Monitor : avoid crackling

[moonshaadow]

When monitoring, switching from one track to another induces crackling.
We should avoid that.

[SpotlightKid]

This can happen, when you switch monitoring between channels with different signal levels. This can cause jumps in the output signal amplitude, causing clicks or pops (or "crackling" as you call it) in the output. This could be solved by a short crossfade when switching monitored channels, something that jack_mixer does not (yet) do (and which would be not trivial to add).

Another possible reason is that the monitor channel output signal clips, because it is always at unity level. If the input signals goes > 0 dBFS, so will the monitor output signal and clip. The solution is of course to lower the level of the monitored channel 8since you can not lower the level of the monitor channel).

With the changes introduced in the last jack_mixer versions, the automatic monitor output channels have few advantages left over just using a normal output channel as a monitor channel.

a) Monitoring output channels
b) Easy switching between the monitored channels with one click

On the other hand, when using a normal output channel as the monitor channel, you can:

a) Set the level of that monitor channel
b) Send the input of several input channels to it (with the "S" button in the control group)
c) Meter the monitor output level.

I plan to additional mouse bindings for exclusive solo, so that b) from the first list above is covered as well.