For some nostalgics it’s not merely enough to emulate the console or computer – they want to emulate the CRT they used it on too!
The nature of cathode-ray tube displays distinctly affected how images are presented by them. For example, its electron gun(s, three inside colour CRTs) sweep from left to right, illuminating one horizontal line and then another underneath. Each line is known as a “scan line”, and a distinct dark gap is formed between them.
These gaps form what looks like a grill over the image. Furthermore, because the phosphors in each scan line are actually struck by the electron guns progressively at a descending angle from the left side of the screen to the right, moiré patterns – visible interference such as ripples or waves – can appear, especially with dithered or half-toned graphics.
Colour CRTs work by having three electron guns, each of which strike phosphors that emit red, green or blue light respectively. This combination of phosphors appears over and over again in a scanline, and our mind combines the brightness of each of these to mix them into a distinct fourth colour.
While LCD and LED displays also use groups of colours to form pixels, these pixels are much smaller, and thus when classic video games are viewed on an LCD or LED monitor or TV, the colours may seem strange or wrong to some players accustomed to the CRT image.
Finally, prior to the era of “flat-screen” CRT displays, video game graphics tended to “balloon” out from the centre, following the curved edges of the tube and creating noticeable distortion in straight lines and rectangular shapes.
All of these effects combine in varying degrees to create the “stereotypical” CRT image – although the final product depended heavily on the model of CRT and how it was calibrated. Some had more distinctive scan lines, others had more ballooning; some used one method of arranging red, green and blue phosphors, others another.
Ultimately, it depended on what brand and model of TV you plugged your console into, or what picture tube they put into the cabinet at the arcade. Therefore, many emulators provide controls to allow users to add and adjust some or all of these effects to their users’ individual tastes.
A “shader” is an intermediate computer program (usually executed by an additional, specialised CPU that is part of modern video hardware called a GPU) that processes video output from a primary program (for example a video game) typically to add visual effects.
Each one has varying options, but most at a minimum emulate the effects of scan-lining on “aperture grille”-style (one method of ensuring accurate phosphor activation by individual red, green and blue electron guns) CRT displays – the most popular type of CRT emulation due to the crispness of the scan lines it creates.
Others can emulate screen curvature, defocussing, colour convergence (or more accurately the lack thereof) and more, although configuring them correctly can prove to be a complex endeavour. It all depends on if you’re motivated enough to expend the effort required to replicate the old Sony TV you (or your Dad) had in your (their) room as a child, or not.
Libretro isn’t a single piece of software, but a series of software “libraries” that emulate various consoles, such as the Nintendo NES, Super Nintendo, Atari 2600, and Sony Playstation; and computer systems such as the Atari ST and Commodore Amiga.
These libraries have a common API, or application interface, which allows for the creation of customised, detached front-end software such as RetroArch.
RetroArch and Libretro are available for Windows, macOS and Android, and can be downloaded from libretro.com
Variations in the personal experiences of vintage video-game players and computer users highlight the impact the quirky natures of cathode-ray tube-based televisions and monitors had on those experiences. CRT emulation allows some to replicate those experiences, while others can experience them for the first time.