Thank you very much ghost.

Bye

I think the problem is turning off tone (setTone 0) while tuner A is yet tuned on high frequency transponder.

In some conditions this bug causes lost lock and image freeze.
I start recording a channel on a high frequency (i.e. 11785H) on tuner A.
Then (while recording) I switch to a channel on another high frequency transponder (i.e. 12360H).
Now I have both tuner A and B busy on two high frequency transponders.
When I switch back to previous channel (11785H), tuner B is correctly closed but (I don't know why) setTone(0) is also called and this causes lost lock of tuner A (LNB is shared) and of course image freeze until setTone(1) is called.

close frontend 1
[prepareClose] running mainloop
[SEC] setTone 0
[SEC] sleep 25ms
set sequence pos 5
[SEC] delayed close frontend
close frontend 1
(0)fe event: status 21, freq 1185724, inversion off, m_tuning 0 N
stateLostLock
OURSTATE: lost lock, trying to retune
(0)tune
RotorCmd ffffffff, lastRotorCmd ffffffff
prepare_sat System 1 Freq 11785000 Pol 0 SR 29900000 INV 2 FEC 3 orbpos 130 system 1 modulation 2 pilot 2, rolloff 0
tuning to 1185 mhz
OURSTATE: tuning
[SEC] set static current limiting
set sequence pos 6
set sequence pos 3
set sequence pos 7
[SEC] invalidate current switch params
[SEC] sendDiseqc: e00000(DiSEqC reset)
[SEC] sleep 50ms
[SEC] sendDiseqc: e00003(DiSEqC peripherial power on)
[SEC] sleep 150ms
[SEC] sendDiseqc: e01038f3
[SEC] sleep 50ms
set sequence pos 3
[SEC] setTone 1

In enigma2 3.1 setTone was not called after closing frontend and there was no problem.
And more, If I do the same test using low frequency transponders, setTone(0) is not called at all.

close frontend 1
[prepareClose] running mainloop
set sequence pos 3
set sequence pos 5
[SEC] delayed close frontend
close frontend 1

Hardware is dm8000 with bcm4501.
I think the same problem is reported in this thread Tuner A mit B verbinden Ruckler

Thanks for the future answer.