Cryptography – Communication system using cryptography – Frequency shift or inversion
Patent
1994-05-03
1996-06-18
Cain, David C.
Cryptography
Communication system using cryptography
Frequency shift or inversion
380 40, 380 19, H04K 104
Patent
active
055286925
ABSTRACT:
A frequency inversion scrambler in a cordless telephone utilizes an integrated high-pass filter (14) between a first stage low-pass filter (12) and modulator (16) to reduce the filter order while maintaining low group delay in the audio signal. The first stage low-pass filter and high-pass filter remove high frequency components and any DC offset from the filtered audio signal. The modulator translates the spectrum of the filtered signal to sum and difference frequencies. A second stage low-pass filter (18) removes the upper portion of the spectrum such that the resulting frequency spectrum is inverted with respect to the original audio signal to prevent eavesdropping of transmissions between the handset and base unit of the cordless telephone. Another frequency inversion circuit (30, 32, 34, 36) in the base unit inverts the frequency spectrum again back to its original state for transmission along telephone lines.
REFERENCES:
patent: 3886313 (1975-05-01), Branscome et al.
patent: 4160875 (1979-07-01), Kahn
patent: 4195202 (1980-03-01), McCalmont
patent: 4652699 (1987-03-01), Akaiwa
patent: 4790009 (1988-12-01), Ishigaki et al.
Bader Scott K.
Hester Richard E.
Atkins Robert D.
Cain David C.
Motorola Inc.
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