cde1023

VGA Ladder

11 posts in this topic

Hi,

I'm building a VGA ladder based on the Arcade Megawing schematics. The use of the four resistors is clear, however I'm wondering:

- what values must be used for C3/C4/C5 and what is their use? Can I skip them?

- for each color, two diodes connect to 3.3V and GND asymetrically. Similarly, what are they for? (overcurrent protection from a misbehaving monitor?) Can I skip them as well?

cde

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Hello,

Go ahead and ignore the diodes and capacitors, they are not necessary and were removed in later revisions of the schematic. If you look at the VGA section of the Arcade MegaWing Hardware Guide it gives a better explanation of what is going on and how to select the resistor values.

Jack.

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Thanks for the fast reply! Indeed I had a look in the meantime at the Terasic DE0 schematics (also a 3*4 bit ladder), and they don't use capacitors or diodes.

Now I don't own the Arcade Mega Wing, but the idea of publishing the design as open hardware is very cool! However looking at the schematics can be sometimes frustrating to not understand the reasons underlying some design decisions. It can be like reading source code without comments or documentation :) Not that it's a bad thing, in fact I see it more like a puzzle to be solved :)

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Agreed, the schematics and Eagle files can cause more questions then they answer many times, hopefully the Hardware Guide fills in the blanks. If it doesn't then a question in the forum should clear it up and let us know that we need to expand on in the Hardware Guide. :)

Jack.

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That was one of the first things I thought of when I was making the VGA circuit, why not use a sigma-delta DAC. I did a lot of research and what I remember coming away from the research with was that the sigma-delta DAC would not be able to change fast enough for video speeds.

I do remember not being totally convinced that was the case though, so I would like to at least try it some day. :) If you get a chance to try it out let us know, I can send you a VGA Wing if you want, then it would just be a quick rework to remove the resistors and jumper the pads to get a direct connection for the sigma delta DAC.

Jack.

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I can probably get by with some wire for now, Though at 640x480 the pixel clock is 25MHz which would mean you could only get about 4-6 times oversampling with the sigma delta at 100-150MHz but I would still like to see how it turns out if I get a chance to try.

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I can probably get by with some wire for now, Though at 640x480 the pixel clock is 25MHz which would mean you could only get about 4-6 times oversampling with the sigma delta at 100-150MHz but I would still like to see how it turns out if I get a chance to try.

I guess all SVGA monitors actually display 50MHz+ color info: you would need a *very* steep low pass filter to filter out the high frequencies while keeping the 25MHz signal. Without having done serious tests, I'd say 10x oversampling (250MHz) + a first order low pass filter is a minimum if you want a reliable image, not depending on the monitor.

It's worth a try, though, I might be totally wrong.

My two cents: use both the R ladder and low frequency sigma-delta, for example to double the precision (oscillate at 100MHz between two values to get a halftone)

For the record: I tried several "flickering" tricks (change pixel value on each frame), but that's quickly painful for the eyes, even if you use a checker pattern.

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Sigma-delta might work for an analogue monitor, but for an LCD monitor I doubt it will work at all.

The ADCs for capturing the VGA signal need to be able to capture full HD/60Hz (approx 150MHz). To do this they have less than 6ns to swing to either end of the input signal's range. This would require 1 or 2 GHz input bandwidth on the ADC,

You would require a multi-Gb/s frequency sigma delta DAC to get even a couple of bits.

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Yes for an LCD it must be able to capture VGA going at that speed, however if the monitor is configured to 640x480 resolution should it not expect the VGA at the 25MHz frequency and as long as the sigma delta can satisfy that then it can work, but yes I guess 1 bit sigma delta is not going to work for anything above minimum resolution.

Perhaps the current 4bit per colour ladder could have a low pass filter shoved on the end and run as a 4 bit sigma delta converter to get something more practical?

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If you can generate three channels of bitstreams at 250MHz, then you might as well go DVI-D and get 24 bit colour - much better than 3.5 bits per channel.

You only need a 125MHz clock if you use ODDR outputs, and I have heard that LVD works OK to drive DVI at lower resolutions / short cable runs, but have never tried it myself. Could try it when I get home though...

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