MOD GUIDE - Universal RGB-to-YPbPr/Component Circuit & Mod [8/24/2014]

[Helder]

My test calls for more resistance on green so still wanna see what happens if you try that idea.

1) Red = 500/.299  = 1672 Ohms
2) Green = 500/.587 = 852 Ohms
3) Blue = 500/.114 = 4386 Ohms

Just pointing out on blue that there was a mistake on the schematic and you wound up building a board with it. If my test/idea is no good, you wanna default back to Steve's generic values and just up resistance on green in your case.

Just to let you know I tried to values with some pots and it was sooo off on every color. I also tried tweaking each pot to see if maybe it would improve but I could never get the colors to even remotely look like they did with the normal resistors in place.

I'm going to try steve's method but I added a pot in the Green input to see if that will help and also increased the 15ohm to 33ohm, I'll post results soon.

[NightWolve]

Ah, thanks for trying!

Alright, just to review here, I trust that the two 10K pots for the Red-Y/Blue-Y lines were set right in the middle to 5k, 5k, etc. when you started it off ? You do have to adjust those pots to achieve the proper tinting especially with a dramatic change in Luma/Y as was done here, and they're not supposed to be exactly the same actually. Strength levels are supposed to be .71 for Red-Y and .57 Blue-Y according to the spec. So, output resistance of Blue-Y would be higher in a normal design, it should be weaker than Red-Y, but in Steve's case with the 10K pot adjustments and no output resistors, you'd fiddle around with just those pots.

Another test would've been to have had just the Luma/Y output from your PCB/board connected to the TV's Composite jack to see if you're getting a good black'n'white picture - that would be the best test for a good black'n'white picture. Should be no excess green, or blue, or red, etc. Then after connecting Luma/Y normally to the YPbPr/Component jacks with Red-Y and Blue-Y also, you'd have to adjust those 10K pots for tinting.

But yeah, I assume in your test you tried plenty of adjusting of those pots and didn't just leave 'em alone, right ? I don't see you mentioning any adjusting of those, just your custom ones on the RGB input lines.

Well, anyhow, that formula was based on that company's op-amp and a sweet spot 324 Ohm feedback resistor, so no expectations it'd apply to the generic transistor with a 500 Ohm feedback resistor. There still should be a way to compute exact resistance values for our Luma/Y, but this might not be it is all.

Also I heard that there are ways to make a similar circuit with an op-amp and some resistors, ever look into this steve?

Yeah, that's what this is that I posted earlier!



Here's where that math info I wanted to test with came from (http://cds.linear.com/docs/en/datasheet/6559f.pdf - good research material from this PDF!):



The company's full page on their circuit titled, "High Speed RGB to YPBPR Converter":

http://www.linear.com/solutions/1312

They're trying to sell you on their chips, LT6559 and LT1395, both op-amps, and that "high speed" RGB-to-YPbPr circuit is an example on how to use them.

http://www.linear.com/product/LT1395 - (Single 400MHz Current Feedback Amplifier)
http://www.linear.com/product/LT6559 - (Triple Video Amplifier)


Self Reminder: I have recently created PCE/TG-16 video testing software that I asked Bonknuts/Tom to produce for me both for flashcarts and as a CD-R for CD drives. All the software does is when it's booted is make an all white screen, all pixels max white, then every time you press button I, it goes to all Red, then all Green, and all Blue, etc. This will allow you to view a steady RGB signal when using an oscilloscope on the HuC6260 video chip on the RGB pins and hopefully help tweak circuits like this one, etc. I will update the thread's OP with info and download links later when I get a chance.

[Helder]

I didn't mess with those pots at first but the difference wasn't major so I didn't bother so much. So that op-amp circuit can be used with any op-amp chip? I have quite a few laying around might give that a go as well. I have some free time today so I will see what my last changes give as results.

[NightWolve]

The exact resistor choices are expertly tweaked based on their chips, so there would be changes with other chips. I dunno what luck you would have. Also, as mentioned earlier, they left H/V Sync up to you. Something like Steve's Sync amp would work I guess, and then you'd connect it to Luma/Y after the 75 Ohm resistor on the final path to the TV.

Oh right, they also assume the RGB signals coming in are 75 Ohm ready, like say from a SCART output source, meaning, 1Vp-p (I guess ?), that is, properly amplified, so if not, you'd have to remove those 3 pull-down resistors at the start, the 80, 86 and 76 ones. Dunno how well this would wind up amplifying weak signals even after that. If you did a basic RGB amp first and THEN fed the lines into this design, then no problem.

Man, can't wait to get going to try this out for myself! My new scope is ready, willing and able, just need to make the time for this project finally. Might just see about using my TurboExpress since my Duo is out of commission waiting for the recap job. Would've been nice to have had a working TG-16 to mess with as well. I won one via NEC's Turbo Titans contest, but it broke on me... The games would load, but the colors were all wildly inverted/crazy, etc. and I guess I just threw it away after I upgraded to a Turbo Duo for the $99 deal back in the 90's.

[Helder]

After tweaking things to where it looks good and getting rid of that Green bleeding the downside is that the Green is lacking or super weak in alot of places. When everything is as it should be with the correct color balance the Green bleeds into the black, below is a picture showing a great example:



Up close (notice the green in the eyes and hand outline):



what do you guys think might need to be done or altered in the circuit to fix it? I tried messing with the 15ohm resistor and all that did was mess with the sync so that's not anything that could fix it. There must be something in the mix that is causing the bleeding but everything else is great.


I tried to find those amps you linked above and they aren't that easy to find unless you get them in China so I looked for something similar and found that the TSH344 and TSH343 used in conjunction could output Component.

Datasheets:

http://www.st.com/web/en/resource/technical/document/datasheet/CD00079538.pdf

http://www.st.com/web/en/resource/technical/document/datasheet/CD00079537.pdf

[thesteve]

what input caps are you using?
i was using 100UF as my scope liked the signal better

[Helder]

I'm using the same 100uf caps as you and the schematic, could changing the cap on the green line affect the bleeding? maybe go to 220uf?

[thesteve]

the bleed is caused by luma sag
either you have a cap reverse biased or need bigger caps
note the inline caps in your scart cable (if used ) will cut the input cap values near in half

[Helder]

I'll try larger caps and I'm using some RCA sockets so that shouldn't be the issue with the Scart. I'll post results soon on different cap sizes.

[Helder]

Well I tried a 470uf and it didn't change anything but one thing I also noticed before and again when I was testing with Sonic 2 is that in certain parts of the level where there seems to be more Blue the green actually changes hue a bit then back to normal once I leave that area. Any other suggestions to stop the bleeding?

[thesteve]

a few sugestions
try it with just luma hooked and see if its still bleeding
try it with 1 input color unhooked and see if it stops bleeding
that should determine where your having an issue

[Helder]

I will try that but before that I changed some of the other resistors to pots on the green line and have had some good results but still bleeds. The only way it doesn't bleed is if I over saturate either Red or Blue but mainly Red. I'm thinking that Red might need some resistor tweaking, I'll post results later today.

Edit: Tried the Luma by itself and I can see a heavy outline on the lettering which is the bleeding green when I connect the other colors one by one. So looks like something on the Green/Luma is the culprit.

Edit 2: It turns out it was the 1k R1 on the Red input that was causing the bleeding, I put a pot on there and tweaked it till the bleeding disappeared and adjusted the other pots to get the correct colors. Now that I know the values I'm wondering how I can remove the 10k pots and put specific numbered resistors in their place and how to rewire them since the pots fed 2 location at a time. I will post the values I used once I documented them all so everyone can use them for the Genesis.

Edit 3: To get no bleeding simply change the Red Input Resistor to 2.4k and the Green Input Resistor to 1.5k. I also have the 100 ohm pot at 45ohms which looks great. One last question so I can refine a new board with these values, can 22uf ceramic caps be used in place of 100uf and 470uf? or do these have to be electrolytic caps or even tantalums?

[thesteve]

if using the sync stripper a 1K resistor from sync input pin to ground is a drastic improvement
dont know how it would effect it running direct chip sync from a PCE

[Helder]

I've never had issues with sync unless this somehow adds to the bleeding on the Red channel?

[thesteve]

It does not, however if red is bleeding, a resistor from the input red to ground will help