Artefacts on playback, backend in KVM

Have a MythTV related problem? Ask for help from other MythTV users here.

Moderator: Forum Moderators

Post Reply
Merlin83b
Junior
Posts: 15
Joined: Fri Feb 07, 2014 3:37 pm
Great Britain

Artefacts on playback, backend in KVM

Post by Merlin83b » Wed Mar 10, 2021 12:48 pm

I've been running MythTV for about 20 years, most of it successfully, most of that time with DVB-T, currently through two USB recceivers. Recently I did some consolidation of services at home onto a server running docker and KVM. I've put my backend into a KVM virtual machine and ever since, recordings are blocky. Disk speeds inside the virtual machine are nice and quick, 233MB/s write, 133MB/s read. There's nothing in the logs to indicate any problems during recording and it's not just playback issue as I've grabbed the .ts file and playback through VLC is also bad.

Playback through the MythTV client looks like this:
IMG_1134.JPG
IMG_1134.JPG (170.99 KiB) Viewed 466 times
As you can see, the network between backend and frontend isn't an issue.

Any clues as to what the issue might be before I give up and run Myth on the host itself rather than in a VM?

Merlin83b
Junior
Posts: 15
Joined: Fri Feb 07, 2014 3:37 pm
Great Britain

Re: Artefacts on playback, backend in KVM

Post by Merlin83b » Wed Mar 10, 2021 1:01 pm

Gordon says the signal quality is decent.
IMG_1136.JPG
IMG_1136.JPG (122.74 KiB) Viewed 465 times

User avatar
kmdewaal
Developer
Posts: 288
Joined: Wed Dec 07, 2016 8:01 pm
Netherlands

Re: Artefacts on playback, backend in KVM

Post by kmdewaal » Wed Mar 10, 2021 5:31 pm

Look in the mythbackend log for lines with the continuity_error_count, like this one:

Code: Select all

<RecordingQuality overall_score="1" key="1435332_2021-02-19T22:06:00Z" continuity_error_count="0" packet_count="488076" />
When the continuity_error_count is 0 you have a perfect recording. Values less than 10 are still OK but higher is suspect.
When your values are very high, which I expect, this means that the USB pass-through (from the VM to the real hardware) is not good enough.
I think that only network tuners such as the HDHomeRun can work reliably in a VM, although this is just a guess, I never tried it.

Merlin83b
Junior
Posts: 15
Joined: Fri Feb 07, 2014 3:37 pm
Great Britain

Re: Artefacts on playback, backend in KVM

Post by Merlin83b » Wed Mar 10, 2021 7:06 pm

Ouch yeah I have high numbers, in the thousands, for all recordings. Damn, guess I’ll be moving the backend out of a VM into the hardware then 🙁

Merlin83b
Junior
Posts: 15
Joined: Fri Feb 07, 2014 3:37 pm
Great Britain

Re: Artefacts on playback, backend in KVM

Post by Merlin83b » Fri Mar 12, 2021 5:23 pm

Migration back to native hardware completed and the problem's solved. Thanks, kmdewaal :)

Post Reply