To Interlace or De-interlace

Interlacing is becoming an important matter these days. Why? Because our TV screens are getting bigger, so the problems it can cause are becoming far more obvious.

So what is interlacing? Back in the early days of TV, the engineers made a sensible decision to save on broadcasting space. Our TV pictures consume the space required for 625 horizontal lines (576 to 585 viewable). Because our power system provided voltage that alternates positive and negative fifty times per second, the TV broadcast system was locked to this (the US uses a 60 hertz system, which partly explains its different TV system).

But sending 625 lines fifty times a second wouldn’t leave room for many TV stations. So only half the lines are sent each time. You get the full 625 lines, but only 25 times per second.

The TV station doesn’t send the first 312 lines, then the second 312 lines. Instead, it sends the first, third, fifth and so on lines, then the second, fourth, and sixth etc. An old fashioned TV would display them in the same order, and typically the second line would be displayed before the first and third, above and below it, faded. All was well.

Unless there is movement in what is being displayed, which is most of the time. Video cameras mostly work the same. They record the first, third etc lines, then one fiftieth of a second later they record the second, fourth etc lines. If your favourite soap star is walking across the screen, she’s going to be in a different place that fiftieth of a second later.

That didn’t matter much when our TV screens were small. But these days many people are buying plasma TVs, and even projectors that can produce 100 inch pictures.

Modern projectors and large-screen TVs incorporate electronics to resolve the problem. This is called ‘deinterlacing’. The idea is that it aligns all those mismatched lines. This is not an easy task, though, and with some equipment it fails occasionally.

The four pictures here show the results. Two of them are of a public service message recently broadcast in Canberra on digital TV over the top of a Sunday Night movie. This scrolled from right to left across the screen. Notice, in the picture where the deinterlacing failed, how the lines stretch to the left and right of the letters in a jagged pattern called ‘combing’. The other version of this shows the deinterlacer working properly. Note, this looks a lot sharper when actually moving.

Worse is when human figures break up due to a failure in the deinterlacing circuitry. Notice how the mouth in the picture should look, then see the confusion of lines in the other picture, thanks to the failure of deinterlacing. The movement in this case is up and down, because the subject was talking.

Not all displays are the same, and the most irritating deficiency of some is their failure to deinterlace the program properly.

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