Why Retro Game Art Was Designed for the CRT Scanline Blur and Phosphor Glow

Load up Sonic the Hedgehog on a Mega Drive and look at the waterfall on Green Hill Zone. On a modern display through a cheap converter, that cascade of blue pixels is a jumble of hard squares with a jagged edge where the transparency should be. On a 1991 CRT, the water appears translucent. The tiles melt into the background. That is not a coincidence. It is a direct result of why retro games were designed for CRTs: the artists used the physical behaviour of the electron beam and the phosphor mask as an intentional part of the art pipeline. The image you saw on a Sony Trinitron or a PVM was not the pixel grid the programmer sent; it was the pixel grid interpreted by shadow mask, scanline gap, and video signal blending. Grasp that process and you see the game as it was meant to look. Miss it and you get a smeared approximation.

How The Scanline Gap Works

The 240p signal that a Mega Drive or SNES outputs runs at a 15.734 kHz horizontal scan rate (NTSC) or 15.625 kHz (PAL). The CRT electron beam sweeps across the screen line by line, and between each line the beam is blanked. That blank line is the scanline gap. On a consumer CRT with 250 to 350 TVL horizontal resolution, the beam spot size at centre is around 0.5 to 1.0 mm. The gap is not a defect. It is a tool.

Retro artists used a technique called dithering to simulate colours the palette did not have. The NES PPU (RP2C02) could display only 25 simultaneous colours out of a 54-colour palette. The SNES PPU (S-PPU1 and S-PPU2) had 256 out of 32768. Still limited. To create the illusion of a third shade between two colours, the artist placed pixels of colour A and colour B in a checkerboard pattern. On a modern display with no scanline gap, those pixels sit edge to edge and the pattern resolves into a sharp, ugly grid. On a CRT, the gap and the phosphor bloom spread each pixel horizontally and vertically. The two colours mix in the eye. The result is a perceived intermediate shade.

Test It Yourself

Grab the 240p Test Suite (Artemio Urbina, 2011). It includes patterns that demonstrate this dithering effect. Load it on a Super Famicom through an RGB SCART cable into a RetroTINK 5X-Pro set to the FirebrandX CRT profile, and the dither pattern resolves into a smooth gradient. Feed the same signal through a generic HDMI converter that treats 240p as 480i. The deinterlacing discards half the vertical resolution, turning the dither into a flickering mess. This failure is common: a scaler chip designed for DVD players sees the 240p signal and applies a bob or weave deinterlace, destroying the scanline gap entirely.

The Mask As A Design Element

The pixel art you see on a modern LCD was designed for a CRT shadow mask or aperture grille. The shadow mask, a sheet of Invar or low-carbon steel with tiny holes, sits in front of the phosphor layer. A consumer CRT shadow mask aperture pitch ranges from 0.6 mm to 0.9 mm. The Sony Trinitron aperture grille uses vertical wires instead of holes, with a pitch of 0.6 mm to 0.8 mm. Each hole or wire slot corresponds to a triad of red, green, and blue phosphor dots.

The electron gun in a colour CRT fires three beams, one for each primary colour. The beams pass through the mask and hit the phosphor dots, which emit light that decays in under 1 ms to 10% luminance (P22 phosphor). That decay time means the phosphor glows and fades between frames, creating a temporal blend that smooths motion. Pixel artists designing for the NES, SNES, or Mega Drive knew the mask would soften the edges of each sprite. A single-pixel line drawn at 256x240 on the NES PPU would not appear as a razor-thin line on screen. It would bloom to roughly the width of one mask pitch, about 0.6 mm at typical viewing distance.

Why Sub-Pixel Detail Fails On LCD

Pixel art from the 8-bit and 16-bit eras contains intentional sub-pixel detail that only resolves through the mask. The SNES sprite limit of 32 per scanline and the Mega Drive limit of 20 per scanline forced artists to economise on detail, and the mask filled in the gaps. On a modern display with no mask, that sub-pixel detail appears as random noise. A RetroTINK 4K with HDR CRT emulation and a FirebrandX profile can replicate the mask effect by drawing a precise grid of dark lines at the correct pitch. A cheap converter simply sharpens the noise.

The waterfall on Green Hill Zone in Sonic the Hedgehog (Mega Drive, 1991) is the single most cited example of a CRT-dependent visual effect. The Mega Drive VDP (Yamaha YM7101) does not have hardware transparency blending for sprites. The waterfall effect is created by alternating columns of blue tiles with columns of the background tile in a pattern. On a CRT, the scanline gap and the phosphor bloom cause the blue and the background to blur together. The result: convincing translucent water. On a modern display through an OSSC with no filter, the alternating columns resolve as sharp vertical stripes. The water looks opaque and striped.

This is not a bug in the emulation or the scaler. It is the absence of the CRT. The calibrator FirebrandX produces the reference CRT profiles used in the RetroTINK 5X-Pro and the MiSTer FPGA. Those profiles apply a scanline mask, a phosphor bloom filter, and a video signal blending simulation that recreates the waterfall transparency. Set your RetroTINK 5X-Pro to the FirebrandX 240p profile, select the composite video filter, and the waterfall resolves correctly. Without that profile, it fails.

The mClassic applies context-adaptive anti-aliasing but no CRT-specific mask. Its retro mode adds a mild scanline effect, but does not simulate the video signal blending or the phosphor decay. The waterfall still looks striped. To see the effect accurately, use a RetroTINK 4K with the FirebrandX profile for HDR CRT emulation and black frame insertion at 120Hz. Anything less, and you are not seeing the game the artist made.

The Signal That Artists Exploited

The composite video signal standard (NTSC RS-170A or PAL ITU-R BT.470-6) encodes luminance and chrominance onto a single wire. The NTSC colour subcarrier runs at 3.579545 MHz; the PAL subcarrier at 4.43361875 MHz. The chroma information is modulated onto the subcarrier and the luma signal carries the brightness. When a scaler decodes composite video, it must separate the two. The S-ENC encoder in the SNES or the CXA1145 encoder in the Mega Drive outputs a composite signal where the chroma subcarrier interferes with the luma at high-frequency detail, creating a pattern called dot crawl.

Retro artists exploited this. On a CRT, the dot crawl pattern on a 4.2 MHz bandwidth NTSC signal blends adjacent pixels together, creating a new colour at the boundary. The NES PPU output voltage levels (0.0V sync, 0.3V black, 1.0V white per RS-170A) mean that the DAC inside the RP2C02 has limited precision. The video signal blending smooths the sharp transitions that the DAC cannot reproduce cleanly. A pixel art edge that looks harsh in RGB becomes soft in composite because the chroma subcarrier blurs the transition over roughly one scanline width.

What To Use In Hong Kong

For a Hong Kong gamer with a Super Famicom, using composite video through a RetroTINK 5X-Pro with the FirebrandX composite filter gives a more authentic image than using RGB SCART. RGB SCART, with its 75 ohm per channel termination and 5 to 6 MHz bandwidth, shows every hard pixel edge the artist intended to be softened by the composite artifact. Avoid the cheap composite-to-HDMI converter from Sham Shui Po. Its comb filter is designed for broadcast video, not games. That comb filter treats the 240p signal as 480i, introduces 2 to 4 frames of lag, and destroys the very blending effect the artist relied on.

Property	CRT Behaviour	Modern Display Behaviour
Scanline gap	Blank line between each horizontal line; creates vertical dither blend	No gap; pixels touch vertically, dither resolves as sharp grid
Phosphor bloom	Beam spot spreads 0.5-1.0 mm; softens pixel edges	No bloom; pixel edges are sharp, sub-pixel detail appears as noise
Phosphor decay	Glow fades in under 1 ms; creates motion blur that masks flicker	Sample-and-hold; motion blur is zero, stroboscopic flicker visible at 60 Hz
Composite video dot crawl	Chrominance interference creates colour blend at pixel boundaries	Comb filter removes dot crawl; hard edges reveal original pixel grid
Shadow mask pitch	0.6-0.9 mm; grid pattern blends colour triads	No mask; each subpixel is individually addressable, no blending

What A FirebrandX Profile Does

The calibrator FirebrandX produces the reference CRT and HDR profiles used in the RetroTINK 5X-Pro, the RetroTINK 4K, and the MiSTer FPGA. These profiles define what the game should look like by measuring the actual output of a Sony PVM-20L5 (800 TVL) or a Sony BVM-D24E1WU (1000 TVL) and creating a mathematical model of the mask, the bloom, and the composite video artifact. The RetroTINK 5X-Pro menu includes these profiles as presets. Select the FirebrandX 240p profile and the scaler applies a scanline mask at the correct pitch, a phosphor bloom filter, and a composite video blending simulation. The Sonic waterfall transparency effect resolves correctly on a modern 4K OLED.

OSSC Versus RetroTINK

The alternative is the OSSC, a line multiplier with no framebuffer and no composite or S-Video input. The OSSC produces razor-sharp pixels with zero lag (measured in microseconds, not milliseconds), but it does not simulate the CRT mask. The OSSC Pro adds a framebuffer and adaptive line multiplication, but still lacks the FirebrandX profiles. For a Hong Kong gamer with a Super Famicom and a Sony PVM, the OSSC is the wrong choice if you want the CRT look on a modern display. Get the RetroTINK 5X-Pro. Set it to 1440p output with the FirebrandX profile.

Avoid The GBS-Control Shortcut

Do not buy a GBS-Control board from a Sham Shui Po shop and expect the same result. The GBS-Control firmware offers motion-adaptive deinterlacing for 480i PS2 games, but it does not include CRT simulation profiles. The scanline effect on the GBS-Control is a generic line overlay, not a FirebrandX-calibrated mask. The image will look like a game with scanlines drawn on top. It will not look like a game running on a CRT.

The single biggest thing newcomers get wrong is assuming any cheap HDMI converter will do the job. You are converting a low-resolution, often interlaced analogue signal into a digital one your TV understands. Every step adds a potential delay or visual artifact. A typical, inexpensive converter from Sham Shui Po treats a 240p game signal as 480i video, applying a deinterlace that discards half the vertical resolution and adds 2 to 4 frames of lag. The Sonic waterfall does not look transparent. The dithering does not blend. The game feels slippery.

Use a lag-free scaler like the RetroTINK 5X-Pro. It costs more but preserves the 240p signal and applies the FirebrandX CRT profile. If you are in a Hong Kong apartment with limited space and a 4K OLED, the RetroTINK 4K is the ceiling of the market, but the 5X-Pro at 1440p is sufficient. Skip the mClassic. It adds 1-2 ms of lag and applies context-adaptive anti-aliasing, but it does not simulate the CRT mask or the video signal blending. The waterfall still looks wrong. Feed the 240p signal through a scaler that knows what a CRT did to it. That is the only way to see the game as the artist intended.