Timothy Miller wrote:
On 3/17/06, James Richard Tyrer <[EMAIL PROTECTED]> wrote:
The extra RAMDAC bits are used for gamma and color correction. You
still have only 8 bits per color in memory.
Understood.
The Philips 3xDAC has 10 bits per channel, and we are grounding the
2 LSBs rather than connecting them to the FPGA.
As I mentioned in another email, we simply haven't got the spare pins
for it.
Perhaps using LVDS driver chips (also to be un-populated on the standard
board) could be use to free up some pins. These would need to be
mounted very close to the FPGA
Short search found Fairchild FIN1102 rated at 800 MB/s 3.3v supply would
accept 2.5v LVC input levels with no extra parts unless you wanted a
series termination resistor. Priced @ $1.20 (1K). Only two channel so
you would need 18 of them. Doesn't sound like a great idea, but it
would work.
If it becomes REALLY important, we can do temporal modulation.
I don't like this idea. Just MHO.
Tech Source and plenty of others have been doing it for ages with
medical LCD displays at 60Hz.
Interesting but ... I think that gamma correction/color matching
software works by loading a palate.
One of the design requirements is that OGA should be very good for
90% or more of desktop users. Very few are going to be overly
concerned about gamma correction at all, let alone how many bits of
precision on the DAC.
Yes, you are correct on this. The only real market for fine control of
gamma and color is pre-press color work. Perhaps most of those people
still use Macs. :-)
I'm not, by any means, saying you don't have a valid point. But I am
trying to determine how critical this is for most users and see how
we could possibly solve the problem given the hardware constraints
we have.
OTOH, if this hardware were available, then Linux color matching
software could easily be changed to support it. Gamma correction
doesn't really work well with only 8 bits.
There's another interesting point that you bring to light. The lower
bits are grounded. If we can only afford the 8 bits from the FPGA,
it would be technically better to connect the lower two bits to the
same signals as the highest two bits.
Probably not needed since the user can simply adjust their monitor's
contrast.
So I talked to Howard about it. He says that we can't even do that
because those signal lines are already connected to two places, so
we'd end up causing too much reflection. Besides, we can compensate
for the tiny drop in peak voltage by adjusting resistors connected to
the DAC.
OTHO, with the HiRes head, you could adjust the black level using the
lower bits. No, that isn't enough (16/4096), but it might be better to
tie them high rather than low.
Isn't there only an amp for the highspeed DAC? Still, it's
important that we get this right too.
Yes, I have now found the Philips TDA8777 which is about the same
as the Analog Devices one except for the sync and blanking.
Do you think this one will do the job?
Yes. They are basically the same except for internal functioning.
--
JRT
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