The data from the people monitoring unit 2 and television channel detection unit 1 is parallel-loaded into a shift register 58 during the 16 blank half cycles and is sent out in serial form at a rate of, for example, one bit per 10 mS. The output from the time slot generator 56 is a 2.5 mS long burst of 51.2 kHz carrier which is gated on or off depending on the data stored in the shift register 58