Another possible video mod for 800XL using FMS6400?

[thomas3120]

Hello,

I know this has been discussed a lot and have read various posts on improving video quality & adding s-video output.

 

Looking at the schematics of the 800XL (only Atari 8 bit computer I have) and the Atari 2600, they seem somewhat similar.

 

In the past I did a video mod (composite & S-video) designed by The LongHorn Engineer and the results (on my 2600) were amazing.

Here is the schematic:

 

It uses a CD4050 IC but also a FMS6400, which is a filter and video summer. The inputs of the FMS6400 are Y(Luma) and C(chroma) and outputs Comp video, S-video.

The only thing(or one thing) I believe that is different in the GTIA (8-bit) is an added Luma signal.

 

FMS6400 Datasheet:

http://www.fairchildsemi.com/ds/FM/FMS6400-1.pdf

 

CD4050 Datasheet:

http://pdf1.alldatasheet.com/datasheet-pdf/view/27058/TI/CD74HC4050E.html

 

Would this possibly work on the Atari 8-bit computers?

 

thomas

[BillC]

IIRC the TIA used in the 2600 has one less Luminance signal than the CTIA/GTIA.

 

low_budget already designed such a video upgrade for the 5200, also compatible with the Atari 8-bits, a couple years back.

http://atariage.com/forums/topic/193474-better-atari-5200-s-video-mod/?hl=+low_budget%20+fms6400

http://atariage.com/forums/topic/198754-current-best-s-video-picture-improvement-mod-for-800xl/?hl=+low_budget%20+fms6400

 

They are even available on eBay from time to time.

eBay Auction -- Item Number: 301236899626

[thomas3120]

Ohh boy..hehe a couple of years too late

 

Thanks for the links though, those didn't come up in my searches.

 

thomas

[kogden]

low_budget's SVideo board works quite nicely, I have 3 of them. MUCH better video output quality but it takes a little effort to install. You'll need some soldering skills.

[thomas3120]

Just want to thank low_budget for providing the video schematics.

Just finished making the board everything works but image is a bit bright (Luma?) Also getting vertical bars/lines.

 

Here's the circuit board:

 

 

Here is a screen capture in Composite:

 

Here it is in S-Video:

 

 

 

Any suggestions on fixing the image?

 

 

Btw, I noticed on the LHE 2600 mod he used 75Ω resistors to ground on the Luma, CV & Chroma outs:

 

Open to suggestions or advice

 

thomas

[Rybags]

75 Ohms is normal for video signals. Did you use something different?

 

I notice Pin 2 of the FMS6400 selects 0 or 6 dB gain and is connected to +5V - if you could have that switchable between that or GND you could try both options in quick succession.

 

All up it looks pretty good though - could you do some pics/cap of just plain hires/text mode?

[thomas3120]

75 Ohms is normal for video signals. Did you use something different?

 

I notice Pin 2 of the FMS6400 selects 0 or 6 dB gain and is connected to +5V - if you could have that switchable between that or GND you could try both options in quick succession.

 

All up it looks pretty good though - could you do some pics/cap of just plain hires/text mode?

 

 

 

No, followed low_budgets v2.2 schematics value by value.

Tried the gain (+5V on Asel) and it got worse and brighter.

 

I tried swapping out GTIA and Sally (CPU?) since I had spares but nothing changed.

 

Here is a video of it in S-Video mode:

http://i239.photobucket.com/albums/ff249/thomas3120/Gaming%20Stuff/th_20140724110142_zps9e506e63.mp4

 

Also, The non cart blue screen with some writting..

 

The vertical lines are exactly the same width all across the screen...really uniform...don't get it :/

 

Any more suggestions are welcome,

 

thomas

Edited July 24, 2014 by thomas3120

[+bob1200xl]

That looks like ground noise. It is very difficult to get a clean signal with point-to-point wiring. Try moving your ground closer to the GTIA ground. Only use one ground, also.

 

Bob

[+Stephen]

It does look like there are the typical 9 DMA refresh lines on the left of the screen as well. It seems to be getting more and more didficult to get a good image with A8 on modern displays. Really need to get my VBXE installed in my PAL machine.

[Bryan]

Possibly the Chorma signal is too strong, or not filtered properly.

 

On the Atari, we only get one Chroma level so the best level for it is a compromise between good color definition near edges and the visibility of the carrier in the picture. The level doesn't affect saturation much because the TV/monitor automatically adjusts to the burst level, which is the same as the color level.

 

However, there's possibly another issue here. The Atari doesn't produce a proper sine-wave for color, it produces a square wave. Without enough low-pass filtering on Chroma, its edges will leak through to the final picture.

 

 

EDIT:

Also, did you bend up the GTIA pins? If so, did you include the 1K pull-ups on your board? GTIA's video pins are open collector.

[BillC]

One thing I disagree with on low_budgets v2.2 circuit is buffering the chroma signal, AFAIK the chroma signal is analog while the 4050 is a digital buffer. His earlier circuits didn't use the 4050 buffer for the chroma signal.

[Bryan]

One thing I disagree with on low_budgets v2.2 circuit is buffering the chroma signal, AFAIK the chroma signal is analog while the 4050 is a digital buffer. His earlier circuits didn't use the 4050 buffer for the chroma signal.

They're all digital signals until they've been conditioned by external circuitry. The buffer isn't really necessary on the 2600 because TIA's pins have sufficient drive for the video circuits. GTIA's outputs do not.

[thomas3120]

I have an oscilloscope (still learning how to use it though).

If someone could give me some suggestions on where to hook up/look at signals, I could post the image results of the scope here.

 

Also what settings on the scope I should use, i.e. Line, Field, Odd, even, Line NO, coupling type, peak detect, average, etc... to use.

 

thomas

[Rybags]

When looking at the Atari's video it's easiest to use software on the Atari to create trigger for scope sweeps.

 

I just programmed in a change of PIA PORTA to transition a bit from high to low once per frame.

If your scope is smart enough to have those functions, you'd probably just want to use "field" although chances are it might start each sweep just after VSync - there's generally not much going on there other than HSync pulse followed by black level, though the colour burst will still be present.

 

I see "Line no." - possibly that is better, could be X lines after VSync. Although on a PAL machine you'd likely have the colourburst different each sweep.

[thomas3120]

When looking at the Atari's video it's easiest to use software on the Atari to create trigger for scope sweeps.

 

I just programmed in a change of PIA PORTA to transition a bit from high to low once per frame.

If your scope is smart enough to have those functions, you'd probably just want to use "field" although chances are it might start each sweep just after VSync - there's generally not much going on there other than HSync pulse followed by black level, though the colour burst will still be present.

 

I see "Line no." - possibly that is better, could be X lines after VSync. Although on a PAL machine you'd likely have the colourburst different each sweep.

 

 

How does is this for composite video signal:

[Rybags]

That's pretty good... can you zoom in so it's only showing the colourburst?

[thomas3120]

That's pretty good... can you zoom in so it's only showing the colourburst?

 

Hope this is better:

 

Btw, Color Burst is:

 

Edited July 28, 2014 by thomas3120

[Rybags]

I think it should be the second one... that first lot is too close to the HSync.

 

In any case, there seems to be a fair amount of noise in that signal.

 

Would you be able to do a scope from the colour signal, and triggered by the HSync?

[thomas3120]

I think it should be the second one... that first lot is too close to the HSync.

 

In any case, there seems to be a fair amount of noise in that signal.

 

Would you be able to do a scope from the colour signal, and triggered by the HSync?

 

Chroma signal?

 

t

[MrMartian]

I think it should be the second one... that first lot is too close to the HSync.

 

In any case, there seems to be a fair amount of noise in that signal.

 

Would you be able to do a scope from the colour signal, and triggered by the HSync?

 

Nope. The color burst is the first part, right after the sync, in the 'front porch'.

 

The 'B' section would be text on the screen.. Depending on line number, possible the 'READY' prompt.

 

Good info: http://www.maximintegrated.com/en/app-notes/index.mvp/id/734

Edited July 28, 2014 by MrMartian