Mailing List Archive

Interlace issues on specific channels
Hi all,
I've been running a split FE/BE Mythtv setup with minimal issues
almost for a year now. On the BE I'm using a HVR-1600 to receive OTA HD via
the ATSC input from a roof mounted antenna. The issue has been that on some
channel, and some channels only I've been getting what appears to be
interlacing/combing effects. I have an example of this effect from the
capture here on start of 60 minutes on CBS (4_2 KPIX-DT in the Bay Area).
The effect is very pronounced during horizontal motion involving
particularly vertical features in the picture.

http://thinkspace.ath.cx/stuff/vlcsnap-2017280.png

The effect is present independent of the player, whether it's on the
Mimimyth FE I normally use, XBMC or VCL on my PC in this case, hence I put
it down to a reception/BE issue regarding the OTA signal quality reception
for these stations. Keep in mind it only happens only on some specific 4-6
OTA channels, Fox (2_1 KTVU-DT) for an example using the same setup is
razor sharp with no such artifacts. Similarly the next higher channel from
CBS, 7_1 (ABC) is razor sharp, but 9_1 has the issue, 11_1 is sharp etc, it
may be alternating but I'm not exactly sure.

I initially put this issue down to a reception/signal quality problem as my
antenna was quite old and a combined VHF/UHF old style design. So recently
I decided to get serious about solving this issue and installed a brand new
DB4 UHF antenna pointed more or less in the direction of the transmission
tower. But still the problem persists. The thing is both CBS (5_1) and
NBC(11_1) are 304-321 degrees orientation and roughly the same distance of
21-25 miles from my place, but NBC doesn't seem to have the issue. Hence it
does not seem to be reception related.

Following this I updated the cx18 driver to a more recent 1.2.0 version and
rescanned all the channel, but no progress. I'm kind of stumped, anyone have
any idea what may be the source of the interlacing issue on the BE?

Many thanks,

Tony
Re: Interlace issues on specific channels [ In reply to ]
On Tue, 2009-09-22 at 08:44 -0700, Tony Sarathchandra wrote:
>
> Hi all,
> I've been running a split FE/BE Mythtv setup with minimal
> issues almost for a year now. On the BE I'm using a HVR-1600 to
> receive OTA HD via the ATSC input from a roof mounted antenna. The
> issue has been that on some channel, and some channels only I've been
> getting what appears to be interlacing/combing effects. I have an
> example of this effect from the capture here on start of 60 minutes on
> CBS (4_2 KPIX-DT in the Bay Area). The effect is very pronounced
> during horizontal motion involving particularly vertical features in
> the picture.
>
> http://thinkspace.ath.cx/stuff/vlcsnap-2017280.png

I suspect you only see this on 1080i video and not 720p.

Firefox says your PNG file is 1920px × 1088px, and since no one sends
1080p OTA AFAIK, you're seeing the effect with 1080i in the example.

You may wish to try some deinterlace filtering to get rid of the
combing. Google turned up this for MythTV:

http://www.mythtv.org/wiki/Using_playback_filters

I don't know if it works.

> The effect is present independent of the player, whether it's on the
> Mimimyth FE I normally use, XBMC or VCL on my PC in this case, hence I
> put it down to a reception/BE issue regarding the OTA signal quality
> reception for these stations.

Hmmm. Well, this is a known effect with 1080i. You must have "young"
eyes; I tend not to notice the effect that much.

Well, WRT to the CX23418, the chip is pretty much a pass through of the
Digital TS from OTA. But the areas to check with the HVR-1600 are:


1. use femon in another window to watch for the uncorrectable block
count increasing occasionally. This would inidcate marginal reception
of a locked station.

2. using the cx18 debug module option (see $ /sbin/modinfo cx18 ), turn
on warnings and look for things in dmesg or /var/log/messages like:


Fell behind! Ignoring stale mailbox with inconsistent data. Lost buffer for mailbox seq no ...

Could not find buf ... for stream TS

Possibly falling behind: CPU self-ack'ed our incoming CPU to EPU mailbox (sequence no. ...) while processing

This would inidcate buffers being dropped or possibly being dropped by
the driver and not being given to the application (i.e. mythbackend) due
to a system level performance issue in the capturing PC.


> Keep in mind it only happens only on some specific 4-6 OTA channels,
> Fox (2_1 KTVU-DT) for an example using the same setup is razor sharp
> with no such artifacts. Similarly the next higher channel from CBS,
> 7_1 (ABC) is razor sharp, but 9_1 has the issue, 11_1 is sharp etc, it
> may be alternating but I'm not exactly sure.

Look for the programming being 1080i vs 720p. In my area:

NBC and CBS: 1080i
Fox and ABC: 720p


> I initially put this issue down to a reception/signal quality problem
> as my antenna was quite old and a combined VHF/UHF old style design.
> So recently I decided to get serious about solving this issue and
> installed a brand new DB4 UHF antenna pointed more or less in the
> direction of the transmission tower. But still the problem persists.
> The thing is both CBS (5_1) and NBC(11_1) are 304-321 degrees
> orientation and roughly the same distance of 21-25 miles from my
> place, but NBC doesn't seem to have the issue. Hence it does not seem
> to be reception related.
>
> Following this I updated the cx18 driver to a more recent 1.2.0
> version and rescanned all the channel, but no progress. I'm kind of
> stumped, anyone have any idea what may be the source of the
> interlacing issue on the BE?

Update things like ffmpeg and any other MPEG or HDTV decoding libraries
used by your apps. Also having a lot of CPU horse power and a good
graphics card might help.

Other than that, look for options to turn on more sophisticated
deinterlacing filters during playback.

Regards,
Andy

> Many thanks,
>
> Tony



_______________________________________________
ivtv-users mailing list
ivtv-users@ivtvdriver.org
http://ivtvdriver.org/mailman/listinfo/ivtv-users
Re: Interlace issues on specific channels [ In reply to ]
On Sat, Sep 26, 2009 at 06:05:50PM -0400, Andy Walls wrote:
> On Tue, 2009-09-22 at 08:44 -0700, Tony Sarathchandra wrote:
> >
> > Hi all,
> > I've been running a split FE/BE Mythtv setup with minimal
> > issues almost for a year now. On the BE I'm using a HVR-1600 to
> > receive OTA HD via the ATSC input from a roof mounted antenna. The
> > issue has been that on some channel, and some channels only I've been
> > getting what appears to be interlacing/combing effects. I have an
> > example of this effect from the capture here on start of 60 minutes on
> > CBS (4_2 KPIX-DT in the Bay Area). The effect is very pronounced
> > during horizontal motion involving particularly vertical features in
> > the picture.
> >
> > http://thinkspace.ath.cx/stuff/vlcsnap-2017280.png
>
> I suspect you only see this on 1080i video and not 720p.
...
> > The effect is present independent of the player, whether it's on the
> > Mimimyth FE I normally use, XBMC or VCL on my PC in this case, hence I
> > put it down to a reception/BE issue regarding the OTA signal quality
> > reception for these stations.
>
> Hmmm. Well, this is a known effect with 1080i. You must have "young"
> eyes; I tend not to notice the effect that much.

is it relevant that the CBS logo in the lower-right does not
appear to show the combing effect? that would seem to indicate
to me that the erroneous converstion is happening at the CBS
station itself before superimposing the logo, no?

--Rob*

--
/-------------------------------------------------------------\
| "If we couldn't laugh we would all go insane" |
| --Jimmy Buffett, |
| "Changes in Latitudes, Changes in Attitudes" |
\-------------------------------------------------------------/

_______________________________________________
ivtv-users mailing list
ivtv-users@ivtvdriver.org
http://ivtvdriver.org/mailman/listinfo/ivtv-users
Re: Interlace issues on specific channels [ In reply to ]
On Sat, 2009-09-26 at 15:33 -0700, Rob Starling wrote:
> On Sat, Sep 26, 2009 at 06:05:50PM -0400, Andy Walls wrote:
> > On Tue, 2009-09-22 at 08:44 -0700, Tony Sarathchandra wrote:
> > >
> > > Hi all,
> > > I've been running a split FE/BE Mythtv setup with minimal
> > > issues almost for a year now. On the BE I'm using a HVR-1600 to
> > > receive OTA HD via the ATSC input from a roof mounted antenna. The
> > > issue has been that on some channel, and some channels only I've been
> > > getting what appears to be interlacing/combing effects. I have an
> > > example of this effect from the capture here on start of 60 minutes on
> > > CBS (4_2 KPIX-DT in the Bay Area). The effect is very pronounced
> > > during horizontal motion involving particularly vertical features in
> > > the picture.
> > >
> > > http://thinkspace.ath.cx/stuff/vlcsnap-2017280.png
> >
> > I suspect you only see this on 1080i video and not 720p.
> ...
> > > The effect is present independent of the player, whether it's on the
> > > Mimimyth FE I normally use, XBMC or VCL on my PC in this case, hence I
> > > put it down to a reception/BE issue regarding the OTA signal quality
> > > reception for these stations.
> >
> > Hmmm. Well, this is a known effect with 1080i. You must have "young"
> > eyes; I tend not to notice the effect that much.
>
> is it relevant that the CBS logo in the lower-right does not
> appear to show the combing effect? that would seem to indicate
> to me that the erroneous converstion is happening at the CBS
> station itself before superimposing the logo, no?

Rob,

I've only every seen the effect in 1080i HDTV with objects that move in
the frame. The CBS logo is always in the same position on the frame, so
it would never show the effect.

However, that does not rule out the possibility that the conversion to
digital at the source is doing a poor job, as you suggest. I would
think CBS has the money and experience to get this sort of stuff right,
though.

Regards,
Andy

> --Rob*
>


_______________________________________________
ivtv-users mailing list
ivtv-users@ivtvdriver.org
http://ivtvdriver.org/mailman/listinfo/ivtv-users