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High Signal Strength Poor Quality Video
PostPosted: Wed May 03, 2006 9:12 pm Reply with quote
speed301
 
Joined: 02 May 2006
Posts: 11




For the past couple of weeks I've been having problems recording the ABC OTA signal on my HD-3000. I need some advice on what could be causing the problems I outlined below.

I consistently get high signal strengths from dtvsignal

Code:

>dtvsignal 17
using '/dev/dvb/adapter0/frontend0' and '/dev/dvb/adapter0/demux0'
tuning to 491000000 Hz
video pid 0x0021, audio pid 0x0024
dtvsignal ver 1.0 - by Jack Kelliher (c) 2002
channel = 17 freq = 491000000Hz
 30db       0%         25%         50%         75%        100%
Signal:     |     .     :     .     |     ._____:_____._____|
Signal: 076 $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$


and azap gives

Code:

> ./azap -r "El Paso, TX"
using '/dev/dvb/adapter0/frontend0' and '/dev/dvb/adapter0/demux0'
tuning to 491000000 Hz
video pid 0x0031, audio pid 0x0034
status 1f | signal 9c28 | snr f36b | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal a830 | snr f3ed | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal a830 | snr f3bd | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal a6e8 | snr f3a7 | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal a6e8 | snr f327 | ber 00000000 | unc 00000000 | FE_HAS_LOCK


But when I use mplayer to try to play streams recorded with getatsc, the video has glitches and the console shows the following messages that seem to correspond to the glitches.

Code:

a52: error at resampling


and

Code:

a52: CRC check failed!


The quality of the other stations corresponds to the signal strength on the other three stations I can pick up (i.e. above 60% = good picture), but ABC is the oddball. Do you think this is my system? Is this something that we see as a result of multi-path? Or maybe it's the station?

Concerning the azap output is it misleading that the snr is high but the picture quality is low?

I'm running Suse 10.0 with the 2.6.13-15.8-default kernel on a P4 with 768 MB.
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PostPosted: Wed May 03, 2006 11:52 pm Reply with quote
kmj0577
 
Joined: 03 Jan 2006
Posts: 57




Might try building mplayer with the patch at http://pchdtv.com/forum/viewtopic.php?t=1048&start=75
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PostPosted: Thu May 04, 2006 6:15 am Reply with quote
speed301
 
Joined: 02 May 2006
Posts: 11




kmj,

Thanks. I'll try that, but I see the glitches when I try to watch the file with xine also. Tells me it's probably not just mplayer.

speed301
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PostPosted: Thu May 04, 2006 9:59 am Reply with quote
kmj0577
 
Joined: 03 Jan 2006
Posts: 57




speed301 wrote:
kmj,

Thanks. I'll try that, but I see the glitches when I try to watch the file with xine also. Tells me it's probably not just mplayer.

speed301

Hmm, so is the video constantly having glitches? What happens if you try to use something to watch it live? For example:
Code:
dvbstream -c 0 -f 491000000 -vsb 8 -o 49 52 | mplayer -

or
Code:
getatsc -dvb 0 <channel> | mplayer -
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PostPosted: Thu May 04, 2006 5:10 pm Reply with quote
speed301
 
Joined: 02 May 2006
Posts: 11




I usually watch live by

Code:

getatsc > tv.ts

In one terminal,

and
Code:

mplayer -vf scale=1024:576 -framedrop -aid 52 tv.ts

in another terminal and I still get the glitches. If I try to play the live stream the way you describe the results are the same.
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PostPosted: Fri May 05, 2006 11:10 am Reply with quote
dieter
 
Joined: 20 Jan 2005
Posts: 43
Location: US




> The quality of the other stations corresponds to the signal strength on
> the other three stations I can pick up (i.e. above 60% = good picture),
> but ABC is the oddball. Do you think this is my system? Is this something
> that we see as a result of multi-path? Or maybe it's the station?

It sounds like this problem is specific to one station, ABC.

If there is a video forum for your geographic area, you could look there.
If the station is having problems there will likely be complaints about it.

Is the ABC transmitter near the other stations' transmitters? (e.g. on the
same hill) Similar height?

Is the ABC station using a similar frequency as the other stations?
If ABC is channel 2 and all the others are fifty-something (or visa-versa),
there could be a difference in propagation. In addition, the characteristics
of receiving antennas vary with frequency. (gain, radiation pattern, VSWR)

Signal strength is necessary but not sufficient.

There is also multipath. Static multipath from buildings, mountains, etc.
Dynamic multipath and Doppler from cars, trucks, airplanes, etc. Varying
signal levels from the wind blowing tree leaves. There is the depression
angle problem if you are close (less than ~5 miles) to the transmission
tower. Interference from other TV stations, FM stations, other radio
sources, electrical devices.

If you are using a preamp or distribution amp it might be overloading.

> For the past couple of weeks I've been having problems

Different temperatures? Trees leafing out?
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Attenuator to fix multipath?
PostPosted: Tue May 09, 2006 9:47 pm Reply with quote
speed301
 
Joined: 02 May 2006
Posts: 11




Dieter,

Thanks for your comments. Antennaweb.org says that I'm 4.6 miles away from the tower. The other stations are at different distances so I assume they're not on the same hill. I live in El Paso and there are mountains all around. Maybe they can cause reflections but I doubt if they are blocking any signal. I did post on my local thread avsforum.com but noone reported any similar problems.

To address your question about the other stations, here my channels.conf file. ABC is listed as KVIA at 491 MHz
Code:

KFOX-DT@:479000000:8VSB:49:52:3
KFOX-WX@:479000000:8VSB:65:68:4
KTSM-DT@:485000000:8VSB:49:52:3
KTSM-2:485000000:8VSB:65:68:4
KVIA:491000000:8VSB:49:52:3
KDBC:497000000:8VSB:49:52:3


If it is multipath do you think maybe a 3 dB pad can attenuate the reflections enough while leaving enough good signal? In the meantime I'll pull out my old communications text book and refresh my memory on this stuff. Thanks :)
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PostPosted: Wed May 10, 2006 9:35 am Reply with quote
dieter
 
Joined: 20 Jan 2005
Posts: 43
Location: US




> I'm 4.6 miles away from the tower. The other stations are at different
> distances so I assume they're not on the same hill.

At 4.6 miles, you might have a problem with depression angle, assuming
that the transmitter is higher than you are. Although this probably
hasn't changed since a couple weeks ago.

If you have good reception of KTSM and KDBC, your problems with
KVIA are unlikely to be due to different frequencies propagating
differently, as they are adjacent channels only 6 MHz apart.
It could be adjacent channel interference, although modern
tuners are supposed to be able to handle adjacent channels.

> there are mountains all around.

You could have multipath from the mountains, although this
probably hasn't changed in the last couple weeks, barring
earthquakes or volcano eruptions. You might be able to adjust
the antenna and put the reflection in a null.

> If it is multipath do you think maybe a 3 dB pad can attenuate
> the reflections enough while leaving enough good signal?

People say this method sometimes works. I've experimented with
attenuators and thus far haven't had any luck. From what I
can figure out, the idea is to drop the reflections down into
the noise, while leaving the main signal above the noise. If
you have a strong signal, you may need more than 3dB to do this.

Cheap enough to try. Digital Orange has 6dB and 10dB
attenuators for $2.64 each. (plus shipping)
http://store.digital-orange.com/db201-406.html
http://store.digital-orange.com/db201-410.html
I think they also had 3dB but only in a bag of 10 or 20.
There are also variable attenuators available. (try google)
A 2-way splitter will drop the level about 3.5 dB.
A 4-way splitter will drop the level about 7.4 dB.

The other thing that attenuators help with is reflections
in the coax due to SWR. If you have splitters, be sure to
terminate unused outputs.

$1.99
http://store.digital-orange.com/db200-075.html

Or you can terminate by running coax to an old tv or vcr
or something.
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PostPosted: Thu May 11, 2006 11:38 am Reply with quote
speed301
 
Joined: 02 May 2006
Posts: 11




dieter,

Again thanks for the insight. Right after I wrote my last response I tried a different antenna and the problems on KVIA disappeared. Previously I had had problems with KVIA and a coax change fixed it.

Both are 2 different Philips/Magnavox branded models. I don't use the amplification feature on either antenna. I modified the "bad" antenna by removing the VHF antennae, with the logic being, "why introduce VHF signals onto the coax and subsequently the card if I don't need them?" The "good" antenna is unmodified.

In any event I have a lot of knowledge just in case I see this problem again. And just for the heck of it I'll try to fix the bad antenna with a pad or some of your other suggestions.
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High Signal Strength Poor Quality Video
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