[GTALUG] war story: fixing an LCD TV

[Lennart Sorensen]

On Fri, Jun 07, 2019 at 12:15:50PM -0400, D. Hugh Redelmeier via talk wrote:
> How would it qualify as HDR?  OK: I know, HDR is anything marketeers
> think that they can get away with.

HDR defines a larger range of brightness.  So without HDR your 8bit
values 0 to 255 (well video rather than computers use 16 to 240 would
cover 0 to 120 nits (or 80 if using sRGB).  In HDR 255 (or 240) would be
10000 nits instead, so the display would interpret the incoming values
differently as a result.  This is why you would tend to get severe color
bands in 8 bit HDR, since your content from 0 to 120 nits now has to be
covered by a much smaller range of your 8 bit values, due to a lot of
values covering the 120 to 10000 nit range.  HDR does use logorithmic
values rather than linear to help a bit, but it really needs 10 or more
bits to get decent gradiants.  Of course no current display can do 10000
nits, but the HDR standards seem to be designed with that as the limit in
the future.  Dolby has a screen that can do 4000 nits, but more typical
high end LCD TVs can do 1500 to 2000 nits, with OLED limited to about
800 nits (but due to having true black and individual pixel control,
the contrast is much higher than the LCD).

> I guess dithering can sort-of add a couple of bits.

Yes 8 bit HDR with dithering is supposed to be quite good actually.
I have never tried doing HDR with a computer to my TV.

> My (cheap, old) Seiki SE39UY04 TV is limited to HDMI 1.4.  So at best
> it can do UltraHD at 30Hz, with 4:2:2 chroma subsampling.  I don't
> know its LCD technology.

Yeah HDMI 1.4 is certainly a big limitation.  2.0 is much better.

>From what I can find, it appears that TV is S-MVA which is supposed to
be similar (but not quite as good) to IPS in viewing angle but have
better black levels.

> So it should be bad.  But for my usage, it seems pretty good.
> 
> - I don't have a lot of dynamic content on my screen, so slow refresh
>   doesn't have a lot of effect.  The mouse cursor movement isn't as
>   smooth as it would be with faster refresh.
> 
> - almost nothing I do exposes the limitations of chroma subsampling.
>   Text is the killer test case but foreground and background for most
>   text differs in luminance (full resolution), not chromanence (reduced
>   resolution).  Some artistic creations have text that renders badly
>   but even then, most artistic creations use quite large text.  In my
>   browsing, I've only encountered this once or twice.

Color text is the biggest problem.  Black or white text generally no
big deal.

> As far as the type of panel, I don't know.  I've googled for the code
> on the panel and get only a few hits, all useless.
> 
> The electronics are sufficiently modular that I wonder if I could
> graft a HDMI 2.0 T-Con board and processor board, to get a better
> system.  I might do it if someone else pioneered.
> 
> Someone else hacked his/her SE39UY04 and discovered that it's running
> Linux and can be modded: <http://www.zeroepoch.com/blog/se39uy04>

Neat.

-- 
Len Sorensen