Northern Telecom Limited

Gernot Eberle

Guy J Chaput

Anthony R Walker

Patrice L Humbert-Droz

William Goodchild

A digital signal processing apparatus is disclosed. The apparatus is particularly suited for detecting a digital signal containing a frequency component in a noisy environment. It uses a distributed arithmetic concept for hardware realization. The use of specific coefficients and hardware arrangement has resulted in a reduction of memory location requirement. The invention finds the main application in the area of telephony in which detection of a variety of signal tones is required for various operations of telephone functions.

Digital signal processing apparatus for detecting a frequency component

Digital FSK demodulators to be used in the telephone environment are disclosed. The demodulator according to one embodiment uses the sequential processing of a digital signal encoded in the fractional two's complement numeric representation. The use of multipliers is largely eliminated, thus enabling extensive use of simple shift registers and adders. Consequently the hardware complexity requirement is greatly reduced, thus resulting in very low cost products.

Digital FSK demodulator

Apparatus for use in a secure data communication system includes a microcomputer that controls a cipher process by which sensitive data is encrypted before transmission. When activated, the apparatus accepts either cleartext or ciphertext and transforms the text to its opposite form according to an algorithm that is past dependent and which is also dependent on a pseudo random cipher key. Ciphertext may then be stored in an information retrieval system or transmitted to a destination terminal where the text is deciphered in real time by a corresponding second apparatus that stores the same key. In either case, security of transmitted and stored data is maintained since the sensitive data is in encrypted form. A set of logical instructions according to the algorithm is stored in a read only memory of the microcomputer which includes an EPROM unit having a data field of predetermined characters. The cipher process is initiated by a control character and starts with predetermined seeds that establish a first origin in the data field for each line of text to be encrypted. This origin is then translated to a second origin of the data field in response to four characters that are randomly generated by a counter. A variable sequence of iterative logical operations transforms cleartext to ciphertext and provides a key stream that is non-linear, random-like and which possesses an encryption period that is substantially longer than the period of any line so that the periodicity of the key stream is never indicated. The cipher keys for each line of text are temporarily stored in a read/write memory to permit editing operations and are erased at the onset of the next line.

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Lanark, Canada

Gernot Eberle