Media regulator Ofcom has given broad approval to the BBC's proposal for a permanent high definition TV channel.

The channel could launch within months on cable and satellite, but Ofcom has recommended delaying its Freeview launch until new technology is ready to support it.

The BBC wants to turn its hi-def pilot channel on satellite and Freeview into a permanent fixture; the sat channel would be a cornerstone of the Freesat service which will launch by March 2008.

There's also a proposal to launch a four-hour overnight hi-def slot on Freeview, for viewers with hi-def PVRs, and to add BBC HD shows to iPlayer.

The BBC HD proposal is currently beiong studied by the BBC Trust to see if it should go ahead, and Ofcom has published its own investigation into the commercial and technological effects it might have on British TV.

Overall, Ofcom's market impact assessment reports: "The BBC HD channel is likely to deliver consumer benefit through increased take-up of HD. This positive impact is likely to be spread across the major TV platforms, including DTT, satellite and cable."

The satellite and terrestrial services would both use MPEG-4 compression, which would need new receivers, and satellite would probably move to DVB-S2, which makes better use of expensive satellite capacity.

But the report warns that launching the overnight service on Freeview in 2008 could harm the development of DVB-T2, which it believes is essential to getting several hi-def channels on Freeview without adding extra capacity after digital switchover in 2012.

Ofcom wants any hi-def channels on Freeview to use DVB-T2 - but the commercial hardware won't be ready until late 2008 at best, if not later.

It's recommended that the BBC HD channel on Freeview "is launched in such a way that it does not create barriers to the delivery of a number of HD services on the DTT platform".

Ofcom also suggested that a quick launch of BBC HD on iPlayer and broadband TV is unlikely because of limited UK broadband capacity - and advised the BBC to consult closely with ISPs and broadband TV providers before it goes ahead.

BBC HD would launch with a four-hour schedule, comprising hi-def versions of current programmes from a mix of genres, and growing to a nine-hour schedule in 2009.

The schedule would be flexible to accommodate live events, and the Freeview service would be a four-hour preview of the following night's pre-recorded programmes, intended for hi-def PVR owners to record between 2am and 4am.

It would be launched by closing BBC Four from 2am and combining other capacity from BBC Parliament and interactive services.

The four-hour schedule would like like this:

The nine-hour schedule would be: