RedShark News

05 Oct

Why you should use Avid DNxHD and Apple ProRes

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Modern cameras are simply amazing. The idea that you can record cinema-quality video on a still-picture camera that costs only a few thousand dollars still seems unreal at times.

But while it’s true that more people than ever can afford to make top-end video, this new capability poses a few questions for the ideal editing workflow.

The pictures from the latest generation of cameras - and that includes DSLRs and dedicated video camcorders - are stunning. No one expected to be able to get pictures this good for so little money.

The quality of the images is all the more surprising when you realise how much they’re being compressed to fit into the internal storage in the cameras. HD video involves almost unimaginable amounts of data. An uncompressed Full HD image (1920 x 1080 pixels) at 30 frames per second generates the equivalent of seventy five copies of War and Peace per second! Cameras compress by as much as seventy times to fit it into in-camera storage.

What that means is that less than two percent of the original data is left in some cases, which makes it even more remarkable that the resulting pictures look so good. Of course, the codec makes sure that the data that’s discarded won’t be missed too much. What remains is the data that really matters.

But even so, when an image is reconstructed from a highly compressed codec, it’s based on extremely fragile information. And sometimes, there’s just too much going on for the codec to cope with, with the result that when it’s decoded, you can see artifacts - areas of the image that have been “damaged” by the compression.

Camera manufacturers know what they’re doing. They don’t want bad images coming out of their cameras, so it’s safe to assume that in most cases, most of the time, highly compressed (whether they’re MPEG-2 or AVCHD - or some other variant of H.264) from a codec inside a camera is going to look pretty good.

Why not stick with these codecs and edit with them?

Well, you can. Most NLEs, including Lightworks, now can handle a wide variety of codecs natively. But this may not always be the best path to take.

There are several reasons why you should look for alternatives to editing with these highly compressed, so-called Long-GOP codecs.

Long-GOP codecs were designed to deliver video. They weren’t ever meant to capture or edit it

The reason that Long-GOP, highly compressed formats were created was to deliver video via DVDs, Blue Ray, Satellite and Terrestrial digital television. And, more recently, through internet downloads. Every single ounce of optimisation that went into these codecs was focused on making the picture look good at a very low bandwidth. And they’re very, very good at their job.

The problem is that almost every technical trick that was pulled to reduce bandwidth made it harder to edit video once it was compressed. The biggest problem is that to be able to see any image at all, the decoder has to reconstruct the image from adjacent frames. Possibly five or six, but equally possibly as many as fifteen. And it has to do this for every video stream - so if you want to preview a dissolve in real-time, your computer’s going have to decode two video clips even though it’s probably struggling to decode one.

All of this puts a big strain on a “normal” computer. In practise it means slow, jerky playback, and fewer effects in real-time. It may also hamper the final quality of your edit, because each time the video is decompressed to carry out an effect, it has to recompress the result, into the same, parsimonious compression scheme.

You can work with Long-GOP video, if you stuff your computer with RAM and it has enough processing power, but this is always going to be second-best to working with a codec that is designed for editing.

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  • When Avids DNxHD codecs first came out we ran them up against our uncompressed DS suite and pushed a whole load of material through comparing the results on our CRT HD grade 1 monitor and you just couldn't see any losses in the DNxHD content.
    The only real issue I've seen has been when DNx185 content has been laid off to HDCAM (with it's horrid sub sampling and compression) and then re-captured as DNx185. Artefact's start to appear and quality quickly falls away depending on the content and the original acquisition codec used.

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  • You mentioned in this article that "Transcoding to an intermediate codec is not your only option." But I was wondering how comparable (in image quality) is transcoding to intermediate codec against capturing in a intermediate codec.

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  • If you capture from uncompressed directly to an intermediate codec, that will be better than transcoding from a Long GOP codec like H.264.

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David Shapton


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