From: Daniel Kahlin (tlr_at_stacken.kth.se)
Date: 2005-11-27 23:14:58
On Sun, 27 Nov 2005, Daniel Kahlin wrote:
> On Sun, 27 Nov 2005, Daniel Kahlin wrote:
>
>> On Sun, 27 Nov 2005, Nicolas Welte wrote:
>>
>>> Look here:
>>>
>>> http://people.freenet.de/x1541/gnd680.jpg
>>>
>>> I replaced R14 with 680 Ohms as well, plus I added an extra 680 Ohm
>>> resistor to GND, from the junction between R14 and R16.
>>
>> Congratulations! :)
>> I saw your description on forum64.de.
>> I understand that you still think the picture is a bit dark. Did you try
>> to replace R12 with 680 ohms aswell?
>> I think it could fix that. By raising the black-level.
>
> Didn't think before I wrote it, but that will actually lower the black-level.
> Might be good though. I added simulations of your fixed DAC
> on my page so you can see for your self.
The only way to determine the value of R12 is probably to measure the
video output.
The signal for PAL should be (in relative voltages):
Sync: -0.307V
Blanking Level: 0V
Black Level: 0V
White Level: 0.714V
The signal for NTSC should be (in relative voltages):
Sync: -0.276V
Blanking Level: 0V
Black Level: 0.054V
White Level: 0.714V
The Luma DAC with R12=330ohms will give the range: (again relative)
Possible Sync Range: 0 to 0.52V
Black Level: 0.52V
White Level: 0.99V
The Luma DAC with R12=680ohms will give the range: (relative)
Possible Sync Range: 0 to 0.27V
Black Level: 0.27V
White Level: 0.79V
As you can see the swing beween the black level and the white level is
only about 0.44V-0.52V compared to the nominal 0.714V. (only simulated
for a resistive load of 75ohms.)
Where the sync will be depends on what the ASIC drives out on the Luma
pins during CSync Low.
This would be fairly easy to check by making the screen all grey and
scoping with a decent scope on the video out. ~5-10Mhz bandwidth.
Reference:
http://zone.ni.com/devzone/conceptd.nsf/webmain/0E5DAD8917692B5D86256F3500549509
/Daniel
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