[mythtv-users] Do HD channels fallback to SD on tuning problems ? [was: multiple transmitters - same channel]

Doug Scoular (dscoular) dscoular at cisco.com
Mon Mar 24 02:10:32 UTC 2014

Hi All,
I changed this thread subject to match more closely to my issue.

On Sat, 22 Mar 2014 04:58:04 +1030, Mark Perkins <perkins1724 at hotmail.com<mailto:perkins1724 at hotmail.com>>

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 couldn't 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 can't 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. SBS HD does succeed in tuning to it's HD channel on occasion just not when I'm composing emails about it ;^)

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.

I've 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's puny
330W. So I guess it's 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:


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<http://ozdigitaltv.com/>:

Gosford Digital TV Channels at North Gosford

Affiliation Callsign Frequency Power (ERP) Pattern Polarisation Coverage
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
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 I'm 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 it's shared by all three.

If you know that Bouddi should definitely be your strongest signal then
use http://ozdigitaltv.com<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 I've had a few
antenna installers round that told me the signal was marginal and the best
I could do was install quad shielded cable 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 we've had all stations and it's
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:


This is the channels.conf it generated:


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:


I¹d still be surprised if mythtv was substituting SBS 1 for SBS HD based
on a poor signal. I'd 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. Maybe there's some new smart HD to SD fallback, who knows ?

Colour me puzzled.



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