SNES-Mini RGB Measurements

Discussion in 'Modding and Hacking - Consoles and Electronics' started by Ultron, Aug 6, 2014.

  1. borti4938

    borti4938 Robust Member

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    Aside from the extra buffer from the THS7374 the THS7314 and THS7374 are almost the same. However, on the board I use a different voltage divider for red compared to green and blue. You can see in the measurements that red has a lower Vpp coming out of the S-CPUN than green and blue. The simple correction with the 1.1k resistor at every R,G,B wire to ground (parallel to the existent ones) won't change that.
    What you can observe is that the color feels a bit different if you compare both methods.
     
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  2. technology4617

    technology4617 Member

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    Would you be willing to draw up a quick diagram of the parts necessary for use with an NTSC SCART cable? I read the readme but am still very confused as to which component is supposed to go where.

    EDIT: Never mind, just learned what an Eagle schematic was. Sorry, this is basically the first time I've touched PCB stuff.

    Well, I don't currently have any issues I'm trying to fix; I just want to make sure I get the best/newest version of the mod done.
     
    Last edited: Jan 30, 2016
  3. borti4938

    borti4938 Robust Member

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    I've updated the bottom silkscreen as well as the components naming in the schematic with their values. Now it should be more clear which component goes where ;)
     
  4. technology4617

    technology4617 Member

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    Ah, that's very clear now, thanks!

    Also, one more thing I was curious about: will using your THS7314 board in a 1CHIP-01 unit improve the picture quality or is it primarily for SNES'es that have the vertical line issue? I've seen a few forum posts that the brightness of a stock 1CHIP unit is too high by default, though I was unsure whether this was actually due to an off-spec signal or people simply preferring a darker signal to aid CRT sharpness.
     
    Last edited: Feb 2, 2016
  5. borti4938

    borti4938 Robust Member

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    It will slightly improve the picture quality.
    The brightness issue comes from the off-spec signal. However, my latest bypass boards corrects it.
    The vertical line issue comes from the DRAM refresh impulse. In 3Chip-models, this refresh impulse can be deactivated by lifting pin 49 at the S-CPU (and set it to Vcc). I neither know side effects nor
    the corresponding pin at the S-CPUN in 1Chip-models. However, the RGB bypass seems to help getting rid of the vertical line.
     
  6. technology4617

    technology4617 Member

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    If you want to keep the stock encoder, will the old THS7314 fix of tying the lines to ground with 750 Ohm resistors fix the issue as well?
     
    Last edited: Feb 6, 2016
  7. Xaranar

    Xaranar Rapidly Rising Member

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    I've ordered your 1CHIP-03/SNES Mini board from OSH Park, and all of the parts from the list on Github. Can't wait to assemble this and solder that bad boy in.
     
  8. Xaranar

    Xaranar Rapidly Rising Member

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    And I've now populated the board and soldered it in to my Mini. I love it, it works great, the only thing that doesn't work is the sync line, it causes all sorts of problems like picture shaking and colours dropping out randomly. I've taken to wiring up the stock sync from the Mini's video chip and it works a treat. Tried different resistor values on the sync line and it seems to get worse with lower resistance.
     
  9. retrorgb

    retrorgb Spirited Member

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    I just realized I never updated the 3-wire mod page on my site with the brightness resistors...and it's been so long since we all worked on it, I'm completely drawing a blank on what needed to be done with that mod. I found the following post and it looks like all I'd need to do is add three 133Ohm resistors from the video line to ground: http://assemblergames.com/l/threads/snes-mini-rgb-measurements.53053/page-2#post-768707

    Am I reading that correctly? Basically this, but the yellow lines would be the 133 Ohm resistors:
    [​IMG]
     
  10. Ultron

    Ultron Spirited Member

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    The 3 wire mod is wrong. The other mod you have on your page is the right one. The correction to the circuit needs to be done before the RGB chip.
     
  11. retrorgb

    retrorgb Spirited Member

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    Thanks for the reply! So, first do the basic 3-wire / 75 Ohm resistor mod, then remove R6, R7 & R8 (bottom of the board near the cartridge pins) and replace them with 133 ohm? Do you happen to have a DigiKey link to the right ones?

    Sorry to keep bugging you, but I've been using the pre-made chips for so long now that I just forgot all about this.
     
  12. Ultron

    Ultron Spirited Member

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    No worries!

    Keep the R6, 7, and 8. Wire up 3 1.2k resistors (Digikey does not have 1.2k 1% tolerance, but Mouser does - Link) from the vias to GND.

    [​IMG]

    Connect the 3 RGB outputs from the S-RGB chip to the Multi-Out. Also connect C-Sync as well. Make sure you have a combination of 75 ohm and 220uF resistors on the output lines of the S-RGB chip, either in the cable or soldered on the board.
     
  13. retrorgb

    retrorgb Spirited Member

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    Oohhhh....this is all making sense now. It's the exact same procedure as when using the THS7314, since it's essentially doing the same thing (changing the brightness before it hits the amp). I'd just post this exact same thing in the guide, but without the blue wires, right?: http://www.retrorgb.com/snesminirgb.html#brightness
     
  14. retrorgb

    retrorgb Spirited Member

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    Okay, that was it, thanks for your help.

    I just finished taking a ton of screen captures. I'll eventually try and get an many SNES revisions as possible, but here's what I have so far, including many different SNES Mini mods and a txt file with a full description of each. These are untouched bmp's without a watermark, so please be respectful and not use them without giving credit. I've had most of my pics stolen and used in other sites, eBay auctions, etc...it's quite annoying: http://www.retrorgb.com/files/SNESVersionCompareJune2016.zip

    I bet the reason I used to think modded Mini's were better then 1CHIP's is because of brightness and both look identical with the THS7374 mod. I'd like to try taking a stock 1CHIP and adding the 750 Ohm resistors to it and see if it's as good as a 3-wire Mini with the 1.2k resistors. Thoughts?
     
  15. Ultron

    Ultron Spirited Member

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    Sorry for the delay in response. I agree with your first point. It was most likely a brightness issue. Try out the mod on a 1CHIP, I would expect the images to be of similar quality with the mod.

    Looking at the pics, I think the Mini 3 wire with the 1.2k resistors looks the best, even better than the THS7314 with the resistors. The THS image does not look as sharp.
     
  16. bigshowpl

    bigshowpl Member

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    Hi, i'm planning to add the 3 x 750 Ohm resistors to my 1CHIP-01 console, but I've seen a comment on another forum that 732 Ohm resistors are actually closer to what we're looking for, can anyone confirm if that's true or not?
     
  17. Bad_Ad84

    Bad_Ad84 The Tick

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    Unless you are buying 0.1% resistors, it really doesn't matter. Even 1% ones could be out by that much.
     
  18. Ultron

    Ultron Spirited Member

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    750 should work fine. Get 1% tolerance.
     
  19. bigshowpl

    bigshowpl Member

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    Thanks for getting back to me! I've tried to do the maths myself and I'm coming up with 820 ohm resistors being closest based on 0.714v being the target would you mind stepping me through the equation to show me where i went wrong (or am right?)

    Thanks!
    Paul
     
  20. Ultron

    Ultron Spirited Member

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    I don't remember if anyone measured the output of the PPU on a 1-CHIP. But if it is the same as the Mini, you would calculate it like this:

    The Mini's output of the PPU DAC circuit is 0.8V at max brightness. The resistance needs to be changed to produce an output of 0.714V.
    The circuit has a 150 ohm resistor (Mini). The current source output by the PPU at max brightness is: 0.8V / 150 ohm = 5.33mA.

    The resistance of the current-to-voltage converter needs to be changed to produce a voltage of 0.714V. So: 0.714V / 5.33mA = approx. 133 ohm.

    So, on a Mini, by adding a 1.2k resistor in parallel with the current 150 ohm installed on the board, we get a resistance of:
    (1.2k * 150) / (1.2k + 150) = approx. 133 ohms. This will give us 0.714V at max brightness.

    I believe the Mini and the 1-CHIP share the same S-CPUN. So, the current source should be the same value at max brightness - 5.33mA.
    So, we are still looking for a combination to give us 133 ohms total resistance. So, 800 ohms would actually be the best value:
    (800 * 160) / (800 + 160) = approx. 133 ohms.

    800 ohm 1/4 watt 1% resistors are hard to find. Try finding something close to it, it should work fine.
     

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