Thanks for the answer. All of this FEL and tv-led goes right over my head. My Ugoos AM6+ should arrive in a couple of days. Was just wondering why the one Ugoos box will work, but anther one won’t. Both have the “J” SoC.
A lot of the confusion stems from the fact there is not only one type of Dolby Vision.
The type most people here are interested in (I would suspect) is the Dolby Vision on UHD BluRay (aka Dolby Vision profile 7). Because the discs need to be compatible with older non Dolby Vision BluRay players they need to retain the original format video (in most cases HDR10 format) - to play on those older devices, for Dolby Vision discs there is a 2nd video stream on the disc containing the “Dolby Vision” data, which needs to be combined with the original video to produce the final Dolby Vision output in the Dolby Vision capable players.
Contrast this to streaming Dolby Vision of say Apple/Netflix content (aka Dolby Vision - profile 5) where there is only one video track which is already in Dolby Vision and no need for the backward compatibility.
Everything that wants to play Dolby Vision needs a licence hence the -J designation for chips that contain that licence, the licence though may be there just to cover the streaming type of Dolby Vision (profile 5), but without the additional hardware/software it may not be able / capable to play the UHD BluRay type of Dolby Vision (profile 7).
Hope that helps clarify a bit and not just add more confusion - Dolby Vision covers quite a variety of solutions and approaches under the hood, it is generally confusing for everyone 
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Thank you, that helps a lot…
One more question, how can I tell if TV-led is actually working?? My TV does NOT show whether it is receiving 8 bit or 12 bit. From my understanding (which is very little), the 8b is true TV-led.
Or do I have to just play some videos and check with my eyes??
My Ugoos turned up today and I restored a backup from an N2 running 20.5 and so far so good. Tried 1 of the profile 7 test videos and it appeared to work fine. My TV (LG C8 OLED) showed it as DV BT2020 so appears correct. So far very impressed but haven’t had much time to try many videos.
I would go so far as to say there’s three levels of compatibility not just 2. Sort of like DTS:X in relation to DTS-HD MA in relation to OG DTS.
Because FEL containing BL-RPU-EL, MEL and profile 8 (also seemingly more common these days on streaming), effectively only containing BL+RPU and of course the base HDR10. The latter two haven’t been an issue for streaming devices, but FEL has.
Do you have an LG TV by chance? If so hit the green button on the remote a bunch of times and you might get a pop-up. The bitrate isn’t always accurate (like it’ll erroneously claim 8-bit when it receives 12-bit 4:2:2 on my LG GX). But the color space is.
Otherwise hit player process info button within CoreELEC. It’s simply pressing ‘o’ with a keyboard connected.
Honestly I don’t think we entirely know why FEL works on a handful of S922X-J based boxes (which requires something like CoreELEC and Android won’t suffice) whereas for S905X4-J based devices despite also running CoreELEC, it won’t work 
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I have a Vizio P65Q9-H1.
Does this information mean anything?? Haven’t received my Ugoos AM6+ yet, so testing on my Dune Homatics running CoreELEC.
DV-STD means it is tv-led.
Taking UHD BluRay again as the example to put this in context: the HDR10 video track (often referred to as the Base Layer or BL) is composed (combined) with the 2nd additional Dolby Vision video track (often referred to as the Enhancement Layer or EL) on the device/box to produce the final output video.
There are two different types of EL found on Dolby Vision disc:
FEL - Full Enhancement Layer
MEL - Minimum Enhancement Layer
FEL contain the additional video data that when combined with the BL gets very close to the original mastering material used to create the disc.
MEL actually contains no useful additional video data.
So what’s the point of MEL? - a reasonable question.
Both EL layer types FEL and MEL also carry HDR Metadata - simply put, data which states how the frame/scene should be considered for HDR - that is the dynamic HDR metadata of Dolby Vision. This Metadata is often referred to as the RPU (Reference Picture Unit).
So Dolby Vision BluRay can be:
BL + FEL (with RPU)
BL + MEL (with RPU)
It is reasonable to state BL + FEL (with RPU) leads to the best quality.
Circling back to tv-led - this is related to the RPU data and where it is processed.
DV-STD (tv-led) the RPU data is transmitted to the TV/Display and the processor on the TV uses that Metadata so adjust the TV / Video data to best fit depending on the capabilities of the TV. So the TV gets the video data and the RPU separately to work out how best to display.
DV-LL (player-led) the RPU is processed on the box/device (altering the video data) before sending to the TV/Display. So the TV only gets video data - it has no knowledge of the RPU and can make no further adjustment based on it, those adjustment are already baked into the video data.
DV-LL is considered not as effective as it knows less about the TVs capabilities / what mode it is in / current brightness etc. when the RPU is used to alter the video data. (There are plenty of videos out there showing this difference in real-world scenarios), Though the exact workings of how the RPU is used will be down to implementation which is somewhat a black-box to us - as it implemented in the TV.
This moves us on to “fake” tv-led.
This occurs when we are in DV-STD mode - but - the RPU is processed at the box/device!
Why would this happen?
My working understanding for that is: the mode the box/device is in (video or graphics) - when the box wants to control graphics on the screen for elaborate menus and other such UI it wants to control the picture to have those self generated graphics look correct, so it stops the RPU data being sent to the TV (which would allow the TV to adjust the picture) and instead again processes the RPU on the box (applying it only to video image data and leaving it’s own generated graphics unaffected) - but wait we are in DV-STD mode so does the TV not expect the RPU data? yes it does and to make this work it is sent an effectively “blank” RPU to to keep the TV happy but carrying no information to actually base picture changes on, so they remain the same.
Long story short we need the software to keep the box in video mode when playing the BluRay video in DV-STD to avoid this “fake” tv-led scenario.
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Yep, also need to factor in the “fake” tv-led that can happen in MEL p7, p8, p5 scenarios.
Great summary. I’ve also considered it like this, in terms of DV PQ, streaming bitrate aside:
P5 = P7 FEL > P8 = P7 MEL
In P5 the BL+EL are encoded together and the RPU is layered ontop. Technically matches the components of P7 FEL. No HDR fallback though as the video data has been muxed together in a proprietary colorspace.
In P5 → P8 conversion, the RPU is extracted from the file, but not the EL. This can then be brought onto a non-DV BL (UHD disk) to create a “hybrid DV remux” if the P5 master (sans RPU) matches the master on the disk. Screencap will easily show that.
Not sure what is functional difference is between P7 MEL and P8. I always thought they had same purpose to hold the RPU.
That’s incorrect. P5 does not equal FEL 12bit. Profile 5 is 10bits but because it uses a more efficient colorspace, it is the equivalent of 11.5bits YCBcr. Only profile 7 FEL can restores the original 12bit master. P7 FEL > P5 > P7 MEL = P8
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This is good to know.
Is there any visual difference on 10bit TVs though? I always thought no functional difference on current panels if you had identically encoded P5 vs P7 FEL.
Yes, having a higher bit depth source can help with banding in poorly encoded BL, but indeed most of the benefits exceed our TVs capabilities. The brightness difference we sometimes see between bluray releases and P5 web-dls is just a studio decision.
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So if P5, P7 MEL, and P8 are all 10 bits, do we know what goes into the 2 extra bits when they are transmitted from devices in a 12-bit form?
Is this done in any special way? Or is it just padded / random the same as 10-bit HDR10 content is when using a 12-bit hdmi format?
Yes, and you can use the DoVi_Baker to merge(the same way the bluray player would) the EL+BL into 16bit RGB screenshots but we just don’t have any TV higher than 10bit to truly display it. But FEL is more than just 12bit. The EL can contain any difference(grain, details, brightness, black, colors) from the 12bit master and the BL encode. Those differences are visible on our 10 bit TVs. Example
Isn’t that tool for adding in the FEL layer? This says it is only needed for FEL files.
I was asking about files in the other profiles
Well you asked about the 2 extra bits which are only possible in P7 FEL so I don’t understand your question then.