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

Daniel Kristjansson danielk at cuymedia.net
Tue Sep 4 03:49:16 UTC 2007

On Fri, 2007-08-31 at 17:22 -0400, Michael T. Dean wrote:
> On 08/31/2007 04:03 PM, Jay R. Ashworth wrote:
> > On Fri, Aug 31, 2007 at 03:39:07PM -0400, Michael T. Dean wrote:
> >   
> >> On 08/31/2007 03:08 PM, Jay R. Ashworth wrote:
> >> What three channels/frequency ID's (and what TV source--i.e. cable)? 
> >> There may be an even easier fix.  That sounds a /lot/ like a frequency
> >> table issue.
> > 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.

> >> In theory, fine-tuning should never be required for any channel.  The
> >> tuner's hardware fine-tune mechanism should be more than enough for
> >> variations from the spec--the resolution of the kernel's tuner module is
> >> (currently) only 62.5kHz.  (And the values you specify for finetune are

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.

> >> The mythfrontend channel editor also allows you to specify a finetune
> >> value (which is, at least, safer than direct DB edits).
> > 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. Some drivers implement a pseudo PLL for
tuners which don't have them ruining all attempts at fine
tuning on the application level. But I don't know of any
tuning modules for analog capture that do this. That malady
has only effected DVB drivers in the past.

-- Daniel

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