Sony BVM-20F1U + Neo-Geo MVS (consolized) saturated/overly bright image

Discussion in 'Neo Geo Discussion' started by 90s Kid, May 26, 2014.

  1. 90s Kid

    90s Kid Active Member

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    Sure, I tried adjusting it - infact if I designate a channel for MVS and adjust whatever is necessary to correct the image - I'm also happy but even adjusting brightness just makes the image darker but equally lossy in detail and just wrong... I think the RGB levels are probably the cause and can be adjusted via the screen, but when I'm adjusting I'm just making it worse...

    EDIT: If it helps, the image looks too "yellow" and bright. Now, brightness can be adjusted sure, but yellow? What R,G or B should be adjusted...?

    Thanks
     
    Last edited: Jun 1, 2014
  2. pza

    pza <B>Site Supporter 2012</B><BR><B>Site Supporter 20

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    You can check your colors with the colortest if you have an unibios installed
    colortst.png

    From what I've read, it's best to install 1KΩ pots on the RGB lines, so you can adjust it the way you want.
     
  3. 90s Kid

    90s Kid Active Member

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    Cool, I'll check that out - see if I can improve things... Is there any services within the UK that can install those pots for me? I'm not too good at that kinda stuff...

    Annoyingly, when you pause the game playing Last Blade 2 - it corrects the colours with its pause effect! How annoying!!!
     
    Last edited: Jun 1, 2014
  4. 90s Kid

    90s Kid Active Member

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    I don't know if this helps perhaps telling me what to adjust...

    no matter what I do, the colour gradients from half way are just a solid block...

    [​IMG]
     
  5. Flash

    Flash Dauntless Member

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    How about just to try to reduce signal levels with something as dumb as a few 75 ohm resistors?
     
  6. 90s Kid

    90s Kid Active Member

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    Thanks, I'm not too good with a soldering iron though, will it be difficult?

    Thanks
     
  7. Flash

    Flash Dauntless Member

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    Very easy - just 75ohm resistors between signal contacts (RGB) and ground. Just like PAL SNES - without resistors image is oversaturated and too bright. It's safe and definitely worth a try.
     
  8. 90s Kid

    90s Kid Active Member

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    Cool, well I'll grab the resistors tomorrow and probably be back to triple check I'm not gonna mash it up lol!

    Thanks
     
  9. Calpis

    Calpis Champion of the Forum

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    The problem is that a MVS doesn't output proper RGB signal levels for a TV/broadcast monitor (for economic reasons arcade monitors use much larger signals and the contrast and black level must be adjusted for each game), nor do they have a linear gamma function (the relationship between signal voltage and monitor brightness isn't linear), so your image is really oversaturated because the signal it's receiving is so large. There's nothing really that can be done about the gamma, but there are two options for delivering a standard signal level to the display:

    -The correct way to consolize a MVS is to attenuate the RGB signals to 0.7V with a voltage divider, then put them through a video amplifier with 75 ohm back-termination resistors. The AES uses 6.8k and 2.2k which roughly yields 0.7V from the DAC's 3.0V signals.

    -The correct way to view MVS on a TV (as in a supergun, from the JAMMA connector, without modifying the board) is to use a 1k linear potentiometer (simulating a voltage divider) as the MVS' DAC's load, taking output from the wiper. The signal from the wiper is attenuated (again you want to adjust for 0.7V) then the signal must be put through a series capacitor into a video amplifier. The video amplifier can be AC coupled with a series input capacitor and resistor biasing network + 75 ohm back-termination resistor and 220 uF series output capacitor. Or if the video amp has a DC restoration/clamp input circuit you can omit the resistor biasing network at the input and the series output capacitor---this will yield a cleaner signal.

    If your BVM can disable the 75 ohm termination resistors at the RGB inputs (many can--it's the Hi-Z or 1k setting), then you might be able to directly connect the MVS and directly view the non-standard signals after significantly adjusting the contrast and brightness controls.

    -----

    The common advice about putting 75 ohm pull-down resistors on the output (the output comes directly from the DAC) is a bad idea. Yes the 75 ohms will be in parallel with the display's 75 ohm termination resistor yielding a 35 ohm load, and the voltage divider formed from the DAC's ~120 ohm output impedance and the 35 ohm load will yield a 0.68V signal.. close enough to 0.7V. The problem is that the DAC is meant to drive a 1-5k ohm load of an arcade monitor, not 120+35 = 155 ohms! 155 ohms means each DAC would have to source 20 mA of current (since the signal is approx 3V), and 10 mA must come from the most significant bit. The chips are only capable of supplying at most 2.5 mA per bit, 1/4 of the most significant bit. This means the signal will have heavy distortion (in the form of dynamically reduced contrast), and more importantly this will physically stress the chip.

    On the other hand if you use a video amplifier as you're supposed to, the DAC won't be stressed, the signal will be impedance matched (very sharp pixel edges without ringing, improved noise figures and no ghosting with long cables) plus the signal will work directly on all calibrated TVs without adjustment.
     
    Last edited: Jun 2, 2014
    flamie666 likes this.
  10. 90s Kid

    90s Kid Active Member

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    Thanks for the detailed information, it sounds best just to buy an AES then. I have an extensive MVS collection though due to owning a cab also... MVS can be played on AES via a converter right? Any drawbacks to this method?

    Thanks
     
  11. blotter12

    blotter12 <B>Site Supporter 2014</B>

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  12. pza

    pza <B>Site Supporter 2012</B><BR><B>Site Supporter 20

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    @Calpis: Interesting read. I almost understand 1% of your post.^^
    What video amp can you recommend/what do you need for the video amp? Something like this?

    @90s Kid: I'd still try the resistor/pot method first, before spending a lot of cash for an AES and Daedalus (I bet 90% of all cMVS- and MAK/Supergun-users don't use a video amp).
     
  13. Itxi

    Itxi Active Member

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    My supergun goes straight from the JAMMA connector to a scart with just a pot on each colour and one on the sync, the colour levels are all fine (with a bit of tweaking).

    It sounds like Calpis has the best solution, but certainly don't need to plug it into anything else between your mvs and the BVM
     
  14. flamie666

    flamie666 Member

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    Calpis thanks for all that info, I'm going to build that voltage divider-amp. Do I need a cap between the resistors and the amp?
    Thank you!
     
  15. retronerd

    retronerd Spirited Member

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    Have you built your oltage divider-amp yet? i want to do the same but need some guidence
     
  16. flamie666

    flamie666 Member

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    Yes I did and it works great! I use it with a PVM and with a Framemeister.
    The circuit is very simple and the results are really nice.
    No color crushing.
    So, the circuit goes like this:
    In series for each color signal, a 6.8K resistor.
    From each resistor to ground, a 2.2K resistor
    Thats the voltage divider part.
    Then from the voltage divider, you go to the THS7314. No caps between needed.
    Then from the output of each color, you go to a 220uf electrolytic cap to its positive leg.
    From the negative leg, you go to a 75ohm resistor and then to the monitor.
    For constructing the THS7314 amp, you can use this guide: http://retrorgb.com/snesminidiy7314.html
    For Sync signal, I used a 75ohm resistor and then to the monitor.
     

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    Last edited: Aug 22, 2016
  17. retronerd

    retronerd Spirited Member

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    Thanks! Do you have a diagram so its more easy to follow? im quite new to this. Thanks
     
  18. flamie666

    flamie666 Member

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    Here I draw a schematic, not pretty but I hope it helps.
    5V for the THS7314
    Correction:
    Looking at the THS7413 datasheet, you don't need any caps at the input or output, so is even simpler
    Here is the schematic:
    Resistor Values:
    R1-R3= 6k8
    R4-R6= 2k2
    R7-R10=75
     

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    Last edited: Aug 22, 2016
  19. retronerd

    retronerd Spirited Member

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    Thanks! do you have an image of the actual board on both sides? thanks
     
  20. retronerd

    retronerd Spirited Member

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    IS THERE NO BENEFIT TO INCLUDE THE CAPS?
     

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