Reading the datasheet, there's only 4 wires carrying the segment data from the VT01 die to the LCD.
Since they specifically call the signals "segments", that implies to me that the LCD interface actually modulates its output on subsequent vertical refreshes, effectively PWMing the segment opacities, rather than the LCD segment drivers themselves driving the segments to intermediate voltages.
It calls out a choice of STN Red/Cyan with "120x2x240" or STN BW with "240x240". Implying to me a fixed pixel clock, and that the same segment signals are either divided among color or just monochrome.
My best guess is that the two LSBs are actually unused. ... and that the display will have a gamma of 1, not the standard PC 2.2
(That said, the hanzi in the dictionary don't look very good without those extra bits...)
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The game "Dragon" in the 14-in-1 games cart
does imply the two extra bits are shadow, as after each level is completed the game fades to $3F before going to solid $00.... although that could just be a vestigial bit from the original 2C02 palette, because it also fades down the $30s bits and then from any color $0x to $3F when becoming darker.
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edit #large: STN screens also have a typical contrast ratio of about 10; you're certainly not going to get nice blacks out of these screens.
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Comparing the original Soccer (JU) to the "World Cup" entry in the games cart, only two entries have those LSbits set in their palettes:
Code:
(face/jersey/hair+pants) (face/jersey/eyes+pants)
pink/red/greenbrown $36 $16 $08 USA CHN $3C $00 $30 white/black/red
pink/blue/redbrown $35 $12 $07 GBR GBR $30 $32 $3C red/red/white
pink/orangypink/dkred $36 $26 $06 FRA FRA $30 $3C $0C red/white/cyan
pink/black/dkred $36 $0f $06 FRG FRG $3C $30 $20 white/red/dkred
brown/paleyellow/black $17 $38 $0f BRA BRA $30 $00 $00 red/black/black
orange/purple/black $27 $24 $0f JPN USA $3C $3F $00 white/white/black
dkpink/palegreen/black $26 $3b $0f ESP ESP $34 $1C $00 pink/cyan/black
and those entries are ones where those two colors would be same and are adjacent in the relevant sprite. Maybe it causes some very subtle difference in color?
Maybe it just sets the PWM phase? A 4-level PWM just requires three phases: ABB where BB is the MSbit and A is the LSbit;
maybe things look different enough to be visually distinct by changing the phase?