Dismiss
InnovationQ will be updated on Sunday, Oct. 22, from 10am ET - noon. You may experience brief service interruptions during that time.
Browse Prior Art Database

NOISE REDUCTION TECHNIQUE FOR RECEIVE CVSD VOCODER

IP.com Disclosure Number: IPCOM000006334D
Original Publication Date: 1991-Dec-01
Included in the Prior Art Database: 2001-Dec-26
Document File: 1 page(s) / 79K

Publishing Venue

Motorola

Related People

Kenneth Fuchs: AUTHOR

Abstract

This invention applies to digital audio transmissions that use CVSD for analog to digital conversion. CVSD modulation produces a serial data stream. The data stream that is produced during silence is called "idle- pattern:' When there are slight disruptions in this idle- pattern the recovered audio will be noisy when it should be silent. This invention eliminates the noise that is heard when there should be silence. This is accomplished by letting the minimum value for the "Step-Size" go to zero in the CVSD demodulator.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 50% of the total text.

Page 1 of 1

0 M

MOTOROLA INC. Technical Developments Volume 14 December 1991

NOISE REDUCTION TECHNIQUE FOR RECEIVE CVSD VOCODER

by Kenneth Fuchs

   This invention applies to digital audio transmissions that use CVSD for analog to digital conversion. CVSD modulation produces a serial data stream. The data stream that is produced during silence is called "idle- pattern:' When there are slight disruptions in this idle- pattern the recovered audio will be noisy when it should be silent. This invention eliminates the noise that is heard when there should be silence. This is accomplished by letting the minimum value for the "Step-Size" go to zero in the CVSD demodulator.

  The following describes the operation of the CVSD demodulator: A receiving device will use CVSD to con- vert a serial bit stream that it receives into analog audio (see figure below). First, an "N-bit-rule" result is low- pass-filtered to produce the Step-Size (N is 3 or more). The N-bit-rule result is a digital value which is "high" if the last N serial bits are identical (all l's or all O's), and is "low" if not. The N-bit-rule result is low-pass-filtered by a filter which has a comer much lower than the data rate. The filter output is called the Step-Size. If the N-bit- rule-result is "high" for many bits in a row the Step-Size will rise to its maximum level and remain there. If the N-bit-rule-result is "low" for many bits in a row the Step-Size will fall to its minimum level and remain there.

ssr,a, bit Jtleam

  To produce analog audio, each Step-size is multi- plied by its corresponding serial bit to produce a new data stream. This new data stream is then low-pass-filtered by a filter which generally has a comer at about one fourth of the data rate. The output of the filter is the reconstructed audio. In some systems the (Step- Size)x(serial bit) stream is integrated before it is low-pass-filtered.

  When the analog audio is flat (no audio present), the serial bit stream is a constant l-O-l-O-l-O-. T...