ViewSonic VG181 Service Manual page 21

ViewSonic Service Manual
July 2000 - Version 1.0 VG181
THEORY OF CIRCUIT OPERATION
The on screen display data can be merged into the video data after the scaling
process. The on-screen display data can also be added as the data is input to the
video port, or as data is input to the graphics port.
The gain function is applied to every pixel and it has the same form as the overlay
functions. Specifically, the pixels are processed as follows :
Output red pixel = ((input red pixel) * RCONT(7:0) / 128) + RBRITE(7:0)
Output green pixel = ((input green pixel) * GCONT(7:0) / 128) + GBRITE(7:0)
Output blue pixel = ((input blue pixel) * BCONT(7:0) / 128) + BBRITE(7:0)
The color look up table replaces each input pixel with a new value based on
register tables stored in the PW364. This function is used to compensate the
inherent gammas of the display device and the data source. It uses piece-wise
linear function to get the output value.
The demultiplexer registers the display pixels before they output to the display.
The pixels can also be set to zero here, or set to the default value. The
demultiplexer can support one pixel per clock or two pixels per clock mode. In
single pixel mode, the data is sent out on pins DRE97:0) , DGE(7:0) and DBE(7:0)
every DCLK. In dual pixel mode, the data is sent out with even pixels on pins
DRE(7:0) , DGE(7:0) , and DBE(7:0) and odd pixels on pins DRO(7:0) , DGO(7:0) ,
and DBO(7:0) .
The PW364 generates the vertical and horizontal timing signals for the display
device, and internal timing signals for the display port portion of the PW364. The
DHS and VHS output signals can be active high or low, depending on the HSPOL
and VSPOL bits. Similarly, DENPOL controls the polarity of the DENR, DENG and
DENB outputs. The DDEN bit enables the DHS, DVS, DENR, DENG and DENB
outputs. The horizontal counter starts with the leading edge of horizontal sync. All
horizontal timing is referenced to this edge.
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