The invention relates to a method and to apparatus for automatically identifying a program including a sound signal, such as a radio program or a television program, in particular, or a program recorded on a medium such as a microgroove disk, a magnetic tape, a compact disk for reading by laser, or a video disk.
Such automatic identification of programs is applicable to measuring the audience of radio stations or television channels, to monitoring the inclusion of advertising programs in television or radio broadcasts, or indeed to keeping an account of royalties due to authors or to performers for public broadcasting of their works.
It has been proposed to add an inaudible coded message to the sound signal of a program to be identified, the encoded message comprising information such as the identity of the broadcasting channel or station, the identity of the program, and possibly the exact date of transmission. A specialized decoder associated with a television or radio receiver serves to extract the encoded message added to the sound signal and to record it in a memory.
To ensure that the encoded message is inaudible on being output from the loudspeaker(s) of a receiver, it must either be at a very low frequency (e.g. 40 Hz), or else it must be at an audible frequency but emitted at a level that is well below the level of the sound signal (in the range -50 dB to -60dB).
The essential drawback of these proposed means is that they require a specialized decoder to be integrated in the television or radio receiver. The characteristics of the loudspeakers in such receivers are such that in general they do not pass signals at frequencies below about 100 Hz. When the encoded message is emitted on an audible frequency but at a level that is well below the level of the sound signal, it is also very difficult to pick up the message using a microphone, particularly since the directivity of a loudspeaker increases with frequency, as does the attenuation of the signal it reproduces.
Another proposed technique consists in using one type of modulation for broadcasting the sound signal and another type of modulation for broadcasting the encoded message. Here again, it is necessary for a specialized decoder to be integrated in the receiver in order to be able to recover the encoded message.