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Digital CCTV Evidence & Aspect Ratio Correction – Part 1

Let’s talk a little more about aspect ratio. Always a lively topic everywhere I go, and regularly misunderstood by industry leading CCTV equipment manufacturers, engineers, and other video professionals. Should we correct, when do we correct, how do we correct, and of course the why. I’ve done a few short posts on the topic in the past (here's one), but this will be in a little more detail. Still writing on the fly, just going to break it down into a few posts over time.

The Basics of Analog Video

We’ll start with a few basic understandings, I hope. First, that in the analog world there are no pixels. Second, that when an NTSC or PAL analog video signal is digitized uncompressed according to the ITU Rec. 601 Standard, the process creates non-square pixels. For uncompressed NTSC that pixel matrix is 720 x 486, and for PAL it is 720 x 576. This is important, as it is the foundation of this entire discussion. There are multiple other video standards that relate to converting an analog NTSC or PAL signal to digital, such as DV, SIF, CIF, and others. The vast majority of these standards, when implemented properly, store the video as non-square pixels.

Unfortunately, to display or print those non-square pixels via a computer properly so that objects depicted aren’t distorted, you’ll need to make those pixels square. No matter how you skin the catfish, this process is going to involve interpolation. No ifs, ands or buts about it.

In the next post, which I’ll get to when I damn well get to it, we’ll talk about Storage Aspect Ratio (SAR), Pixel Aspect Ratio (PAR), and Display Aspect Ratio (DAR). Feel free to let me know if you disagree with anything so far via the comments. Thanks, and be safe out there my friends. More soon!

P.S. - Sorry about the outburst there. I think it's healthy to make myself chuckle occasionally. :)

Links to Articles in DCCTV & Aspect Ratio Series

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