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?”
There is no such thing as a “pixel” in the analog world. When converting analog video to digital, ITU Rec. 601 (http://en.wikipedia.org/wiki/Rec._601) is the standard referenced by most other digital video standards, including all MPEG and ITU H.26X standards. Rec. 601 is our aspect ratio foundation, if you will. In gist, the Rec. 601 standard samples the analog signal and stores the sampled data at what is referred to as the Sampled/Storage Aspect Ratio (SAR); however, this is rarely how the data should be displayed on a computer or printed in order to accurately represent what was recorded. When using Rec. 601 to digitize NTSC video the resulting SAR contains non-square, rectangular shaped pixels. Computers use square pixels. Shit.
If the SAR isn’t the way it’s supposed to be displayed how do we calculate the DAR? That’s where Pixel Aspect Ratio (PAR) comes in, at least if your file format, software, and the equipment used to create the file support it…and they choose to leverage the option and implement it properly. If you’re reading this, you probably already know that’s rare in the world of DCCTV evidence. As I’ve discussed in the past, the AVI file format is still going strong in the world of digital video evidence and it does NOT support PAR, so you’re application(s) may be making some assumptions for you when playing back your AVI file.
There are a slew of tools and resources out there to help you interrogate your media, and there are tricks that can help you make better assumptions than your software, and then validate them. Knowing your digital video standards is one of them.