[mythtv-users] fine tuning on Plextor ConvertX

Brian Wood beww at beww.org
Fri Feb 17 03:18:08 UTC 2006

On Feb 16, 2006, at 7:52 PM, Dale E. Martin wrote:

> Hello!  I'm moving my setup from a combined front-end/back-end to  
> separate
> machines.  I have purchased a Plextor ConvertX to reduce the load  
> on the
> backend system - currently I am using an el-cheapo bttv-based  
> card.  I've
> got the ConvertX working and generally I'm pleased with it.  The  
> picture
> quality is excellent and the load on the machine is negligible  
> while it's
> recording.
> The only issue I'm facing is that channel 5 & 6 aren't coming in  
> well with
> the Plextor tuner.  I had the same issue on the old setup, and I  
> fixed it
> by using the channel editor to adjust the fine tuning.  This  
> resolved the
> problem in the old setup.
> I've made the same adjustments in the new setup and it looks like  
> the fine
> tuning is not being applied - there is no noticeable change.  I was  
> looking
> through the driver source for the wis-g07007 driver and it looks  
> like the
> driver supports fine-tuning, so I was wondering if this was just an
> omission of some sort in mythtv?  (Of course, it looks like it's  
> handling
> the ConvertX as a standard v4l tuner so that would be weird too.)
> Thoughts, experiences, questions etc. welcome!

I don't know the answer to your problem, but I think I know the cause.

I'll bet you're on a cable system, and your operator is not using  
"standard broadcast" channel frequencies.

The standard VHF broadcasting frequency assignments have a gap  
between channels 4 and 5, used for various non-broadcast services in  
the outside world.

The cable industry has a couple of frequency allocation schemes that  
are different from broadcast standard - IRC and HRC (Incrementally  
Related Carriers and Harmonically Related Carriers, in case you're  

Without going into detail, these schemes were developed to try and  
decrease interference on a cable system, and to minimize the effect  
of radiation leakage from broken cables etc (they were not greatly  
successful at either IMHO).

The most obvious difference to normal folks is that channels 5 and 6  
operate on slightly different frequencies from their over-the-air  
equivalents. Other channels on HRC systems are "off" as well, but not  
as far as 5 and 6.

Other than channel tables for dvb cards, I've never seen any  
provision for STD, IRC or HRC in MythTV, I think it is left to the  
controlling card drivers to handle it. I don't know if the Plextor  
has provision to change the tuning table to accomodate non-broadcast  
channel frequencies, I know some devices just relay on very wide AFC  
windows to deal with it.

If you can find any reference to changing your drivers for IRC or  
HRC, and can implement one or the other of them, I'll bet that clears  
up your problem.

If I'm wrong and you're using an antenna, perhaps you are set up for  
HRC or IRC and should not be, but I doubt it, a TV market that has  
both channel 5 and 6 would be unusual (you would have to be between  
two markets and receiving both).

Maybe this will get you pointed in the right direction ???

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