DreamcastHDMI github by chriz2600

citrus3000psi · Joined: 25 Dec 2013 Posts: 648 Location: Indiana

Someone by the name of chriz2600 just posted something pretty interesting on github

https://github.com/chriz2600/DreamcastHDMI

bobrocks95 · Joined: 30 Apr 2012 Posts: 2418 Location: Kentucky

I'll be following this closely!

I don't think robust upscaling is needed for a 480p console, but that's just my opinion. The Dreamcast does have weird video properties that may call for it.

Guspaz · Joined: 06 Oct 2015 Posts: 2368 Location: Montréal, Canada

Well, you'd want a decent way of getting 480i DreamCast content to 480p, and you'd also want some basic scaling to account for the Dreamcast's messed up 640-in-720 framing. By default, the framing won't display the image correctly on a TV regardless of if the TV is set to 4:3 or 16:9, IIRC.

Hoagtech · Joined: 27 Apr 2015 Posts: 691 Location: Bellingham, WA

A full frame output would be amazing @guspaz.

I noticed the game gear had a similar issue which compress images inside the video frame, but Tim developed a workaround by snipping a circuit

. It is part of beharius 's consolised game gear.

I almost feel like the console is half broken until someone figures it out. i played a lot of half life D.C. lately and I'm not too stoked on any of my options available (framemesiter, ossc, vga box, RGBs) it's all meaning less until the frame is displayed correctly.

chriz2600 · Joined: 07 Feb 2016 Posts: 19 Location: Cologne, Germany

Hi, I'm the creator of the DreamcastHDMI project on github.

I recently updated the project to support an "inexpensive" Cyclone IV development board (Waveshare CoreEP4CE6). Everybody with some soldering skills should be able to mod a Dreamcast to output HDMI (with digital sound) for under 30$ (outputting 480p, no scaling, so your TV has to allow custom scaling to prevent squashing of the picture).
Also, i'm currently working on a board to include the FPGA and an ADV7513 HDMI transmitter and some RAM to support 480i (instead of only 480p) and upscaling (to overcome HDTVs Dreamcast scaling problem) as well as a flat flex pcb to be easily soldered to the audio/video DACs. A first test project using the ADV7513 with the mentioned Cyclone IV development board is also available on github.

I will continue make the whole development (including schematics/pcb/software etc.) available under MIT License.

citrus3000psi · Joined: 25 Dec 2013 Posts: 648 Location: Indiana

chriz2600 wrote:

Hi, I'm the creator of the DreamcastHDMI project on github.

I recently updated the project to support an "inexpensive" Cyclone IV development board (Waveshare CoreEP4CE6). Everybody with some soldering skills should be able to mod a Dreamcast to output HDMI (with digital sound) for under 30$ (outputting 480p, no scaling, so your TV has to allow custom scaling to prevent squashing of the picture).
Also, i'm currently working on a board to include the FPGA and an ADV7513 HDMI transmitter and some RAM to support 480i (instead of only 480p) and upscaling (to overcome HDTVs Dreamcast scaling problem) as well as a flat flex pcb to be easily soldered to the audio/video DACs. A first test project using the ADV7513 with the mentioned Cyclone IV development board is also available on github.

I will continue make the whole development (including schematics/pcb/software etc.) available under MIT License.

Thanks for the update!

bobrocks95 · Joined: 30 Apr 2012 Posts: 2418 Location: Kentucky

chriz2600 wrote:

Hi, I'm the creator of the DreamcastHDMI project on github.

I recently updated the project to support an "inexpensive" Cyclone IV development board (Waveshare CoreEP4CE6). Everybody with some soldering skills should be able to mod a Dreamcast to output HDMI (with digital sound) for under 30$ (outputting 480p, no scaling, so your TV has to allow custom scaling to prevent squashing of the picture).
Also, i'm currently working on a board to include the FPGA and an ADV7513 HDMI transmitter and some RAM to support 480i (instead of only 480p) and upscaling (to overcome HDTVs Dreamcast scaling problem) as well as a flat flex pcb to be easily soldered to the audio/video DACs. A first test project using the ADV7513 with the mentioned Cyclone IV development board is also available on github.

I will continue make the whole development (including schematics/pcb/software etc.) available under MIT License.

I take it that the version out now can't linedouble either? So games that don't support 480p essentially won't output any video?

citrus3000psi · Joined: 25 Dec 2013 Posts: 648 Location: Indiana

bobrocks95 wrote:

I take it that the version out now can't linedouble either? So games that don't support 480p essentially won't output any video?

Yes, from what I'm reading the game has to support VGA. A VGA cable has to be plugged in (or correct pins grounded) for the HDMI device to work.

bobrocks95 · Joined: 30 Apr 2012 Posts: 2418 Location: Kentucky

Hmm, retrorgb also mentions some games that don't support RGB either... I wonder how that could be handled. I don't even see why or how that could happen.

Joelepain · Joined: 12 Sep 2012 Posts: 157

Guspaz wrote:

and you'd also want some basic scaling to account for the Dreamcast's messed up 640-in-720 framing

Does HDMI support VESA 640x480@60p standard ? If yes, problem solved, it's up to the TV/monitor to be respectful of the standard.
If no, I can't even imagine how it's possible to do a proper scaling of 640 to 720 samples per line that satifies every one (both the pixel-perfect clan and the I-don't-care-if-it's-a-little-blurry-as-long-as-it-works clan), and all that on a cheap fpga.

bobrocks95 · Joined: 30 Apr 2012 Posts: 2418 Location: Kentucky

Joelepain wrote:

Guspaz wrote:

and you'd also want some basic scaling to account for the Dreamcast's messed up 640-in-720 framing

Does HDMI support VESA 640x480@60p standard ? If yes, problem solved, it's up to the TV/monitor to be respectful of the standard.

If I understand right, the Dreamcast is 640x480 embedded inside a 720x480 frame. So if the TV processed it as 640x480 it's going to squash the aspect ratio as it condenses it all (all meaning including the pillarbars) to fit the width. So just telling the TV it's 640x480 won't solve anything.

Joelepain · Joined: 12 Sep 2012 Posts: 157

bobrocks95 wrote:

If I understand right, the Dreamcast is 640x480 embedded inside a 720x480 frame. So if the TV processed it as 640x480 it's going to squash the aspect ratio as it condenses it all (all meaning including the pillarbars) to fit the width. So just telling the TV it's 640x480 won't solve anything.

If I understand right, the dreamcast internally renders at 640x480. So all the "embedding" is done at the DAC level I suppose. So if you're taping the digital signal, you should have the proper 640x480 active aera of pixel untouched.
Now the real question : is the internal render of dreamcast meant for square pixel aspect ratio ? If yes, then problem solved. If no, i don't think there is any scaling algorithm on earth (or at least runnable on a "cheap" fpga and better than the scaling engine of a TV) that can do this aspect ratio convertion properly. A choice has to be made : 1:1 pixel perfect and wrong aspect ratio or soft image but right aspect ratio.

chriz2600 · Joined: 07 Feb 2016 Posts: 19 Location: Cologne, Germany

Joelepain wrote:

If I understand right, the dreamcast internally renders at 640x480. So all the "embedding" is done at the DAC level I suppose.

Unfortunately not, the DDR-Pixel Clock is directly derived from 13.5 Mhz crystal of the DC. So the pixel clock is 27 Mhz (54 Mhz via 12 bit interface) and timings are already 480p@59.94Hz on the digital side.
The best way would be to create a real 640x480p signal from the "inner" 640x480 pixels of the 720x480p signal.
For this you need a 25.2/1.001 (25.174825) MHz signal to match the refresh rate of 59.94 Hz. But the FPGA's PLL is not capable of providing a 25.2/1.001 MHz clock needed for VGA with 59.94Hz from 27/54 Mhz, so I will need to try an external PLL to generate 25.2/1.001 Mhz.
As soon I get my hands on such a PLL, I will try to recode the inner 640x480 pixels of the 720x480p signal to 640x480p, which (I tested some HDTVs already) will be displayed with a pixel ratio of 1:1.

Woozle · Joined: 24 Jun 2015 Posts: 185 Location: Florida

Have you considered using a phase accumulator to generate a 25.174Mhz clock. A phase accumulators running off a 250Mhz clock with a 32 bit counter should give you a nice enough clock for VGA. Final quality would probably depend on how the display samples the vga signal.

meneerbeer · Joined: 26 Mar 2015 Posts: 50

Woozle wrote:

Have you considered using a phase accumulator to generate a 25.174Mhz clock. A phase accumulators running off a 250Mhz clock with a 32 bit counter should give you a nice enough clock for VGA. Final quality would probably depend on how the display samples the vga signal.

That will create a lot of jitter. I have used this method a few times before. On the Xilinx FPGAs, the DCM would not lock properly to it for the HDMI clock generation. With some trickery you can get this to work for some cases. However, this is by far not a stable solution.

Edit:
Oh wait, you were saying VGA.

Joelepain · Joined: 12 Sep 2012 Posts: 157

chriz2600 wrote:

...

Yes pixel-clock was the only problem I was thinking of, but I'm a complete ignorent in these kind of electronics. Glad you clarified this point.
Have you tried to ask Marq's what he's doing on the OSSC for this ? because depending of the sample rate setting of the vga input, the dvi outputs 720x480 or 640x480. But I suppose the output of the OSSC is more or less dependant of the pixel clock (or sync to be accurate) of the input.

Woozle · Joined: 24 Jun 2015 Posts: 185 Location: Florida

meneerbeer wrote:

Woozle wrote:

Have you considered using a phase accumulator to generate a 25.174Mhz clock. A phase accumulators running off a 250Mhz clock with a 32 bit counter should give you a nice enough clock for VGA. Final quality would probably depend on how the display samples the vga signal.

That will create a lot of jitter. I have used this method a few times before. On the Xilinx FPGAs, the DCM would not lock properly to it for the HDMI clock generation. With some trickery you can get this to work for some cases. However, this is by far not a stable solution.

Edit:
Oh wait, you were saying VGA.

I wasn't thinking when I wrote that. I saw a VGA clock rate and assumed it was for VGA output, when the thread is about DC HDMI.

I agree, it wouldn't be a good idea to use that method with the FPGA DCMs. But, if the HDMI Tx chip only needs the pixel clock and the TX chip can tolerate a clock generated by a phase accumulator, then maybe it would work?

Do you know of any good external PLLs?

chriz2600 · Joined: 07 Feb 2016 Posts: 19 Location: Cologne, Germany

Woozle wrote:

I agree, it wouldn't be a good idea to use that method with the FPGA DCMs. But, if the HDMI Tx chip only needs the pixel clock and the TX chip can tolerate a clock generated by a phase accumulator, then maybe it would work?

I will definitely give this a shot!

Woozle wrote:

Do you know of any good external PLLs?

Here is my plan:
I ordered an IDT ICS664-03 Digital Video Clock Source, for which the datasheet claims to have exact (0ppm) multiplication ratio for the possible clock outputs. I want to use this to create 74.25/1.001 (74,175824) Mhz from the dreamcast's 54 Mhz clock. To get 25.2/1.001 Mhz I want to use the FPGA PLL which can achieve 25.2/1.001 Mhz via a divider of 165 and a multiplier of 56.
As both 720x480p and 640x480p have 525 vertical lines (and because of that the same horizontal frequency) I want to use 2-port RAM inside the FPGA (with two clocks) to buffer one horizontal line and then output as 640x480p.

Unseen · Joined: 25 May 2014 Posts: 624

chriz2600 wrote:

Unfortunately not, the DDR-Pixel Clock is directly derived from 13.5 Mhz crystal of the DC. So the pixel clock is 27 Mhz (54 Mhz via 12 bit interface) and timings are already 480p@59.94Hz on the digital side.
The best way would be to create a real 640x480p signal from the "inner" 640x480 pixels of the 720x480p signal.

Have you tried to cheat by just extending the blanking areas so the active part of the line is reduced to 640 pixels?

chriz2600 · Joined: 07 Feb 2016 Posts: 19 Location: Cologne, Germany

Unseen wrote:

Have you tried to cheat by just extending the blanking areas so the active part of the line is reduced to 640 pixels?

Yes, I've tried that, and for some devices e.g. a Eizo Foris FS2333 monitor it worked great, but for others (e.g. Panasonic VT-30 Plasma) it did not worked at all, especially not switching to 1:1 pixel ratio.

Wolf_ · Joined: 25 Jun 2016 Posts: 383

chriz2600 wrote:

Joelepain wrote:

If I understand right, the dreamcast internally renders at 640x480. So all the "embedding" is done at the DAC level I suppose.

Unfortunately not, the DDR-Pixel Clock is directly derived from 13.5 Mhz crystal of the DC. So the pixel clock is 27 Mhz (54 Mhz via 12 bit interface) and timings are already 480p@59.94Hz on the digital side.
The best way would be to create a real 640x480p signal from the "inner" 640x480 pixels of the 720x480p signal.
For this you need a 25.2/1.001 (25.174825) MHz signal to match the refresh rate of 59.94 Hz. But the FPGA's PLL is not capable of providing a 25.2/1.001 MHz clock needed for VGA with 59.94Hz from 27/54 Mhz, so I will need to try an external PLL to generate 25.2/1.001 Mhz.
As soon I get my hands on such a PLL, I will try to recode the inner 640x480 pixels of the 720x480p signal to 640x480p, which (I tested some HDTVs already) will be displayed with a pixel ratio of 1:1.

Have you thought about reaching out to Kevtris at AtariAge? The NES had a similar issue but he was able to underclock the system from 60.9fps to 60fps and get it hdmi compliant. It would be nice if the same method worked for the dreamcast.

What I'm most worried about are the games that can not be run in or be forced to run in vga mode. If this hdmi mod only works off the vga mode then it would make the 49 games that can't be hacked to run in it unplayable on that console permanently which for me is a dealbreaker as the Akura hdmi box is probably about the same performance but can be unplugged if you want to run any of the games that won't display in vga mode.

Spoiler: show

Another feature I would like to see is Linedoubling with zero lag as the ossc does it for 1080p or 4k output as the ossc doesn't have hdmi input meaning that to upscale a hdmi dreamcast we would have to try and use hdmi in on the xrgb mini and I've heard that it doesn't work well with hdmi in signal upscaling.

I know it will be a good bit of work but to make this mod something that lasts the test of time and doesn't cause a bunch of hassle removing later to upgrade it then at the very least meeting the hdmi sync standard and not bricking the console for 49 games would be required.

chriz2600 · Joined: 07 Feb 2016 Posts: 19 Location: Cologne, Germany

Wolf_ wrote:

Have you thought about reaching out to Kevtris at AtariAge? The NES had a similar issue but he was able to underclock the system from 60.9fps to 60fps and get it hdmi compliant. It would be nice if the same method worked for the dreamcast.

I think, this problem was different. The dreamcast video output is correct in regards to pixel timing, so it easily can be converted to "real" 480p, but this leads to wrong pixel ratio when scaled by HDTVs. The analog signal tweaks this by using some non-standard timing for hsync and vsync. For "correct" VGA output, you need a 400/429 clock ratio to convert one line of 480p to one line of VGA.
In the new version on github I achieved this clock conversion by the help of an external PLL (ICS664-3).

Wolf_ wrote:

What I'm most worried about are the games that can not be run in or be forced to run in vga mode. If this hdmi mod only works off the vga mode then it would make the 49 games that can't be hacked to run in it unplayable on that console permanently which for me is a dealbreaker as the Akura hdmi box is probably about the same performance but can be unplugged if you want to run any of the games that won't display in vga mode.

Luckily, this mod is non-destructive. Analog output and HDMI work simultaneously.
I also added 480i/240p to 480p line-doubling in the lastest version.

For some details on the the latest version see here: https://github.com/chriz2600/DreamcastH ... /README.md

Wolf_ · Joined: 25 Jun 2016 Posts: 383

chriz2600 wrote:

Luckily, this mod is non-destructive. Analog output and HDMI work simultaneously.
I also added 480i/240p to 480p line-doubling in the lastest version.

So does this mean that to play every game you would need to use analog output for the 49 games that won't run in vga mode as well as a hdmi connection for everything else?

citrus3000psi · Joined: 25 Dec 2013 Posts: 648 Location: Indiana

chriz2600 wrote:

Luckily, this mod is non-destructive. Analog output and HDMI work simultaneously.
I also added 480i/240p to 480p line-doubling in the lastest version.

For some details on the the latest version see here: https://github.com/chriz2600/DreamcastH ... /README.md

Do you have a schematic of this new design?

bobrocks95 · Joined: 30 Apr 2012 Posts: 2418 Location: Kentucky

chriz2600 wrote:

For some details on the the latest version see here: https://github.com/chriz2600/DreamcastH ... /README.md

Awesome work! Is the roadmap at the bottom up to date? i.e. do you still need a VGA cable plugged in for the system to detect it?

I'll eagerly await someone to make a board for this! Or maybe it's better to wait for a flex cable... How many pins on the GPU need to be connected?

Wolf_ wrote:

chriz2600 wrote:

Luckily, this mod is non-destructive. Analog output and HDMI work simultaneously.
I also added 480i/240p to 480p line-doubling in the lastest version.

So does this mean that to play every game you would need to use analog output for the 49 games that won't run in vga mode as well as a hdmi connection for everything else?

Sounds like HDMI + S-Video is what you'd need to cover all possibilities.

Aren't there games you can trick into VGA by flipping a switch after starting them though? I wonder how that could be recreated...

citrus3000psi · Joined: 25 Dec 2013 Posts: 648 Location: Indiana

bobrocks95 wrote:

I'll eagerly await someone to make a board for this! Or maybe it's better to wait for a flex cable... How many pins on the GPU need to be connected?

While I wait for a schematic. I'll see what it might take to build a QSB.

bobrocks95 · Joined: 30 Apr 2012 Posts: 2418 Location: Kentucky

citrus3000psi wrote:

bobrocks95 wrote:

I'll eagerly await someone to make a board for this! Or maybe it's better to wait for a flex cable... How many pins on the GPU need to be connected?

While I wait for a schematic. I'll see what it might take to build a QSB.

Bless you citrus!

Link83 · Joined: 21 May 2013 Posts: 312

chriz2600 wrote:

I think, this problem was different. The dreamcast video output is correct in regards to pixel timing, so it easily can be converted to "real" 480p, but this leads to wrong pixel ratio when scaled by HDTVs. The analog signal tweaks this by using some non-standard timing for hsync and vsync. For "correct" VGA output, you need a 400/429 clock ratio to convert one line of 480p to one line of VGA.
In the new version on github I achieved this clock conversion by the help of an external PLL (ICS664-3).

Luckily, this mod is non-destructive. Analog output and HDMI work simultaneously.
I also added 480i/240p to 480p line-doubling in the lastest version.

For some details on the the latest version see here: https://github.com/chriz2600/DreamcastH ... /README.md

Nice work chris2600

So does this now output as 640x480p over HDMI with the correct pixel ratio?
Also do you have any other features planned that you would like to add that are not listed in the readme?

I would be interested to know if the FPGA could support a "trick VGA mode" that switches to the 31kHz video mode just after the games IP.BIN has been read, or perhaps even some way to use a controller button combination to switch video modes?

chriz2600 · Joined: 07 Feb 2016 Posts: 19 Location: Cologne, Germany

Wolf_ wrote:

So does this mean that to play every game you would need to use analog output for the 49 games that won't run in vga mode as well as a hdmi connection for everything else?

No, the current implementation supports line doubling of 480i content to 480p. You only need HDMI.

citrus3000psi wrote:

Do you have a schematic of this new design?

Yes, I've updated the github site.

bobrocks95 wrote:

Is the roadmap at the bottom up to date? i.e. do you still need a VGA cable plugged in for the system to detect it?

Updated roadmap is now available. You don't need a VGA cable, but you have to attach a switch to pin 6/7 of the video connector. Automatic mode detection (480p/480i) is done by the FPGA.

bobrocks95 wrote:

I'll eagerly await someone to make a board for this! Or maybe it's better to wait for a flex cable... How many pins on the GPU need to be connected?

There are a total of 20 pins to connect to, including video, audio and mode switching.

Link83 wrote:

So does this now output as 640x480p over HDMI with the correct pixel ratio?

Yes, on all HDTVs/monitors I've tested so far. Also the specification defines a pixel ratio of 1:1 for 640x480p@59.94/60 Hz.

Link83 wrote:

I would be interested to know if the FPGA could support a "trick VGA mode" that switches to the 31kHz video mode just after the games IP.BIN has been read, or perhaps even some way to use a controller button combination to switch video modes?

As this mod taps only in the video circuit, there is currently no way to detect controller events. But the switch needed for mode selection operates exactly as those on VGA boxes, so it should be possible to start in 480i mode and then flick the switch to go to 480p mode.

Wolf_ · Joined: 25 Jun 2016 Posts: 383

chriz2600 wrote:

Wolf_ wrote:

So does this mean that to play every game you would need to use analog output for the 49 games that won't run in vga mode as well as a hdmi connection for everything else?

No, the current implementation supports line doubling of 480i content to 480p. You only need HDMI.

So does this mod work off the vga and the av out lines or does it attach at some kind of central video generating area?

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