Cryptography – Electric signal masking – By signal having discrete frequency component
Reexamination Certificate
1998-06-08
2002-07-16
Barron, Jr., Gilberto (Department: 2132)
Cryptography
Electric signal masking
By signal having discrete frequency component
C380S241000, C380S275000, C725S018000
Reexamination Certificate
active
06421445
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to apparatus and methods for including codes in audio signals and decoding such codes.
For many years, techniques have been proposed for mixing codes with audio signals so that (1) the codes can be reliably reproduced from the audio signals, while (2) the codes are inaudible when the audio signals are reproduced as sound. The accomplishment of both objectives is essential for practical application. For example, broadcasters and producers of broadcast programs, as well as those who record music for public distribution will not tolerate the inclusion of audible codes in their programs and recordings.
Techniques for encoding audio signals have been proposed at various times going back at least to U.S. Pat. No. 3,004,104 to Hembrooke issued Oct. 10, 1961. Hembrooke showed an encoding method in which audio signal energy within a narrow frequency band was selectively removed to encode the signal. A problem with this technique arises when noise or signal distortion reintroduces energy into the narrow frequency band so that the code is obscured.
In another method, U.S. Pat. No. 3,845,391 to Crosby proposed to eliminate a narrow frequency band from the audio signal and insert a code therein. This technique evidently encountered the same problems as Hembrooke, as recounted in U.S. Pat. No. 4,703,476 to Howard which, as indicated thereon, was commonly assigned with the Crosby patent. However, the Howard patent sought only to improve Crosby's method without departing from its fundamental approach.
It has also been proposed to encode binary signals by spreading the binary codes into frequencies extending throughout the audio band. A problem with this proposed method is that, in the absence of audio signal components to mask the code frequencies, they can become audible. This method, therefore, relies on the asserted noiselike character of the codes to suggest that their presence will be ignored by listeners. However, in many cases this assumption may not be valid, for example, in the case of classical music including portions with relatively little audio signal content or during pauses in speech.
A further technique has been suggested in which dual tone multifrequency (DTMF) codes are inserted in an audio signal. The DTMF codes are purportedly detected based on their frequencies and durations. However, audio signal components can be mistaken for one or both tones of each DTMF code, so that either the presence of a code can be missed by the detector or signal components can be mistaken for a DTMF code. It is noted in addition that each DTMF code includes a tone common to another DTMF code. Accordingly, a signal component corresponding to a tone of a different DTMF code can combine with the tone of a DTMF code which is simultaneously present in the signal to result in a false detection.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide coding and decoding apparatus and methods which overcome the disadvantages of the foregoing proposed techniques.
It is a further object of the present invention to provide coding apparatus and methods for including codes with audio signals so that, as sound, the codes are inaudible to the human ear but can be detected reliably by decoding apparatus.
A further object of the present invention is to provide decoding apparatus and methods for reliably recovering codes present in audio signals.
In accordance with a first aspect of the present invention, apparatus and methods for including a code having at least one code frequency component with an audio signal having a plurality of audio signal frequency components, comprise the means for and the steps of: evaluating an ability of a first set of the plurality of audio signal frequency components to mask the at least one code frequency component to human hearing to produce a first masking evaluation; evaluating an ability of a second set of the plurality of audio signal frequency components differing from the first set thereof to mask the at least one code frequency component to human hearing to produce a second masking evaluation; assigning an amplitude to the at least one code frequency component based on a selected one of the first and second masking evaluations; and including the at least one code frequency component with the audio signal.
In accordance with another aspect of the present invention, an apparatus for including a code having at least one code frequency component with an audio signal having a plurality of audio signal frequency components, comprises: a digital computer having an input for receiving the audio signal, the digital computer being programmed to evaluate respective abilities of first and second sets of the plurality of audio signal frequency components to mask the at least one code frequency component to human hearing to produce respective first and second masking evaluations, the second set of the plurality of audio signal frequency components differing from the first set thereof, the digital computer being further programmed to assign an amplitude to the at least one code frequency component based on a selected one of the first and second masking evaluations; and means for including the at least one code frequency component with the audio signal.
In accordance with a further aspect of the present invention, apparatus and methods for including a code having a plurality of code frequency components with an audio signal having a plurality of audio signal frequency components, the plurality of code frequency components including a first code frequency component having a first frequency and a second code frequency component having a second frequency different from the first frequency, comprise the means for and the steps of, respectively: evaluating an ability of at least one of the plurality of audio signal frequency components to mask a code frequency component having the first frequency to human hearing to produce a first respective masking evaluation; evaluating an ability of at least one of the plurality of audio signal frequency components to mask a code frequency component having the second frequency to human hearing to produce a second respective masking evaluation; assigning a respective amplitude to the first code frequency component based on the first respective masking evaluation and assigning a respective amplitude to the second code frequency component based on the second respective masking evaluation; and including the plurality of code frequency components with the audio signal.
In accordance with yet another aspect of the present invention, an apparatus for including a code having a plurality of code frequency components with an audio signal having a plurality of audio signal frequency components, the plurality of code frequency components including a first code frequency component having a first frequency and a second code frequency component having a second code frequency different from the first frequency, comprises: a digital computer having an input for receiving the audio signal, the digital computer being programmed to evaluate an ability of at least one of the plurality of audio signal frequency components to mask a cods frequency component having the first frequency to human hearing to produce a first respective masking evaluation and to evaluate an ability of at least one of the plurality of audio signal frequency components to mask a code frequency component having the second frequency to human hearing to produce a second respective masking evaluation; the digital computer being further programmed to assign a corresponding amplitude to the first code frequency component based on the first respective masking evaluation and to assign a corresponding amplitude to the second code frequency component based on the second respective masking evaluation; and means for including the plurality of code frequency components with the audio signal.
In accordance with a still further aspect of the present invention, apparatus and methods for including a code having
Graybill Robert B.
Hassan Sayed
Jensen James M.
Lynch Wendell D.
Perelshteyn Michael M.
Arbitron Inc.
Barron Jr. Gilberto
Flanagan, III Eugene L.
St. Onge Steward Johnston & Reens
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