[mythtv-users] Problem: LiveTV ok, recordings are crappy

Nick Morrott knowledgejunkie at gmail.com
Sun Jun 22 14:29:08 UTC 2008

On 22/06/2008, Paolo Asioli <paolo.asioli at libero.it> wrote:
>  Hi, Nick and Brian.
>  I have two DVB-T Hauppauge USB Stick. They worked perfectly until a week or
> so ago I upgraded my Ubuntu and with it MythTV.
>  I think something happens when I record from a schedue instead of the
> recording from LiveTV.
> I've just made this test: started watching a show on LiveTV (perfect) for 5
> mins
> stopped watching, copied the mpeg to my PC (plays perfectly on VLC)
> started a recording on the same channel, same show
> the scheduled recording shows the usual signs of "damage" (strips box shaped
> of green "noise")
> if I copy the file to my PC the file shows the same corruption when payed in
> VLC No error shows in the console where I launched the frontend where I
> launched mythfrontend and no errors show in the log or in dmesg.
>  Something strange happens when I launch the recording: when I watch LiveTV
> the load is fairly high but wait % is very low and stays most of the time to
> 0% and the mythbackend uses 15-20% of the CPU, while when recording from
> schedule mythbackend uses around 10% CPU and there's always a >0% of wait
> state processes.
>  It's like when the recording is scheduled the process recording gets a
> lower priority than when watching LiveTV and can't keep up with the MPE2
> stream coming from the DVB-T card.


As you have 2 DVB-T tuners, it's highly likely that LiveTV and
scheduled recordings are using different tuners. In LiveTV (where you
don't see any problems) change to the other tuner's input (you can do
this via the OSD menu) and see if you start to see the same types of
problems you see in your scheduled recordings. Under normal
circumstances, MythTV will use the first capture card input configured
(highest recording preference) for scheduled recordings, and the last
input configured (lowest recording preference) for LiveTV.

Based on your report, it is very likley that the signal levels to the
two tuners are different, and that the tuner for LiveTV has a stronger
signal than the tuner being used first for scheduled recordings. A
simple test is to schedule 2 recordings at the same time on different
channels (and on different multiplexes if you have multirec enabled to
make sure different physical tuners are used). Another test is to
watch LiveTV and enable Picture in Picture when both tuners will be
used at the same time, and see if you have any problems.

When using DVB/ATSC tuners, picture breakup/green blocks/freezes/audio
pops are indicative of poor signal quality - so this needs to be
checked first. If you swap over the RF flyleads into your 2 USB
tuners, do you now see perfect recordings but poor LiveTV?


Nick Morrott

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