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Annoying Aspects of CCTV (Part 2)

Following on the heels of my previous post I can’t ignore some other shortcomings in ways that CCTV footage is displayed by the machines sold in our market.

My point this time is that pictures are often displayed as too narrow. Ok, let’s not lose too much sleep over this one as the effect is not dramatic, but my beef is that it shows a disregard or ignorance of correct technique by those who design the CCTV equipment. This is usually when showing digital video on a computer display. Here’s the explanation:

Woody from Toy Story

Narrow headed Woody. © Disney

The normal 4:3 picture when digitized (D1 sampling) is 576 pixels high and 720 pixels wide. Yes, for those good at quick mental arithmetic, 4:3 would strictly mean that the 576 picture lines (from a PAL camera) would dictate that there be 768 pixels horizontally. For reasons of how best to digitally sample TV signals the world some time ago agreed that the sampling process will be designed to actually give 720 pixels. So, to avoid distortion when the picture is displayed they agreed that each pixel will be rectangular. If each pixel is about 7% wider than it is tall then we re-establish the correct 4:3 picture on the screen. Voila!

However, many bits of CCTV software simply display these rectangular pixels as square ones on a computer screen. So, the result is a display where images that are squashed sideways by 7%!

Back to the police radio: “the suspect is an IC1 male wearing yellow and with a really narrow head….Yeah, looks like Woody off that Toy Story movie”?!

The solution to this wrong practice? Be aware of the issue and ask your supplier to show you that their machine does not have this shortcoming. It’s quite likely that they will have this problem, if what I’ve seen around the market is anything to go by! However, the more we as independent CCTV consultants pressure them to get it right the likelier it is that they’ll put some effort into fixing their products when they realise that thier customers not as dumb as they assume.

Until next time; stay focused!

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