A method and a device for detecting an unoccupied transmission channel in a multi-channel transmission system having a plurality of transmission channels for an audio and/or video transmission, as it is frequently installed in seats of vehicles, trains, airplanes or in auditoriums and conference rooms. When expanding or modifying the transmission system, each receiving module until now had to be uninstalled and appropriately adapted in case of unoccupied transmission channels. This is very labor-intensive and costly. According to the invention, an encoding of the unoccupied transmission channels using an identification is provided, which is detectable by the receiving modules. Upon detection of the identification, the respective transmission channel is automatically blocked for the selection. A complicated disassembly and a corresponding manual adaptation of the receiving modules may therefore be dispensed with.