Abstract: A system for detecting a first type of signal processing having been applied to audio signals that employs an encoder for imposing upon the audio signals, in a predetermined relationship, first coding signals robust against the first type of signal processing, and second coding signals vulnerable to contamination by noise when subjected to said first type of signal processing. A detector is conditioned to reject signals contaminated by the noise. A comparator compares the relationship between first and second coding signals as received in order to detect variation in the predetermined relationship, and thereby to discern whether unauthorized signal processing of the first type has been applied to audio signals received by way of the communications channel. The second coding signals are robust against other types of signal processing.

Abstract: A system for labelling and subsequently identifying a multi-frequency signal, includes means for inserting a code signal into a multi-frequency signal, signal distribution means, signal receiving means, code extraction means, and monitoring means to determine which parts of the frequency spectrum will at least partly mask the code signal at a given time using predetermined criteria. The means for inserting a code signal includes means for eliminating one or more frequency ranges being located in a part of the frequency spectrum that will at least partly mask the code signal, the location of the frequency ranges being eliminated from the said multi-frequency signal varies with the frequency content of said multi-frequency signal.