[mythtv-users] quality question.. ways to make it better?

Bruce Markey bjm at lvcm.com
Thu May 20 16:39:14 EDT 2004


Johnny Lee wrote:
> Bruce, So what kind of card do you currently use? I've played only with my

I have two old WinTV model 401 (rev 02), two AverTV, one WinTV GO,
and had a TV Wonder VE that went bad. I also have a PRV-250 that I
only use for testing and have never used in production for a
number of reasons (no CC, post processing of commercial flagging,
driver stability issues, audio issues, and matching picture setting
to the other cards to name a few). I suppose I'm out of step in
saying that I prefer the software encoders over the hardware
encoders but, hey, I do ;-).

The default saturation can be very different from one model of
card to the next. This is why the AGC is important so that it
normalizes the chroma level and I do now get nearly identical
results from each card when using the same settings.

> color/brightness/contrast settings and I seem to like everything default
> other than changing the contrast to 25000. I currently run a Winfast Deluxe
> BT878A card w/ dbx stereo. So what does the filter do exactly? I am going to
> run it now to see if I can see a difference or not...

It remaps the luma and chroma values to always fall within the
normal ranges for the ITU 601 standard (aka CCIR 601). The adjust
filter is needed because there is a known problem with the bt
chipset. Here is a comment from videodev2.h:

/* broken BT878 extents (601, luma range 16-253 instead of 16-235) */

Can you spot the bonehead typo that lead to the hardware design
flaw? ;-)

My thanks to Andrew Mahone for taking on the task of writing the
adjust filter to fix this. By default the adjust filter remaps the
luma range 16-253 down to 16-235. However, the input range can
be set in the first two parameters and it gets remaped to the
normal range. In the settings I sent of 34:253 the distance is
the same as the output (219) so there is no scaling, it just
uses the high range of the card then is moved down. The brightness
seems high because slightly higher than black needs to be about
35. We won't see anything in the 16-34 range.

The main trick in these setting is stretching the chroma range
relative to the luma so that there is more color density at
higher luma (bright). Without this on bttv, dark red bleeds and
bright red looks pale and pinkish. by stretching the range, the
bright red becomes more solid without dark red bleeding.

Taken to an extreme, things look too vivid to be true. It's like
The Ultimate Highlight on Sunday night Sportcenter. Things are
more colorful but not overdriven as if the saturation was simply
turned up. If saturation is the color width, this would be the
color height.

Stretching the chroma is done by narrowing the range for the
values in the forth and fifth parameters thus stretching them
wider on output. Chroma isn't like luma. A higher number doesn't
simply become more color. If you move just the bottom or just
the top, the color would shift to green or to purple. For the
colors to be balanced, the distance from the bottom to 0 has to
be the same as the distance from the top to 255. The best way to
check this is to be sure the two numbers add up to 255. So, 2:253
would work but probably pale. 50:205 would look surreal.

When it is pushed too far or not nearly enough, big changes make
very little difference. When it is close to right, changing by
even 1 or 2 in the values makes an obvious difference.

> I was playing with my colors,brightness,contrast after adding these filter
> settings you gave and this is what I got that looked almost identical to
> what my TV was showing...
> 
> update channel set contrast=30000,brightness=37500,colour=32768,hue=32768; 

If you're happy with that then that's great. I can't see your set
so I can't know for a fact, however, I would suspect that your
white and near white would be pushed out of the range and you
would lose detail on the high end. Channel surf until you find
an outdoor scene with sunny clouds in the sky. I suspect you would
lose the detail of the folds and see just white blobs like an
overexposed photograph.

If so, you may want to try setting this contrast value lower and
either turn up the brightness and contrast of your TV set or use
the "F" key for the XV controls (there is a checkbox in the playback
settings that must be set first). Increasing the  XV brightness and
contrast may get you the brighter image you are looking for without
distorting the high end while recording.

--  bjm



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