HDMI-to-VGA adapter

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  • #23225
    James-F
    Participant

    I’m actively researching the wobble/jitter problem some folks have had and I’m going to be sending MLIG some hardware to try in the near future. On the NES it could be that a dejitter mod is needed but I’m not sure.

    I do not see any wobble or phase issues when using with a PC (NVidia GTX) in any resolution, the image is crystal clear.
    Maybe it’s a power issue, maybe timing issues?

    Can you test the HDMI->VGA adapter if it clips the dark shades with full scale 0-255 HDMI output?
    I have one of these adapters from ebay and it seems that it clips the darker shades from the OSSC HDMI 0-255 output.
    Maybe these HDMI->VGA adapters require HDMI limited range 16-235 to work properly?

    Quoted myself from another thread, and answering my own question after some testing;
    Any consumer device that uses HDMI is generally designed for YCbCr Limited Range, that includes DVD/Blu-Ray players and TVs.
    These video conversion devices will behave much better in YUV/YCbCr limited range since the color processing inside them is in YUV limited range anyway.
    That includes both this HDMI-to-VGA adapter and the Scart-to-HDMI adapter from ebay.

    Changing HDMI to limited range 16-235 on the PC fixes the gray scale when using this adapter.
    Moreover, this adapter accepts HDMI YCbCr or RGB color space without any problem, but only shows correct gray scale in limited range.
    Saying again, the majority of video related consumer HDMI devices are YCbCr Limited Range 16-235.

    I wonder what range the RetroTink 2x outputs? I know it is YCbCr422 color space.
    I know the OSSC outputs RGB full scale 0-255 so it will clips the very dark and white shades.

    #23230
    James-F
    Participant

    I think I know why the wobbles appear.

    I short, the output from the analog-to-digital conversion from the OSSC or RetroTink2x is not perfect since it was sampled from an analog signal and not generated digitally from a PC GPU for example.

    Every time there is an analog to digital sampling stage there is phase and clock adjustments, basically, how often and where to sample.
    When the OSSC samples the RGB input it has H.samplerate for “clock” and Phase that the user can change.
    When sampling low analog horizontal resolution like 320 or 256 with high H.samplerate the image will be sharper and more stable.
    Sampling any resolution with exact sampling rate for that resolution is what an LCD monitor with VGA input does, or the OSSC in Optimized mode.

    When the HDMI-to-VGA dongle converts the digital signal to analog it expects perfect digital signal but because this signal is a sampled signal by the RetroTink2x it is far from perfect and already has phase and clock misalignments.
    Therefor the VGA output from this dongle already has these pre-sampled imperfections on the output.

    When you input this VGA signal into the OSSC or a VGA monitor, it is samples AGAIN and converter to digital.
    In THIS stage you get the wobbles if the clock and phase is not perfectly set (optimized mode) by the OSSC or your LCD VGA input.
    To mitigate the wobbles when sampling the VGA input on the OSSC ALWAYS use optimized mode and adjust H.samplerate (clock), till the wobbles disappear.

    Use the Checkerboard pattern in the 240p suite to adjust the H.samplerate when adjusting Optimized mode, or lets the LCD VGA-input monitor automatically adjust the clock+phase with this Checkerboard pattern on screen if you are using the VGA input on an LCD screen.

    Using this method I was able to reduce the wobbles to minimum so they are practically non visible.

    #23246
    mikechi2
    Participant

    Hi James – I think you are spot on! In addition, I think as Matt told me earlier, a LPF would help. so we should really be using the SCART or RCA jacks. I’m going to test as soon as I have a RetroTINK back in my hands…

    #23575
    James-F
    Participant

    Got the RetroTink 2x today, it works great.

    Now about using RT2x with the VGA adapter and OSSC;
    The RT2x outputs exactly 828 total pixels, so you have to sample the right amount of pixels in generic 4:3 modes for shimmer free image.
    That means that when the OSSC multiplies the resolution you have to set H.Samplerate in multiplies of 858.

    For correct H.Samplerate in the OSSC, multiply 858 by 1.5 for x3, by 2 for 4x, etc..
    2x 858
    3x 1287
    4x 1716
    5x 2145

    The VGA adapter will only map correctly with Limited range HDMI signal which the RT2x outputs.
    Also, the adapter has severe chroma clipping/distortion with very bright colors (especially cyan), test with SMPTE Color Bars 100% and high output console like 1Chip SNES.
    This chroma distortion does not happen when connecting the RT2x directly to a display.

    #23641
    demolition
    Participant

    Where do you change the H.Samplerate for 4:3 generic mode?

    #23642
    BuckoA51
    Keymaster

    Same place, Sampling opt->adv. timing.

    #23647
    demolition
    Participant

    Hooray, I learned something new today – I wanted to change the adv. timing menu, but was confused which mode I was supposed to select since there was no ‘312p’ mode to choose from. What I have now learned is that it starts on the currently active mode so you should not go left/right if you want to tune the current mode. 🙂

    So in this case, it started on the 1280×288 mode and now I am able to fine tune it.

    I have managed to set up a 1600×1200 output mode which is native for the 20″ monitor I am using and the image quality is really nice now. Apart from a very slight horiz judder, I see no noise and the colors and sharpness is close to perfect, so I am very happy indeed. This is with a standard PAL C64 with Lumafix as my C64 Reloaded exhibits some high frequency noise that I cannot seem to get rid of (I assume it is SMPS noise from its converters).

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