While capturing
(with mplayer or any other program) you can change the active input channel with this command in
the terminal:
v4l2-ctl -d /dev/video0 -i 1
The -d option of the v4l2-ctl command selects the videodevice, it is not needed if there is only one (/dev/video0) in the system.
The -i option selects/ changes the input channel of your device.
The v4l2-ctl utility is part of the v4l-utils package.
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With the "Qt V4L2 test utility" program (qv4l2) you can test switching the inputs.
In this program there is a drop down menu where you can select
the different inputs while capturing.