[mythtv-users] pcHDTV signal strength issue

David George david at thegeorges.us
Wed Jun 30 10:02:21 EDT 2004

On Wed, 30 Jun 2004, Doug Larrick wrote:

> David George wrote:
> > I also went into the channel editor and changed the freqid as per messages
> > I found in the pcHDTV forum and in the Myth mailing list.  I got kinda
> > conflicting info on what to use for the freqid.  For example in
> > Charleston, CBS broadcasts NTSC on channel 5 and ATSC on channel 47.  I
> > tried putting 47 in the freqid, but then found other messages that seemed
> > to imply that the freqid field should be 5-1.  I have tried both and get
> > the same 13% signal strength.  This is on a channel that dtvsignal says is
> > about 85%.
> Freqid should be 47-1 for that channel (though the '-1' is not important
> unless the channel has subchannels).

Yep, tried 47-1 also.  The two highest strength stations here have
subchannels.  The rest of the local stations aren't quite up to power yet
and have a low signal strength even with dtvsignal.

> > I am running FC2 with kernel.org 2.6.7 with kraxel v4l2 and pcHDTV
> > patches.  I am running MythTV CVS as of maybe 2 days ago.
> Does dtvsignal take a while to stabilize for you?  You might try
> increasing the amount of time Myth samples the signal strength (one of
> the settings in Mythfrontend).

Nope, dtvsignal takes about two seconds to get to its max reading.  I see
24%, then 66%, then 88%, then it stabilizes.  From the backend log it
looks like it is waiting 5500ms for the signal strength.

> On my system, I have to use one of the utilities that sets the channel
> (dtvsignal, etc.) once before running Myth for the first time after a
> reboot.  Haven't had time to look into it.

Tried that also.

> Signal strength can vary from hour to hour and day to day.  Make sure to
> do your debugging in the same session as you're seeing failures.

Yep, made sure of that.  I did the dtvsignal and then went into myth and
scheduled a one-time record of something on an HD channel that was
currently playing.

> You might also disable the signal strenth check in Myth (requires
> rebuilding) to see if it's giving bad results -- though it uses the same
> code as dtvsignal so I'm not sure why it should.

I will try that.  I am a coder so I will dig into this further.  After
spending an evening on it, I figured I would post a message and see if I
was missing something.  Hopefully tonight I will be able to spend some
time changing the sample time and removing the signal strength check if

Also, I did fix the VIDIOC_S_STD problem.  The latest (2.6.7-1) kraxel
patches changed the V4L2_STD_ATSC_* values.  So I changed
libs/libmythtv/videodev2_myth.h from:

#define V4L2_STD_ATSC_8_VSB     ((v4l2_std_id)0x01000000)
#define V4L2_STD_ATSC_16_VSB    ((v4l2_std_id)0x02000000)


#define V4L2_STD_ATSC_2_VSB     ((v4l2_std_id)0x02000000)
#define V4L2_STD_ATSC_4_VSB     ((v4l2_std_id)0x04000000)
#define V4L2_STD_ATSC_8_VSB     ((v4l2_std_id)0x08000000)
#define V4L2_STD_ATSC_16_VSB    ((v4l2_std_id)0x10000000)

These values were found in


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