When Capcom decided to move its Marvel series' sprites from the CP-S II games to the soon-to-be-a-standard NAOMI platform five years ago with its popular Marvel vs. Capcom 2, some realized that something just wasn't right with the game's graphics. More than a matter of design (which was also there), there was a problem in the execution. The game was presented at a high resolution (640 x 480 pixels), but the sprites weren't displayed accordingly, since they had been drawn for a much lower resolution display originally, the one of the old CP-S II hardware, and they had to be upscaled in order to preserve the sprites' size and aspect ratio. Even worse, every other graphic material in the game, including backgrounds, were actually designed at hi-res, emphazising the denaturalized sprites' presentation, which had been anti-aliased too in order to mask the inherent pixelation.

Sadly, this way of conceiving 2D graphics became common use, not only on arcade games which made use of pre-drawn sprites, but also for arcade-to-home ports for that new wave of 128-bit consoles, and even original games which weren't "ports" at all. Today, it is odd the case of an originally low-res game which is displayed at its design resolution, but there actually are more people concerned about the subject, hence a new term has appeared to name the phenomenon: fake low-res. So let's try to explain here what is it all about in order to make it accessible for more people to see the differences by themselves.

The truth is that Capcom didn't invent the thing with its MVC2. Some years before, an arcade fighting game supported by Sega's ST-V system called Astra Super Stars had already displayed low-res-designed characters at a hi-res on-screen presentation and with hi-res-designed backgrounds. Previously, some 2D games for the N64 (Tactics Ogre 64, Yuke! Yuke! Trouble Makers...) had already used a hi-res display for low-res-designed graphics. But even before than that, fake low-res was a common way of graphic presentation of computer games, such as the arcade ports for Sharp's X 68000 computer. The X 68000 could output true low resolutions at 15 kHz, but, for some reason, some developers chose the 31 kHz display for their low-res games. 

First technicality here - the monitor's operating speed. Display resolutions depends on the monitor's operating speed. Basically, a CRT monitor draws a maximum number of horizontal lines per second (clock speed), and, according to it, the CRT monitor is classified as a low-res monitor or a hi-res one (although there also were extended-resolution and medium-resolution monitors at their time they were abandoned pretty soon). Given this way of drawing the image, that is, by horizontal lines, any picture displayed by a CRT monitor is visually divided into that - horizontal lines, which implies there's a blank space between every couple of drawn lines. That space, which takes the form of a thin black line, is informally called scanline, and, obviously, the lower the resolution (and the bigger the monitor) is, the more noticeable the scanlines are. It's important to understand that the old (and not-so-old) sprite-based video-games were conceived with that in mind, that is, the graphic artists did draw the games' graphics knowing that the picture would get interpolated black lines, hence they were implemented for this type of presentation.

Getting back to the ciphers, a low-res monitor displays 262 horizontal lines on screen (that's the vertical resolution) which are refreshed (updated) 60 times per second. 262 lines by 60 times per second means that a low-res monitor draws 15,700 lines per second in total. That is a frequency of 15 kHz. So that's why whenever someone talks about 15 kHz displays he's refering to low resolutions. Although the truth is that a low-res monitor never actually displays the whole 262 lines. A given number of those lines just refers to the time the monitor needs to restart to draw the lines from the top of the screen once it reached the bottom. Actually, a low-res monitor will display a maximum of 240 lines on screen. That's why the vertical resolution of so many games is of 240 pixels, though 224 and even 192 pixels were also quite of a standard, depending on the year and that moment's CRT technology.

Conventionally, those monitors capable to draw 480 horizontal lines (that is, they're able to work at 31 kHz) or more are high resolution monitors. Your standard CRT TV works at 15 kHz but it doesn't at 31 kHz, while your standard CRT VGA monitor for your PC, just the contrary. Due to how a CRT works, which opposes to the digital display of a modern set (TFT monitors, plasma TV's...), there's a wide range of horizontal resolutions and pixel's proportions (yeah, pixels don't have to be square) which can be displayed at full screen on a CRT monitor, hence the different resolutions which both, arcade games and home consoles have been using since their beginning. If you're really interested in the subject, check out this site, which explains it in detail.

Indeed, CRT's are so versatile that a 15 kHz monitor/TV is able to display a hi-res picture. That's a process which involves converting a natively 31 kHz picture into a 15 kHz one and it's called interlacing. It does exactly that - filling the blank space (scanlines) between the lines by doing a second scan. As they're two different scans done consecutively, the process generates some visual inconsistencies, like flickering, and will never give the same image quality to the one you'd get from a true 31 kHz (VGA) display. Even so, some home systems (like the Nintendo 64) work only on interlaced modes, that is, they output the picture at hi-res but to be displayed only at 15 kHz. It was the only way to get hi-res games work at that moment's standard TV's. Though one comes to think if hi-res was really necessary for that result and given the memory limitations, but hey. So, and this is the important thing here, 15 kHz does not imply necessarily low-res, but also interlaced hi-res.

But what about the opposite. What if you want to adapt a low-res (15 kHz) picture for a 31 kHz display. Well, that involves upscaling. And, since it has to be done by the renderer hardware, it will be digital upscaling, your on-screen presentation will be pixel-doubled, and, technically, the result is a true hi-res picture. To illustrate:

The point is that some popular hi-res systems (DC, PS2, NAOMI and Atomiswave, etcetera), like the X 68000 at its moment, are hybrid systems - They are able to output true low-res modes. That's not all - The X 68000 was sold with a multi-sync monitor, but some systems like the PS2 are intended to be played through a 15 kHz monitor, so any hi-res-rendered game (which includes the fake low-res ones) needs to be interlaced for a 15 kHz display. 

Why do this, then? As I mentioned, different systems do often use different resolutions. If you're porting, for instance, a CP-S game like Street Fighter II to the Dreamcast you'll find out that the 15 kHz resolution modes of the latter have little to do with the CP-S resolution. Indeed, the CP-S (and its successors) made use of a quite odd resolution (384 x 224). Its aspect ratio was still of 4:3, though, since the pixels were prominently oblong. Dreamcast's low res mode is of 320 x 240, not very suitable to reproduce the original graphics at full screen without altering the aspect ratio and keeping part of the actual area out of the screen. While this is likely the most edgy example one could think of, it serves well to explain the main reason behind the phenomenon, and is also applicable to the games which just use old sprites on new hardware (like The King of Fighters Neo Wave, to name one) or emulated iterations with (almost) no actual resolution discrepancies between the target console and the original system. Basically, game developers don't want to waste their efforts on redrawing graphics, re-adapting the game area and adjusting aspect ratios in order to get a full-screen display when they just can leave it on the hardware's hands. By using the hardware's scaling features at 640 x 480 pixels of display resolution, they get the picture (whatever the resolution and pixel's shape it originally had) to fill the whole screen, thus, preserving the aspect ratio and the visible area, but also denaturalizing the game's look, adding refresh rate issues and introducing an abrasive problem of visual (im)precision usually stressed by hardware filters. Blatantly enough, most of these fake low-res ports (due to the fact they're usually PS2 games) are not 31 kHz-compatible despite being rendered internally at a 31 kHz resolution. That is, they take a low-res game, convert it into a hi-res one to finally interlace the picture for a 15 kHz display. Hilarious at best, if you ask.

Actually, all this dissertation is only of relevance for those into RGB monitors, should I add. It's totally pointless to care about fake low-res and, at the same time, using diluted outputs such as composite or S-Video. Scanlines are a must if we're speaking about visual precision and true low resolutions, and those are only effective with an RGB signal. Obviously, it will also be useless for the ones who use non-CRT monitors, since any low-res picture will automatically be upscaled by the apparatus, no matter how it is output by the gaming system. So now you know, this is another effect of our digital era, full of standardization discrepancies and lacking of people who actually do care a bit about visual art, visual honesty.