arlier Quadra machines. However, one of the main goals of these CPUs (and perhaps the #1 goal) was to reduce the cost of the machines
relative to the Quadra 700 and 950, while still providing the same (or better!) performance. While it was not possible to significantly reduce cost while maintaining the exact feature set of the earlier Quadras, most of the Quadra video features were carried through to the Centris 610/650 and Quadra 800. The main exceptions are support for Apple convolution (flicker reduction) for NTSC and PAL, and support for 24 bits per pixel (bpp).
The video hardware for all three CPUs, the Centris 610 (C610), Centris 650
(C650) and Quadra 800 (Q800), is virtually identical. The only exception is
that the C610 only requires 100 ns VRAM, while the C650 and Q800 require 80 ns
VRAM.
The maximum supported pixel depth is 16 bpp. This is not a matter of the
amount of VRAM in the machine - it is a hardware limitation The RAM/DAC
used by all three machines simply does not include the hardware required to do 24 bpp on any display. The same is true for the lack of support for Apple
convolution - the hardware necessary to do this is not present in the machine.
24 bpp support was dropped for a number of reasons:
1) Cost reduction. It is still relatively expensive to provide the 24 bpp
support offered by the Q700 and Q950. It would not have been possible to hit
the price point of the C610 while still offering 24 bpp.
2) Marketing research data has shown that a very large percentage of Quadra
users do not use the onboard video, but rather use an accelerated video card
capable of driving a 2-page display at 24 bpp. Since it was not reasonable to
burden the price of every C610, C650 and Q800 with the cost of a 2-page 24 bpp
frame buffer, the 24 bpp feature was dropped altogether (with a net result of a substantial decrease in cost).
3) 16 bpp offers most of the advantages of 24 bpp, without much of the cost
and at a higher level of performance. QuickTime MooV playback is optimized
for 16 bpp. Also, for casual browsing of image data (e.g., scanned images or
PhotoCD pictures) 16 bpp is quite adequate and offers better performance with
fewer system resource requirements than 24 bpp.
Convolution support was dropped primarily for cost reasons, and also because it is very rarely used. NTSC and PAL timing support are still offered, however. The C610, C650, and Q800 do support all the monitor types supported by the Q950. This includes support for a 1024 x 768 resolution on 19-inch displays (which was not provided by the Q700).
One area that was positively impacted was performance. At the same processor
clock speed, the video section of these new machines outperforms the video
section of the older Quadras. (I.e., video performance on the C650 is better
than the Q700, and on the Q800 is better than the Q950.) Due to an improved
video memory controller design, one wait state was removed from many of the
frame buffer access cycles. This results in reduced memory access time
overall, and therefore improved performance. The graphics tests in Speedometer (version 3.11) show an improvement of roughly 6-10% over the earlier Quadra machines (each running System 7.1).
Supported Display Configurations and Monitor ID Codes
The Centris/Quadra frame buffer determines what type of display is attached to
the video connector by examining the state of 3 sense line pins. The following chart details how these three pins must be wired for each of the supported display types. For each supported display, the screen resolution (horiz. pixels X vertical pixels), dot clock frequency, and the vertical and
horizontal scan rates are listed.
Basically, the Centris 610/650 and Quadra 800 support any display, whether from Apple or from another vendor, that meets one of the following specifications:
Extended sense codes will be examined if the following sense code is detected:
1 1 1
NOTE 1 on above monitors: A sense pin value of 0 means that the pin should be
grounded to the C&VSYNC.GND signal; a value of 1 means do not connect the
pin.
NOTE 2 on above monitors: sense pins 4, 7, and 10 are referred to as SENSE0,
SENSE1, and SENSE2 in pinout tables for the video connectors.
NOTE 3: The terms 'underscan' and 'overscan' are used to describe the active
video resolution for NTSC and PAL modes. Underscan means that the active
video area appears in a rectangle centered on the screen with a black surrounding area. This ensures that the entire active video area always is displayed on all monitors. Overscan utilizes the entire possible video area for NTSC or PAL. However, most monitors or televisions will cause some of this video to be lost beyond the edges of the display, so the entire image will not be seen.
EXTENDED SENSE CODES:
NOTE for extended sense codes: A sense pin pair value of 0 means those pins
should be tied together (as opposed to grounding the pins to pin 11); a value
of 1 means do not connect the pins. Do _not_ wire any of these pins to ground.
