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

Sun Mar 23 03:09:46 UTC 2014

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