Alister Chapman talks about HD to SD downconversion

Categories: Miscellaneous 2 Comments

HD-to-SD

Ever since the release of the XDCAM EX cameras users have been having problems getting good lookin SD pictures out of downconverted HD. Why is this and what can be done about it? This is an issue that effects all high resolution HD cameras and is not unique to the EX’s. There are two key issues.

The first is the way basic software converters handle fields in interlace material and the second is the amount of information inan HD image that must in effect be discarded to get a SD image. At first glance you would think that starting off with lots of picture detail would be a good thing, but in this case it’s not.

Let’s see if I can explain.

Imagine that you have something in you HD picture that over 4 pixels goes from light to dark, in Hd you get a gradual transition from light to dark and all looks good. Now what happens when you take those 4 pixels and convert them to SD. The 4 pixels become just 2 and instead of a stepped change from light to dark the picture now goes instantly from a light pixel to a dark pixel. If these pixels were the edge of a moving object, as it moved the pixels would be switching instantly from on to off and unless the object moved at exactly one pixel per frame you will get a flickering effect. Clearly our nice gradual transition from light to dark has been lost and if there is any motion we may now be seeing flickering edges. Niether of these look good.

So what can be done? Well the best way to improve the SD down conversion is to soften the HD image before it is down converted to prevent this single pixel light to dark switch from happening. You need to end up with an SD image where you go from full light to full dark over at least 3 pixels to prevent flicker (Twitter).

How much you will need to soften you HD by will depend on how sharp it is to start with. Simply turning down the cameras detail settings can be a big help, but even then the best results are often obtained by applying some kind of blur filter in post production. In FCP i find the flicker filter works quite well.

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2 comments on this post

  1. Terry Wilson says:

    I think it needs to be pointed out that this might only be a potential issue if you’re shooting interlaced material. I for one have only ever shot 1080P, edited in both HD and SD timelines and outputted SD for DVD & Web, and the picture has always been superb, and a massive improvement over DV & HDV. 1080P from an EX-1 that is….

  2. HD Warrior says:

    I tend to agree with you Terry I mainly shoot 720p and the SD pictures look fantastic. It’s only when I rarely shoot 1080 50i that the SD pictures don’t look as good.

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