VIDEO DAC BRIGHTNESS ISSUE

[PaulJ]

@exxos , lot of stuff to digest here. With only a cap connect it see's some dc voltage from the chip while the analog inputs are not connected. When the analog inputs are selected via the blanking signal the charge on the cap is essentially seen as a dc level. It also acts like a dc level. The screen gets pretty dark as if a higher dc value is injected. Its a small cap so the meter would discharge it to quickly to be seen. The caps on typical application are in series and would block any dc component applied to the analog input letting the ac video component pass and be referenced to the 16th digital step via the clamping component internal to the chip.

Personally I think this is going to be an up hill battle. Have you though of using your mux chips to set the blanking level on the current mono inputs and enable that side of the mux via the blanking signal. I did try switch the mux while in medium res to mono, There was mono video coming from the video chip being feed to the mux but the screen turns to 100% black with no detectable video on the screen. I assume I am using it improperly but it appeared to me that I should have seen something on the screen even if it was just garbage. Anyway all this needs additional investigation.

[exxos]

It would need 3 way MUX chips to do blanking there.. Or another row of chips to blank all the digital colours. I was trying to avoid using more chips by doing the blanking on the DAC. This was tested by people on the DAC prototype boards last year. I'm sure there's a way to solve this without adding more chips. Its a shame the DAC doesn't have a bias output voltage .

Switching to mono for blanking would need the mono signal bypassing to 0v. Maybe a 1k resistor in series and a fet to clamp it may work. But would need logic to switch the MUX on blanking as well anyway. I guess it could be done..but I don't know if the MUX chips could switch fast enough either.

Maybe @PhilC or @Icky can try the caps and see what results they get ?

[PaulJ]

@exxos , what was the expectation for the mono jumper. When I try it I get a black screen. The mono side of the mux is receiving what should be video. What is the expectation for this capability? I just noticed your up pretty late.. like 3:00 AM in GB?

[exxos]

@exxos , what was the expectation for the mono jumper. When I try it I get a black screen. The mono side of the mux is receiving what should be video. What is the expectation for this capability?

It does the same as the UBE adapter. It switches mono to the rgb lines so you can use a VGA monitor on mono. Its unlikely to work on a TV or low res monitor. I think @Icky posted a image of it in mono mode somewhere ?

Edit: I can't check right now.. but the ST should switch the mono detect pin on the ST with that jumper as well..

[PaulJ]

@exxos , what was the expectation for the mono jumper. When I try it I get a black screen. The mono side of the mux is receiving what should be video. What is the expectation for this capability?

It does the same as the UBE adapter. It switches mono to the rgb lines so you can use a VGA monitor on mono. Its unlikely to work on a TV or low res monitor. I think @Icky posted a image of it in mono mode somewhere ?

Edit: I can't check right now.. but the ST should switch the mono detect pin on the ST with that jumper as well..

Have to take a look then.

[PaulJ]

@exxos , @Icky , checked into the mono jumper a little. When in medium resolution and I put the mono jumper on I get no mono signal out of the video shifter and the screen is black. When I use the mono adaptor I get mono out of the shifter and put the mono jumper on I get mono video through the mux and DAC but the monitor is using conventional analog mono. So how is the jumper toggling mono detect? I've looked around and can't find any connect from the RGB_SW and MONO_ON.

[exxos]

I'm just looking into that now in fact..

[exxos]

It looks like I have forgotten to wire mono detect up... The jumper I think should have had a pull up (which is on a SIL anyway) and the jumper to pull up the MUX and MONO_DET.. So when the jumper was "LO" it would switch the MUX and mono dect.. But the muxes are are not wired up to allow that as RGB and MONO are the wrong way around :roll: Will fix that on REV F.

All is not a loss though. It would need a logic level N-mosfet or open collector buffer, so when the jumper is HI and selects MONO, it will turn on the FET and pull down MONO_DET on the monitor port.

[btl]

I also did icky's version of resistors only, and i am quite happy with it:

20191222_174234.jpg

didn't have a 10k, so i did a 4.7k / 2.2k combo, close enough for now.

[exxos]

I also did icky's version of resistors only, and i am quite happy with it

didn't have a 10k, so i did a 4.7k / 2.2k combo, close enough for now.

It may give better results if you just leave those 3 DAC pins disconnected..