10/07 Kingswood Warren in the news

It seems that the journalists from CNet Crave have been visiting my department to have a look at my colleagues' prototype system for doubling the capacity of a DVB-T multiplex (the technology used by Freeview) to try and make terrestrial HDTV practical. The technology makes use of the fact that you can transmit two independent signals on the same frequency if you polarise them 90° apart - something that's been known for about a century, but hasn't been practical until recently, with the development of MIMO techniques.

They are currently squeezing three HD transmissions and one Standard Definition TV channel into a single multiplex, the HD channels at a bitrate of 15Mb/s - plenty for the most recent version of Dirac (although as far as I know nobody's proposing that codec for transmission) and h.264 codecs ought to be looking pretty good at that bitrate by the time any hypothetical MIMO technology could be deployed country-wide. (The current BBC HD trial service started broadcasting using h.264 at 19Mb/s back in 2006, and at that time it really needed a bit more than that, in my opinion. It is in the nature of video codec implementations to improve with time, however, as their creators find ways to make better use of the data-reducing tools that the codec standards provide.)

No technology is ideal, and this one would require everyone to upgrade their aerials again, as well as their set-top boxes, but to get HD into the very limited bandwidth available terrestrially requires compromises, and this technology is a very interesting option.

06/07 A little more evidence

In a recent post, I was talking about why televisions refresh their picture at the same rate as the mains frequency. As far back as 1939, I said, flicker caused by the mains-driven studio lights beating against the picture-taking frequency of the studio cameras was being cited as a reason. Supporting (if anecdotal) evidence comes from "An unreliable and wholly unofficial history of BBC Television Centre", by Martin Kempton, which recounts how studios TC6, 7 & 8 were equipped in the 1960s with kit to allow them to create programmes in the American NTSC standard. This involved running the cameras (and other equipment) at a field rate of 59.94Hz, whereas the studio lights were still driven at the UK mains frequency of 50Hz. Martin reports that flicker caused by beating of this kind was indeed a problem when the studios were operated in "NTSC mode". It seems that few NTSC programmes were made at TVC, in part because of operational issues such as this one, but also because high-quality "standards converters" for turning PAL programmes into NTSC ones were entering operational use at around the same time as these studios were commissioned (including the "field-store" converter developed here at Research Department).

As we investigate the effects of higher frame rates on video quality, this is something we will have to bear in mind...