The appliance comprises an audio port, which is usable as an interface for testing of the appliance. In a preferred embodiment, the appliance comprises stereo output ports, one port for a reception of digital test signals from a control computer and the other port for a transmission of digital signals from the appliance to the control computer. For the separation of the test signals and the audio signals a test adapter is used which is coupled between the appliance, the control computer and an amplifier for the audio signals. The test signals are modulated onto a carrier signal having a frequency of above 20 kHz, for example a frequency within the frequency range of 1-10 MHz. This allows a serial connection, which is compatible with the widely used RS232 link for set-top boxes.