STICKY: Using a Gert VGA 666 adapter - [HOWTO]

[Burngate]

Isn't that why an R-2R DAC is to be prefered? And if so, why wasn't it chosen?

[gregeric]

Hooray, Gert's back

[Gert van Loo]

Isn't that why an R-2R DAC is to be prefered? And if so, why wasn't it chosen?

Two (and a half) reasons:
1/ It uses a lot more components.
2/ The R-2R solution assumes a very high impedance follower (almost no load on the network)
The VGA input impedance of your monitor is 75 ohms.
2.5/ I have to do some calculations but my gut feeling says you never get anything for nothing.
I suspect the R-2R solution also falls over if you start looking at the resistor precision.

[james-at-lo-tech]

I've released a 24-bit VGA board today - see here.

With the 6-bits per channel, the colours in the current design already show 'banding' (In some places the colour jumps are clearly visible).

The banding I think is because of the GPIO output impedance; in my testing it can be reduced somewhat by selecting 16mA drive (after boot, since the overlay can't). Tabulating all possible values, discontinuities can be seen assuming 32R output impedence. Reducing the resistor values by 16R with 16mA selected pretty much gets rid of it. But 888 isn't the main feature of my VGA board, rather ESD protection, buffering, and noise immunity - and CE Mark.

0.1% resistors are pretty readily available in 0603 format but I'm yet to build a board with them. It looks OK already with 1%

[Burngate]

I was lying in bed last night, thinking about it, and I think reason 2.5 must be right.
In fact, I did some calculations in my head, and convinced myself that the errors would be identical.
Of course this morning I can't reproduce it.

[Gert van Loo]

Wikipedia to the rescue (Yes, I am too lazy to calculate it myself).

The R–2R ladder operates as a string of current dividers, whose output accuracy is solely dependent on how well each resistor is matched to the others. Small inaccuracies in the MSB resistors can entirely overwhelm the contribution of the LSB resistors. This may result in non-monotonic behavior at major crossings, such as from 01111 to 10000.

[john.t]

Hello,

i have been able to make my own vga666, and tested it and worked fine for my vga output. But i would also like a dual display via vga and hdmi. Ive heard that its possible. How do i do this?
thank you very much.

john

[john.t]

Wikipedia to the rescue (Yes, I am too lazy to calculate it myself).

The R–2R ladder operates as a string of current dividers, whose output accuracy is solely dependent on how well each resistor is matched to the others. Small inaccuracies in the MSB resistors can entirely overwhelm the contribution of the LSB resistors. This may result in non-monotonic behavior at major crossings, such as from 01111 to 10000.

Hi gert

i have been able to make my own vga666, and tested it and worked fine for my vga output. But i would also like a dual display via vga and hdmi. Ive heard that its possible. How do i do this?
thank you very much.

john

[Gert van Loo]

I would also like a dual display via vga and hdmi.

I have no idea, honest. I just gave the hardware to Dom and he did something in software.
But then I just typed 'dual display' in the search box and apart from this entry
another hit came up: viewtopic.php?f=28&t=154067&p=1101328&h ... y#p1025113

[john.t]

I would also like a dual display via vga and hdmi.

I have no idea, honest. I just gave the hardware to Dom and he did something in software.
But then I just typed 'dual display' in the search box and apart from this entry
another hit came up: viewtopic.php?f=28&t=154067&p=1101328&h ... y#p1025113

So do you mean that if i use vga666 that theres no possible way to make hdmi work?even if i dont use them simultanously?
Its for our company actually .we are trying to have a test image generator via hdmi and vga in one hardware..it doesnt have to work together at the same time..

[Gert van Loo]

Sorry, misunderstanding.
Yes, you can have two displays, e.g. one playing video and one for disk top.
But AFAIK you can have only a desktop on one at a time.
I also think you can't switch desktop dynamically, only by editing the set-up (config.txt).

But as I said, I have no idea of hoe the software works.
I agree with you, a bit more information on the software site of dual display would be nice....
I have put in an inquiry to the proper authorities.

[john.t]

Thank you for the explaination.
Its more clear to me now..i dont need the desktop actually i only need the rpi to load my test image that i already have..but not only vg but also via hdmi.in other words i switch between vga and hdmi to test a video hardware if his video output still works..
Ps i accidentally clicked the "report this post".im using a smartphone and i clicked way too fast and my smartphone isnt that fast..so just ignore if there's any notification about reporting..
Thank you again for the useful info
John

[john.t]

Any news on software / firmware to support dual display (HDMI + VGA)?

Im also having this problem. Have you able to solve it? i only can use vga not hdmi .

[Gert van Loo]

I have PM-ed a moderator about this. That is all I can do as it is SW.

[mahjongg]

Moderator Dom had this to say about future improvements planned for dual display use (both official LCD on DSI, and Gerts VGA666 solution).

Currently the linux framebuffer (console / X) can only run on a single display and can't be switched without a reboot. The display used can be configured from config.txt.

The other display can only be driven through dispmanx. omxplayer is one example that can output to either display.
Some games and emulators use dispmanx for better performance so they would be possible.

But in general dual display use is very limited.

Proper linux dual display support will only come with the experimental arm side graphics driver.
In theory once https://github.com/raspberrypi/linux/pull/1813 is merged then the official display will be supported as a standard linux/X framebuffer,
and in theory whatever X multi-monitor support that exists will work with LCD display and HDMI.

Currently I've not got the PR working with the official display, but presumably Eric has and we'll work out the magic soon.

hope this helps...

[james-at-lo-tech]

it would transform the use of this board for desktop (thin client) purposes, e.g. Citrix, WTware etc

[john.t]

If anyone using Gert's adapter would like to use both hdmi & vga simultaneously, I've found this to work ok:

My relevant settings in

config.sys

enable_dpi_lcd=1
display_default_lcd=0


Open terminal (visible screen being hdmi, as display default for DPI is set to 0 above)


wow thanks for this info!

i was wondering if i could auto run a python code which generate an image via omxplayer too?i need to be able to show my image during boot up so it will automatically load my py code.ive been able to do tha via hdmi and vga666 separately but i wanted to be able to show my image without editing my config txt everytime i switches uotputs
thnks
john

[email protected] ~ $ omxplayer --display 4 pi2-clkpwm.webm.mp4

Obviously choose your own video file.

Below is terminal output for the poor quality attached jpeg.

[email protected] ~ $ omxplayer --display 4 pi2-clkpwm.webm.mp4

Invalid framerate 1000, using forced 25fps and just trust timestamps
Video codec omx-vp8 width 1280 height 800 profile -99 fps 25.000000
Subtitle count: 0, state: off, index: 1, delay: 0
V:PortSettingsChanged: [email protected] interlace:0 deinterlace:0 anaglyph:0 par:1.00 layer:0
V:PortSettingsChanged: [email protected] interlace:0 deinterlace:0 anaglyph:0 par:1.00 display:4 layer:0
have a nice day

I actually have the DPI setting for 1024*768, but have since tried another old vga monitor, capable of 1280*1024 & it seems happy to play high res' files.

VGA666 is currently displaying a BBC HD recording & cpu usage hasn't risen beyond 10%.

666dualmonitors.jpg

[CostasVav]

I have just got my Gerts VGA666 plugged in and working. I initially tested on a RPi3 with very good results. When I was attempting to fit the RPi3 into my project, I accidentally fried it (it touched a live 110VAC wire... Don't ask).

I then tested it on an old RPi A+ (all I have laying around). The quality is very poor, surprisingly. There is smearing/ghosting of the image to the right (about 10 pixels worth) and a slight flicker (in the 20-40Hz range).

Is the drop in quality due to the RPi A+ or did something else fail? Is the A+ too slow to drive the VGA signal?

Thanks!

[rpdom]

Perhaps you have damaged some of the resistors on the VGA666 board.

Is the A+ too slow to drive the VGA signal?

All models of Pi (other than the early ones with 26 pins) are fully capable of driving the VGA666 adaptor without any effort. It is all done in hardware. I've used it on a Pi Zero and a B+ (same speed as A+) with excellent results.

[mario439]

Hi.
Is there a Browser that works like omxplayer?
I only want to display 2 Browser windows on 2 displays.

I tried with Chromium an the command line parameter "--display" but "chromium-browser --display 5" gives me an error.
"[3312:3312:0204/223546:ERROR:browser_main_loop.cc(271)] Gtk: cannot open display: 5"

greetz from Germany

[alkemir]

Hi,

adding my two cents:

I recently programmed a screen cloner which works with the GERT666. You can find more details here:

https://github.com/alkemir/gertCloner

This will get us to dualscreen in mirror mode, but not extended mode yet.

I was wondering (maybe Dom can help us out) if it would be possible to configurate a big screen for X;
say 2048x768 and set the viewport for the HDMI output to just the left portion of it (0,0 to 1024,768) and then
the right portion (1024,0 to 2048, 768) could be cloned using my approach to the VGA screen. In this way
it should be (in principle, if I understand correctly) possible to finally bring extended dualscreen for the RPi.

Regards

Bryan

[CostasVav]

So I plugged the Gert666 into a new RPi 3B and the image quality was much better than the RPi A+. Nothing else was changed. It still wasn't good enough for me (still a slight noise level), so I switched to an HDMI-to-VGA adapter instead for best quality.

There must be something about the A+ giving off a little more interference than a 3B.

[walterav1984]

Since VGA24bit and RGB 15khz SCART to CRT arcade solutions are now available.

How far away is a GPIO2Component with analog YUV (YpBpR) for high quality interlaced SD (maybe HD since the VGA also support HD resolutions)?

https://en.wikipedia.org/wiki/YPbPr

Can we use the Gert VGA 666 hardware but just needs a different firmware overlay in the raspberry and use a other DB15 pinout?

[james-at-lo-tech]

So I plugged the Gert666 into a new RPi 3B and the image quality was much better than the RPi A+. Nothing else was changed. It still wasn't good enough for me (still a slight noise level), so I switched to an HDMI-to-VGA adapter instead for best quality.

There must be something about the A+ giving off a little more interference than a 3B.

The cheap adapters appearing from the hobby PoC board typically have little (if any) attention in their implementation to EMC either in terms of radiated emissions or susceptibility to radiated emissions. Also they provide no ESD protection.

[CostasVav]

So I plugged the Gert666 into a new RPi 3B and the image quality was much better than the RPi A+. Nothing else was changed. It still wasn't good enough for me (still a slight noise level), so I switched to an HDMI-to-VGA adapter instead for best quality.

There must be something about the A+ giving off a little more interference than a 3B.

The cheap adapters appearing from the hobby PoC board typically have little (if any) attention in their implementation to EMC either in terms of radiated emissions or susceptibility to radiated emissions. Also they provide no ESD protection.

This was a PiSupply board. They did not provide the exact resistors shown in Gert's design. E.g. a 520ohm instead of a 499. Maybe source of issue.