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HD progress makes interlace redundant

28 June 2007

Broadcasters should forget about shooting interlaced HD if they want to deliver high quality at low bitrates, according to the EBU at this week’s HD Masters conference in London. In its tests, the EBU found that the lower the transmission bit rate, the better 720p-originated pictures were liked. “To give the same perceived quality picture, 1080i needed about 5Mbps more bandwidth than 720p,” said EBU Senior Engineer Hans Hoffmann, writes David Fox.

In general, at all rates of compression (except uncompressed), 720p looked slightly better than even 1080 50p on LCD screens and about equal on plasma displays – and gave reasonable results at even 6Mbps when 1080 50i was deemed to be worse than uncompressed SD images. Because of the amount of extra detail needing to be compressed, 1080 50p required about 3Mbps extra for a picture judged to be about 70% the quality it delivered uncompressed.

Hoffmann concluded that, “We should avoid interlaced in production and emission for HDTV,” and that “720p/50 in emission provides the best bandwidth-quality trade-off for displays up to about 50 inch.”

However, broadcasters are not so keen on 720. “Progressive makes a hell of a lot of sense, and you [should] go for the highest standard, which is 1080 at the moment,” said Paul Kafno, managing director, Park Pictures, and one of Europe’s HD pioneers.

As far as the BBC is concerned, there is “only one production standard”, and currently 90% of its programmes are at the top of the standard, which is 1920×1080 at 25p. “You can’t get higher quality than that, just better motion,” explained Andy Quested, principal technologist, BBC Production. For him, 1080 50p will be the target for sport or anything that they want to look like video.

“720 is not HD. We can not sell it, so we can not use it,” with the exception of Panasonic’s Varicam and HDX900, added Quested. But the BBC has to get special dispensation for these cameras (from co-producers like Discovery), and even these will progress to 1080p soon (with Panasonic’s move to 100Mbps AVC).

Of course, interlaced (at 1080 50i) still has a place in sports and OB production. It is what OB companies have now and handling 1080 50p requires new technology and/or mezzanine compression techniques. Those are emerging now, but Hoffmann warned: “Do not expect a low-cost implementation in 1080 50p end to end for another three years.”

It is the cost of upgrading to 50p, especially so soon after equipping studios for 1.5Gbps HD, that makes Andy King, head of technology, BBC Resources, “very nervous, very scared. _If 50p is pushed early, it’s going to cause real problems with paying off our [existing] investment in HD.

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