Naked Science Forum
Non Life Sciences => Technology => Topic started by: thedoc on 18/12/2012 18:41:44
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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?
Read a transcript of the interview by clicking here (http://www.thenakedscientists.com/HTML/content/interviews/interview/1000068/)
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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 :)
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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.
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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.
Note than analogue was also compressed in some ways as well, the colour signal varied quite slowly across the scanline, because it was found that the human eye wasn't very sensitive to that, but the brightness varied much more quickly. So this isn't just digital being crummier, in fact in most cases it's able to achieve far better results.
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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.
TV has been compressed since the start in the UK in 1936 by means of interlacing which I always considered a mistake as most domestic receivers paired lines to some extent reducing the vertical resolution to less than 190 lines.
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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.
Analogue systems are all inherently a compromise to get acceptable quality. PAL is not quite as good as NTSC in an ideal situation but NTSC is rarely ideal and used to be nicknamed "Never The Same Colour". Digital processing in a TV is also much more easily cost reduced with technological advances than are analogue systems.
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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.
Edit: correction - two AAA batteries. It shows how often I don't have to change them that I was able to forget how many there are in it.
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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.
Compare to digital broadcast, 2 single 5kW signals carries all the TV and radio channels that took 1MW to broadcast, and there is only one licence fee spread over all the channels to pay. A win from the stations parts, and now they have a monopoly so that any new entrants are locked out, as they are the incumbents. A new station has to put up their own towers and such, not share infrastructure, or have the rules set for them along with price and content they can broadcast.
Digital also means a more expensive receiver, and a more power hungry one, no way you can use a long wire antenna and get radio broadcasts with no batteries like AM is capable of. It may be capable of better quality, but most digital channels are compressed so that they can put the maximum number in a single multiplex, so overall it is actually worse than AM radio, just without the noise of all the lightning in the background. Digital TV is the same, look at a scene with high motion, like F1 in car views, where the grass is a uniform green colour block, while with the analogue you can see the grass blades moving providing it was never compressed along the way.
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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.
With digital radio, the BBC World Service is uncomfortable to listen to because of the low bit rate, though it's infinitely better than trying to hear it on shortwave.
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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.
syhprum - Interlacing was essential with analogue TV as non-interlaced 25 FPS video is unwatchable on a CRT display - you also get twice as much movement rendition for a slight sacrifice of some vertical resolution.
graham.d - most engineers would disagree that NTSC pictures look better - lower resolution and a colour space that is noticeably more red that what we're used to in Europe.
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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.
Apparently PAL has a self calibrating colour signal- whereas NTSC's colour tends to wander, and it's rather more bandwidth given to it as well, PAL also has slightly more lines.
Properly set-up studio monitors should be more or less the same though.
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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.
I calibrate studio monitors every day and (as per my BBC training!) set the white point to 6500k (actually, 6504k since they discovered the Planck constant needed re-calculating in the 70s!) BUT American engineers set their white point to 5500k - in fact CRTs that have "NTSC phosphors" are damned near impossible to line-up for UK studio use.
Nowadays the whole world has unified on D65 for HD working but in standard def days American TV monitors (properly calibrated) did look very different colour wise to ones set up for UK use.