Freeview HD: Ofcom strikes fear into our HD-loving hearts

In a post on its blog, the BBC has announced that we'll start to see some high-definition services appear on Freeview from the end of this year. It looks very much like these channels will be broadcast in 720p, and will comprise the usual terrestrial stalwarts in HD form. Expect BBC HD, ITV HD, 4HD and Five HD to launch as and when it becomes technically possible. We've got a slight issue with this, though.

Freeview HD is something we've been looking forward to for some time. We think it's about time the majority of people in the UK had access to a source of HD that doesn't involve a subscription to a cable or satellite TV service. Ofcom, to its credit, has been reasonably proactive about making this happen, and it now looks like we'll see a Freeview HD service launch around the end of this year or the start of next. The Granada region goes first, with London predicted to be last, in 2012.

But our concern is that Ofcom isn't being realistic about HD. Firstly, its document is called Digital Television: Enabling New Services -- Facilitating Efficiency on DTT, which sounds like it was written by someone with a degree in horse-dung management from the University of Utter Failure. Secondly, the contents are alarming -- you only have to look at DAB to hear what happens when you try to cram loads of stations into a small space.

What's got us so concerned? Well, firstly, Ofcom suggests that it would prefer material to be broadcast in a progressive format -- which is good -- but, because 1080p is too bandwidth-hungry, the solution is to use 720p -- which isn't so good. 720p is fine, and a very worthy upgrade to SD, but, with most TVs now being 1080p -- even those smaller 32-inch sets -- we can't help think that 1080i would be a much better choice of broadcast resolution. 1080i broadcasts fit well with 1080p screens -- the resolution is a 1:1 match -- and a good TV shouldn't struggle to de-interlace the two fields of information. 720p isn't really as much of a step forward as we think is necessary, and it's not a format that any UK satellite broadcaster has opted for.

1080i comes under criticism because interlacing is seen as an old-fashioned compression system. In fact, interlacing is a really clever way of compressing a video signal. By sending only half the amount of data, splitting each frame in two, you essentially halve the amount of bandwidth needed. Putting the interlaced information back together should be reasonably easy on modern TVs, most of which have pretty good de-interlacing abilities. After all, broadcast SD is all interlaced, and, if you don't notice any artefacts during EastEnders, then your TV is more than capable of dealing with 1080i.

Ofcom is trying to shoehorn the maximum possible number of channels into the HD multiplex, Multiplex B (a multiplex is a collection of digital sub-channels that exist within the space usually used by a single analogue channel). Ofcom claims that, in 2009, it should be possible to have three channels on Multiplex B. It thinks four should be possible in 2010. Ofcom also thinks that, by 2012, five HD channels could be crammed into the same space currently being filled by five SD channels.

At this point, we should says that a 64QAM, DVB-T2 multiplex should have a data rate of 31.4Mbps. That would mean that each channel would be allocated a maximum data rate of 6.28Mbps. For the sake of comparison, BBC HD on Freesat/Sky HD has a resolution of 1080i and a data rate of 16.38Mbps. BBC HD on iPlayer has a bit rate of 3.2Mbps, and is 720p. Of course, iPlayer content looks good on a PC, but has far too many artefacts to be practical on a large-screen TV -- believe us, we've tried it.

There's a suggestion that statistical multiplexing is the answer, but we predict something of a fight between the broadcasters over using that technology. Statistical multiplexing works by dynamically assigning bandwidth to channels as they need it. For example, if BBC HD is showing tennis and ITV HD is broadcasting a talking heads piece, BBC HD would need more bandwidth, so it would be assigned more bandwidth. That's fine, until BBC HD is showing Wimbledon, ITV HD is showing football and 4HD is playing an action movie. Who gets the bandwidth then? Ofcom thinks it's worth doing, because it estimates a further 25 per cent bandwidth saving could be made.

We're also worried about audio quality. Ofcom proposes that it would be possible to transmit Dolby Digital sound at 160kbps, instead of the currently used rate of 384kbps. That's less than half of the rate chosen for digital satellite. While lower bit rates are fine for stereo programmes, if you try and put 5.1-channel surround sound into 160kbps, you're likely to hear plenty of nasty compression artefacts.

Ofcom also suggests that broadcasters should "frequently upgrade encoders to ensure maximum efficiency". The BBC has something to say about this, refusing to just continually renew hardware, because, it says, it's simply not good value for licence fee payers. We agree with both here the BBC and Ofcom here. Money being no object, we think encoders should get frequent upgrades, but, in these tough times, there simply isn't an unlimited pot of money knocking around.

What would we do? Well, converting Multiplex B to HD with DVB-T2, 64QAM and an MPEG-4 codec is a solid plan. We'd like to see no more than three services on that multiplex though, and they should be 1080i for the time being, with 384kbps Dolby Digital sound. Three channels on a 31.4Mbps multiplex is still pushing it slightly. Ideally, there should just be two.

It doesn't seem unreasonable that Freeview needs to become more of a premium platform -- no-one needs those shopping channels, so let's clean it up. Ofcom should restrict DTT licences from now on, and keep only the channels on Freeview that actually provide some sort of public service. And, no, we don't think flogging jumpers with a wolf knitted on the front is a valuable public service. And no-one needs ITV2 Plus 1 or E4+1 - at least, no-one with a DVR does. There's room for HD on Freeview, we should just be slightly more restrictive about who and what we give capacity to. We want HD on Freeview, but we want it to be more like Blu-ray HD and less like YouTube HD.

This article is based on two Ofcom documents. The first is Digital Television: Enabling New Services -- Facilitating Efficiency on DTT, and the second is a ZetaCast report commissioned for Ofcom called Review of DTT HD Capacity Issues. Both are available from the Ofcom Web site as PDF documents. We'd welcome your thoughts on HD via Freeview. Use the comments section below, or our user forums.