RAMDAC ==>
Random Access Memory Digital-to-Analog Converter
<hardware> (RAMDAC) A combination of three fast DACs with a small SRAM
used in graphics display adapters to store the colour palette and to generate
the analog signals to drive a colour monitor. The logical colour number from the
display memory is fed into the address inputs of the SRAM to select a palette
entry to appear on the output of the SRAM. This entry is composed of three
separate values corresponding to the three components (red, green, and blue) of
the desired physical colour. Each component value is fed to a separate DAC,
whose analog output goes to the monitor, and ultimately to one of its three
electron guns (or equivalent in non-CRT displays).
DAC word lengths range usually from 6 to 10 bits. The SRAM's wordlength is three
times the DAC's word length. The SRAM acts as a colour lookup table. It usually
has 256 entries (and thus an 8-bit address). If the DAC's word length is also 8
bits, we have a 256 x 24-bit SRAM which allows a selection of 256 out of
16777216 possible colours for the display. The contents of the SRAM can be
changed while the display is not active (during display blanking times). The
SRAM can usually be bypassed and the DACs can be fed directly by display data
(for true colour modes).
(1996-03-24)
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Access Memory Digital-to-Analog Converter »
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