Browse Prior Art Database

LOW LEVEL PSTN NOISE SUPPRESSION BY QUASI-COMFORT NOISE INSERTION

IP.com Disclosure Number: IPCOM000008717D
Original Publication Date: 1998-Jun-01
Included in the Prior Art Database: 2002-Jul-05
Document File: 3 page(s) / 153K

Publishing Venue

Motorola

Related People

Michael Loos: AUTHOR

Abstract

This publication describes a method of sup- pressing the low level noise present in the outbound audio path of a code excited linear prediction (CELP) type based digital cellular system during a subscriber to landline call. The method is a hybrid between a "straight" CELP encode and an encode using voice activity detection based comfort noise insertion. These VADKN approaches are used when transmission bandwidth is desired to be reduced during non-speech segments, while a straight encode is typically used when bandwidth is not constrained. The straight CELP encode can result in low level system (PSTN or other) back- ground noise being emphasized which, with volume amplification by the subscriber unit (SU), can result in an objectionable user perceived noise.

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MOTOROLA Technical Developments

8

LOW LEVEL PSTN NOISE SUPPRESSION BY QUASI-COMFORT NOISE INSERTION

by Michael Loos

OVERVIEW

  This publication describes a method of sup- pressing the low level noise present in the outbound audio path of a code excited linear prediction (CELP) type based digital cellular system during a subscriber to landline call. The method is a hybrid between a "straight" CELP encode and an encode using voice activity detection based comfort noise insertion. These VADKN approaches are used when transmission bandwidth is desired to be reduced during non-speech segments, while a straight encode is typically used when bandwidth is not constrained. The straight CELP encode can result in low level system (PSTN or other) back- ground noise being emphasized which, with volume amplification by the subscriber unit (SU), can result in an objectionable user perceived noise.

  The basic approach used is to detect low noise periods and to replace the output of the CELP encode with a special set of CELP parameters which, when decoded by the SU, results in "smooth" synthetic background noise. This approach does not use the full VAD/CN scheme which can be employed as many of those approaches lacks the ability to handle the low-level background noise satisfactorily. Rather, it mimics the behavior of a comfort noise system by selecting CELP

parameters (LPC codes, excitation codes, and gain codes) which produce an output resembling the out- put from a CN decode. By using the CELP encode parameters directly the method achieves a high degree of control over the synthesized background noise spectrum and, more significantly, energy.

  There are a number of advantages for the proposed method versus either the normal bit exact CELP encode or the encode with VAD/CN. These include:

. Elimination of objectionable "crackly" low

level noise.

* Precise control of background noise energy.

* Low memory usage requirement.

This approach is termed quasi-comfort noise and abbreviated QCN.

ALGORITHM

  The algorithm will be detailed in the sections below. Its description is based upon a generalized CELP coder with the following general parameters: frame energy, frame LPC codes, subframe excita- tion codes, and subframe excitation gains.

INTRODUCTION

A basic flow chart for the approach is given below. The main blocks of the algorithm are:

* Low noise decision-This involves making a decision as to whether or not the input is the low background noise case and, if so, results in the QCN parameter calculation and substitution.

* Subframe gain update (or initialization) and quan- tization-This section updates the gain histomry and generates a QCN gain code for substitution.

. LPC update (or initialization) and quantization- This section updates the LPC history and generates the LPC codes for substitution.

. Excitation code randomization-This section selects random excitation code words for substi- tution.

These individual steps will be detailed further on the nex...