Monday, December 17, 2007

Digital tuners in an HD world

This should be the third and final part of the whole television saga.
Part 1: HD/SD what's it all about
Part 2: LCDs vs Plasmas (vs CRTs)

So I've covered what HD actually is and how it differs from SD, I've discussed the merits of LCD and Plasmas, so what's left? Well the bit that everyone really doesn't think about, but surprisingly makes a TV a TV - the tuner.

A TV tuner is simply described as a bit of kit that takes the hodge-podge of broadcast signals and filters them all out except the one you're interested in. For the old TVs this is an analogue tuner for analogue signals, but now we're entering a digital world you need a digital tuner.

For the early-birds the easiest way of getting this was using a separate set-top box, basically this was a digital tuner that took the signal and just pumped it out to your analogue telly. You could hook this up to your normal aerial input, but I'm sure most just connected via SCART leads.

So how does that affect HD broadcasts? Well if you're getting your programmes via cable or satellite (or internet) it doesn't, you can stop reading now unless you want to have a good laugh at your poor terrestrial cousins.

Starting simply digital broadcasts need two things to work - an agreed method of broadcasting and how it's actually broadcast. Think of it as writing a postcard, the postcard is the 'means' and you writing on it in English is the 'how'; the broadcaster sends you the 'postcard' and you read it in 'English'. For digital transmissions the means is called DVB-T (Digital Video Broadcasting - Terrestrial) and the how is a compression technique called mpeg2 (you probably already know about mp3 or more formally mpeg1-layer3). So far all terrestrial tuners can understand DVB-T and mpeg2 and all SD (and some HD) broadcasts use this; so far.

Progress ever progress, see HD needs more information and the compression of mpeg2 just doesn't cut it a new 'how' is needed - welcome to mpeg4. OFCOM are looking to whip out one of the digital groups and replace it with HD broadcasts in mpeg4 as early as 2009 and rolling on to 2012, after that... well might as well switch all the groups over.

So the 'postcard' you'll receive will now be 'written' in Klingon, which you don't understand. Wait it gets better in conjunction with this OFCOM is pushing the switch to DVB-T2; so now you'll be getting an email in Klingon, when you've haven't got an email address.

What all this means is that the TV or set-top box you buy now, won't receive the Freeview HD channels unless it can be updated not only to mpeg4 but to DVB-T2. The good news is that OFCOM announced that such equipment will be availible to buy... from 2009...when they start to switch everything over... and after (and during) the time they're turning off the analogue signals.

Suddenly the reason they're starting at the top of the country and working down to London makes sense, by the time the analogue is turned off in London the DVB-T2/mpeg4 TVs and boxes will have been available for a while and will be cheaper. For everyone else cough up now and later.

Addendum - as a service to my readers (all five of them) I poked my head into our local Currys and Comets, which I'm guessing would be the first stop for most people. I can only shake my head in disbelief - all the TVs on the floor display in Currys are being fed by an aerial input, that's a non-HD source, so the TVs are upscaling an SD broadcast. I was told that the TVs on the back wall were HDMI fed, but I don't what they were using as source material because it was blocky and awful (and had Vince Vaugn in it)

Grabbing a shirt I asked about DVB-T2/mpeg4, as far as he knew none of the TVs or set-top boxes were compatible and he had no knowledge as to the possibilities of an upgrade in the future. I left for Comet.

Slightly better this time, the ones on the right were being fed by an aerial, the ones in the middle by component (still not HD), and the ones on the left by HDMI (praise the HD gods). The big ones on the wall were sealed off, but looked HD. The big Phillips LCD TV was showing their own demo video, bikini-clad girls lying down, chopping up lemons and limes, things moving about in slow-motion; remember what I said about fast-moving objects on LCD screens - not the best video to judge such qualities by.

I grabbed a jacket this time and hit him with the same question - "Not a problem, all the TVs with 1080 on them can handle that" Which would be clever as at the time I write this DVB-T2 as a standard doesn't exist yet.

[Addendum - link from Charles on p.o.t. on a coincident thread. Yep in the US they're getting $40 off the purchase of a set-top box, while we pay full price and pay a licence fee to the BBC who are part of the Freeview consortium - neat]


Anonymous said...

I still feel utterly confused… Oh well!

FlipC said...

It is and purposefully so (from their end not mine), but in all seriousness what's leaving you confused, what do you want to know? If I have an answer for you I'm more then happy to help.

Anonymous said...

So… what is "HDMI"?

And why does interlaced even exist in digital form any more? Why not just make all digital signals progressive?

FlipC said...

High-Definition Multimedia Interface or in simple terms High-Def SCART, links your box to the telly. They also have to have HDCP (High-Bandwidth Digital Content Protection) or the box will down-size your output picture to stop you recording broadcasted films and duplicating them a 1,000+ times on your Blu-RAy/HD-DVD recorder.

As for the existence of interlaced standards, it's all in the bandwidth dude. 720i50 is really 720i25 instead of one full frame every 50th of a second you get one every 25th. Think of the file size of your animations at 50fps and 25fps.