Palette LUT — NTSC / PAL switching

What the LUT stores

hdl/palette_lut.sv is a 256-entry × 24-bit RAM. The address is the 8-bit Atari hue:luma value coming out of color_resolver (bits[7:4] = hue 0..15, bits[3:1] = luma 0..7, bit[0] mirrors luma’s LSB and is otherwise ignored). The data is straight RGB888.

The TMDS / DVI scan-out path reads palette_lut[resolved_color] once per pixel-clock and feeds the 24-bit result into the encoder.

Why two reference palettes

Atari’s NTSC and PAL hardware encode the colour-burst phase differently. The hue → carrier-phase mapping is rotated, the saturation is slightly different, and PAL’s alternating-line phase reversal kills the chroma-distortion issue NTSC has. Result: the same hue:luma byte produces visibly different colours on a real NTSC vs PAL Atari. Programs that target one region picked their hue values to look right under that region’s palette.

To stay colour-accurate, we need both reference palettes available.

Two ways to load

The LUT is fully writeable at runtime via the chiplet-ext registers:

$D483 PAL_R    write red byte
$D484 PAL_G    write green byte
$D485 PAL_B    write blue byte
$D486 PAL_IDX  write 8-bit Atari index — commits {R,G,B} into entry IDX

Two delivery options for the boot defaults:

1. Synth-time bake-in. Pass INIT_FILE to palette_lut:

   palette_lut #(.INIT_FILE("hdl/palette/atari_ntsc.hex")) u_lut (...);

   Two reference tables ship in the repo:

   • hdl/palette/atari_ntsc.hex — NTSC-shaped colour wheel
   • hdl/palette/atari_pal.hex — PAL-shaped colour wheel

   Pick whichever matches the dominant target region. The other one ships in firmware as a 768-byte blob and gets paged in via the chiplet-ext writes when the user toggles region.

2. Pure firmware-driven. Leave INIT_FILE empty (LUT comes up black at /G_RST), and have the PS-side boot firmware push 256 entries via writes to $D483-$D486 during the cold-boot sequence. Region toggle = re-push the other table (≈ 1 ms at 1.79 MHz bus).

We default to the synth-time bake-in since it gives a sensible power-on display before any firmware has run.

File format

atari_ntsc.hex / atari_pal.hex are plain $readmemh-compatible hex files: 256 lines, one 24-bit RGB888 per line, MSB = R, LSB = B. Header lines (//) are tolerated by $readmemh so the files are self-documenting.

The two reference tables in this repo are derived approximations based on the NTSC and PAL hue-rotation tables widely quoted in the Atari hardware manuals + Altirra. They aren’t claimed to be pixel-exact reproductions of any specific real silicon’s analogue colour-burst output — the chiplet-ext write path lets you replace them with whatever your favourite reference says.