[mythtv-users] Volume control for BT878 card w/loopthrough

Mark Boyum circushair at gmail.com
Wed Apr 18 17:25:53 UTC 2007


On 4/16/07, Bruce Markey <bjm at lvcm.com> wrote:
>
> Yan Seiner wrote:
> > I've yanked the PVR150 from my system and plugged in a BT878 card I've
> > had laying around.  I have to loop the sound from the BT878 card to my
> > sound card.
>
> Good move. I've had a PVR-250 for years that I've yanked in
> the past but currently is installed and configured as my
> last choice with lower input priority.
>
> > The problem is that the sound comes in way, way too loud - about 4x
> > louder than the digital feeds.  I have not found any way to get myth to
> > control the sound from /dev/dsp...
>
> $ alsamixer -V capture
>
> Move the slider sub-captioned "Capture" (you must have been here
> before if you got capture working in the first place). I've found
> in the past that 100% can be raspy so I set mine at 90%. You can
> have this open in a shell window and adjust while you use "Y"
> to switch between cards until you match your digital feeds.
>
> > I would welcome any suggestions....
>
> "Any"? I'm much happier with my software encoded MPEG-4 files
> than the MPEG-2 files from a hardware encoder. However, the
> bttv defaults aren't good so most people assume that ivtv is
> better when they compare the picture 'out-of-the-box'. Here are
> some settings that I use:
>
> In my /etc/modprobe.d/misc (or wherever you set modprobe options)
>
> options bttv chroma_agc=1 combfilter=2 full_luma_range=1 coring=1
> video_nr=0 vbi_nr=0
>
> AGC is automatic gain control which normalizes the chroma level
> so you get more consistent color saturation.
>
> The combfilter fixes the multi-color flickering you may see on
> small white lettering or striped shirts.
>
> The full_luma_range expands the values to 0-255 rather than 16-253
> (no equivalent for ivtv). This can improve contrast and overall
> detail.
>
> The coring makes a hard cut off for the black level and can remove
> low level noise and artifacts in dark areas.
>
> "video_nr=0 vbi_nr=0" force the driver to take /dev/video0 and
> /dev/vbi0 to defeat the evil known as udev.
>
>
> Xawtv (from the bttv dev) includes some other useful utilities.
> I use "v4lctl list" to see driver attributes. This can also be
> used to set attributes and I do so in a wrapper script to start
> mythbackend:
>
> ...
> v4lctl -c /dev/video0 setattr 'mute' off > /dev/null 2>&1
> v4lctl -c /dev/video0 setattr 'uv ratio' 52 > /dev/null 2>&1
> v4lctl -c /dev/video0 setattr 'whitecrush upper' 253  > /dev/null 2>&1
> v4lctl -c /dev/video0 setattr 'luma decimation filter' on > /dev/null 2>&1
> ...
>
> The "'mute' off" is because of an issue that came up it the
> kernel a few revs back and I want to be sure that the driver
> is unmuted.
>
> The "uv ratio" is like tint from purple to yellow rather than
> red to green. I find that 52 makes yellow, gold and flesh tones
> look more natural.
>
> The color and hue are good at 50% (32768) but contrast and
> brightness are way off. I find that I need to set the contrast
> to mid to low 20,000s (around 40%) and the brightness around
> 40,000 (like 60%). YMMV but for analog my cable today I am
> using 26000, 40200 and for s-video from a digital cable box,
> 26350, 43150 with the attributes listed above (coring and full
> luma range will have a big impact on the best contrast and
> brightness settings).
>
> I also use the "quickdnr" filter for a handful of channels that
> have noticeable interference or noise. This cleans up the image
> before ffmpeg MPEG-4 compression and not only makes the picture
> cleaner but makes the compression more efficient.
>
> --  bjm


I don't know about the original poster, but I just wanted to give you a
quick thanks for the bttv tweaks.  I'm currently using an old Athlon 1400
Backend system with a 80GB storage drive to simply record our favorite shows
each week.  When we miss an episode we just hookup the S-Video output of an
old Windows laptop to the TV and use MythTV Player to watch it.

The image quality, skin tones and general color dullness, was the biggest
hurdle for the WAF.  Hopefully that will no longer be an issue!

Thank you to the Mythtv developers and the wonderful community around them.
You've given me a way to utilize old hardware destined for the scrap heap
and a project/hobby to help me learn Linux.

-Mark
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