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Interlace or Progressive – Who Chooses?

Why interlace is evil and is banished from the HEVC specification

By Bruce Devlin | 09/23/14

Regular readers of this blog – or just people who have met me and been slightly startled when I start foaming at the mouth – will know that I am obsessive to the point of mania on one topic: Interlace is evil. Repeat after me: interlace is evil.

Back when television first became a practical proposition, in the 1930s, we did not have electronics in the way we would recognize them today. Most of the processing was done in big circuits using glass bottles which our American friends call ‘tubes’ but the rest of the world gives the rather more helpful name ‘valves’.

I like the name valve because it tells you what they do: like a bathroom tap you can turn the current on and off. And like a bathroom tap, you cannot do it very quickly. So the concept of high-speed electronics was simply impossible back then. You had to find ways to make those relatively slow devices do what you needed.

The Interlace Compromise

Those early television pioneers had to find a compromise between the need for at least 50 pictures a second to create convincing movement and at least 400 lines in the picture to give reasonable resolution, and the difficulty of working with electronics that would not go that fast.

The workaround they came up with was to give us not 50 pictures a second but 50 half pictures a second. First they wrote the odd numbered lines, then they wrote the even numbered lines. The two were interlaced together and, hey presto, the eye was fooled.

But at the risk of stating the very very obvious, we no longer need to work around slow electronics because we can have all the processing power our hearts desire in a single chip. We could banish interlacing forever, today.

Why am I so passionate about banishing interlacing? Because we can do stuff to content downstream that can break the sequence. Obviously you have to have matching odd and even fields if the picture is going to look right. The problem is even greater when you create 30 frames a second (60 fields a second) material from 24 fps film or film-like originals – then it is really easy to get the sequence wrong and make the pictures look horrid.

HEVC The Death of Interlace?

The bright spot on the horizon is that the next big thing in digital video compression – HEVC or H.265 – has very little support the concept of interlacing. All HEVC streams that are Highly Efficient are progressively scanned. I cannot tell you how pleased I am by this. Finally, one of the unnecessary problems of our industry and our age is on the way out.

But for now we have to deliver content over a number of platforms using a number of codecs, and most of that content will have been originated using interlaced equipment. Content owners have to deliver material to those delivery platforms – some of which will be run by broadcast engineers who understand the problem, and some may be run by people who do not – and they have to trust that the material will not get messed up between them and the viewer.

For now, broadcasters are stuck with interlaced formats, although we all hope that there will be a move towards HEVC delivery sometime soon, and not just because it means we can transmit progressive television.

Online services have the option of selecting the delivery mode, and they should be encouraged to go for modern codecs for progressive display whenever possible.

Content owners can gently encourage this by delivering their content to each outlet in pristine condition in the delivery codec and resolution. An intelligent platform like the AmberFin iCR will sort out errors that have arisen in the interlaced sequence, and deliver a clean and artefact-free output, at the target resolution and codec.

The message, though, is clear. We now watch content on progressive displays (CRTs are fading fast from the memory). Let’s do all we can to ensure that everything upstream of the viewer is also progressive.

Featured in: Dalet AmberFin | Transcoding |

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By Bruce Devlin

With 30 years in the industry, Bruce looks after Media Technology for Dalet. An engineer who designed antennas, ASICs, software, algorithms, systems and standards, Bruce is best known for being @MrMXF and you can get his book on Amazon ☺.

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