Equipment for labelling audio signals with identification information has an encoder which inserts the binary information into two very narrow notches of centre frequencies 2883 and 3417 Hz, between semi-tones in the tonic scale to minimise music breakthrough into the decoding circuits, and to ensure that no fundamental frequencies in the tonic scale will be excluded in the reproduction. The notches are derived from a 3-stage biquad filter, and are approximately 50 dB deep and 150 Hz wide at the top.
The encoder includes a wide bandpass circuit consisting of a 1 KHz highpass filter 10 and a 6 KHz lowpass filter 11 introduced to ensure that the code insertion level is not determined by frequencies, either high or low, which do not adequately mask the code frequencies.
The code amplitude is kept a fixed level below the programme, initially adjustable by a suitable control.
The code sequence has an addressing pre-amble consisting of a simultaneous burst of both the lower and higher frequencies for a period of 8 digits, followed by a message portion of 40 bits formed of an appropriate stream of the two frequencies.