Svideo core for first prototype
Waiting the Chinese sun to rising, I decide to proceed with the "video mod" of our v 1.0 board. So I desolder the "vsync" pin 12 of video dac, and solder it again using a "kynar cable" to a near via, which drive directly to F15 of FPGA.
So, when you have ready some core to try it, let me know it.
#1 Updated by foft over 4 years ago
Panos tried out a new core with composite only - on green using the sync pin. Looks brighter but just black and white - on several TVS.
I think its black and white because...
Basically I re-scaled Y to use the full DAC range. Composite is made by basically Y + chroma. Since Y is now a larger range (say 0-255 instead of 140-255) I need to scale chroma similarly. So basically chroma is 50% of the voltage it should be...
#3 Updated by foft over 4 years ago
Setting this up in the simulator so I can understand chroma better.
The problem seems to be that I've used all the space for luma and chroma is a sine wave +-255. For some reason it only allows space of 32 though - which is odd! Anyway I need to allow some space of the chroma in composite video - and rescale it larger.
Reading video demystified this isn't really to spec. I think changing R5 might actually yield more 'correct' results. In any case I think we might be able to get good results from the sync-on-green approach so will continue to try that first.
#8 Updated by foft over 4 years ago
Worth a read for some background: http://codeandlife.com/2012/10/09/composite-video-decoding-theory-and-practice/#more-1032
After this its 'Video Demystified' for the details
#9 Updated by foft over 4 years ago
Added traces of Atari basic from the scope, to see the voltage level differences.
Eclaire is driving my monitor, Atari is through a 66ohm resistor (little low, so voltage is suppressed a little).
Still, a stark difference!
#10 Updated by foft over 4 years ago
We have a range of 0-1v on the DAC. 0-0.3V sync and 0.3V-1.0V is set by changing the dac level. So trouble is its not really enough. Might still need to change R5 on the V2, so perhaps we need a jumper for 'composite level'/'rgb level' as well as vhdl changes. Still sure we can make it might better with pure vhdl.
#13 Updated by foft over 4 years ago
So fixed saturation (except gray), just phase shifted for the cols. Then add luma. Might try that rather than generic rgb component.
#17 Updated by Stephen over 4 years ago
Maybe is time to think about implementing one external RGB -> composite/s-video converter, fit directly to VGA output connector.
But the question still remain for me if people really need it.
My thoughts (coming from NTSC land). We haven't had TVs or monitors with S-Video connections available new for probably 15 years now. Also, s-video will not allow artifacting. So I would say that composite is good to have for games which rely on artifacting. VGA and HDMI will cover any other device. It's a safe bet to say I'll be using mine via HDMI 99.9% of the time.
Also, I will pretty much be running in PAL mode all the time with this device.
#20 Updated by Stephen over 4 years ago
For anyone with composite/svideo setup. Please can you tell me what you think of svideo_gtia6.sof vs the previous version.
This is generated more the 'atari way' - i.e. off hue/brightness with phase shifts, rather than being based on the RGB palette.
Hate to be a dunce - where is the documentation for cabling s-video from this board? I have a Commodore 1084S monitor as well as a 20" Sony PVM which I can test.