Video DAC

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Revision as of 23:14, 16 May 2020 by Monk (talk | contribs) (Added a note to the schematic and corrected the 74ls06 reference)
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Resistor array forming the video DAC on a MV1FZS.

The NeoGeo uses a discrete video Digital-to-Analog Converter made of resistors fed by two 74LS273 latches which store the color value from the palette RAM and 74LS05 inverters. This is controlled by either NEO-B1, NEO-GRC or NEO-GRZ.

Video DAC on a AES board.

Aes viddac schematic.png

Note: The schematic has a typo and shows a 74LS06 instead of 74LS05 but both can be used.

Each color component has its own set of 7 resistors. 6 of them are used for the actual color value (5 + 1 common bit). The last (SHADOW signal) is related to the REG_SHADOW and REG_NOSHADOW registers (system latch).

Arcade

The output isn't lowered like on the AES. It goes straight to the JAMMA edge as a 0~3.5V signal for the high-impedance inputs on arcade monitors.

Home

The output is divided by 2200 / (6800 + 2200) = 4.1 to obtain a ~0.9Vp-p max signal, which feeds the video encoder for amplification and conversion to composite.