Naked Science Forum
Non Life Sciences => Technology => Topic started by: paul.fr on 10/11/2007 22:44:23
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???
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It's in way the audio is transmitted, i.e. the modulation.
"FM" stands for "frequency modulation", and the frequency of the radiowave is varied in an analog kind of way in exact proportion to the instantaneous amplitude of the analog audio signal to be transmitted. In the UK, the radio frequency is between 87.5 and 108.0 MHz, and the modulation allows the carrier to instantaneously deviate from its nominal by up to +/-75kHz, if I remember right. FM broadcasts began in the 1960's, and are nice and simple. There's some clever tricks to enable a stereo-difference signal to be transmitted alongside the basic mono signal, which is nice... but does tend to cause stereo hiss especially if the signal is a bit weak. It's pretty good, but you have to retune your radio (if on a national station) as you drive around the country (although RDS can do this automatically). Although fairly resistant to interference, you will get some disturbances to the audio. You may also get nasty distortion if you've got a strong reflected signal coming to your aerial via a different path, or "flutter" if there's a reflected signal off a moving object such as a bus or aeroplane. Receivers are fairly simple, and personal/portable sets can run for many many hours on small batteries. There is no time-lag between the audio transmitted and received, save for the limits of the speed of light (radio waves).
"DAB" stands for "Digital Audio Broadcasting", and is somewhat complicated(!) Firstly the audio is digitally compressed and coded using a technique similar (in fact a forerunner to) mp3 coding. This is a lossy codec and reduces the data rate to something like 96kbits per second to 224kbits per second if you're lucky. It's a perceptual codec and in 40ms (or so) blocks chucks away (or codes more coarsely) audio-frequencies which you're unlikely to hear due to them being masked by other sounds. The digital audio datastream from several programmes are then bundled up together in a multiplex and "digitally" modulated onto a carrier at around 220 MHz (in the UK). The digital modulation is known as COFDM (Coded Orthogonal Frequency Division Multiplexing), which uses Fourier transforms and is rather clever. Effectively what this does is to code the signal onto a few hundred sub-carrier signals whose amplitude and phase are varied according to the data to be coded. Each subcarrier has a relatively slow data rate, and only changes at the "symbol rate" (several hundered microseconds). Long and the short of it is that this modulation scheme is extremely efficient (in terms of data transfer / amount of radiofrequency spectrum - in fact very close to the theoretical maximum), and robust to multipath (signal reflection) effects. This robustness to multipath means you can actually transmit a national programme multiplex from multiple transmitters across the whole country on the same frequency simultaneously - provided all the transmitters are accurately synchronised. This is called a Single Frequency Network (SFN) and is clearly also good for making efficient use of the radio spectrum. The digital COFDM modulation usually incorporates redundancy and error-correction mechanisms, so that any modest interference can be perfectly corrected for.
Since there is a fixed data-rate in a multiplex, the broadcaster can trade off the number of stations in a multiplex against the audio quality (data-rate per programme). Many engineering people will complain that in Britain the audio is overcompressed (too low a data rate) so it looses definition and clarity. Hence the claim "DAB sounds worse than FM (with a good aerial)"... which may often be true.
DAB receivers are much more complicated, and the batteries still don't last very long in personal receivers. [:(]
Both the (mp3-like) digital audio compression and (to a lesser extent) the COFDM modulation, and their subsequent decoding, introduce significant delays in the audio path from studio to home receiver. This means that the time-signal "pips" will be late on digital receivers, and you'll get an appreciable echo if you listen to the same programme on FM and DAB simultaneously.
Almost every digital data-communication system these days seems to use COFDM, including digital terrestrial TV, WiFi, broadband ASDL modems. A Google search on COFDM will reveal lots, but you could do worse than visit Wikipedia: http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing
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Thanks for that, short :-) explination Andrew
Note: is everyone called Andrew lately?
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I luff DAB radios...BUT....find that FM on certain channels still sounds better !
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Note: is everyone called Andrew lately?
Well, I've always been called Andrew [;D]
(Except when I'm called William because officially that is my first name!)
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FM on certain channels still sounds better
Yes - it certainly does. Have you looked at the bit rate used by some of the digital channels? Reception may be free from interference but it is heavily distorted by the original coding. 'Digital' is not synonymous with 'high quality', by any means.
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FM on certain channels still sounds better
Yes - it certainly does. Have you looked at the bit rate used by some of the digital channels? Reception may be free from interference but it is heavily distorted by the original coding. 'Digital' is not synonymous with 'high quality', by any means.
Absolutely agree !!...yes..the bit rate on some digital channels is appalling.....
I have a Tivoli DAB/FM radio....when I switch between the two....it's a whole level of audio immersion when it goes from DAB to FM.
...of course...If one has a a pocket DAB radio where the speaker is so tiny...it's not so evident..but still is audibly discernible though.
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Many of the channels are only in mono - including THE JAZZ. Ridiculous.
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To have JAZZ in Mono is heresy against doctrine !!.....particularly Jazz !!
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Now I know why my new DAB just dosn't sound the same as the good ol' radio.
Mono - most channels?!? LOL - now it makes sense.
I've read techminds post probably about 10 times just now but I think I'll give up... I gather it's all rather complex and for a general non-techie like moi it's kind of hopeless.
But one thing I still would like to know - not in great detail but just plain english please if possible:
DAB radia will eventually go interactive... or so I've heard.
Now just how is that supposed to work?
be gentle to a technically challenged person
[;D]
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Now I know why my new DAB just dosn't sound the same as the good ol' radio.
Mono - most channels?!? LOL - now it makes sense.
I've read techminds post probably about 10 times just now but I think I'll give up... I gather it's all rather complex and for a general non-techie like moi it's kind of hopeless.
But one thing I still would like to know - not in great detail but just plain english please if possible:
DAB radia will eventually go interactive... or so I've heard.
Now just how is that supposed to work?
be gentle to a technically challenged person
[;D]
Think of it like this...
radio with pictures,
radio with video display
radio with the ability to communicate with the radio station via the radio itself !
radio with facility to download and keep the songs you're listening too
radio with the facility to shop
radio with games to play whilst you listen...the list goes on !
Sounds like a small version of digital TV !!
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The trouble with so-called interactive services is that, unless you have a communications channel back to the program source, it really only can involve selecting from a range of alternative sets of data / sound / pictures. That requires a lot of extra channel capacity or reduced quality in the 'wanted' signals.
That is the beauty of using the Internet as your program source. It does imply a pretty huge server capacity and consumes a great deal of energy on a national basis.
Radio broadcasting of signals is fantastic value in terms of energy consumed per viewer.
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It DOES sound like a small version of digital TV.... all coming from a radio?
See - there's why I can't quite wrap my head around it. I also see it more limited like sophiecentaur describes it, choosing from a RANGE of data/sound/picture sets .
Like he says: that's the beauty of using the internet as your program source.
Won't it be more like the internet takes over so to speak rather than 'interactive' DAB?
Yes it does imply a huge server capacity but is that not really already in place (more or less, or at least easily 'upgrade-able'?)
Energy consumed per viewer internet vs dab radio... hmmmm now *that* is an interesting concept/idea I've never thought about.
Could you elaborate?
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Energy consumed per viewer internet vs dab radio
Not a fair comparison if you take it further because 'broadcasting' will always have a limited capacity for number of programmes available and a server-based system is more flexible. But ignoring that and assuming that you need the same information capacity - bandwidth.
Take analog TV as an example; for years there was the option of cable reception in a few areas where there was really poor reception.
At Crystal Palace, there are UHF Klystron Transmitters, consuming something like 1MW of electrical power for each TV channel.
The antenna is a large 'aperture' and has a vertical radiation pattern which beams most of the radiated signal in vertical directions where it is wanted and reduces the amount of power delivered upwards. The more complex the antenna, the more efficient the coverage can be.
Several million homes / receivers can receive usable signals from this transmitting station. That's much less than one watt used for each receiver. The signal would also be available for many more receivers in the area for free.
A wired system for so many receivers over such a large area as Greater London would need a complex distribution system with repeaters and distribution amplifiers. A conservative estimate of power required to feed each receiver would be, perhaps 10W.
The efficiency of a broadcast system is, potentially, even higher when you go to satellite transmission a few kW transmitter power serving 60million people. The capital equipment is a bit pricey but Murdoch finds it worth while!
Internet connections are not free, in terms of energy - several watts per terminal, at least. DAB power consumption will be a lot lower, per channel, than for TV.
You pays yer money and you takes yer pick.
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At Crystal Palace, there are UHF Klystron Transmitters, consuming something like 1MW of electrical power for each TV channel.
The antenna is a large 'aperture' and has a radiation pattern which beams most of the radiated signal in vertical horizontal directions where it is wanted and reduces the amount of power delivered upwards or into the ground. The more complex the antenna, the more efficient the coverage can be.
Basically all true (I've corrected the accidental mistakes), but although the Effective Radiated Power ERP may be 1MW per programme, the actual transmitter power will only be high tens of kilowatts. The "ERP" is the equivalent power you'd need to put into a simple dipole antenna to get the same signal strength as in the main beam of the fancy antenna.
So broadcasting is even more efficient than you originally claimed [:)]
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Those were not accidental mistakes. The vertical radiation pattern is at least as important as HRP - particularly if you are in the middle of your service area. Main stations are invariably in the middle because it is not possible to have a large horizontal aperture array at several hundred metres up a mast - so you can't have a narrow horizontal beam. I don't know of a main station with what you would call a highly directional HRP.
VRP has to be tailored to be in a vertical range from zero in the horizontal direction to a peak about 30 degrees below. In fact, a lot of the power would be pointed 'into the ground', because that is where the receivers are (or 10m above). There aren't many receiving antennas above the horizon, looking from the top of Xtal Palace mast - just distant ones where you cause interference.
Yes the EIRP is about 1MW - but that's not really relevant to the running cost per head. What counts is the electricity bill.
The actual power input approaches 0.5MW - there are four klystrons: two for vision and two for sound . The vision klystrons are 80KW peak sync power and are run in linear (ish) mode) which is not very efficient. The Sound klystrons are under run and take a bit less.
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Those were not accidental mistakes. The vertical radiation pattern is at least as important as HRP - particularly if you are in the middle of your service area. Main stations are invariably in the middle because it is not possible to have a large horizontal aperture array at several hundred metres up a mast - so you can't have a narrow horizontal beam. I don't know of a main station with what you would call a highly directional HRP.
VRP has to be tailored to be in a vertical range from zero in the horizontal direction to a peak about 30 degrees below. In fact, a lot of the power would be pointed 'into the ground', because that is where the receivers are (or 10m above).
Ok, sorry. I claim we were both correct, but I misinterpreted your writings. I should have been less quick to offer "corrections". The energy is wanted in an essentially horizontal plane (granted with some energy heading downwards, for the benefit of those living right under the mast), but this means shaping the vertical radiation pattern.
Sorry I got my wires (or EM waves?) crossed. [8D]
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OK techmind - terminology rules, as usual.
It's quite amazing how good value broadcasting is, though. The 'wasted' power could be used for millions more receivers - unlike the wasted power lost in copper.
I taught an A level Physics option a while ago and set a problem in which the students had to calculate the losses in a cable of various lengths - a good quality, low loss cable .
Then they calculated the losses for a radio link .
Plotting the two sets of results on a distance - loss graph, the cable loss goes up linearly dB per km but the radio link loss just increases 6dB every time you double the length.
At short distances the cable wins but, once you get to the distance of equal loss, the radio link just gets better and better
Space communications really benefits from the inverse square rule. Just as well, as there's no wired option!
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Also I like FM too, but with DAB radios you are 7 seconds behind the analogue
ones. I have counted them and it has been noted in Parliament too.
It might be due to the newness of the DAB facilities.
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DAB takes up much less spectrum space per audio channel than FM. To achieve this, the sound signal needs to be coded and bit-rate reduced, quite savagely. In the receiver, this all has to be undone and an audio signal needs to be produced. This processing all takes quite a long time; hence, the delay. The FM transmitter produces its modulated signal in a few hundredths of a second and the receiver takes the same sort of time.