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1088XLD - Custom A8 Computer in 1050 Drive Case

Alternative Atari Motherboard Piggybacks 1088XEL tech Repurposing a 1050 case

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#301 mytek OFFLINE  

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Posted Sat Oct 27, 2018 3:38 PM

Re-visiting the revised video output section.

 

In order to better ascertain if my simple transistor buffer design would work, I built a little test board to take the place of the UAV on one of my 1088XEL's.

 

ALT-VB_board.JPG

 

I just noticed I stuffed a 1K resistor for R1 instead of the spec'ed 1.5K resistor.

Hmm... after checking on a scope, the output looks good, so I think I'll keep it that way.

 

scope.png

 

After firing it up I noticed that there were evenly spaced vertical lines in the S-Video display. Argg :mad:​ the dreaded vertical banding problem has reared it's ugly head !!!

 

Example of what those vertical lines looked like (image courtesy of this POST) ...

 

vertical_banding.jpg

 

This is caused by chrominance bleeding over into the luminance line. I had thought by adding an independent transistor buffer stage for the composite video output prior to injecting the chrominance signal, that I would avoid this problem. Well I guess the answer to the million dollar question is "No it didn't".

 

So for now I eliminated the entire composite output circuit, but the real culprit is capacitor C2. Just clipping this out of the circuit solved the issue.

 

ALT-VB_TestBoard_schema.png

 

And now for the comparison test results (These are not video captures. Taken with a camera looking directly at an LCD monitor)...

 

Text.jpg

 

Color.jpg

 

The S-Video results are very good indeed. And for all practical purposes looks identical to the UAV's video output in this regard. This I am happy about  :) .

 

In order to solve the chrominance bleed-over caused by adding the composite video output circuit, would likely take a true video buffer amplifier IC to isolate that part of the circuit from the S-Video outputs. Unfortunately those video buffer ICs have gotten pretty scarce and expensive in a single supply thru-hole device. So I'll need to put on my thinking cap to see what alternative approaches could be used to satisfy the requirements. for the time being I like the S-Video results, and will be keeping that part of the circuit in my 1088XLD design.



#302 Kyle22 OFFLINE  

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Posted Sat Oct 27, 2018 9:28 PM

It looks MUCH better than my original 800 on this 24" Gateway monitor. It has a little bit of vertical banding, but not that bad.



#303 mytek OFFLINE  

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Posted Sun Oct 28, 2018 7:01 AM

The actual onscreen video looked even better then my sample photos. Although after many attempts and fiddling with camera settings, I was finally able to capture something close to what it looked like in reality on my Sharp LCD monitor.



#304 Kyle22 OFFLINE  

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Posted Sun Oct 28, 2018 6:47 PM

It looks MUCH better than my original 800 on this 24" Gateway monitor. It has a little bit of vertical banding, but not that bad.

To be clear, I mean MY 800 has more vertical banding than yours.

:)

 

Edit: typo.


Edited by Kyle22, Sun Oct 28, 2018 6:48 PM.


#305 mytek OFFLINE  

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Posted Sun Oct 28, 2018 10:00 PM

To be clear, I mean MY 800 has more vertical banding than yours.

 

Yeah I kinda figured that's what you meant  :) .

 

At first I thought the banding I was seeing in my bread boarded prototype was just attributable to the long leads connecting everything together, so that was the main reason I laid out a test board to be sure. Anyway it turned out to not be the cause, and was simply due to the circuit design and the inability to fully isolate the composite color injection from the luma output. So what I need is better isolation in the buffer amplifiers, and I think I found a possible solution that is relatively cheap to implement. ST electronics has the TSH343 triple video buffer chip that might do the trick. It's very inexpensive, selling for less then $2 in quantities as few as one piece, and it runs off a single 5v power supply. The only issue, is that it only comes as a surface mount device, but at least it's in the larger SOIC package which even I with my poor close-up vision can solder.

 

I have some samples of that buffer chip coming.



#306 BillC OFFLINE  

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Posted Tue Oct 30, 2018 2:11 AM

 

Yeah I kinda figured that's what you meant  :) .

 

At first I thought the banding I was seeing in my bread boarded prototype was just attributable to the long leads connecting everything together, so that was the main reason I laid out a test board to be sure. Anyway it turned out to not be the cause, and was simply due to the circuit design and the inability to fully isolate the composite color injection from the luma output. So what I need is better isolation in the buffer amplifiers, and I think I found a possible solution that is relatively cheap to implement. ST electronics has the TSH343 triple video buffer chip that might do the trick. It's very inexpensive, selling for less then $2 in quantities as few as one piece, and it runs off a single 5v power supply. The only issue, is that it only comes as a surface mount device, but at least it's in the larger SOIC package which even I with my poor close-up vision can solder.

 

I have some samples of that buffer chip coming.

Prior to the introduction of Bryans' UAV mod there was an upgrade that used a video buffer IC, the FMS64XX. The TSH343 is meant for component video

http://atariage.com/...a-new-video-mod

 

The FMS64XX chips buffer chroma/luma, and also create a buffered composite signal. They are discontinued and no longer available from major suppliers like Digikey or Mouser, but are available from eBay at reasonable prices.

https://www.ebay.com...=p2045573.m1684

 

 

Correction: The FMS6406 is still available from Digikey and probably other suppliers as well, the price is higher than the FMS6400 through eBay.

https://www.digikey....CSXCT-ND/820964


Edited by BillC, Tue Oct 30, 2018 2:25 AM.


#307 _The Doctor__ OFFLINE  

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Posted Tue Oct 30, 2018 2:44 AM

1.98 to clean up a boat load of mess sounds good to me. looks like it's got everything works for PAL and NTSC and with and can deal with stereo as well. Nice 1 to use

https://www.fairchil.../FM/FMS6406.pdf



#308 mytek OFFLINE  

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Posted Tue Oct 30, 2018 11:36 AM

Although the FMS64xx looks to be a good chip for this usage, I still like the TSH343, especially considering that it's still in production and I can source it from several vendors. The fact that it was intended for component video doesn't negate it's usage for either S-Video or composite which from an analog standpoint are not all that different. They're both using an SOIC-8 package, so no advantage/disadvantage there. But the far better pricing and availability speaks volumes for using the TSH343. Also I believe the TSH343 is better suited for direct coupling of the inputs, not requiring inline capacitors. Since I'll be doing the chroma/luma mixing for composite video myself, the built-in composite video mixing of the FMS64xx is not required (although that is a nice feature).

 

So I read the entire thread that BillC linked to (thanks for that). And one thing that I saw as an operational concern, was the repeated mention of vertical banding being seen via S-video on LCD displays. This appeared all the way through the rather lengthy development thread, and still seemed to be the case even after the last revision. So that doesn't bode well, since my first attempt at this yielded a fully functional simple design that for all intents and purposes fulfilled the criteria for excellent quality video output, with the one caveat being vertical banding issues which made it a no go in my book. I would not think this would have been an issue with the FMS64xx design, but how to explain all the reports that said vertical banding persisted no matter the revision level of the circuit.

 

Anyway my mission is to create clean S-Video and composite video output in the 1088XLD, as well as do a major cost cut in doing so. So that criteria is going to guide my journey and the methods employed to get there. Not saying I'll end up using the TSH343 in the final design, but for the time being it holds a lot of promise to help create what I am after.

 

Thanks again BillC for pointing me to that alternate video development thread. It made for a very informative and good read  :) .



#309 _The Doctor__ OFFLINE  

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Posted Tue Oct 30, 2018 1:23 PM

to kill the banding didn't wasn't it a matter of attenuating the input signal.... if not see what attenuation of the output does... I do know there were a number of crt's back in the day where they looked great with a weaker signal...



#310 mytek OFFLINE  

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Posted Tue Oct 30, 2018 3:34 PM

to kill the banding didn't wasn't it a matter of attenuating the input signal.... if not see what attenuation of the output does... I do know there were a number of crt's back in the day where they looked great with a weaker signal...

 

The vertical banding occurs when some of the chroma signal bleeds into the luma circuit. Yes attenuation of the chroma will also influence how pronounced the banding is, but that is not the way to fix the problem correctly. Bottom line is you need to prevent the bleed-over in the first place, and the best way to do that is to have complete electrical isolation where the luma and chroma are mixed to create composite video. Essentially you don't want the mixing of the chroma to contaminate the independent luma destined to go to the S-Video output.

 

Edit: CRTs are not affected as much by the chroma leaking into the luma, but for some reason the LCDs really see it. It is primarily an S-Video thing.



#311 mytek OFFLINE  

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Posted Thu Nov 1, 2018 5:51 PM

Chroma Bleed-Over into S-Video Luma Solved on Video Test Design

 

ALT-VB_rev1_1_schema.png

Changed Q2 to NPN darlington transistor (MPSA13) for the composite output, which gave me additional isolation from S-Video luminance circuit. Also adjusted a few values to render a better picture. The MPSA13 is obsolete, although Jameco has over 3,000 of them in stock. However the currently manufactured BC517 should also make for a good substitute.

 

Composite Video

 

Composite.JPG

 

The UAV has an ever so slightly better picture than what we see here, but the difference is relatively small. The UAV's composite video is a tiny bit sharper, and the text is slightly whiter in color.

 

S-Video

 

S-Video.JPG

 

My S-Video results actually look a tiny bit better than the UAV (at least in my 1088XEL). With the UAV I see a very small gray bar on the left hand side just prior to rendering the blue background, whereas my circuit does not have this. I tried adjusting the UAV's trim pot, but I couldn't get rid of the gray bar. However this bar is very small, and not all that noticeable. Overall both the UAV and my S-Video circuit look very good.



#312 _The Doctor__ OFFLINE  

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Posted Thu Nov 1, 2018 6:15 PM

Chroma Bleed-Over into S-Video Luma Solved on Video Test Design

 

attachicon.gifALT-VB_rev1_1_schema.png

Changed Q2 to NPN darlington transistor (MPSA13) for the composite output, which gave me additional isolation from S-Video luminance circuit. Also adjusted a few values to render a better picture. The MPSA13 is obsolete, although Jameco has over 3,000 of them in stock. However the currently manufactured BC517 should also make for a good substitute.

 

Composite Video

 

attachicon.gifComposite.JPG

 

The UAV has an ever so slightly better picture than what we see here, but the difference is relatively small. The UAV's composite video is a tiny bit sharper, and the text is slightly whiter in color.

 

S-Video

 

attachicon.gifS-Video.JPG

 

My S-Video results actually look a tiny bit better than the UAV (at least in my 1088XEL). With the UAV I see a very small gray bar on the left hand side just prior to rendering the blue background, whereas my circuit does not have this. I tried adjusting the UAV's trim pot, but I couldn't get rid of the gray bar. However this bar is very small, and not all that noticeable. Overall both the UAV and my S-Video circuit look very good.

Now this one belongs on all the bases...

My kind of mod!

+1 MyTek

 

Gratz,

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Einstein seal of approval !


Edited by _The Doctor__, Thu Nov 1, 2018 6:17 PM.


#313 mytek OFFLINE  

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Posted Fri Nov 2, 2018 8:35 AM

Now this one belongs on all the bases...
My kind of mod!
+1 MyTek
 
Gratz,
_Doc Brown__
Einstein seal of approval !


Thanks Doc :).

Yeah sometimes persistence pays off. It is in my nature to never give up (quoting Galaxy Quest). Now if I only had that time machine that we all talk about. Because I would travel back to the beginning, and hand this circuit to the creators of the A8 for reference. I know they did wonderful things creating the custom VSLI chips that powered the A8, but I think they were often confused when it came to designing the video output circuits.

Just to be clear... Bryan's UAV board is still the winner when it comes to composite video output quality. However when it comes to S-Video, we appear to be in the same league.

 

EDIT: It's also nice to keep it to the through-hole theme I always pursue in my projects, thus making it possible for more DIY types to build my creations.



#314 mytek OFFLINE  

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Posted Fri Nov 9, 2018 7:35 PM

MIDI DIN Interface board revision V1.2 (hopefully this is the final version)

 

MIDI-DIN-INTFC_V1_2_schema.png

 

Changed to ON-OFF-ON switch (3 position) and routed trace to 3rd position so that you now have these 3 different possibilities...

  1. MIDI-OUT only to feed internal Synth (software application can optionally echo MIDI-IN)
  2. External Synth only (Internal Synth disconnected)
  3. Auto Mode (MIDI-IN or MIDI-OUT feed internal Synth automatically dependent upon activity)

Updated manufacturing files and OSH Park link in this thread: http://atariage.com/...53#entry4145545

 

Note: V1.1 boards are still usable, only requiring one added jumper wire and different switch.






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