Taking a look at the ALSA code now; at a glance the loopback timer comes from include/linux/timer.h -- it doesn't look like there's any immediate dependency on

Taking a look at the ALSA code now; at a glance the loopback timer comes from include/linux/timer.h -- it doesn't look like there's any immediate dependency on a hardware interface or indeed any other device in the system. And I do recall using loopback without a hardware interface. I would predict that the difference is actually in the virtualisation, the implementation of that timer may be very different to a physical or bare-metal machine.