Retropie video output to G07 Electrohome crt

mahoneyt944 · 19 May 2021, 09:34

Just watched this video below and I'm in a similar situation where I want to wire my rpi3b+ with official image to a freshly rebuilt G07 Electrohome crt monitor. From what I can tell, in the video it shows a rpi3 going to a HDMI to VGA adapter then the VGA cord is split into Red, Green, Blue, Ground, Vertical sync, and Horizontal sync. This is then connected to the monitor. Then via the settings in config.txt, the video output is set in a way the crt accepts.

My understanding is my crt and the one used in the video are 15khz signals. But VGA would typically be 31khz? So how is this working exactly? Does the settings in config.txt reduce it to use 15khz?

The wiring starts around 7 mins in and config settings in the video description.

mitu · 19 May 2021, 11:45

@mahoneyt944 said in Retropie video output to G07 Electrohome crt:

But VGA would typically be 31khz? So how is this working exactly? Does the settings in config.txt reduce it to use 15khz?

I think so. Looking at the configuration for the HDMI signal, it seems to be using
CEA Mode 5 (480i@60hz - 240p), which has a 15Khz Hsync (horizontal sync frequency), while a VGA video mode (640x480@60hz / DMT mode 4) has 31Khz Hsync.

mahoneyt944 · 19 May 2021, 12:19

@mitu assuming you meant CEA mode 6? Any thoughts on this vs CEA mode 8?

mitu · 19 May 2021, 12:33

@mahoneyt944 said in Retropie video output to G07 Electrohome crt:

@mitu assuming you meant CEA mode 6? Any thoughts on this vs CEA mode 8?

Yes, I meant 6. I think in the video you can see that the poster tried the CEA mode 8 (had it in the config) and maybe it didn't work (or perhaps CEA 6 is better for the converter).

mahoneyt944 · 19 May 2021, 21:21

@mitu I'm curious how much the HDMI to VGA adapter plays a role in this to work. I have one from the pihut and a unbranded one which looks almost identical to try.

I also have a HDMI to VGA cable which is just a direct pin to pin connection type of thing. I think it may work since we are manipulating the signal sent from the pi in software but I have my doubts. Worth a try, otherwise not a very useful cable.

jamrom2 · 20 May 2021, 02:24

@mahoneyt944
This is a great video. Thanks for posting it .

I don't think the HDMI to VGA is a direct pin to wire to pin set up. Every cable I've seen when I bought one for my C64 Maxi HDMI to my old 4:3 19" Dell VGA had a small output conversion PCB on the VGA end of the cable. I'm pretty sure what he did is the only way to do it.

mahoneyt944 · 20 May 2021, 06:02

@jamrom2 @mitu

I started my first attempt at this and I got a picture. Still needs work though to be acceptable but I can make things out and I am running / navigating retropie. Yes, the HDMI to VGA adapter is needed. The direct pin to pin cord I have is useless as expected. My guess is the adapter isolates the RGB signals we grab from.

Sync wise, my g07 monitor seems to prefer separate negative horizontal and vertical syncs as opposed to composite in the video.

Mode 6 seems like it's where it needs to be, though I need to test others. It fills the entire screen and I can see things more clearly. Mode 8 would display but it was extra small. Seems to prefer interlaced over progressive scan in this regard.

I had to adjust the screen rotation to 3. Nothing special about this though just how the monitor sits in the cabinet.

Right now I'm hunting down any other resolution settings that may need to be set while adjusting the ones I have in the config.txt. There is also all the adjustments on the crt itself which adds loads of complexity.

I think the next step is to set the HDMI signal boost levels properly and verify with a volt meter it's in spec. The g07 manual specifies the color signals between 0 - 4V and the syncs to be between 2 - 4V. I have no idea where there are now.

mahoneyt944 · 20 May 2021, 16:17

Voltages are in spec. I seem to be caught between pixel encoder settings atm. At full rgb I get the right colors but some of the images are glitched/ color bleed. With limited rgb I get a very clear image but the colors are faint.

mahoneyt944 · 21 May 2021, 04:54

@mitu if you watch the video around 19 minutes he talks about setting it to low res in the pop up menu, which is a setting outside of the config.txt that I've already done. I'm using an official build so I'm not sure how I could verify this on my build too. Seems like that might be what I need to do with the image I'm getting.

Right now if I set the pixel encoding to 1 "limited rgb" the image clarity is much higher but I get less vibrant colors, faint looking in fact, and black shows as gray etc. With pixel encoding set to 2 "rgb full" I get better colors but some text is choppy and hard to read, the colors seem to bleed more too.

Is there other low res settings I could try out?

mitu · 21 May 2021, 05:04

@mahoneyt944 said in Retropie video output to G07 Electrohome crt:

@mitu if you watch the video around 19 minutes he talks about setting it to low res in the pop up menu, which is a setting outside of the config.txt that I've already done. I'm using an official build so I'm not sure how I could verify this on my build too. Seems like that might be what I need to do with the image I'm getting.

That seems to a configuration related to the Attract Mode theme used in the video. Attract Mode is available in RetroPie also, you can install it as an alternate front-end, but it's not so well documented as EmulationStation.

What you can try is to load a low-res theme, specially designed for 240p modes - they're quite a few of them available. Looks for themes with 240p names and try a few of them - it should improve the frontend experience.

Ultimately, you'd choose what video mode is best for emulation (I guess full RGB ?) and tune the front-end based on it.

mahoneyt944 · 21 May 2021, 14:23

@mitu I think the issue I'm having is the color encoding. G07 monitors were made in 1981 where the color palette was very basic.

hdmi_pixel_encoding=2...... I get more accurate colors but colors are bleeding and some graphics are color glitched. some text is not very readable.

hdmi_pixel_encoding=1...... I get a crisp clear readable picture but the colors are wrong. black shows as grey for example.

oddly the rasp-config menu / retropie menu looks good in both encodings.

Is there a way to set rgb manually or change palettes? G07 is considered standard res /low res monitor.

mitu · 21 May 2021, 16:25

@mahoneyt944 said in Retropie video output to G07 Electrohome crt:

Is there a way to set rgb manually or change palettes? G07 is considered standard res /low res monitor.

Not sure if there's such a thing as a 'palette' for an analog signal, but no, have no idea. You may want to ask in the raspberrypi.org/forum forum and provide some pictures.