homerun57
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Joined: Tue Sep 09, 2014 9:29 am

Symptom: Green blocks when playing tv live stream

Tue Sep 09, 2014 9:46 am

Hi Everyone,

I'm running openelec 4.0.7 (booting from usb) on my rpi. I'm using xbmc and tvheadend to watch satellite feed and I'm having some problems with the image quality on some SD channels:

http://tinypic.com/r/2nupr9i/8

I have no problems with HD channels or any other type of high def media.
I've tried disabling hardware acceleration and it doesn't change a thing.
The channels that I have problems with are all being transmitted in the same frequency and their format is mpeg-2.
So far I've been able to reproduce this problem on a number of devices all of them running linux or android.

Since I have no problems when running under Windows, is it safe to assume that this is a problem with the codec?
Any thoughts?

Thanks

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redhawk
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Re: Symptom: Green blocks when playing tv live stream

Tue Sep 09, 2014 11:07 am

Are the green blocks always located at the bottom of the screen or do they randomly appear elsewhere??

Do you know what resolution the mpeg-2 stream is using ??

Richard S.

homerun57
Posts: 5
Joined: Tue Sep 09, 2014 9:29 am

Re: Symptom: Green blocks when playing tv live stream

Tue Sep 09, 2014 11:19 am

The green blocks are always at the bottom and I have the resolution set to 720p... but you mean the resolution of the source?!
If I disable the hardware acceleration I have the impression that I have less green blocks but depending on the type of image sometimes it covers almost half the screen (mostly during commercials).

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redhawk
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Re: Symptom: Green blocks when playing tv live stream

Tue Sep 09, 2014 11:46 am

Green lines in mpeg-2 streams is quite a common problem https://www.google.co.uk/?q=mpeg-2+gree ... green+line although I have no idea why it happening, however the following article regarding MPEG-2 Artifacts maybe of some interest http://www.ciscopress.com/articles/arti ... 1&seqNum=3
If the compression ratio is too high, there are sometimes simply not enough bits to encode the video signal without significant loss. The better encoders will progressively soften the picture (by discarding some picture detail); however, poorer encoders sometimes break down and overflow an internal buffer. When this happens, all kinds of visual symptoms—from bright green blocks to dropped frames—can result. After such a breakdown, the encoder will usually recover for a short period until once again the information rate gets too high to code into the available number of bits.
Since all the Pi can do is decode mpeg-2 in hardware there is very little you can do to correct the problem the issue is with the source encoder generating non-standard or bad data.

Richard S.

homerun57
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Joined: Tue Sep 09, 2014 9:29 am

Re: Symptom: Green blocks when playing tv live stream

Tue Sep 09, 2014 11:55 am

That's just too bad... :( I guess I'll have to wait until all the channels go HD... :)

Thanks a lot Richard!

dom
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Raspberry Pi Engineer & Forum Moderator
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Re: Symptom: Green blocks when playing tv live stream

Tue Sep 09, 2014 3:00 pm

Can you record a sample using tvheadend and check the green blocks still occur?
If so can you upload the sample (e.g. to dropbox or google drive) and I can have a look.

homerun57
Posts: 5
Joined: Tue Sep 09, 2014 9:29 am

Re: Symptom: Green blocks when playing tv live stream

Wed Sep 10, 2014 4:27 pm

I've uploaded a sample here:

https://www.dropbox.com/s/dp96y7bnsd0bm ... 9.mkv?dl=0

When I play it locally I can see the green blocks, but if I use the web player you can only see a blur

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redhawk
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Re: Symptom: Green blocks when playing tv live stream

Wed Sep 10, 2014 5:22 pm

What hardware you are using to pick up the satellite feed??

As far as I can tell from the sample file it's corrupted, the frames extracted from the transport stream demuxing process are incomplete hence the errors always appear at the bottom of the screen.
Some video players can detect problems and compensate by blurring from previous frame or neighboring pixels but in the case of the Pi it's probably just goes green due to lack of decoding data.

The bottom line is there's nothing wrong with the Pi the issue is with the software used to rip the mpeg-2 data out of the satellite feed.

Richard S.

homerun57
Posts: 5
Joined: Tue Sep 09, 2014 9:29 am

Re: Symptom: Green blocks when playing tv live stream

Wed Sep 10, 2014 5:37 pm

I use a dvbsky s960 and a pctv 460e. I have the same problem with both.
As for the software... could tvheadend be the problem?!
This only happens with a few channels.

dom
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
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Re: Symptom: Green blocks when playing tv live stream

Wed Sep 10, 2014 6:03 pm

homerun57 wrote:I've uploaded a sample here:

https://www.dropbox.com/s/dp96y7bnsd0bm ... 9.mkv?dl=0

When I play it locally I can see the green blocks, but if I use the web player you can only see a blur
As redhawk says, the file is clearly corrupted in the web browser. Perhaps there are better error concealment mechanisms
but I think you need to address the problem at the source (possibly the dvb dongle or tvheadend).

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