A marking signal 1 identifying the owner of a recording and formed by binary elements passes from a data store 2 to code generator 3, which translates each like binary element into a pseudo random sequence of binary elements, to produce an encoded marking signal and then, via low pass filter 4 and multiplier 5, passes to multiplier 6 with an audio signal 7 input to effect amplitude modulation thereof. After filtering by high pass filter 8, the signal is added to the audio signal channel by circuit 9. Decoder 11 (also Fig. 3) enables extraction of the marking signal for identification. The arrangement facilitates proof of ownership of illegal re-recordings and automatic logging of playback time for royalty purposes.