to achieve. Static is also a problem associated with AM and not FM; FM has a good rejection of static due to the limiting in the discriminator/demodulator circuits - even during a fierce electrical storm, FM will hardly be affected by lightning strikes nearby. DAB on the other hand will break up into a garbled mess of gobbledegook when lightening strikes nearby. DAB with it's poor forward error correction can't handle severe variable multtipath reception either. The DAB lobby will say that it can, but in the real world it doesn't. I live in Southend-on-sea, Essex. my local DAB site at the Maitland House TX site puts out just over 1 kWatts on it's multiplexs, but at my location from the site 2.5 miles away, the signal is affected terribly by effects from nearby evergreen trees, wet roof tiles and aircraft using Southend Airport. I have an Antiferrence 3 element yagi on a 20 foot mast on top of my roof some 25 feet above ordanance datum, 28 metres above sea level. With this set up the reception should be astounding, but no, it's just like any other DAB reception available on either expensive or cheap receivers, very variable, upset by nearby conditions, etc., etc.. With my FM reception using a 2 element vertical yagi I can receive local and distant stations as far away as the south coast and into Norfolk and Oxfordshire, without any problems, except for an increase in noise when listening in stereo.
Now, if the broadcasters were to use the AAC digital system with 320 kbps on DAB as well as their internet streams then maybe the change to DAB would be an option. But using an outdated system like we have in the UK is totally insane. It's almost like saying 405 line (377 visible lines) picture quality is better than the 1125 line (1080 visible lines) standard HD picture that we have nowadays. There almost was a conversion to 405 line color TV in the UK in the mid 1960s, but luckily they saw the light in the UK then and adopted the German developed PAL system. By the way, why should DAB be the norm for portable everyday radio broadcasting anyway - If the BBC can produce a higher quality sound for internet users, then why not use it over the airwaves as well? After all the BBC are funding this from the license fee aren't they? Not everyone wants to use a PC or other portable device to receive streamed audio over the internet; radio should be for all to receive equally well and in hi-fi.
DAB is not the way forward, just ask the majority of people who end up returning their radios to the stores that they have purchased them from. In my current employment I am also involved with selling all forms of audio technology and I can honestly say that a good 70% of customers have returned their DAB radios with reception issues. As for in car reception, well just don't go there, if your experiences of receiving non-stop un-interupted DAB in a car are good, then I would be supprised, if not shocked by anyone saying that it works without fail. Analog fades gracefully at great range from a TX site, but DAB just stops dead or gets totally corrupted. Just try and receive the national BBC multiplexes in Southend without any problems and you'll see what I mean by my comments. Oh, and what is wrong with using an anlog audio processor anyway? As a designer of such units myself, I find the way some folks condem them to be rather patronizing indeed. Properly set up processors sound great on pop music and rock music formats.
If you've ever set up a VHF FM or medium wave AM transmitter and acheived a great sound on air then you'll know how much pride us terchnicians try to get the best out of any broadcast medium. You do have to impose some form of gain riding AGC platform at least to smooth out studio operator gain errors and to provide a consistent level to the modulators. Too much audio will hit the big "6" on a peak program meter and that then starts to hit the saftey limiter guard band. If limiters and compressors were not used on radio and TV broadcasting the effets would be disaterous, not only for the listeners but the the transmitters and other spectrum users as well. And by the way, audio compressors are most certainly used on DAB, I actually own such a device that works on FM; AM; DAB; IBOC and DRM, plus one that I am currently designing as well. The fact is that the saftey limiter ratio is even tighter on DAB as any overshoot would totally destroy the audio quality on a digital signal. It's a very rare thing to find a broadcaster that doesn't use a form of audio processing. Radios 3 and 4 have softer processing than most other networks, but they do employ tighter and more punchy processing at drivetimes each day.
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Sunday, 27 February 2011
By the way I realise you probably consider me to be a tedious DAB hater but I find no merits to the path the government 9not this one but a previous one) has put us on for this.
Thanks for the site for other areas but I am afraid you will not convince me on DAB.
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No Brian, with respect Sir I'm not delusional at all. Having spent since the end of 1969 in electronics as a radio, and television engineer, antenna engineer and a lot of that time on various broadcast stations, including offshore broadcasters, I think that I know a fair bit about the subject of RF propagation. Maybe (and please don't take this the wrong way) it is yourself that can't handle the fact that DAB is a pointless waste of money and time.
Also when an argument is not going your way don't be as quick to use words such as "delusional" at fellow contributors on here to what is after all an intelligent and interesting debate. I for one would not insult anyones intteligence by calling them "delusional" so why should you?
Multipath reception is not a major problem on FM due to the way in which the system works. I could write pages about why, but it would not serve any purpose at all. AM radio suffers multipath due to the way in which the signal is delayed by skywaving, a problem on the HF bands and the medium and long wave bands at night.
The skywave effect is a problem as you know because an amplitude modulated signal has to be in precise phase relationship all of the time to receive a correctly demodulated signal, any delays in reception caused by skywave reception causes the delayed signal adding or subtracting from the ground wave signal causing massive iregularities in the carrir and its sidebands and the result is distorted phasey audio. The caputure effect on FM will negate the effect of phase cancelleation/addition distortion on FM and even a phase modulated carrier wave as well. The effects of mobile flutter on FM are I'm sure what you are confusing multipath reception with. The mobile flutter effect whilst on the move in a vehicle is caused by local obstructions that momentarily block and absorb (Attenuate) the line of sight path of the signal as you pass by them.
This is what makes me laugh at the so called "experts" that write these reports that have no experience in the real world of what they are talking about. They base their arguments on biased opinions to prove whatever they want in favor of the answer they are looking for.
I mentioned that multipath reception does exist on FM, but it also exists on all other forms of broadcasting using a freely radiating antenna, it can't be helped, it's just one of those things that happens. However, the effects of the phase changes that happen with multipath reception on FM are almost all cancelled out by the way in which the discriminator works in the receiver. It demodulates changes in the carrier frequency and not in amplitude. There are small ammounts of amplitude detection in any discrimator, but this is usually below the limiting threshold and would only tend to show up with a very very weak signal, from a great distance, i.e: way below line of sight or during a troposheric lift.
As most listeners will be listening to signals with field strengths way above 50 dB/uV or even more, then any localized multipath would be not a problem. If we all had to listen to AM instead of FM on band 2 then yes, multipath would cause some serious phase distortion with reception. With DAB when the error correction gives up due to bad phase errors caused by multipath reception, the effects are very noticeable indeed!
Having had one of my HF drivers (tweeter) blow out on my KEF speakers here at home was not funny at all when a passing plane caused such a glitch in reception due to multipath on my BBC Radio 7 DAB reception that the resultant spike in the audio took out the HF drive unit. Anyone who has experienced a DAB period of garbled reception knows what I mean.
I think that DAB has been ill conceived, using yesterdays technology, which was probably OK if only a handfull of broadcasters used a multiplex, had a sensible bit rate of lets say 320 or 440 kbps and a high sampling frequency rate, a better forward error correction, had enough field strength and that it offered a trully hi-fi choice for the listener. Then I believe that DAB would be a real success indeed and not a one trick pony that it is at the moment. After all, the public can be fooled about DAB for some of the time, but not all of the time, remember the Ford Edsell car anyone?!
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Monday, 14 March 2011
Friday, 16 September 2011
The digital radio switchover has been put back, and the powers that be say people are not taking up digital radio nearly as fast as expected. I think the reason for the low take up is that coverage is so bad in most out of town areas. I live in Castleford West Yorkshire, and get variable levels of reception in different rooms of my house. People want to get a radio, tune the various station groups, and listen to the radio. But that not the case, is in many areas digital signals are weak to the point that the receiver much be really carefully placed in order to get a signal. Listeners are used to turning on an FM radio and listening to their favourite station, they don't want all the messing about in order to get the digital set to work!
Digital radio is great for all ages. It can provide music and programmes of every type. But the public won't take to it if it doesn't work as well as the radio they already have. People don't want to go back to the 1960s/70s, with radio antennas fitted to almost every chimney stack!
My point? Increase the power and the take up will be massive, carry on as we are and normal people won't be interested.
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Thursday, 16 February 2012
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