People are often surprised when I tell them Standard Definition (SD) Video can look gorgeous in their Home Theater. The sad truth, alas, is that MOST people have never ever seen SD Video "done right"! The problem is, at these resolutions (and data rates), there is NO margin for error. ANY mistake in the content creation or video processing -- any corner cut in the setup at any stage -- WILL produce image defects that viewers can not ignore. And such defects, once introduced, can NOT be corrected by any sort of subsequent image enhancement processing. The critical information has been permanently lost. At best you can blur such defects to make them less annoying.
This post discusses the technical details of image "Aspect Ratio", as found on traditional, SD-DVD movie discs, with tips for avoiding some common mistakes which will reduce your SD viewing quality.
First let's get past the "Why bother?" question. Even at this date there are tons of wonderful movies and TV shows which have not been released on Blu-ray or UHD discs, and quite possibly never will be! SD Video on home media has been sold in a variety of forms, including cassette tape cartridges and Laser Disc for example, but far and away the most successful physical media format for SD Video has been the traditional SD-DVD optical disc. (These days, SD Video can also be "streamed", but the same considerations apply -- along with the distinct possibility that what you are streaming has ALREADY been damaged in the process of creating the low-data-rate streaming file being sold to you.)
By the time SD-DVDs were launched, movies had already been made in a large variety of image shapes. Way back in 1932, the Academy of Motion Picture Arts and Sciences standardized the Academy Aspect Ratio: 1.375. That is the image was 1.375 times as wide as it was tall -- just a little wider than square (which would be 1.000).
The first TVs adopted basically the same shape -- an aspect ratio of 4:3, which is 1.333... times as wide as it is tall. Movie makers countered in the 50s by making movies much wider to attract audiences back into theaters. There are a variety of such "Scope" film formats -- some of them ridiculously wide. (Which also introduced problems in where to place the speakers!)
By the time SD-DVD optical discs launched in the late 90s, TV makers were already in the process of countering back. They were getting ready to launch HDTVs, which had an aspect ratio of 16:9 -- 1.777... Scope movies which were wider than that would play with black "Letter Box Bars" above and below the image, padding the image to the 16:9 shape of the TV. Traditional TV programs which were narrower than that would play with "Pillar Box Bars" to the left and right of the image.
And so SD-DVD optical media was designed to be a TRANSITIONAL format -- able to serve BOTH such uses on traditional 4:3 TVs and the newfangled, 16:9 HDTVs.
In NTSC Television markets (such as here in the US), standard definition DIGITAL video was thus defined as an intermediate shape. Each frame of video on the disc is made up of 480 lines of 720 pixels each. 720 by 480, if you do the math, is an aspect ratio of 1.5, precisely.
So how do you get a traditional 4:3 (= 1.333) movie or TV show and a new-fangled 16:9 (=1.777...) widescreen movie or TV show to work in an on-disc pixel matrix which is NEITHER of those?
The cunning plan was to use Non-square Pixels!
That is, each SD-DVD disc would include info saying whether the content on the disc was intended to be 4:3 or 16:9 -- when VIEWED. If the content was intended to be viewed as 4:3, then the pixels coming off the disc were to be INTERPRETED as rectangles that were taller than they were wide! If the content was intended to be viewed as 16:9, then the pixels coming off the disc were to be INTERPRETED as rectangles that were wider than they were tall!
"Interpreted" here means how that set of pixels was to be rendered on a given TV. For TVs that were fundamentally Analog in nature -- such as traditional Cathode Ray Tube TVs -- the appearance of a given content pixel on screen was controlled by the timing and length of the ray sweep and the spacing between lines of rays as the rays impinged on the light emitting phosphors of the screen. For the newfangled, digital HDTVs, the trick was to realize there were lots more pixels on the HDTV screen than just the 720x480 array of SD pixels. A 1080p TV for example would have 1080 lines of 1920 pixels each. So the circuitry in the TV would light up the correct set of adjacent, smaller, HD pixels to emulate the non-square shape of the SD content pixels. A process called "up-scaling" of 480 video to 1080 display.
I should mention at this point that HDTV pixels are SQUARE, as you can see from the math. That is, 1920/1080 = 1.777... = 16:9.
So what if the movie you are playing is WIDER than widescreen? I.e., has a shape which is wider than 16:9? In that case the content on disc would include Letter Box Bars top and bottom to pad the movie shape to 16:9 -- the widest allowed format on the disc itself.
Which now brings us to the rather critical topic of Anamorphic vs. non-Anamorphic DVDs! You see, studios were not required to put their widescreen movies on disc as 16:9. They could put them on disc as 4:3, and just include MORE Letter Box Bar black pixels top and bottom to pad the wide move to that squarer, 4:3 shape.
Why would studios do this? Because when SD-DVDs first launched in the market it was still the case that almost all the TVs out there were 4:3. So why not author discs that would fit that 4:3 shape precisely?
The problem is -- think back to Non-square pixels -- whether the content on disc is authored as 4:3 or 16:9, the data actually written onto the disc is still made up of video frames which are always and only 720x480 pixels. So if you relegate a bunch of those 720 lines to the role of Letter Box Bars you've basically wasted a bunch of pixels! They are no longer carrying useful content. They are just padding.
However, on a 4:3 TV, that's the best you could do anyway. When a widescreen or wider than widescreen movie is viewed on a 4:3 TV it is NECESSARILY going to look like a thin rectangular strip with large amounts of black (the Letter Box Bars) above and below.
But the problem is even worse when you play such a disc on a 16:9 HDTV. The TV sees the content coming off the disc as 4:3 -- because that's how the disc describes itself. And so the TV wants to display that as a 4:3 image in the middle of the screen with Pillar Box Bars padding the left and right sides with black. But if the content being played is actually a widescreen movie (or wider), the content ALSO includes Letter Box Bars -- in the content on the disc -- padding the top and bottom of that movie to the 4:3 shape.
And so you see a tiny widescreen movie playing in the middle of your 16:9 screen with black around ALL FOUR sides!
Typical HDTVs have Zoom features to correct for this, but the problem is you've already wasted the limited resolution on the disc by relegating a large chunk of lines top and bottom to be nothing but Letter Box Bars.
The answer of course is not to buy such a disc for playback on your 16:9 TV. Instead, what you want is to buy a disc that has been authored to use the 720x480 pixels per frame as a 16:9 image!
And that's the first takeaway from this discussion. Because the trick is, how do you know whether a given SD-DVD disc is authored that way or not?
The answer is, an SD-DVD disc authored as 16:9 will be labeled either as "Anamorphic" or "Enhanced for 16:9 TVs".
When the studios started making such discs, there was confusion about how to label them. Sometimes the 16:9 version was labeled "Widescreen" and the 4:3 version was labeled "Full Screen". But studios had ALREADY been in the practice of labeling widescreen (and wider than widescreen) movies as "Widescreen" even when they were actually authored on disc as a 4:3 image! "Widescreen" in this context simply meant the studio had not cropped off the left and right sides of the widescreen movie to bung it into the squarer, 4:3 hole on disc.
So if you are using a 16:9 TV -- as most of us are these days -- the safe thing to do is to look for the terms "Anamorphic" or "Enhanced for 16:9 TVs" when choosing an SD-DVD disc. A disc labeled only "Widescreen" may or may not actually have been authored as 16:9.
And again, the point here is to get increased resolution from your SD Video movie. How? Because more (or even ALL) of the 720x480 pixels per frame are actually being used to hold real movie content. All due to the magic of non-square pixels!
Which brings us to the opposite side of this problem. Suppose you want to play an older movie, or a traditional TV show which is inherently 4:3 on your 16:9 TV?
Some people prefer to use a Zoom mode to stretch the 4:3 image to fill their 16:9 screen. But understand that there is no free lunch here. If you Zoom the image both horizontally and vertically you can fill the screen left to right, but a significant portion of the movie will be "cropped" -- lost beyond the top and bottom edges of your screen.
And if you stretch the 4:3 image only horizontally -- to avoid such cropping -- you will have distorted the image. Circles now look like wide ovals.
So folks who are particular about viewing movies as they were intended to be seen always want to preserve the Original Aspect Ratio -- which in this case means adding Pillar Box Bars left and right to pad the 4:3 content out to the 16:9 shape of the TV.
And here's the second important takeaway from this discussion:
You want to make sure you ONLY add the Pillar Box Bars as PART of the process of up-scalling the SD 4:3 Video image to the higher resolution of your HD or 4K TV!
Why? Again we have to go back to how Digital SD Video works. Whether the image is 4:3 or 16:9, the SAME 720x480 pixels are used -- along with the flag describing the intended viewing shape.
This is true not only on disc, but also on the HDMI cable carrying any SD Video to your TV.
That is, if you send 480i or 480p video to your TV -- expecting the TV to do the up-scaling to 1080p, or 4K, for display -- the data on the HDMI cable is still 720x480 pixels per frame REGARDLESS of whether you are sending a 4:3 or 16:9 image!
Why would you do this? Well you may believe your TV is better at up-scaling than your source device. For example, typical cable and satellite TV set top boxes are notoriously Godawful at video processing. So you might want to have them send any SD resolution TV channels you are watching to the TV as 480i to let the TV do the heavy lifting of video processing.
And having decided to do that for those, the temptation is to do the same with your other source devices.
Now here's the problem. Most source devices able to output SD Video on HDMI will offer a setting which controls whether pillar box bars are included in the output.
Consider what happens if you turn that On while the output resolution remains set at either 480i or 480p. The content is a matrix of 720x480 pixels flagged as 4:3. If the source device is going to add the Pillar Box Bars, that means converting that content to 16:9 while reserving a bunch of pixels on the left and right sides to make up the Pillar Box Bars.
But the resulting, 16:9 output is STILL a matrix of 720x480 pixels! You've just changed the flag saying how those pixels will be interpreted for viewing -- from rectangles that are taller than they are wide to rectangles that are wider than they are tall -- while also setting a bunch of those pixels on the left and right side of each line to Black!
Think about that. This means you are taking 720 pixels of content data across any given line of the movie and SQUEEZING it into far fewer pixels in the center of the line -- because the outer pixels on each line now have to be set to Black.
And that means you have CLOBBERED Horizontal Resolution!
Indeed, the problem is even WORSE for color than it is for gray scale details. Why? Because for technical reasons I'll discuss in some future post, color information is only recorded across each line HALF as often as gray scale information.
This is not a small thing. If you see SD Video that's been sent to your TV this way you will almost certainly decide it looks terrible.
So what to do? Instead of telling the source device to add the pillar box bars, let the source device send your 4:3 SD Video across the HDMI cable AS still 4:3. Which means the TV now has to take on the job of adding the Pillar Box Bars.
But the TV is also up-scaling that 480 video to 1080p or 4K. And so it has LOTS MORE pixels available across each line! Plenty of pixels to hold all the information from the original 720 pixels across each SD Video content line with enough still left over to become Pillar Box Bars.
Now, there HAVE BEEN examples out there of HDTVs that screw this up. That is they have the pretty egregious bug of adding the Pillar Box Bars while the video REMAINS at SD resolution, and then scaling the result of THAT up to 1080p. But most TVs won't screw this up.
Meanwhile, if you are OK with your source devices doing the upscaling to 1080p or 4K, then they can ALSO, safely add the Pillar Box Bars. So for example, if you are playing a 4:3 SD-DVD disc and want the player to output 1080p or 4K video to your TV, it is perfectly OK to have the player also add the Pillar Box Bars. Just as with TVs, there HAVE BEEN examples of disc players (for example) that screw this up, but most players will get it right.
So to summarize:
1) Non-Square SD Video pixels are your friend. Use them wisely!
2) When buying a widescreen (16:9) or wider than widescreen movie on SD-DVD, be sure to look for labelling that says it is either "Anamorphic" or "Enhanced for 16:9 TVs" -- which both mean the same thing. And,
3) If you decide to play 4:3 SD Video content and have the source device send that video as 480i or 480p to your TV -- for example by selecting "Source Direct" or "Native" output in typical source devices -- be SURE the source device is not ALSO trying to add the Pillar Box Bars to that SD Video output!
--Bob