[mythtv-users] multiple transmitters - same channel

[myth]

> 
> Given you may be receiving weak signals from quite a number of
> transmitters given your location and that mythtv only keeps the highest
> frequency then I suspect that selective scanning will be the way to go
> and will clear a lot of this up.
> 
As the original poster, thank you to all that responded.  I can say this and the response from Karl Dietz is the key to resolving the issue for me.

It seems that mythtv channel scanner will insert the last scanned entry.  So where you have more than one transmitter transmitting the same network then the last one scanned wins.  As mythtv scans from low frequencies to high frequencies then in this case the highest frequency wins.  Where you don't want this to be the case you need to do a single transport scan and enter the frequency of the transport that you wish to use.

The way I chose was the following method;
* I researched all the potential transmitters and associated frequencies in my area.  (For Australia there is a really good tool where you enter your postcode/address and it provides this information together with estimated quality of reception. http://myswitch.digitalready.gov.au/) (Of course your antenna setup, the direction/s it points and sensitivity of your tuners all have a significant impact as well.)
* Where there was multiples of the same channel, I did a single transport scan for that frequency and then tried to evaluate the quality.  I then re-scanned the same channel but different frequency and compared the difference.  (As I use HDHomeRun devices, I could also use the quality estimates in their native application simultaneously to mythtv playing the channel.)
* Once you have decided on the frequencies you want to use, make sure they are scanned last.(Using the single transport scan.)  Then clean up your channel table, as you may have the same channels repeated because of all the re-scanning.
* One item to note, is that "the same channel"  is not always so intuitive. For instance SBS and ABC are broadcast using the same callsign and most likely content but may not have the same transport_id and network_id.  For instance, SBS Illawarra and SBS Sydney can appear as two different channels on mythtv as their id's differ, however Sydney-Gore Hill and South-West-Sydney have identical id's and you must use the manual separation method I describe here.
-Andrew