Digi' tv quality

[empip]

Will we notice the difference...

http://www.ukfree.tv/fullstory.php?storyid=1107051356

...The Ofcom plans for HD on Freeview will mean a drop in picture quality for everyone starting with the Granada switchover Ofcom's proposals to bring three (and eventually four) HD services to Freeview will come at a price. The plan will require the BBC to vacate multiplex B when the Granada region switches over in 2009, and to move all but one these services to multiplex 1:

BBC one
BBC TWO
CBBC/BBC three
CBeebies/BBC FOUR
BBCi 301
BBCi 302
BBC News Interactive
BBC Radios 1, 1Xtra, 2, 3, 4, 5 Live, 5 Live Sports Extra, 6 Music, 7, Asian Network
Two of BBC Radio Wales, Cymru, Scotland, Radio nan Gaidheal, Ulster and Foyle (not England)
BBCi Text service (the “digital Ceefax” service)
BBC Parliament

To do this, the BBC will change the transmission mode of Multiplex 1 from 16QAM to 64QAM. This increases the capacity by 33% from 18Mb/s to 24Mb/s...

I have been reading that following 'switchover' digital transmission power will be increased, apparently leading to better reception for most... Will the above changes (if factual) have a more noticeable effect than a stronger signal??

 

[Steve]

Talking about power, you got me thinking. How much electricity does a television mast / transmitter consume?

 

[libby lou lou]

Why don't you ask in the plumbing section ?

you thinking of giving tv up for the evoiroment

 

[Inky Pete]

Will we notice the difference...

....not in the outlying US islands!!!

 

[Agile]

Talking about power, you got me thinking. How much electricity does a television mast / transmitter consume?

That all depends on the power of the transmitting station.

A main station like Crystal Palace or Sutton Coldfield on the UHF services will have a transmitter providing a peak power of say 400kW into the antenna. But the average power is less that 280kW.

However as the transmitters are not 100% efficient it might take about 1 MW input power to produce that RF power output.

But thats only for one TV channel and there are five at most sites.

The original question addresses a different problem. Thats the available bandwidth for the video signal. A full SD transmission has 5.5 MHz but some of the sat channels are only 2.5 MHz. The available bandwidth can vary from time to time depending on the demands on the system.

To fit HD into the Freeview transmissions may mean reducing bandwidth on other channels. The transmission power has little to do with that.

The transmission lines for the RF output is a bit like plumbing with an inner conductor of about 75mm and an outer of 150mm all in rigid aluminium.

Tony

 

[Space cat]

On analogue TV, picture quality depends on signal strength. As you get further away from the transmitter the signal gets weaker but background noise doesn't. First the colour goes and then the picture disappears into snow. One of the few advantages of analogue TV is that you can still get useful information out of very weak signals.

The available bandwidth (5.5MHz) sets the upper limit on horizontal resolution but whether you get that resolution or not depends upon the quality of the TV itself. Vertical resolution is fixed by the number of scan lines. Reduced bandwidth leads to loss of resolution and a fuzzier picture. This is what you get from video tape. Colour resolution on both live TV and tape is much lower. Colour TV exploits a quirk in our vision whereby a crisp monochrome image crudely painted over with colour still looks sharp. You can see the blurred colours if you look carefully. It's especially noticeable where green meets magenta.

Digital TV is different. Picture quality does not degrade with falling signal strength until you reach a critical level. At this point you lose the sound and the picture breaks up into blocks or else it just goes blank. It's a bit like putting increasingly dirty discs into your computer. They work perfectly until the the error correction algorithms can't cope and you get an error message.

Reducing the bandwidth of a digital signal doesn't affect the resolution in the same way as it does on analogue TV. That's because digital TV transmits only the changes between one frame and the next. What you will get is more of those curious artifacts whereby fast moving objects seem to jump or leave trails behind them. They're rare at the moment - or maybe I just don't see them! Increasing the signal strength will not compensate for loss of bandwidth but it will enable more people to watch the degraded picture!

 

[Agile]

Reducing the bandwidth of a digital signal doesn't affect the resolution in the same way as it does on analogue TV. That's because digital TV transmits only the changes between one frame and the next. What you will get is more of those curious artifacts whereby fast moving objects seem to jump or leave trails behind them. They're rare at the moment - or maybe I just don't see them!

Whilst that can be true its not the whole story!

I dont pretend to understand all aspects but based on the viewed picture when the bandwidth is reduced on sat channels they reduce the video bandwidth but keep the refresh rate about the same so the degredation is primarily in the definition rather than the motion.

My limited information comes from my own observations and from the owner of the Baby Channel as I maintain his gas boiler!

I saw a red Peugeot 405 estate reg M247 LYO in very good condition for anyone who recognises that series of company cars! I think mine was M404 LYO !

Tony Glazier

 

[Space cat]

-- when the bandwidth is reduced on sat channels they reduce the video bandwidth but keep the refresh rate about the same so the degredation is primarily in the definition rather than the motion.

Reducing the video resolution is one way of getting your bandwidth down before you even begin to compress the data. Will the Beeb do this on Freewiew? I don't know. I'd be even more interested in what will happen to the quality of the new HD TV channels as they try to squeeze more of them into a limited space. Excessive compression is one of the things that killed off DAB radio.

 

[Agile]

Excessive compression is one of the things that killed off DAB radio.

I was not aware that DAB has been "killed off" !

I am aware that the audio quality is agreed to be poorer than FM in a good reception area.

I am also aware than some of the smaller broadcasters have closed as they could not attract enough advertising to pay for the transmission costs.

What has disappointed me is that the receivers have been developed as lo-fi kitchen units most in mono or effectively in mono. I expected they would have developed as a hi-fi tuner.

With the poor receivers in use its not surprising they fit in more channels when no one is listening to them with quality reception.

Tony

 

[Space cat]

I am also aware than some of the smaller broadcasters have closed as they could not attract enough advertising to pay for the transmission costs.

Maybe that's what I heard. It was on the news a few months ago and I wasn't listening all that carefully. The reason given was that sales of DAB receivers were disappointingly low and advertising depends upon having an audience. No listeners; no pay!

There was no way I was going to pay the outrageous prices of the first DAB radios and I never even considered a hi-fi tuner because I can feed radio from my TV digibox into the audio system. The news item suggested that DAB might do better if car manufacturers could be persuaded to fit the radios as standard but it wasn't looking good. I think the deliberate overcrowding of the available bandwidth (leading to loss of quality) might have been the last straw. Oddly enough, I'm now seeing DAB receivers in the shops at more realistic prices but is this a case of too little too late?

 

[Agile]

I find them in a lot of kitchens in London when I do boiler repairs.

I would guess that perhaps 20% of kitchens have them but they usually stay on one channel.

I also have one of them in my kitchen too!

Do you have any idea of the audio quality on satelite radio channels?

Tony

 

[dave.m]

Serious question
Do transmission signals get weaker if more people turn on the their TV?
I mean, is it a bit like the power stations, do they have to boost a signal if there is something like a World Cup Match on or does the signal strength only get weaker by distance travelled.

 

[Space cat]

Do you have any idea of the audio quality on satelite radio channels?

Sorry, I can't help there. I remember that the old analogue Astra satellites used to broadcast radio on a par with terrestrial FM. It was slightly inferior because it was compressed (in amplitude) before transmission then expanded in the receiver.

But that was analogue. I've never had a digital satellite receiver. For expert advice on audio topics you could have a look on www.hydrogenaudio.org.

Do transmission signals get weaker if more people turn on the their TV?

As a physicist I have to say yes but the effect is so small that you can ignore it. I remember one exception to this. A few decades ago I heard of somebody who lived close to a high power transmitter. He filled his loft with large coils and extracted about 4 kW of free power. Unfortunately this affected the signal strength in the immediate vicinity and he got done.

 

[empip]

I decided on impulse, to buy a Bush freeview digi box - £20.
Waltzed it home, sorted the connections to dvd player, video and tv (5yr old 'monster' Sony),
Tongue in cheek, set off the channel search, when it looked like halfway through the motions with nothing found, I wandered off to the kitchen thinking it'll have to be the 20 yr old aerial out of loft and a roof top jobby.

Blimey, wandered back into lounge, channel search had completed upmteen channels plus radio - as per the norm... All working quite grandly it transpired, except the odd 'snap' from sound which also occasionally gets out of synch with the pictures.. Not often or for long enough to spoil the entertainment. ( generally screen menu displays 100% quality : 70% strength - is this meaningful / normal??)

My next Q is... would there be any advantage if say 3 aerials were positioned in loft - (extra tv's in house) - actually linking them together, would there be any advantage?

 

[dave.m]

Thanks, SC that's answered my question.
Cheers
dave