[mythtv-users] mythtv-users Digest, Vol 132, Issue 42

Thomas Mashos thomas at mashos.com
Sun Mar 23 16:17:34 UTC 2014

On Sat, Mar 22, 2014 at 8:09 PM, Doug Scoular (dscoular)
<dscoular at cisco.com> wrote:
> Hi Mark,
> On Sat, 22 Mar 2014 04:58:04 +1030, Mark Perkins <perkins1724 at hotmail.com>
> wrote:
>>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 couldn¹t get alt-F7 to bring up the signal meter either. I even tried
> changing the binding to a plain old underscore but still no joy.
>         I get SBS 1 at about 75% to 76% according to the signal monitor that
> briefly flashes up when I change channels from a different multiplex.
> Unfortunately when I select SBS HD it does that weird thing and tunes to
> SBS 1 instead so I can¹t tell you what mythtv signal level SBS HD is
> getting for the moment. The worst signal I get is 72% for some of the
> Seven network channels.
>>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.
>         I live at Church Point and there¹s a hill behind the house that does a
> pretty effective job of blocking any signals coming from the South or West.
>         I¹ve never known which transmitter I am actually tuned to but I do get
> Newcastle news and advertising content so it must be one of the
> transmitters to the North of me.
>         Newcastle appears to be a far more powerful transmitter than Bouddi or
> Gosford. Newcastle is 250kW as opposed to Bouddi 650W-1.25kW and Gosford
> 330W. So I guess it¹s a question of how much the Newcastle signal degrades
> compared to the nearer Bouddi and Gosford transmitters.
>         If I do try to dvbscan using any of:
> /usr/share/dvb/dvb-t/au-Sydney_North_Shore
> /usr/share/dvb/dvb-t/au-Sydney_Kings_Cross
>         No channels are found due to the hill methinks.
>         I tried to make up my own au-bouddi and au-gosford dvbscan files but only
> got successful tuning to the ABC channels which share frequencies with the
> Newcastle transmitter. In fact, the Bouddi and Gosford transmitters have
> completely identical frequencies according to http://ozdigitaltv.com:
> Gosford Digital TV Channels at North Gosford
> Affiliation     Callsign        Frequency       Power (ERP)     Pattern Polarisation    Coverage
> Map
> SBS     SBS34   571.500 Mhz     330.00 W        Omnidirectional Horizontal      sbs.com.au
> ABC     ABC37   592.500 Mhz     330.00 W        Omnidirectional Horizontal      abc.net.au
> Seven   NEN59   746.500 Mhz     330.00 W        Omnidirectional Horizontal      ­
> Seven   ATN61   760.500 Mhz     330.00 W        Omnidirectional Horizontal      ­
> Nine    TCN62   767.500 Mhz     330.00 W        Omnidirectional Horizontal      ­
> Ten     TEN65   788.625 Mhz     330.00 W        Omnidirectional Horizontal      ­
> Nine    NBN68   809.500 Mhz     330.00 W        Omnidirectional Horizontal      ­
> Bouddi Digital TV Channels at Bouddi
> Affiliation     Callsign        Frequency       Power (ERP)     Pattern Polarisation    Coverage
> Map
> SBS     SBS34   571.500 Mhz     1.27 kW Directional     Vertical        sbs.com.au
> ABC     ABC37   592.500 Mhz     1.25 kW Directional     Vertical        abc.net.au
> Seven   NEN59   746.500 Mhz     650.00 W        Directional     Vertical        ­
> Seven   ATN61   760.500 Mhz     1.27 kW Directional     Vertical        ­
> Nine    TCN62   767.500 Mhz     1.27 kW Directional     Vertical        ­
> Ten     TEN65   788.625 Mhz     1.27 kW Directional     Vertical        ­
> Nine    NBN68   809.500 Mhz     650.00 W        Directional     Vertical        ­
>         So it looks like I¹m picking up Newcastle in preference to either Gosford
> or Bouddi since I cannot tune to any of the above frequencies except the
> ABC (592.500MHz) and it¹s shared by all three.
>>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).
>         I am not really as concerned about which transmitter has the strongest
> signal (and my guess is that it¹s Newcastle) but more about why my mythtv
> SBS HD channel seems to switch to SBS 1 sometimes.
>>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.
>         I know my signals are relatively weak, 72% to 76%, as I¹ve had a few
> antenna installers round that told me the signal was marginal and the best
> I could do was install quad shielded and a new antenna and that might set
> me back $600 with no guarantees of actual improvement. So I did quite a
> bit of research over a year ago and installed the quad shielding and a new
> recommended antenna myself. Since then we¹ve had all stations and it¹s
> pretty rare to see any artifacts in the transmissions.
>         I have included the stderr output of dvbscan with the au-Newcastle file
> for background here:
> https://www.dropbox.com/s/t5bk4z4f7cwer6c/scan.stderr
>         This is the channels.conf it generated:
> https://www.dropbox.com/s/solzd1ghk4fa8dg/channels.conf
>         And I also found this nice perl wrapper around femon, found at
> http://www.linuxtv.org/wiki/index.php/Testing_reception_quality, that
> gives more human readable output than femon:
> https://www.dropbox.com/s/fgatpumulzxvm1x/femon-daemon-output.txt
>         I¹d still be surprised if mythtv was substituting SBS 1 for SBS HD based
> on a poor signal. I¹d have thought that it would simply look up the
> channel frequency for SBS HD, try to tune to it and fail rather than
> somehow know it could tune to SBS 1 for the same content but at Standard
> Definition.
>         Colour me puzzled.
>         Cheers,
>         Doug
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Thomas Mashos

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