4G and Freeview

[Resurrection Man]

Why would they tweak the bitrate dependent on the programme? I can understand it if they then, at the same time, opened up another channel to use the freed up spectrum but they don't. Do they get charged by the tx company on a per bit basis?

[Dave2002]

Why would they tweak the bitrate dependent on the programme? I can understand it if they then, at the same time, opened up another channel to use the freed up spectrum but they don't. Do they get charged by the tx company on a per bit basis?

Because SD channels are transmitted on a multiplex which can take say 3 to 10 video channels. If there are a small number all can be in high (SD) quality. If there are more TV channels on the same multiplex then the quality of each channel will be reduced. The broadcasters might, for example, have one high quality programme on at the same time as another few programmes which are less demanding - games shows, cartoons, news etc. If they tried to broadcast several demanding programmes such as nature programmes at the same time, then they would all have their perceived quality reduced.

The BBC seems to adjust the quality, and hence the bit rate depnding on the quality of the source material so as to give what they feel is an optimum balance for all the channels on each multiplex. I believe they may even use "in progamme" tricks, which could arguably be for artistic reasons,, but may also be influenced by technology. News programmes for example often show a newsreader against a blurry impressionistic static background. This is going to be much less technically demanding than if the whole screen image has to be in sharp focus with lots of detail, or if the background is moving. It may be less distracting, and artistic, but it also reduces technical demands.

Commercial broadcasters also tend to have more TV channels per multiplex, and this reduces the quality available on each, but it does give them more exposure.

HD channels will tend to occupy most of each multiplex. Radio stations can be packed in. Obviously it's possible to get more radio programmes on each multiplex.

[Resurrection Man]

I see where you're coming from. I found this chart that makes things very clear...just look at the number if channels on Mux 1 and B both before and after switchover !

http://www.ukfree.tv/allchannelsmuxes.php

[Dave2002]

Thanks - also interesting! I didn't realise the capacity on the multiplxes has been increased, notionally to 40 Mbps. The "before" capacity is shown as 24 Mbps for each multiplex, though in fact I thought it was closer to 18 Mbps with the error correction and protection switched on. Is the 40 Mbps of the new configuration similarly affected? I suspect that if all five HD channels on BBCB broadcast high quality content at the same time, that some additional loss in quality would occur.

Does anyone know what is going to happen during the Olympics? Aren't there supposed to be more channels (many more?)?

[Anna]

Does anyone know what is going to happen during the Olympics? Aren't there supposed to be more channels (many more?)?

Info from BBC Press Office here:

http://www.bbc.co.uk/mediacentre/mediapacks/bbc2012/gamestime/london-games.html

http://www.bbc.co.uk/mediacentre/mediapacks/bbc2012/gamestime/digital.html

Edit: Any many thanks to mangerton for clarifying about Gaelic in Scotland, very interesting.

[Gordon]

Thanks - also interesting! I didn't realise the capacity on the multiplxes has been increased, notionally to 40 Mbps. The "before" capacity is shown as 24 Mbps for each multiplex, though in fact I thought it was closer to 18 Mbps with the error correction and protection switched on. Is the 40 Mbps of the new configuration similarly affected? I suspect that if all five HD channels on BBCB broadcast high quality content at the same time, that some additional loss in quality would occur.

Does anyone know what is going to happen during the Olympics? Aren't there supposed to be more channels (many more?)?

Off the top of my head the net bit rate of the 64QAM mode in DVB-T1 using 7 microsecond guard interval and 2/3 error rate is 24MBit/s. The 40 MBit/s quoted refers to DVB-T2 [see the chart posted above - at the bottom of that chart its states that the 40 MBit/s is for T2] which is only used currently for HDTV in PSB3 [if we did that in PSB1 it means that T1 receivers, most at DSO, couldn't get services]. After DSO the powers were boosted for DVB-T1 and the extra signal to noise used for supporting 64QAM in some areas, that previously couldn't, giving more bit rate. 18 MBit/s 64QAM comes from changing the error control rate to the more rugged 1/2. 64QAM can provide 50% [ratio 6:4] more bit rate than 16QAM other things being equal. 16QAM is more rugged than 64 and so can tolerate a leaner error control rate eg 3/4 for 18 MBit/s. 16QAM can't achieve 24MBit/s. Swings and roundabouts.

The charts I posted earlier were dated well after switchover and so must be current.

[Dave2002]

Info from BBC Press Office here:

http://www.bbc.co.uk/mediacentre/mediapacks/bbc2012/gamestime/london-games.html

http://www.bbc.co.uk/mediacentre/med...e/digital.html

Looks as though the 24 Olympic live streams will either be Internet only, or spread across all transmission methods.
Since some TVs and PVRs are now Internet connected presumably thay can argue that all 24 streams are available on TV.

Additionally there will be some red button channels (301 etc.). What is the impact of the red button channels on multiplexes? Are they lower quality and lower bit rate?

[Resurrection Man]

At some cost to those who are blind or partially sighted....http://www.bbc.co.uk/blogs/rogermose..._freeview.html

Digital 'quirk', my a*se. Fundamental laws of physics/quantum mechanics/mathematics/stochastic algorithms.

[Gordon]

At some cost to those who are blind or partially sighted....http://www.bbc.co.uk/blogs/rogermose..._freeview.html

It's worth quoting this blog in more detail:

But on Freeview unfortunately we've discovered a quirk of the digital age, which is that TV platforms can struggle and go "blocky" when we play out the same pictures on multiple channels - especially when the pictures have a lot of activity in them.

Over the past month a lot of technical effort has gone into trying to finding a solution.

Audio only option

But we've reluctantly accepted that on Freeview we can't broadcast the same pictures, as had been planned, on BBC One standard-definition; channel 301 (which would have accommodated the two audio choices); and on the BBC News Channel, with some pictures also appearing in the News on BBC Two.

The result is that we're having to reduce the Freeview 301 offering to audio-only, which means the AD service will be there - but not the "no commentary" option, which won't be available on Digital Terrestrial services.

We know this will disappoint some people, and we've tried really hard to find a solution.

But we have to follow this course to protect the picture quality on BBC One where the overwhelming majority will be watching.

And, as I say, if you use cable or satellite or click on our website then you will find all the services as previously advertised - and Audio Description will still be on Freeview albeit as sound only.

Digital 'quirk', my a*se. Fundamental laws of physics/quantum mechanics/mathematics/stochastic algorithms.

Quite. This could/should have been foreseen a long time ago. This problem arises because the BBC multiplexes are made up of many video services which compete for the limited bit rate space. When a service gets less bit rate than it needs it shows in the blockiness referred to in the blog.

A system called Statistical Multiplexing is used [granted US patent by my old company in 1998] to allow servics to share bit rate. Without it fewer services could be fitted in. The SM system shares the space dynamically so that short term peaks of demand in one service [eg when there is lots of movemnet] "borrow" space from others whose demand is low at that time [eg they have still or slow moving pictures]. The average bit rate per video service is then reduced. SM works on swings and roundabouts principle and only works well if the separate video services are not the same programme. In the situation that the BBC intended, using SM doesn't work. If they must have the same programme in some channels, one partial solution is to delay each of those services with respect to each other in the multiplex so that the short term peaks of demand do not co-incide in time. This is a bit tricky to do but not impossible if they had hought about it in advance.

To call it a "quirk" is a bit disingenuous to say the least! It is certainly a well knoiwn feature of SM and should have been anticipated long ago during the planning of the coverage of the Olympics. One wonders that some engineer somewhere did point it out but some producer oik ignored the advice.

[mangerton]

To call it a "quirk" is a bit disingenuous to say the least! It is certainly a well knoiwn feature of SM and should have been anticipated long ago during the planning of the coverage of the Olympics. One wonders that some engineer somewhere did point it out but some producer oik ignored the advice.

Gordon, thanks for posting. That was most interesting, even although I have absolutely no interest in the olympic farrago.

Your use of the word "disingenuous" is much more charitable than I would be. I don't think the BBC employs engineers any more, and therein lies the problem. Birtist PR andmarketing types now rule the roost. A better distribution of the capacity in the multiplexes would go a long way to solving the problem. Do we really need all these "plus one" channels?

[Resurrection Man]

Thanks for that, Gordon. My mind is reeling at the sort of technology that can do that in real-time. Dedicated DSP chips, at a guess.

[Gordon]

Thanks for that, Gordon. My mind is reeling at the sort of technology that can do that in real-time. Dedicated DSP chips, at a guess.

Yes indeed for the video coders/multiplexers used by full scale broadcasters. These are harder to build because they have to do more work than decoders; the often try several coding options at once and choose the best. In decoders [your set top box or integrated TV] dedicated chips may be used rather than DSPs although they are an option. Devices called Field Programmable Gate Arrays [FPGA] are also used in coders which are not quite the same as DSPs. The tasks involved are often shared among several devices.

For each full HDTV [1920 x 1080 progressive] video service the processing rates have to be 5 times faster than standard definition and the pixels come at the processors at about 1.5 GigaBits/second. After the number crunching that is reduced to about 10 MegaBits/second a ratio of 150:1, and that includes the audio too! When we first began exporting our video coding equipment back in 1992 we had to get an export licence because the equivalent MIPS fell into the category that also covered supercomputers and were considered the same as military hardware!!! Clearance from DTI, as was, and GCHQ were needed!!

[LeMartinPecheur]

A tad off-topic but still about Freeview so I crave indulgence! Down here in the SW we've been fully digital for a good while. Even before analogue departed we'd invested in some Humax add-on boxes to give us some nice extra channels and better radio. At switchover all was fine.

But now I can no longer get Film4 on the Humax add-on box despite running the retuning programme, yet it's still there on the Humax digital TV in the next room. How can that be? Could it be that the digital TV has a rooftop aerial where the Humax freeview box runs off a set-top aerial? The TV aerial at Caradon Hill is line-of-sight and the box tells me it's getting a good signal.

[mangerton]

A tad off-topic but still about Freeview so I crave indulgence! Down here in the SW we've been fully digital for a good while. Even before analogue departed we'd invested in some Humax add-on boxes to give us some nice extra channels and better radio. At switchover all was fine.

But now I can no longer get Film4 on the Humax add-on box despite running the retuning programme, yet it's still there on the Humax digital TV in the next room. How can that be? Could it be that the digital TV has a rooftop aerial where the Humax freeview box runs off a set-top aerial? The TV aerial at Caradon Hill is line-of-sight and the box tells me it's getting a good signal.

Could be, but it seems rather unlikely if you're los to the transmitter and getting a good signal. I'd try swapping the boxes and seeing what happens then.

Let us know how you get on.

[LeMartinPecheur]

Could be, but it seems rather unlikely if you're los to the transmitter and getting a good signal. I'd try swapping the boxes and seeing what happens then.

Let us know how you get on.

mangerton: can't swap a box I'm afraid. The set that still has Film4 is an integral Humax digital TV. The one that hasn't is an analogue set with a Humax Freeview box. Now I think about it we have two of the latter: my daughter at the other end of the house (not quite so line-of-sight onto the Caradon mast!) has another Humax box that is similarly 'stationally challenged'. It too works off a set-top aerial.