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.

This brief (approx. 9min.) introduction into the world of digital video evidence is intended to provide law enforcement professionals with a better understanding of the basic concepts and related issues.  It was geared towards first responders, who in many cases are the ones initially seizing video evidence.

I posted a new page last week under our Professional Community menu called Free DME & FVA Tools. Hope you find it helpful.

It’s sometimes difficult for traditional Computer Forensic (CF) examiners to understand why they should treat video and multimedia any differently than other types of digital evidence. After all, a bit is a bit, and a byte is a byte. Right? CF examiners are typically highly trained and highly technical people. If anyone is going to understand how to recover and interpret multimedia data, one would think that a traditional CF examiner would be at or near the top of your go-to list. The problem with this assumption is that multimedia data is fundamentally different than most other types of data, and in more than one way.

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.

Medical experts, accident reconstruction experts, and other types of experts are frequently asked to interpret data from images that were obtained from video evidence. When these experts have no training or background in processing multimedia evidence, and/or make no effort to consult someone who does, bad things generally happen. Really bad things, like having all of their evidence thrown out of court, for one.

Magnet Forensics launched the evolution of DVR Examiner today, Magnet WITNESS.  WITNESS includes all of the capabilities of DVR Examiner plus many new features, essentially creating a single solution to acquire, review, analyze and report on all of the video evidence from your case, regardless of source.  Proprietary DCCTV systems and files, cloud CCTV sources such as Ring & Arlo, MP4 & AVI exports from other sources such as in-car and body worn systems.

Unlike DVR Examiner, WITNESS includes the ability to create sub-clips, create synchronized previews, convert proprietary DCCTV files and more.  Learn more at https://www.magnetforensics.com/products/magnet-witness/

"Who designed this user interface, Stevie Wonder?" Actual statement from a LE technician and point well taken when it comes to proprietary DCCTV players. They're often horribly designed, and like all multimedia players/editors/tools regardless of who makes them, they are time & resource dependent (e.g. hardware resources, drivers, frameworks, codecs, etc.).

Until about two years ago, it wasn't that often that I’d encounter Variable Frame Rate (VFR) video evidence, unless of course the case included video recorded by mobile phones. Times, they are a changing my friend.

DVR/NVR manufacturers are leveraging the advantages of VFR more often these days. And why shouldn't they be, especially when we’re seeing even the high-end professional video recording equipment start to use VFR more frequently. Throw in the want/need to get to Ultra HD before the next guy, and suddenly you've got a decent argument for VFR.