Frequency inversion scrambler with integrated high-pass filter having autozero to remove internal DC offset
US5528692A · kind A · utility
Assignee
Inventors
Key dates
| Filing date | May 3, 1994 |
| Grant date | Jun 18, 1996 |
| Priority date | — |
| Expiry date | May 3, 2014 |
Classification
- Technology area (CPC H)Electricity
- CPC primaryH04K1/04
- WIPO fieldTelecommunications
- WIPO sectorElectrical engineering
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.
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.