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Let's talk about video compression

Video compression is a fact of life: Video takes up space - a LOT of space. I have a 10 minute video from a New York trip that takes up almost 2GB of space on my machine. It's really easy to fill up the whole of your hard drive with video.

But here's the thing that I find astonishing: The video that we download from our camcorders onto your hard drives to edit has already gone through video compression and is already compressed by a factor of 5. That's right, the 2GB New York file is actually 10GB of video that is compressed.

Why is it compressed?

It's all to do with being able to process the data quickly and efficiently - as well as writing the video data to a thin tape in your camcorder. The digital video (DV) specifications call for a video compression ratio of 5:1. What this means is that in order to take the 10Gb of video file I've taken and put it onto a small DV video tape it has to be squeezed into a space 5 times smaller than itself. This can only mean one thing: something is being lost as a result.

This is what's known as 'lossy' video compression. It reduces file size but at the expense of something else. Usually you don't realise that something is missing because the compression programs are really clever. But you have lost something none the less.

In normal, everyday editing you don't miss this compressed footage. It looks alright on your camera, it plays alright on your PC, so whats the problem?

The problem is that 95% of your video footage is not going to be watched from the tape or from big files on your computer. What you're going to do is edit the footage together so you can do something like burn it onto a DVD or upload it to Youtube.

Guess what? In order to do that you have to compress your footage. That's right, the original footage - which has already been compressed by a factor of 5 when you dropped it onto your DV tape - is now going to be compressed even further - sometime by a factor of 50 or 100 times.

What's going to happen? You're going to lose quality.

Youtube
How many times have you watched a Youtube video and thought 'The quality on that is awful'? Usually it's a result of the video compression used to create the file. Let me give you an example:

Follow this link: Rockstar. This is the video for Nickleback's 'Rockstar'. (For copyright reasons I can't display it on my website but Youtube can sort that out!). Right at the very beginning, the very first picture you see is a close up of the young boy who takes a deep breath and (in Chad Kroger's voice) says 'I'm through with...'. Watch it a couple of times. What do you see? Can you see those large, square 'chunky' things that appear whenever the boy takes a breath? What about whenever the picture cuts from one person to another? Can you see the same things there? These are compression artefacts/artifacts. These are the result of taking things that are compressed and compressing them down further.

The Youtube video compression is doubly bad because they are compressing the file down to a certain physical size as well as compressing it down to fit a certain rate of data transfer. Both of these together are a nasty combination!

DVD's
Let's talk DVD's now. The 2GB file of New York I mentioned earlier is about 10 minutes long. But I have in my collection a version of 'The Abyss' by James Cameron that has almost 6 hours of film on one DVD. Now it is a professional DVD which will store nearly 9GB of data, but still: they appear to have way more footage on their than you would expect, and the quality is still excellent. How can this be?

DVD's are compressed using a format called MPEG2. This works by taking a snapshot of a frame, storing that in it's entirety and then on every following frame they only look at the changes from the previous frame. This way they can compress lot's more into the same space.

Point to note:
The thing to remember here is that the compression cannot be reversed. Imagine the scenario where you have a DVD with some footage that you want. You can use on of the DVD tools to extract the video from the DVD, edit it using a free editing tool and then save it back to Youtube. But it's going to look awful. Here's why:

Original footage 1:1 (as shot on your camera)
Compressed DV 5:1 (as stored on the camera tape)
Edited and saved to AVI format: 8:1 (as created from your editor)
Compressed from AVI to DVD : 12:1 (as an example - when loaded to DVD)
DVD transferred to AVI for editing : 1:1 (When ripped from DVD)
Compressed from AVI to DVD : 12:1 (as an example when loaded back onto DVD after editing)

Thus your original 1GB of video has been compressed many many times before it ends up on your DVD. Each compression will lose quality.

This is why you need to remember the key mantra of video production

Do not compress until the final possible stage !

This means to keep your video as raw as possible and only to compress at the latest possible point before you put it into your end format, whether this is DVD or Youtube.

Click here to return from video compression to technical editing

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