Getting good SD from an HD camera.

This a recurring question that I get asked about time and time again. The main problem being that the SD pictures, shot with an HD camera look soft. So why is this and what can be done about it?

Well there are several issues to look at. First there is camera optimisation. Sadly what works for HD doesn’t always work well for SD. Secondly there is the downconversion process. If your shooting HD and simply outputting SD using the cameras built in downconverter than you really don’t have many options but if your using a software downconverter you may be able to improve the results your getting.

Starting with the camera, what can you do? Well first off let me say that a camera optimised for HD will always be a compromise when it comes to SD. As the native resolution of HD cameras increases then the problem of getting good looking SD actually gets worse. The problem is that a good high resolution camera will normally only have a very small amount of artificial sharpening via the detail or aperture circuits, because in HD it will look nice and sharp anyway. SD cameras and the SD TV system with it’s inherently low resolution and soft pictures has always relied very heavily on detail enhancement to try and make the pictures appear sharper than they really are. When you take the minimal additional sharpening of an HD camera and downconvert it to SD it all but disappears, the end result is a soft looking picture. There is no easy fix for this, you can either add additional extra thick detail correction edges to the HD pictures, which risks spoiling the HD image or you can add additional detail correction in post production. On a Sony camera the thickness of the detail correction edges is controlled using the “frequency” setting. Setting this to a negative number will thicken up the detail edges, very often you need to go all the way to -99 to get an appreciable difference. As an alternative you can add extra sharpening or detail correction in post, after the downconversion process. This is the way I would go whenever possible as I don’t want to compromise my HD pictures for the sake of the SD images.

The second issue is the quality of the downconversion. A simple rescale from HD to SD rarely works well as it can create a lot of aliasing. Aliasing is the result of taking too much detail and trying to record or represent it with too few pixels. See this article for more on aliasing. Imagine a diagonal line running through your image.

Diagonal Line Sampled in HD

Diagonal Line Sampled in HD

If you sample it at a high resolution, with your HD camera then the line looks reasonably good as you can see in the diagram to the left.

Simple SD Downconversion

Simple SD Downconversion

If you then take that HD captured edge and simply scale it down to SD, you quarter the number of samples and the end result is a jagged, stepped line. Not pretty. In addition, if the line moves through the image it will flicker and “buzz”. This is far from ideal.

Same Line, Blurred Before conversion to SD

Same Line, Blurred Before conversion to SD

A better approach is to blur the HD image before down converting using a 4 pixel (or similar) blur, or to use a downconversion programme that will include smoothing during the conversion. The final image shows the kind of improvement that can be gained by softening the image before down conversion. The blur around the edges of the line soften it and make it appear less jagged. This will result in a much more pleasing SD image.  Next you then add in some detail correction to restore the apparent sharpness of the image and viola! A decent looking SD image from an HD source. In compressor to get a good downconversion you need to activate the advanced scale tools and use the “better” or “best” scaling options.

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8 Responses to Getting good SD from an HD camera.

  1. Andy Taplin says:

    I’m still watching broadcast TV on my trusty CRT and I notice that increasingly factual TV which is shot HD, presumably mostly XDCAM and HDCAM, exhibits terrible aliasing – so little or nothing has been done to make the SD transmission look better.

    Dramas look great, even when shot digitally, so there is something they are doing in post that the TV guys are not.

    Kind of makes a nonsense of the broadcaster’s stipulation to use certain equipment when they let the SD transmissions go out looking so bad.

  2. alisterchapman says:

    I think drama tends not to suffer as it will normally have a shallower DoF so less in focus stuff to aliase. Factual tends to use default camera settings, which generally have high levels of HD detail correction, so no hope at all for SD viewers.

  3. alisterchapman says:

    I think drama tends not to suffer as it will normally have a shallower DoF so less in focus stuff to aliase. Factual tends to use default camera settings, which generally have high levels of HD detail correction, so no hope at all for SD viewers.

  4. roman says:

    Since I’m working with Avid MediaComposer, can anyone recommend a blur-setting that works best for downconverting HD-material to SD in Media Composer?

    Thanks!

  5. Dave says:

    Alister, question – for applications that need SD footage only, have you any experience of using an NX5 in SD mode?

  6. Dave says:

    Alister, question – for applications that need SD footage only, have you any experience of using an NX5 in SD mode?

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