[mythtv-users] Realtime Fine Tuning -- just a pipe dream?

Daniel Kristjansson danielk at cuymedia.net
Wed Sep 5 16:35:36 UTC 2007

On Tue, 2007-09-04 at 21:43 -0400, Jay R. Ashworth wrote:
> On Mon, Sep 03, 2007 at 11:49:16PM -0400, Daniel Kristjansson wrote:
> > > > 4, 11, and 72.  :-)
> > > OK, not likely a freq table issue.  :(
> > FYI The other reason channels can be off is because cable operators
> > are supposed the shift the frequency of any channel interfering with
> > other regional broadcasts up or down a bit.
> IRC and HRC, yes.

No, in addition to using a different frequency table, Cable plant
operators are supposed to apply small offsets whenever there is
interference, regardless of the frequency table in use. These are
small enough that most tuners in TV's will not even notice.

> > Most analog tuners have a PLL that locks on to the visual carrier
> > frequency. So the frequencies in MythTV for analog tuners is actually
> > the approximate location of this visual carrier. Unfortunately the most
> > popular tuners for computers, the Conexant CX and BTTV chip families,
> > do not appear to not be very good at locking onto the VC so you need
> > to give it a pretty close estimate. Also the resolution is only 62.5kHz,
> > this low resolution is not common to all tuners, which is why MythTV
> > uses exact frequencies now instead of the closest BTTV chip tunable
> > frequency. When there is a good PLL using the correct frequency will
> > only mean a slightly faster tune, when there isn't using the correct
> > frequency will mean a better picture.
> But there's no way to extract the PLL offset -- much less the AGC
> level, so you could figure out if your signals are twisted -- from the
> tuner, right?

There is no way to do it in the API, but if the PLL is working you
can measure the time it needs to tune after switching from a different
frequency. By switching back and forth to a reference frequency you
can eventually find the actual frequency based on the time it takes
the PLL to converge on the frequency. But this isn't really practical.

> > > > Sure, but again; not interactive.
> > > Right.
> > 
> > It's supposed to be interactive, the vagaries of hardware
> > implementations may mean this is not the case, or there may
> > simply be a bug.
> It *is* supposed to be interactive?

I'm sorry, it appears that patch never made it into SVN. It would have
been under Ctrl-G if it were there.

-- Daniel

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