Numbers are bandied about in the film and television industry far more often than politics. From my experience it seems that if you don't have a definitive answer to a question you can confuse the issue by quoting numbers.

My esteemed boss, David Hague, flicked me an email recently asking for an explanation of what the numbers 4:2:0 and 4:2:2 and 4:4:4 meant as far as video was concerned. I shot back an answer that I thought would keep him quiet for month or two but I was sadly mistaken.

Used in the context of video these numbers represent colour space. Let me explain.

Once upon a time we had film and then when David was a boy we got television. As many of us older folk know, television in its infancy was black and white. Yep! No colour at all. Then the God's descended from the heavens and man invented colour television. A by-product of that invention was colour space.

Colour space as the name implies is a component of the television broadcast signal, Chroma, as opposed to the black and white component, Luma. Back in the old days and even today some people can only receive black and white television so we need to keep these components separate. So how do we keep broadcasting to the black and white TV's?

Well, we create a process signal called YUV (ah! I've heard of that). "Y" represents the black and white component and "U&V" represents the colour component. When we transmit this signal the black and white television ignores the colour component. But it is not as easy as that. If we try to transmit all those samples we can't. Too fat, so we have to make it much skinnier.

Let me explain how we do that. The YUV signal is made up of the "Y" sample (luminance) and chroma Red (Cr) and chroma blue (Cb). This gives us YCrCb, more easily thought of as YUV but as you have worked out the colour green is nowhere to be seen. So where do we put the green? 60% goes in the "Y" channel and 20% in each of the "U" and "V" channels.

Okay, so this doesn't tell us how to reduce the samples. What we need to understand is that the eye sees black and white better than colour. To fool the audience we're going to leave the "Y" signal alone and play only with colour. What's a sample you say? Well, in PAL land we have 720 horizontal samples (and 576 lines). Get the picture? For starters we're going to reduce the "UV" sample by half.

Of our sample (720) we're going to leave a full 720 luma samples per pixel, remember, people see black and white better, but halve each of the colour channels to 360 samples per pixel. This leaves a ratio for YUV of 4:2:2 (aha!!!)  - 720 Luma + 60% green, 360 Chroma Red + 20% green and 360 Chroma Blue + 20% green. You know, no one misses that colour. It still looks great when you see it on television.

Like I mentioned earlier there are other sampling rates like 4:2:0 (DV) and 4:4:4 and NTSC has a 4:1:1 sample rate. If you want to know more about these I suggest you email david@australasiancamcorder.info and he can pass them back to me!

Sample rates are all about getting a lot of information into a small area (like DV tape) so whilst it's good to understand the principle it's simply nice to know information.

By John Westwood

Formerly a banker come accountant, John Westwood, in his early twenties, made his first documentaries and sold them to the ABC. After twenty years in the finance industry and a long involvement in amateur film and video clubs John turned professional and created Redgum Television Productions in 1984. Since then he has made over 3000 documentaries with more than 200 broadcast in seven different countries.

Although camera work is a key starting point for most videographers John has a bent for scriptwriting and editing and has won international acclaim in North America, the UK and a number of Asian countries. More commonly known as Redgum, John still has a strong involvement in the amateur club scene and can be found between documentary shoots trolling the local video clubs sharing experiences with budding filmmakers or relating that experience through the pages of the auscam magazine.

Currently, John is working on a commission to produce five one hour documentaries for an international agency. This type of work will keep him busy for two or three years and add another element to his industry based training regime for budding documentary makers. John can be contacted through www.redgumtv.com.au