Video From Scratch

[ivop]

Hi all,

 

This is a project I have been working on on and off for the past five years. Recently I redid everything in KiCad as I wanted to learn the package and move away from Eagle, and thought this might be a nice time to share it with all of you.

 

 

The zip-file contains:

 

- schematic (both .sch and .pdf)

- netlist (.net)

- components (.cmp)

- board layout (.brd)

- LTSpice4 simulation (.asc)

 

Basically, I redesigned the complete video output circuitry of the Atari. A few of the goals I had in mind:

 

- use common, through-hole components so almost everybody can make and maintain it

- abide by 1.0 Vpp video signal standard (black/blank levels, 75 ohm impedance, et cetera)

- use R2R ladder instead of non-uniform 4 resistor DAC Atari used

- fit on a 5x5cm board (smallest size for ITead, Seeed, ...)

- understand everything that is going on inside the circuit

- output separate Y and C signals (CVBS can be done externally if it's really needed)

- big power and ground planes for shielding and decoupling

- and of course have a clear image

 

I have gone through several revisions through the years, build some on perf board, did simulation with LTspice (works with Wine on Linux, too), even had SMD boards made at one point (which violated my first goal ) and this is what I finally ended up with.

 

My test machine is an Atari 800XL with _all_ the video circuitry removed. To use this board, it's not necessary to replicate that. Just lift the GTIA's LUM0-3, CSYNC and COLOR pins out of its socket and solder wires or pin-headers onto them. Be sure to feed a _clean_ 5V and GND to the board and a video signal will "appear" at the other four pin headers at the bottom.

 

It is licensed under Creative Commons Attribution-ShareAlike 4.0 International

All comments, if not too harsh , are welcome! If you see ways to improve the circuit, please let me know. I have no plans yet to do a run of these boards. Perhaps later. Or perhaps somebody else sees an opportunity.

 

Regards,

Ivo

 

vfs-rev5.zip

[Stormbringer]

Would this board work on 400/800's?

[foft]

Very cool. + handy for gtia replacement experiments:)

 

I don't see why it wouldn't work on 400 or 800. It just connects directly to lifted gtia pins.

Edited March 20, 2015 by foft

[Simius]

The output stages are incorrectly calculated. Output impedance will be 117,8ohm instead 75ohm (R21||R23 + R24).

Edited March 20, 2015 by Simius

[Kyle22]

Would this board work on 400/800's?

I don't see the need for it on the 800, they already have nice video.

[OX.]

I'd like to know how the picture quality compares to the other s-video mods out there and if it eradicates the vertical lines problem, any before and after pics?

[ivop]

@stormbringer: as foft says, there's no reason to assume it won't work for the 400/800 or for any other GTIA based Atari.

 

@simius: aw, that's happened between revisions when I wanted to use standard, easy to get resistor values and totally forgot about impedance :/ It does not matter for picture quality though and the output stage forms a voltage divider with the 75 ohm resistor in the display device. The signal does have a slightly lower current though... Do you know about a not-too-intrusive solution?

 

@OX.: I don't have any before pictures, but it was terrible. I'll see if I can make a few photos tomorrow of this circuit connected to a C= 1084.

[+Stephen]

I don't see the need for it on the 800, they already have nice video.

Not entirely true - on any "modern" display, the vertical banding is terrible with my 400s and 800s. On an old school CRT, yes, the video is great.

[OX.]

I was thinking more on the lines of how it looked using it with an lcd screen rather than 1084/crt's as most of the s-video mod's look fine with crt's but have had a varying range of success on lcd's.

[Irgendwer]

Would be cool if this could be a GTIA daughter board (just plug it into the GTIA socket and put the GTIA itself into a socket on the board)...

 

[ivop]

@OX.: Sadly, I have only one very old JVC 32S60 LCD television that won't sync on anything build before 1995. Nintendo 64 is the oldest machine it can deal with. Its HDMI implementation is very broken, too. Perhaps I can try it on an LCD screen next time I'm in Maarssen (C64/MSX/Atari meeting) next month.

 

@irgendwer: yeah, that would be ideal for the casual user. don't think it'll still fit a 5x5cm board though

[FifthPlayer]

Does this mod work with NTSC as well as PAL A8s?

[Magic Knight]

I was thinking more on the lines of how it looked using it with an lcd screen rather than 1084/crt's as most of the s-video mod's look fine with crt's but have had a varying range of success on lcd's.

 

I agree, CRT's love the native output from the GTIA (abeit some signal conditioning before it hits the Monitor), but LCD tv's input circuitry these days have variable results ive noticed (with some tests ive done on several types of boards).