Vocal’s Line / Network Echo Canceller

Figure 1: Line/Network Echo Cancellers remove audible echoes caused by electric signal reflections from a telephone hybrid circuit VOCAL’s Line/Network Echo Canceller is used in telecommunication applications to remove audible echoes caused primarily by electric signal reflections from the telephone hybrid circuit (a.k.a. telephone line transformer). Telephone hybrid circuits convert 4-wire interface into 2-wire interface in POTS/PSTN systems. Historically, the introduction of the 4-wire-to-2-wire interface was motivated by the need to reduce the cost of copper loops connecting the POTS telephone line facility (access network) to individual subscribers. Figure 1 illustrates a typical line echo canceller operating at the near-end of the voice connection. Signals travelling from the far end to the near end are reflected from the hybrid circuit, as a result of inadequate impedance matching between the telephone line and the hybrid terminals facing the telephone line. Although hybrid circuits are equipped with balance circuits that are selectable, ideal impedance matching can hardly be achieved due to line makeup/telephone set variations, which are not under telephone company control. These variations may occur even during the telephone call. Therefore, one of the requirements for the Line/Network Echo Canceller is that the adaptive filter, whose role is to rapidly identify the echo path (and specifically, the echo path impulse response), enters in the state of adaptation as soon as the voice channel between the near end and far end is open. Figure 2: Typical building blocks of a Line/Network Echo Canceller Typically, the echo path impulse response of the hybrid circuit alone is relatively short and seldom exceeds 8-12 milliseconds. However, additional signal delays, due to signal paths linking the trunk facility and the line facility, have to be accommodated. This is why often Line/Network Echo Cancellers offer echo span coverage as large as 128 milliseconds. Typical building-blocks of Line/Network Echo Canceller are illustrated in Figure 2. The Adaptive Filter is one of the most important blocks. The internal structure of the adaptive filter as well as the adaptation algorithm are critical from the viewpoint of the echo canceller adaptation speed and depth characteristics. Double-talk detector block controls the adaption mechanism: if the near-end signal (Sgen) is present, the double talk halts the filter adaptation and thus prevents the filter from divergence. The Nonlinear processor removes traces of residual echo from the error signal. The delay block (which often is optional) shifts in time the input signal to the adaptive filter in order to compensate signal delays between the echo canceller ports (Rout and Sin) and the ports of the line hybrid circuit. VOCAL’s Line/Network Echo Canceller offers reconfigurable echo path coverage of up to 128 ms. Features Fully compliant with ITU G.168-2015 RecommendationsRapid convergence and re-convergence ( for echo path changes)Double talk detectionNo divergence during double talkAdjustable echo tail length to 32 msec for full complianceAdjustable echo tail length to 128 msec with restricted complianceProper operation during facsimile and low speed (<9.6 kbit/s) voice-band data transmissionsDisables automatically in the presence of ANSamExcellent tone rejection and narrow band stabilityNon-linear processing (NLP) with Comfort Noise Generator (CNG)Tone detector and hold release logicDC Offset CorrectionBulk Echo Delay EstimatorUtilizes Normalized LMS (NLMS)Supports multiple channelsSupports user callable functionsEasily portable, re-entrant and re-locatable code Operational Characteristics Exceeds those set forth by the ITU-G.168 Recommendations Fast Convergence with NLP Disabled: LRin – LRES > 20 dB in 1 secondSteady State Convergence with NLP Disabled: LRin – LRES > 30 dBFast Convergence with NLP Enabled: LRin – LRET > 20 dB less than 1 secondSteady State Convergence with NLP Enabled: LRin – LRET > 55 dBFast Re-Convergence for Echo Path Changes: LRin – LRET > 20 dB less than 1 secondSteady-State Leak Rate: Increase Residual Echo < 10dBNo generation of echo of when echo path is open, ∞ ERLComfort Noise Generator tracks background noise level changesDTMF Transparency Design Constraints The standard VOCAL echo canceller operates on 16-bit signed input values, and returns 16-bit signed values.A variable internal excitation delay is available to provide full echo canceller tail length in the presence of known system latencies. Standards Compliance VOCAL’s LEC/NEC software solutions are designed to pass G.168-2015 requirements using any echo path models defined by G.168-2015 as well as customer models, provide they fit to the echo path coverage of the echo canceller using G.168-2015 testing. VOCAL’s line echo cancellation modules pass the standard ITU G.168 Test Suite: Test No. 2 – Convergence and Steady state residual and returned echo level testsTest No. 2A – Convergence and Re-convergence tests with NLP enabledTest No. 2B – Convergence and Re-convergence tests with NLP disabledTest No. 2C – Convergence test in the presence of background noiseTest No. 3 – Performance under conditions of double talkTest No. 3A – Double talk test with low cancelled-end levelsTest No. 3B – Double talk test with high cancelled-end levelsTest No. 3C – Double talk test under simulated conversationTest No. 4 – Leak rate testTest No. 5A – Infinite return loss convergence testTest No. 5B – Re-convergence during call transfer testTest No. 6 – Non-divergence on narrow-band signalsTest No. 7 – Stability testTest No. 9A – Comfort noise test with step change of background noise levelTest No. 9B – Comfort noise test with dynamic change of background noise levelTest No. 10 – Facsimile test during call establishment phaseTest No. 10A – Canceller operation on the calling station sideTest No. 10B – Canceller operation on the called station sideTest No. 14 – Performance with V.Series Low-speed Data ModemsTest No. 15 – PCM offset testTest No. 16 – DTMF Transparency Test Proprietary G.168 Line Echo Canceller Test Compliance As an experienced leader in this field, VOCAL has identified shortcomings in the G.168 standard and has implemented several additional test sets to guarantee world-class performance of our G.168 line echo canceller software in the harshest acoustic environments. All of our line echo cancellation modules must pass these VOCAL specific tests in addition to the standard ITU G.168 Test Suite, as well as undergo voice testing in our labs. VOCAL Test 2D – improved echo power testVOCAL Test 3D – double talk in the presence of tonesVOCAL Test 5D – improved line change detectionVOCAL Test 7D – improved narrow band rejectionVOCAL Test 9D – improved comfort noise trackingVOCAL Test 9E – comfort noise tracking during singletalkVOCAL Test 9F – comfort noise tracking during doubletalkVOCAL Test 9G – comfort noise transitionsVOCAL Test 21 – ANSam detection More Information LEC Subsystems Double-TalkNon-Linear Processing in Echo CancellationTone Detectors in Echo CancellersHigh Pass Filters in Echo Cancellers More Information on VOCAL’s categories of Echo Cancellation Solutions Acoustic Echo CancellationLine and Acoustic Echo Cancellation CombinedLine/Network Echo CancellerVoIP Echo CancellationEcho Cancellation in VoIP NetworksEcho in Voice over IP NetworksEcho Paths for Hands-Free TerminalsEcho Cancellation in Cellular NetworksWideband Acoustic Echo CancellationMultichannel Acoustic Echo CancellationNon-Linear AECLine and Acoustic Echo DiagnosticsListing of VOCAL’s Echo Cancellation literatureEcho Cancellation DesignLine echo canceller with bulk delay compensationFar-end line echo cancellerDigital network echo canceller Platforms VOCAL’s optimized Echo Cancellation software is available for the following platforms. Please contact us for specific Echo Cancellation supported platforms. Processors Operating Systems Texas Instruments – C6xx (TMS320C62x, TMS320C64x, TMS320C645x, TMS320C66x, TMS320C67x), DaVinci, OMAP, C5xx (TMS320C54x, TMS320C55x) Analog Devices – Blackfin, ADSP-21xx, TigerSHARC, SHARC PowerPC, PowerQUICC MIPS – MIPS32, MIPS64, MIPS4Kc ARM – ARM7, ARM9, ARM9E, ARM10E, ARM11, StrongARM, ARMCortex-A8/A9/A15/A3x/A5x/A7x, Cortex-M3/M4/M7/M33 Intel / AMD – x86, x64 (both 32 and 64 bit modes) Linux, uClinux, BSD, Unix Microsoft Windows ACM / RTC / CE / Mobile Apple iOS / iPhone / iPad & MacOS VOCAL LANsEND Google Android Green Hills Integrity Micrium μCOS Symbian Wind River VxWorks eCOS / eCOSPro