run
dbus-monitor --system | grep tvout
plug the cable with tv ( device ) connected
if a good av input is connected it will show string "tvout " else string "tvoutandbthsp"

possible faults:
- wrong cable
- wrong cable pinout . try with original cable. cable on the market sometimes have ground and av output swapped ( f... iphone legacy )
- wrong av input socket impedance .must be 75 ohm

correct detection and work of n900 can be checked by putting an resistor of 75 ohm across yellow output of n900 cable

0 ohm mean headphones
75 ohm mean av out
more than that mean mike