1     Scope
 2     References
 3     Terms and definitions
        3.1     acoustic echo:
        3.2     cancelled end:
        3.3     combined loss (ACOM):
        3.4     comfort noise:
        3.5     composite echo:
        3.6     convergence:
        3.7     convergence time:
        3.8     echo canceller:
        3.9     echo path:
       3.10     echo path capacity (Δ):
       3.11     echo path delay (td):
       3.12     echo return loss (ERL) (AECHO):
       3.13     echo return loss enhancement (ERLE) (ACANC):
       3.14     electric echo:
       3.15     H register:
       3.16     leak time:
       3.17     non-cancelled end:
       3.18     nonlinear processor (NLP):
       3.19     nonlinear processing loss (ANLP ):
       3.20     pure delay (tr):
       3.21     residual echo level (LRES):
       3.22     returned echo level (LRET):
       3.23     open echo path:
 4     Abbreviations
 5     Test signals
 6     Characteristics of echo cancellers
        6.1     General
        6.2     Purpose, operation and environment
        6.3     External enabling/disabling
        6.4     Tests and requirements for performance with input signals applied to the send and receive paths
                  6.4.1     Transmission performance
                  6.4.2     Echo canceller performance
                  6.4.3     Nonlinear echo paths
 7     Characteristics of an echo canceller tone disabler
        7.1     General
        7.2     Detector characteristics
        7.3     Guardband characteristics
        7.4     Holding-band characteristics
        7.5     Operate time
        7.6     False operation due to speech currents
        7.7     False operation due to data signals
        7.8     Release time
        7.9     Other considerations
 8     NLP's for use in echo cancellers
        8.1     Scope
        8.2     General principles and guidelines
                  8.2.1     Function
                  8.2.2     Suppression threshold
                  8.2.3     Control of NLP activation
                  8.2.4     Frequency limits of control paths
                  8.2.5     Signal attenuation below threshold level
                  8.2.6     Testing of NLP's
Annex A - Description of an echo canceller reference tone disabler
        A.1     General
        A.2     Disabler characteristics
                  A.2.1     Tone detection
                  A.2.2     Phase reversal detection
        A.3     Guardband characteristics
        A.4     Holding-band characteristics
        A.5     Operate time
        A.6     False operation due to speech currents
        A.7     False operation due to data signals
        A.8     Release time
Annex B - Description of a reference NLP
        B.1     General
        B.2     Suppression threshold (TSUP)
        B.3     Static characteristics of activation control
        B.4     Dynamic characteristics of activation control
        B.5     Frequency limits of control paths
        B.6     Testing
Annex C - Composite Source Signals for Testing of Speech Echo Cancellers: Signal, Description and Analysis
        C.1     Introduction
        C.2     Composite Source Signal - General Considerations
                  C.2.1     General Description of the Different Sequences
                  C.2.2     Calculation and Analysis Using a Composite Source Signal
        C.3     Bandlimited Composite Source Signal With Speech Like Power Density Spectrum - Practical Realization for Measurements of Echo Cancellers
                  C.3.1     Composite Source Signal for single talk
                  C.3.2     Bandlimited Composite Source Signal to simulate double talk
        C.4     Appropriate Analyses to Determine Convergence Characteristics of Speech Echo Cancellers Using the Composite Source Signal
                  C.4.1     Calculation in the Frequency Domain
                  C.4.2     Calculation in the Time Domain
                  C.4.3     Level Calculations According to the Active Speech Level P.56
Annex D - Echo-Path Models for Testing of Speech Echo Cancellers
        D.1     Introduction
        D.2     Echo-Path Models from Network Hybrid Simulator
        D.3     Echo-path models measured from telephone networks in North America
Appendix I - Guidance for application of echo cancellers
        I.1     Scope
        I.2     Echo control in the PSTN
                  I.2.1     PSTN Transmission Planning
                  I.2.2     Echo Suppressors
                  I.2.3     Echo Cancellers
                  I.2.4     Responsibilities of Modem Manufacturers and End Users
        I.3     Application Rules and Operational Constraints
                  I.3.1     Public Network Transmission Planning
                  I.3.2     Delay Considerations
                  I.3.3     Provisioning of the Echo Path Capacity and Echo Path characteristics
                  I.3.4     End User/Manufacturer/Private Network Transmission Planning
        I.4     Effect of Cancellers on Voice and Data services
                  I.4.1     Interaction with Voice-band Data
                  I.4.2     Interaction of Echo Control with Facsimile Transmission
        I.5     High-Level Speech
                  I.5.1     Introduction
                  I.5.2     Double Talk Detection and Activity Detection
                  I.5.3     Effects of low bit rate coders
                  I.5.4     Effects of a Non-Linear Echo Path
                  I.5.5     Guidelines for Rout usage in Echo-Cancellers
        I.6     Network and Service Evolutionary Considerations
                  I.6.1     Bit Transparency of Echo Cancellers
                  I.6.2     Non-Linearities and time variant Effects in the Echo path
                  I.6.3     Voice Compression Between Tandem Cancellers
                  I.6.4     Tandeming of Echo Cancellers
                  I.6.5     Convergence Speed
                  I.6.6     Acoustic Echo Control and Environments
                  I.6.7     New Circuit Switched Service
                  I.6.8     Comfort Noise
        I.7     Special DCME/PCME Networking Considerations
                  I.7.1     Detailed Interaction
                  I.7.2     Possible Solutions
        I.8     Considerations regarding echo canceller performance during double talk
                  I.8.1     Introduction
                  I.8.2     Double talk parameters
                  I.8.3     Analysis of Technical Parameters that Influences Performance Under Double Talk Conditions
                  I.8.4     Conducting the Double Talk Tests 3A and 3B Without Inhibiting the Adaptation
                  I.8.5     Subjective and Objective Echo Canceller Testing
        I.9     Guidelines on the use of parameters for testing echo cancellers
Appendix II - Measurement Methods for Characteristics of Echo Paths and an example for  the North American Network
       II.1     Introduction
       II.2     Measurement Procedure
       II.3     Test Signal
       II.4     Computation of Impulse Response
       II.5     Analysis of Echo-Path Characteristics
                 II.5.1     Dispersion Time
                 II.5.2     Magnitude Response of Echo Path
                 II.5.3     Echo-Path Characteristics from Measurements in North America
       II.6     Conclusions
Appendix III - Multiple Tail Circuits

ITU-T Recommendation G.168