I stand corrected. First, I did find a reference to Pixel Aspect Ratio (PAR) and down sampling uncompressed NTSC video in the joint Forensic Imaging and Multi-media Glossary (PDF) published by the IAI and LEVA, the final version of which was released in July, 2006. Second, in an effort to simplify this discussion, I’ve over simplified the Interpolation Methodology I described in Part 2. I will be expanding on that in this post, in far more detail than I had originally intended.

I was hoping I’d be able to wrap this series up with this post, but it’s clear to me now that I won’t be able to. There’s simply too much to cover and I’m certain that at least a few are still scratching your heads (like I was), wondering why we shouldn’t just rely on the pixel matrices to calculate Pixel Aspect Ratio (PAR) from analog sources. It is critically important to understand that I am approaching this topic from a forensic perspective, with the goal of standardizing the methodology used for forensic processing, interpretation, and presentation.

Assumptions can be dangerous things. Let’s say you assumed your software was displaying your AVI video file at the proper aspect ratio (ratio of width-to-height). Well, chances are pretty good you’d be wrong…no matter what software you’re using. What!? So the perp in my video evidence may be thinner and taller OR shorter and pudgy-er!? Yep.

Fact – the AVI file format does not provide a standardized way to report aspect ratio. Due to this shortcoming, applications cannot calculate the proper Display Aspect Ratio (DAR) if it is supposed to be different than the Storage/Sample Aspect Ratio (SAR). Therefore, applications "assume" a square Pixel Aspect Ratio (PAR) and display the video at it's SAR, even if that's not correct...which is generally the case when dealing with DCCTV evidence.

Moral of this story? Don’t assume.

Understanding video standards is fundamental to aspect ratio correction. Back in the predominantly analog days we had three main standards referenced or used for most video recordings; NTSC, PAL, and SECAM. Then in the early ‘90’s came the first digital multimedia frameworks to reach the average consumer; QuickTime and, shortly thereafter, Video for Windows (VfW).

Today we have dozens of multimedia frameworks, digital video and digital display standards, all of which lead to a great deal of confusion regarding the plethora of acronyms and what they truly mean. AVC or H.264? HEVC or H.265? CIF or SIF? Don’t even get me started on the profiles and parameters available for each standard, as the combinations are truly mindboggling. When it comes to proper Display Aspect Ratio (DAR) though, it really boils down to “Are the originally recorded pixels square or non-square?”

I probably should’ve just dropped the mic after the last post, but we’re going to continue on. I’m not one for dropping names, and in this case I don’t have to either. Everyone has gotten this wrong at some point, and I mean everyone. The people working on related standards; the people making the world’s leading non-linear professional editing systems; the people who make a living professionally processing and transcoding video; the people making multimedia playback software; the people making DCCTV systems; the people making operating systems; and yes, even forensic video and digital evidence technicians and analysts. We’re all human, my friends. It is a long, convoluted, complex process with its very foundation based on sampling an analog signal.

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.

Several other things I should be doing on a Saturday morning, but I find myself anxious to continue this discussion. Maybe it’s because although multiple industry Best Practice documents talk about correcting Aspect Ratio, none of them discuss the proper way to do it. It could also be my new coffee maker, which I'm hypothesizing has increased my caffeine intake substantially, although I have not increased my coffee intake. Who knows. Anyway, let’s start by recapping Part 1.

On vacation, but thought I'd comment on this topic before getting on the Harley to go run some errands in the cold Pacific Northwest. As pointed out in someone else's recent blog post, MPEG-4 can leverage what is referred to as a Sample Aspect Ratio (SAR)...not to be confused with Storage Aspect Ratio (SAR) or Signal Aspect Ratio (SAR). It's important to note that in the case of MPEG-4, the Sample Aspect Ratio is the Pixel Aspect Ratio (PAR); they are one and the same.

It's also important to note, again, that regarless of any of these numbers, the shape of the samples from an analog source ARE NOT DEFINED BY THE NUMBER OF LINES.

Oh, one more thing...most multimedia NLE and encoding applications provide precise control of all of these settings.  Just an FYI.  All the best my friends.