[mythtv-users] multiple transmitters - same channel

Fri Mar 21 18:28:04 UTC 2014

>    I¹ve experienced a fair bit of weirdness coming to a head with the recent
> Sydney rescan.
> 
>    Before the Sydney rescan I noticed that SBS HD came and went, no mythtv
> rescan, just disappeared and then came back seemingly once every couple of
> nights.
> 

When you tune to SBS HD it would be interesting to know what the reported signal strength on that channel is. It flashes up for a few seconds on first tune otherwise press alt-F7 (according to the key bindings wiki, although it wouldn't work for me). It's not a definitive signal strength but would be interesting.

>    I didn¹t really mind since the Standard Definition SBS channel was always
> present. My guess was that this was something that shepherd (a really
> great Australian tv_grab_au implementation) and mythfilldatabase were
> somehow conspiring to do.
> 

My brother is in Sydney as well and is seeing some odd behavior as well. My current guess is that maybe some of the transmitter broadcast strengths have also been changed in advance of the retune. Just a guess.

>    Since it was approaching rescan time I decided to be brave and that I
> would delete all my channels in mythtv setup (on 0.27, mythbuntu 12.04.4
> LTS) in the hope that this might cure the above behaviour. I know that I
> don¹t get the main Sydney channels and my antenna points North towards
> Bouddi/Newcastle (Bouddi being nearer as the crow flies) and I get central
> coast news and crappy adverts (no offence Newcastle) rather than Sydney
> news and ads. 
> 
>    So I had looked up the dates for the Bouddi retune (Thursday, 30th
> October 2014) on the outside chance that I was actually tuning to the
> Newcastle (Monday, 8th September 2014). Imagine my surprise when all my
> SBS channels refused to tune on the Sydney retune day, last Tuesday (18th
> March 2014)! All I got was (TL) rather than (TLMS).
> 

Based on a recent thread I noted that mythtv only keeps the last frequency scanned for a channel. Plus antennas often work reasonably well backwards, so given you mentioned pointing north to Bouddi I wondered whether the signal from Sydney (south) might be higher frequency. However they are both the same 571.5MHz (referring to SBS). But this is not the frequency from your MySQL-foo below (599.5MHz). I did a bit of googling and found that SBS transmit on 599.5MHz from Newcastle, which at a guess is going to be exactly where your antenna is pointing (north), just much, much, much further away and therefore much weaker.

If you know that Bouddi should definitely be your strongest signal then use http://ozdigitaltv.com to get the exact frequencies you need and then only do partial scans on those. Alternatively, if you've got the time selectively scan the frequencies on all your possible transmitters and see which is going to be the strongest. I believe that the reported signal strength number may not be particularly accurate as an absolute number but is excellent for relative comparisons (ie which signal is stronger as reported by the tuner).

Also note that some info may be out of date with the retune stuff, might be worth cross-checking the frequencies from Ozdigitaltv with each of the stations, Google will normally bring up the answer without too much trouble. Ozdigitaltv has Sydney on 571.5MHz but SBS (http://www20.sbs.com.au/transmissions) has it on 184.5MHz, I have no idea which one is correct as of the retune.

>    So I brought forward my plans, used mythconverg_backup.pl to make a
> backup, deleted all channels and did a full rescan. SBS 1, 2, 3, NITV and
> HD all came back with everything else (but with much higher channel
> numbers: 849, 850, 851, 852 and 853).
> 
>    However, the problem is still present; SBS HD disappears and reappears
> while the rest appear consistent (not that I watch much live tv other than
> SBS news).
> 
>    Any thoughts on where I should start looking for the culprit ?
> 
>    I has mysql foo ;^)
> 
> mysql> select callsign, channum, transportid, networkid, frequency from
> channel, dtv_multiplex where channel.mplexid = dtv_multiplex.mplexid and
> callsign like 'SBS%' or callsign like 'NITV%';
> +----------+---------+-------------+-----------+-----------+
> | callsign | channum | transportid | networkid | frequency |
> +----------+---------+-------------+-----------+-----------+
> | SBS ONE  | 849     |         848 |     12802 | 599500000 |
> | SBS TWO  | 850     |         848 |     12802 | 599500000 |
> | SBS 3    | 851     |         848 |     12802 | 599500000 |
> | NITV     | 852     |           2 |        57 | 536500000 |
> | NITV     | 852     |         547 |      4112 | 592500000 |
> | NITV     | 852     |         848 |     12802 | 599500000 |
> | NITV     | 852     |        2058 |     12812 | 690500000 |
> | NITV     | 852     |        2480 |     12930 | 704500000 |
> | NITV     | 852     |        1792 |      4121 | 585500000 |
> | SBS HD   | 853     |         848 |     12802 | 599500000 |
> 
> 
>    It¹s weird that the channels went AWOL on the night of the Sydney retune
> rather than the planned Bouddi or Newcastle ones. That doesn¹t explain why
> SBS HD is occasionally AWOL or, for that matter, why I have so many NITV
> entries. I confess I don¹t know much about channum versus transportid
> versus network id versus frequency.
> 

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.

>    Any clues much appreciated.
> 
>    Oh, and apologies for co-opting this thread. I can change the subject if
> you iike.
> 
>    Cheers,
> 
>    Doug
> 
> 
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