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.

1080 for sure, that's in the name.  They're all the same thing folks, and I'll be darned if I can find any formal video specification referencing any of them, so they share that too.  Is it all just marketing BS?  No, but surely the confusion has been leveraged by some DCCTV manufacturers, resellers, and the like to their benefit.

A couple of years ago I did a series of posts on aspect ratio correction of DCCTV recordings; more specifically DCCTV recordings captured by analog CCTV cameras, and correcting Display Aspect Ratio (DAR) for forensic interpretation. I pointed out in my posts and the subsequent Video & Display Standards Chart, that the one exception that I'm aware of where a DCCTV recording captured by an analog CCTV camera shouldn't be displayed 4:3, was the relatively new 960H format.

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?”

* Updated with Corrected Images & Explanations. 

After the break you'll find several images of a bogus Person of Interest (PoI) that were recorded by a DCCTV system. Two different analog CCTV cameras with built-in IR illuminators were connected to the black-box, h.264 DVR. These JPG images were exported from the DVR’s proprietary player. All of these images exported at 704 pixels by 480 pixels. When the recorded video is played back via the proprietary player it is displayed at 630 x 455; however, analysis of the proprietary file and exported AVI files reveals both of those contain a 704 x 480 video stream.

Your task, should you choose to accept it, is to:

• Describe the PoI’s clothing items from these images as you would for producing a BOLO. Note any issues that may affect your description.
• Identify the single most important correction that should be made to these images prior to printing. (BONUS - Why does this correction need to be made, and what tipped you off to it?)

If you’ve taken one of my recovery classes or attended one of my presentations on the topic at a LEVA conference or other event, you may have seen these examples.

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.

Some more offline comments and testing recently have raised a critical issue; the use of Square Pixel Sampling and Non-Square Sample Formats. Before that though, let's talk about validation testing. When we get to this level of detail especially, it is important to validate our processing, tools, and complete process. Correcting Aspect Ratio is certainly no exception. In fact, I think we would all agree that when this level of detail really matters, you must validate. Lots of great resources out there about that topic.

A great overview on the history of aspect ratio. I was going to link it to our library, but apparently I have to update the code for the Vimeo videos. Ugh. I'll get to it, just seems my to-do list is a bit like the NTFS $MFT...it only grows, never shrinks! ;)

The Changing Shape of Cinema: The History of Aspect Ratio from FilmmakerIQ.com on Vimeo.