Artefacts on playback, backend in KVM

[Merlin83b]

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 (170.99 KiB) Viewed 972 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]

Gordon says the signal quality is decent.

IMG_1136.JPG (122.74 KiB) Viewed 971 times

[kmdewaal]

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]

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]

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