Zidoo 1619 Black Level issues explained...

Discussion in 'HDD Media player(RTD 1619DR)' started by Markswift2003, Feb 11, 2021.

  1. Markswift2003

    Markswift2003 Well-Known Member SUPER Administrator Beta test group Contributor

    I thought I'd start a new thread with this since I want to point Zidoo here after the New Year.

    Apologies if this ends up like an essay, but I thought it important enough for a full explanation.

    So there are 3 separate issues with video levels over all the HDR modes:

    1. Black and White Clipping - this is where black never reaches black and white never reached white so you get black/white crush.

    2. Raised Black Floor - This is where the whole video level is raised so blacks become grey and the image looks washed out.

    3. Raised Video background layer - This is where the system generated background layer on which the video is overlaid is raised above black.

    So, to tackle these one at a time with test results at the end:

    1. Black and White Clipping

    This is the easiest to understand.

    For 8-bit video (HD BluRay) there are 256 luminance (brightness) levels and for video these are limited to 16-235.

    For 10-bit video (UHD BluRay) there are 1024 levels, but again for video they are limited to 64-940.

    (The full ranges are 0-255 and 0-1023 respectively, but as I say, video always uses the limited range)

    For simplicity and by convention, the Zidoo settings just list these as 0-255 and 16-235

    So for 10-bit, black is 64 and white is 940. For HDR it gets more complicated because of the PQ (Perceptual Quantiser) gamma curve and tone mapping, but suffice to say that for a 1000nits signal, white is at approx 723, and this is the signal I used in the tests.

    So, for 1000nits HDR signal, if black doesn't reach 64 and white doesn't reach 723 then we lose information and the bigger the gap, the more is lost, resulting in "crushed" blacks and "blown" whites.

    For an SDR signal, white must reach 940 but remember that doesn't mean it's brighter than 723 above - that's all down to the display - these are video signal levels we're talking about, not display brightness.

    As can be seen in the tables below, the only modes that have correct clipping are Realtek SDR.2020, VS10 HDR and VS10 LLDV.

    Realtek HDR to be fair is pretty good and because of the way PQ works, you'd never notice in real world content.

    2. Raised Black Floor

    This is where the whole video level is raised with respect to black - so instead of black starting at 64 as it should, it starts at some value above that, say 100, and so black is actually grey and all shades above black have a lighter shade relative to that raised black floor.

    This means the whole picture looks far too light and washed out similar to this:


    From the tables below, we can see that Black Floor is raised in VS10 HDR and VS10 SDR. The effect is more pronounced with VS10 SDR since that uses Power Gamma rather than PQ gamma and Power Gamma comes out of black quicker than PQ.

    3. Raised Video Background Layer

    When playback starts, video is overlaid on a background layer which must be set to video black (64).

    So, for example, if you play a video that has a smaller vertical resolution than the native resolution, the "black bars" will appear black.

    This is important where 1.85:1, 2.20:1, 2.35:1 or 2.40:1 videos have been cropped which is common practice in encoding.

    So for example a 3840x1600 or a 1920x800 video must be overlaid on a black background in order that the black bars top and bottom are not visible.

    As shown in the tables below, this video background layer is not true black for VS10 HDR and VS10 LLDV.

    This results in grey "Black Bars" top and bottom, and although the video background level is not far above black, it is very noticeable in low light.

    These graphics are exaggerated to demonstrate what I mean:

    Zidoo Raised BG Graphic.png
    And you get this when viewing cropped video:

    Zidoo Raised BG Graphic#2.png
    As I say, these images exaggerate the effect.

    4. Test Results

    These are the results of testing with Masciola and Mehanik Clip patterns. Mehanik also provides a 3840x1600 cropped black clip pattern (3840x1600) to test the Video Background Layer.

    The figures in red show what the values should be.

    As stated previously, these test patterns are 1000nit HDR and these results are with a Zidoo Z9S set to 16-235:

    As can be seen, the only mode that has correct levels, with no raised black floor or video background is SDR.2020 which is only really useful to owners of certain projectors.

    As stated in the previous thread by @tosh123 by setting the HDMI Level to 0-255, we can mitigate the raised black floor for VS10 HDR and VS10 SDR but it does not fix either clipping or Video Background.

    So these are the results for VS10 modes set to 0-255:

    There is no point showing results for the Realtek engine at 0-255 since that behaves correctly as far as HDMI Level goes.

    In both cases, VS10 LLDV is usable as long as you use full frame video (or can set masks)

    In the second case (0-255) VS10 HDR is usable, again as long as you use full frame video (or mask)

    Although VS10 SDR doesn't look too bad in the second case, black clip is a bit nasty at 76 and white is clipped badly (although this may be to do with the SDR conversion and you'd only lose specular highlights so it's not as bad as it looks on paper)

    VS10 SDR is certainly a whole lot better than Realtek HDR>SDR since the colour conversion of BT.2020 to Rec.709 in Realtek SDR is terrible.
    Last edited: Feb 11, 2021
    pcristi, leonkoum, skelton and 7 others like this.
  2. ammar11

    ammar11 Well-Known Member

    Great analysis, Mark.
    Markswift2003 likes this.
  3. rozel

    rozel Well-Known Member

    Superb work @Markswift2003 , I am learning a great deal thanks to your help :)
    Markswift2003 likes this.
  4. afss

    afss Active Member

    Thanks Mark, you are awesome ! This makes it crystal clear to understand and to explain Zidoo what the problem is !
    Markswift2003 likes this.
  5. Gilles

    Gilles New Member

    Excellent ! thank you very much.
    I hope than Zidoo technical staff can now correct these issues as soon as possible.
    Markswift2003 likes this.
  6. Whitfield

    Whitfield Active Member

    Nice work, Mark. I'm sure Zidoo would be able to get a grasp on B/W level issues with this.
    It can be me not fully comprehending, but are the notes in the test results correct/in the right place?

    DELUCAS Well-Known Member

    If this issue resolves my 4K material looking washed out and colourless on my 1080p Monitor
    It be great if it gets addressed .
  8. Markswift2003

    Markswift2003 Well-Known Member SUPER Administrator Beta test group Contributor

    Not quite sure what you mean?
  9. Markswift2003

    Markswift2003 Well-Known Member SUPER Administrator Beta test group Contributor

    4K material will look washed out on a 1080p SDR display if the colour is not converted from BT.2020 to Rec.709 and gamma is not tone mapped - in other words an "HDR to SDR" conversion.

    Currently, the best way to do this is:

    Set <HDR> to "Dolby Vision VS10 Engine"
    Set <Colour Settings/HDMI Range> to "0-255"

    Assuming the display has an SDR only EDID, the VS10 engine will convert all HDR content (including DV) to SDR and the HDMI range setting brings the black floor back down.
    DELUCAS likes this.

    DELUCAS Well-Known Member

    Thanks Mark
    Will try them settings ( it seems ive changed quite a few over the 6 month period ;)

    will report back if it helps
  11. Markswift2003

    Markswift2003 Well-Known Member SUPER Administrator Beta test group Contributor

    Maybe work through the basic settings listed in the first part of the sticky and then change the HDR and HDMI Range so you know everything else is correct...
  12. Whitfield

    Whitfield Active Member

    In the first (Level 16-235) list, behind "SDR Rec.709 limited", shouldn't there be "Raised Black Floor", instead of behind "Dolby Vision VS10 Engine, HDR", or am I missing something here?

    Edit: I'm also wondering if, where white levels don't reach 940 (235) "B/W clipping" would be more apropriate?
    Last edited: Feb 12, 2021
  13. Markswift2003

    Markswift2003 Well-Known Member SUPER Administrator Beta test group Contributor

    No - the raised black floor only happens with VS10 HDR and VS10 SDR.

    "SDR Rec.709 Limited" uses the Realtek engine and doesn't suffer the black floor issue.
  14. Whitfield

    Whitfield Active Member

    Why does "SDR Rec.709 limited" then show Black Level 76, where, as you stated, in red, it should be 64? (Same as "Dolby Vision VS10 Engine, SDR" btw)
    Am I not seeing something or misinterpreting that value?
  15. Markswift2003

    Markswift2003 Well-Known Member SUPER Administrator Beta test group Contributor

    That's the Black Clip, not Black Floor:

    For SDR, the luminance range in the video signal goes from 64 (black) to 940 (white). If the signal is not passed with that full range it is said to be clipped - so the black value of 76 has clipped black before it gets to 64.

    This means that all values in the video from 64 to 76 will show as black and therefore the deep greys from 64 to 76 are lost - this is "black crush".

    The black floor is the actual video level that the black signal of 64 is actually output at - so if the black floor is raised then although the video signal is outputting black at 64, the whole luminance range is shifted up so the whole image becomes lighter.

    So now, black, despite being at 64, is displayed as grey (in the old analogue days we used to call this the "pedestal").
  16. Whitfield

    Whitfield Active Member

    Thanks for this extra explanation, Mark. It made me understand now.
    Appreciate your effort and time.
  17. mattmarsden

    mattmarsden Active Member

    So Mark, assuming I'm using a full frame video, using VS10 HDR mode in 0-255 will give near perfect output as I'm not worried about the background levels?
  18. Markswift2003

    Markswift2003 Well-Known Member SUPER Administrator Beta test group Contributor

    No problem - I know it takes a bit of getting your head around but that's why I wrote the thread - it's so fundamental to video presentation and it's such a shame that the Zidoo doesn't fit the specs properly yet.

    It may be that the black clip is intrinsic to the Realtek SOC because the Z9S has exactly the same figures for Black Clip in the Realtek engine but I know the white clip for VS10 SDR can be fixed because it used to be 940 in previous firmware.

    At that stage White Clip for VS10 HDR 1000nit was incorrect at 817 and has since been fixed so I wonder if that fix triggered the error now present in VS10 SDR.

    It has to be said that white clip for HDR to SDR conversion is complicated because it depends on the nits level of the source video, but it looks to me like they've set a hard ceiling at 1000nits, which to be fair would cover the vast majority of content.
    Last edited: Feb 12, 2021
    Whitfield likes this.
  19. Markswift2003

    Markswift2003 Well-Known Member SUPER Administrator Beta test group Contributor

    If you have an HDR10 (not DV) display and use "Dolby Vision VS10 Engine" then yes - everything is converted to HDR and levels are correct.

    If you use "Dolby Vision VS10 Engine for HDR" then HDR content will be correct but SDR content will not be because in that case the HDMI Level needs to be 16-235. This is another reason why this needs fixing ;)

    Also, with "Dolby Vision VS10 Engine for HDR" I'm not sure what happens if you play Dolby Vision Profiles 4, 5 and 8 - I would expect they get processed by the VS10 engine to VS10 HDR like they do with "Dolby Vision VS10 Engine" but I'm not 100% sure...
    Last edited: Feb 12, 2021
  20. DrGiggles

    DrGiggles Member

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