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.

Abstract: A method of scrambling a compressed audio and/or picture signal to provide a scrambled signal for distribution, the information content being recoverable from the scrambled signal using descrambling means (10) under the control of authorized recipients of the scrambled signal, is described. The method consists of or includes: a) encoding the information content using a wavelet filterbank having a tree structure, b) distributing the scrambled signal to authorized recipients, and c) decoding the information content using the corresponding inverse wavelet filterbank. The wavelet filter used, the wavelet filterbank tree structure, the number of decomposition stages in the tree structure and/or the time for which a given filter or tree structure or number of decomposition stages is used can be chanced in an apparently random fashion to improve security whilst keeping the scrambled signal recognizable. Progressively increasing distortion with time is also described.