ADAPTIVE ECHO CANCELATION in .NET

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ADAPTIVE ECHO CANCELATION
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Table 10.2 Partial C code for acoustic echo cancelation
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if (farFlag == 1) // There is far-end speech { if ((nearFlag == 0) || (trainTime > 0)) // Receive mode { /* Receive mode operations */ if (trainTime > 0) // Counter is no expire yet { trainTime--; // Decrement the counter if (txGain > 0.25) txGain -= rampDown; // Ramp down farEndOut = (float)(txGain*errorAEC); // Attenuate by 12 dB } if (errorAECpowM<clipThres) // If ERLE > 18 dB { // Enable center clipper farEndOut = comfortNoise; // and inject comfort noise } else // If ERLE < 18 dB { if (txGain > 0.25)txGain -= rampDown; // Ramp down farEndOut = (float)(txGain*errorAEC); // Disable center clipper } // Attenuated by 12 dB if (farInPowM < 16000.) // Signal farEndIn is reasonable { /* Update AEC coefficients, otherwise skip adaptation*/ temp = (float)((mu*errorAEC) /(spkOutPowM+saveMargin)); // Normalize step size for (k=0; k<AECorder; ++k) { /* Leaky normalized LMS update */ AECcoef[k] = (float)(leaky*AECcoef[k] + temp*AECbuf[k]); } } } else // Double talk mode { /* Double-talk mode operation */ if (txGain > 0.5) txGain -= rampDown; // Ramp down if (txGain < 0.5) txGain += rampUp; // Ramp up farEndOut = (float)(txGain*errorAEC); // Attenuate 6 dB } } else // No far-end speech { // Transmit mode operation if (nearFlag == 1) { if (txGain < 1) txGain += rampUp; farEndOut = txGain*microphoneIn; // Full gain at transmit path } else // Idle mode operation { if (txGain > 0.5) txGain -= rampDown; // Ramp down if (txGain < 0.5) txGain += rampUp; // Ramp up farEndOut = (float)(txGain*microphoneIn); // Attenuate 6 dB } }
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EXPERIMENTS AND PROGRAM EXAMPLES
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2 0 0.5 1 1.5 2 2.5 3 (a) Far-end speech signal 3.5 4 4.5 5 104
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0 Echo from far-end speech Near-end speech 2 2.5 3 (b) Near-end mic input 3.5 Echo from far-end speech
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2 2.5 3 3.5 (c) Acoustic echo canceler output
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Figure 10.27 Experiment results of acoustic echo cancelation: (a) far-end speech signal; (b) near-end mic input; and (c) acoustic echo canceler output
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5. Using the knowledge learned from previous experiments, write an assembly program to replace the adaptive ltering function used by this experiment. 6. Convert the rest of the experiment to xed-point C implementation. Pay special attention on data type conversion and xed-point implementation for C55x processors.
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Table 10.3 Files
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ADAPTIVE ECHO CANCELATION
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File listing for experiment exp10.7.3_intrinsicAec Description C55x project le C55x linker command le Main program Initialization function Double-talk detection function Major module for LMS update and ltering NLP function Utility function of long division Header le Header le for using intrinsics Header le needed for intrinsics Data le of far-end signal Data le of near-end signal
intrinsic_aec.pjt intrinsic_aec.cmd fixPoint_leaky_lmsTest.c fixPoint_aec_init.c fixPoint_double_talk.c fixPoint_leaky_lms.c fixPoint_nlp.c utility.c fixPoint_leaky_lms.h gsm.h linkage.h rtfar.pcm rtmic.pcm
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10.7.3 Acoustic Echo Canceler Using C55x Intrinsics
This experiment shows the implementation of a xed-point acoustic echo canceler. We use the normalized LMS algorithm presented in 7. In addition, we add an NLP function to further attenuate the residue echoes. The les used for this experiment are listed in Table 10.3. Fixed-point C implementation of leaky NLMS algorithm using intrinsic functions has been discussed in Section 7.6.3. Using the same technique, the DTD can be implemented in xed-point C using the C55x intrinsics. Table 10.4 lists portion of the C program for far-end speech detection. In the program, the function aec_power_estimate( ) is used to estimate the signal power. The variable dt->nfFar is the noise oor of the far-end signal. If the signal power dt->nfFar is higher than the noise oor, the speech is detected.
Table 10.4 Partial xed-point C code for far-end signal detection
// Update noise floor estimate of receiving far-end signal // temp = |farEndIn|, estimate far-end signal power temp32a = L_deposit_h(lms->in); dt->farInPowS = aec_power_estimate( dt->farInPowS,temp32a,ALPHA_SHIFT_SHORT); if (dt->nfFar < dt->farInPowS) { // Onset of speech, slow update using long window dt->nfFar = aec_power_estimate( dt->nfFar,temp32a,ALPHA_SHIFT_MEDIUM); } else { dt->nfFar = aec_power_estimate( dt->nfFar,temp32a,ALPHA_SHIFT_SHORT); } // Threshold for far-end speech detector temp32b = L_mult(extract_h(dt->nfFar),VAD_POWER_THRESH); temp32b = L_add(temp32b,dt->nfFar); temp32b = L_add(temp32b,L_deposit_h(SAFE_MARGIN));