modules/audio_processing/aec/echo_cancellation.c

/*
 *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

/*
 * Contains the API functions for the AEC.
 */
#include "webrtc/modules/audio_processing/aec/include/echo_cancellation.h"

#include <math.h>
#ifdef WEBRTC_AEC_DEBUG_DUMP
#include <stdio.h>
#endif
#include <stdlib.h>
#include <string.h>

#include "webrtc/common_audio/signal_processing/include/signal_processing_library.h"
#include "webrtc/modules/audio_processing/aec/aec_core.h"
#include "webrtc/modules/audio_processing/aec/aec_resampler.h"
#include "webrtc/modules/audio_processing/aec/echo_cancellation_internal.h"
#include "webrtc/modules/audio_processing/utility/ring_buffer.h"
#include "webrtc/typedefs.h"

// Maximum length of resampled signal. Must be an integer multiple of frames
// (ceil(1/(1 + MIN_SKEW)*2) + 1)*FRAME_LEN
// The factor of 2 handles wb, and the + 1 is as a safety margin
// TODO(bjornv): Replace with kResamplerBufferSize
#define MAX_RESAMP_LEN (5 * FRAME_LEN)

static const int kMaxBufSizeStart = 62;  // In partitions
static const int sampMsNb = 8; // samples per ms in nb
static const int initCheck = 42;

#ifdef WEBRTC_AEC_DEBUG_DUMP
int webrtc_aec_instance_count = 0;
#endif

// Estimates delay to set the position of the far-end buffer read pointer
// (controlled by knownDelay)
static int EstBufDelay(aecpc_t *aecInst);

int32_t WebRtcAec_Create(void **aecInst)
{
    aecpc_t *aecpc;
    if (aecInst == NULL) {
        return -1;
    }

    aecpc = malloc(sizeof(aecpc_t));
    *aecInst = aecpc;
    if (aecpc == NULL) {
        return -1;
    }

    if (WebRtcAec_CreateAec(&aecpc->aec) == -1) {
        WebRtcAec_Free(aecpc);
        aecpc = NULL;
        return -1;
    }

    if (WebRtcAec_CreateResampler(&aecpc->resampler) == -1) {
        WebRtcAec_Free(aecpc);
        aecpc = NULL;
        return -1;
    }
    // Create far-end pre-buffer. The buffer size has to be large enough for
    // largest possible drift compensation (kResamplerBufferSize) + "almost" an
    // FFT buffer (PART_LEN2 - 1).
    aecpc->far_pre_buf = WebRtc_CreateBuffer(PART_LEN2 + kResamplerBufferSize,
                                             sizeof(float));
    if (!aecpc->far_pre_buf) {
        WebRtcAec_Free(aecpc);
        aecpc = NULL;
        return -1;
    }

    aecpc->initFlag = 0;
    aecpc->lastError = 0;

#ifdef WEBRTC_AEC_DEBUG_DUMP
    aecpc->far_pre_buf_s16 = WebRtc_CreateBuffer(
        PART_LEN2 + kResamplerBufferSize, sizeof(int16_t));
    if (!aecpc->far_pre_buf_s16) {
        WebRtcAec_Free(aecpc);
        aecpc = NULL;
        return -1;
    }
    {
      char filename[64];
      sprintf(filename, "aec_buf%d.dat", webrtc_aec_instance_count);
      aecpc->bufFile = fopen(filename, "wb");
      sprintf(filename, "aec_skew%d.dat", webrtc_aec_instance_count);
      aecpc->skewFile = fopen(filename, "wb");
      sprintf(filename, "aec_delay%d.dat", webrtc_aec_instance_count);
      aecpc->delayFile = fopen(filename, "wb");
      webrtc_aec_instance_count++;
    }
#endif

    return 0;
}

int32_t WebRtcAec_Free(void *aecInst)
{
    aecpc_t *aecpc = aecInst;

    if (aecpc == NULL) {
        return -1;
    }

    WebRtc_FreeBuffer(aecpc->far_pre_buf);

#ifdef WEBRTC_AEC_DEBUG_DUMP
    WebRtc_FreeBuffer(aecpc->far_pre_buf_s16);
    fclose(aecpc->bufFile);
    fclose(aecpc->skewFile);
    fclose(aecpc->delayFile);
#endif

    WebRtcAec_FreeAec(aecpc->aec);
    WebRtcAec_FreeResampler(aecpc->resampler);
    free(aecpc);

    return 0;
}

int32_t WebRtcAec_Init(void *aecInst, int32_t sampFreq, int32_t scSampFreq)
{
    aecpc_t *aecpc = aecInst;
    AecConfig aecConfig;

    if (aecpc == NULL) {
        return -1;
    }

    if (sampFreq != 8000 && sampFreq != 16000  && sampFreq != 32000) {
        aecpc->lastError = AEC_BAD_PARAMETER_ERROR;
        return -1;
    }
    aecpc->sampFreq = sampFreq;

    if (scSampFreq < 1 || scSampFreq > 96000) {
        aecpc->lastError = AEC_BAD_PARAMETER_ERROR;
        return -1;
    }
    aecpc->scSampFreq = scSampFreq;

    // Initialize echo canceller core
    if (WebRtcAec_InitAec(aecpc->aec, aecpc->sampFreq) == -1) {
        aecpc->lastError = AEC_UNSPECIFIED_ERROR;
        return -1;
    }

    if (WebRtcAec_InitResampler(aecpc->resampler, aecpc->scSampFreq) == -1) {
        aecpc->lastError = AEC_UNSPECIFIED_ERROR;
        return -1;
    }

    if (WebRtc_InitBuffer(aecpc->far_pre_buf) == -1) {
        aecpc->lastError = AEC_UNSPECIFIED_ERROR;
        return -1;
    }
    WebRtc_MoveReadPtr(aecpc->far_pre_buf, -PART_LEN);  // Start overlap.

    aecpc->initFlag = initCheck;  // indicates that initialization has been done

    if (aecpc->sampFreq == 32000) {
        aecpc->splitSampFreq = 16000;
    }
    else {
        aecpc->splitSampFreq = sampFreq;
    }

    aecpc->skewFrCtr = 0;
    aecpc->activity = 0;

    aecpc->delayCtr = 0;

    aecpc->sum = 0;
    aecpc->counter = 0;
    aecpc->checkBuffSize = 1;
    aecpc->firstVal = 0;

    aecpc->ECstartup = 1;
    aecpc->bufSizeStart = 0;
    aecpc->checkBufSizeCtr = 0;
    aecpc->filtDelay = 0;
    aecpc->timeForDelayChange = 0;
    aecpc->knownDelay = 0;
    aecpc->lastDelayDiff = 0;

    aecpc->skew = 0;
    aecpc->resample = kAecFalse;
    aecpc->highSkewCtr = 0;
    aecpc->sampFactor = (aecpc->scSampFreq * 1.0f) / aecpc->splitSampFreq;

    // Sampling frequency multiplier (SWB is processed as 160 frame size).
    aecpc->rate_factor = aecpc->splitSampFreq / 8000;

    // Default settings.
    aecConfig.nlpMode = kAecNlpModerate;
    aecConfig.skewMode = kAecFalse;
    aecConfig.metricsMode = kAecFalse;
    aecConfig.delay_logging = kAecFalse;

    if (WebRtcAec_set_config(aecpc, aecConfig) == -1) {
        aecpc->lastError = AEC_UNSPECIFIED_ERROR;
        return -1;
    }

#ifdef WEBRTC_AEC_DEBUG_DUMP
    if (WebRtc_InitBuffer(aecpc->far_pre_buf_s16) == -1) {
        aecpc->lastError = AEC_UNSPECIFIED_ERROR;
        return -1;
    }
    WebRtc_MoveReadPtr(aecpc->far_pre_buf_s16, -PART_LEN);  // Start overlap.
#endif

    return 0;
}

// only buffer L band for farend
int32_t WebRtcAec_BufferFarend(void *aecInst, const int16_t *farend,
                               int16_t nrOfSamples)
{
    aecpc_t *aecpc = aecInst;
    int32_t retVal = 0;
    int newNrOfSamples = (int) nrOfSamples;
    short newFarend[MAX_RESAMP_LEN];
    const int16_t* farend_ptr = farend;
    float tmp_farend[MAX_RESAMP_LEN];
    const float* farend_float = tmp_farend;
    float skew;
    int i = 0;

    if (aecpc == NULL) {
        return -1;
    }

    if (farend == NULL) {
        aecpc->lastError = AEC_NULL_POINTER_ERROR;
        return -1;
    }

    if (aecpc->initFlag != initCheck) {
        aecpc->lastError = AEC_UNINITIALIZED_ERROR;
        return -1;
    }

    // number of samples == 160 for SWB input
    if (nrOfSamples != 80 && nrOfSamples != 160) {
        aecpc->lastError = AEC_BAD_PARAMETER_ERROR;
        return -1;
    }

    skew = aecpc->skew;

    if (aecpc->skewMode == kAecTrue && aecpc->resample == kAecTrue) {
        // Resample and get a new number of samples
        WebRtcAec_ResampleLinear(aecpc->resampler, farend, nrOfSamples, skew,
                                 newFarend, &newNrOfSamples);
        farend_ptr = (const int16_t*) newFarend;
    }

    WebRtcAec_SetSystemDelay(aecpc->aec, WebRtcAec_system_delay(aecpc->aec) +
                             newNrOfSamples);

#ifdef WEBRTC_AEC_DEBUG_DUMP
    WebRtc_WriteBuffer(aecpc->far_pre_buf_s16, farend_ptr,
                       (size_t) newNrOfSamples);
#endif
    // Cast to float and write the time-domain data to |far_pre_buf|.
    for (i = 0; i < newNrOfSamples; i++) {
      tmp_farend[i] = (float) farend_ptr[i];
    }
    WebRtc_WriteBuffer(aecpc->far_pre_buf, farend_float,
                       (size_t) newNrOfSamples);

    // Transform to frequency domain if we have enough data.
    while (WebRtc_available_read(aecpc->far_pre_buf) >= PART_LEN2) {
      // We have enough data to pass to the FFT, hence read PART_LEN2 samples.
      WebRtc_ReadBuffer(aecpc->far_pre_buf, (void**) &farend_float, tmp_farend,
                        PART_LEN2);

      WebRtcAec_BufferFarendPartition(aecpc->aec, farend_float);

      // Rewind |far_pre_buf| PART_LEN samples for overlap before continuing.
      WebRtc_MoveReadPtr(aecpc->far_pre_buf, -PART_LEN);
#ifdef WEBRTC_AEC_DEBUG_DUMP
      WebRtc_ReadBuffer(aecpc->far_pre_buf_s16, (void**) &farend_ptr, newFarend,
                        PART_LEN2);
      WebRtc_WriteBuffer(WebRtcAec_far_time_buf(aecpc->aec),
                         &farend_ptr[PART_LEN], 1);
      WebRtc_MoveReadPtr(aecpc->far_pre_buf_s16, -PART_LEN);
#endif
    }

    return retVal;
}

int32_t WebRtcAec_Process(void *aecInst, const int16_t *nearend,
                          const int16_t *nearendH, int16_t *out, int16_t *outH,
                          int16_t nrOfSamples, int16_t msInSndCardBuf,
                          int32_t skew)
{
    aecpc_t *aecpc = aecInst;
    int32_t retVal = 0;
    short i;
    short nBlocks10ms;
    short nFrames;
    // Limit resampling to doubling/halving of signal
    const float minSkewEst = -0.5f;
    const float maxSkewEst = 1.0f;

    if (aecpc == NULL) {
        return -1;
    }

    if (nearend == NULL) {
        aecpc->lastError = AEC_NULL_POINTER_ERROR;
        return -1;
    }

    if (out == NULL) {
        aecpc->lastError = AEC_NULL_POINTER_ERROR;
        return -1;
    }

    if (aecpc->initFlag != initCheck) {
        aecpc->lastError = AEC_UNINITIALIZED_ERROR;
        return -1;
    }

    // number of samples == 160 for SWB input
    if (nrOfSamples != 80 && nrOfSamples != 160) {
        aecpc->lastError = AEC_BAD_PARAMETER_ERROR;
        return -1;
    }

    // Check for valid pointers based on sampling rate
    if (aecpc->sampFreq == 32000 && nearendH == NULL) {
       aecpc->lastError = AEC_NULL_POINTER_ERROR;
       return -1;
    }

    if (msInSndCardBuf < 0) {
        msInSndCardBuf = 0;
        aecpc->lastError = AEC_BAD_PARAMETER_WARNING;
        retVal = -1;
    }
    else if (msInSndCardBuf > 500) {
        msInSndCardBuf = 500;
        aecpc->lastError = AEC_BAD_PARAMETER_WARNING;
        retVal = -1;
    }
    // TODO(andrew): we need to investigate if this +10 is really wanted.
    msInSndCardBuf += 10;
    aecpc->msInSndCardBuf = msInSndCardBuf;

    if (aecpc->skewMode == kAecTrue) {
        if (aecpc->skewFrCtr < 25) {
            aecpc->skewFrCtr++;
        }
        else {
            retVal = WebRtcAec_GetSkew(aecpc->resampler, skew, &aecpc->skew);
            if (retVal == -1) {
                aecpc->skew = 0;
                aecpc->lastError = AEC_BAD_PARAMETER_WARNING;
            }

            aecpc->skew /= aecpc->sampFactor*nrOfSamples;

            if (aecpc->skew < 1.0e-3 && aecpc->skew > -1.0e-3) {
                aecpc->resample = kAecFalse;
            }
            else {
                aecpc->resample = kAecTrue;
            }

            if (aecpc->skew < minSkewEst) {
                aecpc->skew = minSkewEst;
            }
            else if (aecpc->skew > maxSkewEst) {
                aecpc->skew = maxSkewEst;
            }

#ifdef WEBRTC_AEC_DEBUG_DUMP
            (void)fwrite(&aecpc->skew, sizeof(aecpc->skew), 1, aecpc->skewFile);
#endif
        }
    }

    nFrames = nrOfSamples / FRAME_LEN;
    nBlocks10ms = nFrames / aecpc->rate_factor;

    if (aecpc->ECstartup) {
        if (nearend != out) {
            // Only needed if they don't already point to the same place.
            memcpy(out, nearend, sizeof(short) * nrOfSamples);
        }

        // The AEC is in the start up mode
        // AEC is disabled until the system delay is OK

        // Mechanism to ensure that the system delay is reasonably stable.
        if (aecpc->checkBuffSize) {
            aecpc->checkBufSizeCtr++;
            // Before we fill up the far-end buffer we require the system delay
            // to be stable (+/-8 ms) compared to the first value. This
            // comparison is made during the following 6 consecutive 10 ms
            // blocks. If it seems to be stable then we start to fill up the
            // far-end buffer.
            if (aecpc->counter == 0) {
                aecpc->firstVal = aecpc->msInSndCardBuf;
                aecpc->sum = 0;
            }

            if (abs(aecpc->firstVal - aecpc->msInSndCardBuf) <
                WEBRTC_SPL_MAX(0.2 * aecpc->msInSndCardBuf, sampMsNb)) {
                aecpc->sum += aecpc->msInSndCardBuf;
                aecpc->counter++;
            }
            else {
                aecpc->counter = 0;
            }

            if (aecpc->counter * nBlocks10ms >= 6) {
                // The far-end buffer size is determined in partitions of
                // PART_LEN samples. Use 75% of the average value of the system
                // delay as buffer size to start with.
                aecpc->bufSizeStart = WEBRTC_SPL_MIN((3 * aecpc->sum *
                  aecpc->rate_factor * 8) / (4 * aecpc->counter * PART_LEN),
                  kMaxBufSizeStart);
                // Buffer size has now been determined.
                aecpc->checkBuffSize = 0;
            }

            if (aecpc->checkBufSizeCtr * nBlocks10ms > 50) {
                // For really bad systems, don't disable the echo canceller for
                // more than 0.5 sec.
                aecpc->bufSizeStart = WEBRTC_SPL_MIN((aecpc->msInSndCardBuf *
                    aecpc->rate_factor * 3) / 40, kMaxBufSizeStart);
                aecpc->checkBuffSize = 0;
            }
        }

        // If |checkBuffSize| changed in the if-statement above.
        if (!aecpc->checkBuffSize) {
            // The system delay is now reasonably stable (or has been unstable
            // for too long). When the far-end buffer is filled with
            // approximately the same amount of data as reported by the system
            // we end the startup phase.
            int overhead_elements =
                WebRtcAec_system_delay(aecpc->aec) / PART_LEN -
                aecpc->bufSizeStart;
            if (overhead_elements == 0) {
                // Enable the AEC
                aecpc->ECstartup = 0;
            } else if (overhead_elements > 0) {
                // TODO(bjornv): Do we need a check on how much we actually
                // moved the read pointer? It should always be possible to move
                // the pointer |overhead_elements| since we have only added data
                // to the buffer and no delay compensation nor AEC processing
                // has been done.
                WebRtcAec_MoveFarReadPtr(aecpc->aec, overhead_elements);

                // Enable the AEC
                aecpc->ECstartup = 0;
            }
        }
    } else {
        // AEC is enabled.

        EstBufDelay(aecpc);

        // Note that 1 frame is supported for NB and 2 frames for WB.
        for (i = 0; i < nFrames; i++) {
            // Call the AEC.
            WebRtcAec_ProcessFrame(aecpc->aec,
                                   &nearend[FRAME_LEN * i],
                                   &nearendH[FRAME_LEN * i],
                                   aecpc->knownDelay,
                                   &out[FRAME_LEN * i],
                                   &outH[FRAME_LEN * i]);
            // TODO(bjornv): Re-structure such that we don't have to pass
            // |aecpc->knownDelay| as input. Change name to something like
            // |system_buffer_diff|.
        }
    }

#ifdef WEBRTC_AEC_DEBUG_DUMP
    {
        int16_t far_buf_size_ms = (int16_t)(WebRtcAec_system_delay(aecpc->aec) /
            (sampMsNb * aecpc->rate_factor));
        (void)fwrite(&far_buf_size_ms, 2, 1, aecpc->bufFile);
        (void)fwrite(&aecpc->knownDelay, sizeof(aecpc->knownDelay), 1,
                     aecpc->delayFile);
    }
#endif

    return retVal;
}

int WebRtcAec_set_config(void* handle, AecConfig config) {
  aecpc_t* self = (aecpc_t*)handle;

  if (handle == NULL ) {
    return -1;
  }

  if (self->initFlag != initCheck) {
    self->lastError = AEC_UNINITIALIZED_ERROR;
    return -1;
  }

  if (config.skewMode != kAecFalse && config.skewMode != kAecTrue) {
    self->lastError = AEC_BAD_PARAMETER_ERROR;
    return -1;
  }
  self->skewMode = config.skewMode;

  if (config.nlpMode != kAecNlpConservative && config.nlpMode != kAecNlpModerate
      && config.nlpMode != kAecNlpAggressive) {
    self->lastError = AEC_BAD_PARAMETER_ERROR;
    return -1;
  }

  if (config.metricsMode != kAecFalse && config.metricsMode != kAecTrue) {
    self->lastError = AEC_BAD_PARAMETER_ERROR;
    return -1;
  }

  if (config.delay_logging != kAecFalse && config.delay_logging != kAecTrue) {
    self->lastError = AEC_BAD_PARAMETER_ERROR;
    return -1;
  }

  WebRtcAec_SetConfigCore(self->aec, config.nlpMode, config.metricsMode,
                          config.delay_logging);
  return 0;
}

int WebRtcAec_get_echo_status(void* handle, int* status) {
  aecpc_t* self = (aecpc_t*)handle;

  if (handle == NULL ) {
    return -1;
  }
  if (status == NULL ) {
    self->lastError = AEC_NULL_POINTER_ERROR;
    return -1;
  }
  if (self->initFlag != initCheck) {
    self->lastError = AEC_UNINITIALIZED_ERROR;
    return -1;
  }

  *status = WebRtcAec_echo_state(self->aec);

  return 0;
}

int WebRtcAec_GetMetrics(void* handle, AecMetrics* metrics) {
  const float kUpWeight = 0.7f;
  float dtmp;
  int stmp;
  aecpc_t* self = (aecpc_t*)handle;
  Stats erl;
  Stats erle;
  Stats a_nlp;

  if (handle == NULL ) {
    return -1;
  }
  if (metrics == NULL ) {
    self->lastError = AEC_NULL_POINTER_ERROR;
    return -1;
  }
  if (self->initFlag != initCheck) {
    self->lastError = AEC_UNINITIALIZED_ERROR;
    return -1;
  }

  WebRtcAec_GetEchoStats(self->aec, &erl, &erle, &a_nlp);

  // ERL
  metrics->erl.instant = (int) erl.instant;

  if ((erl.himean > kOffsetLevel) && (erl.average > kOffsetLevel)) {
    // Use a mix between regular average and upper part average.
    dtmp = kUpWeight * erl.himean + (1 - kUpWeight) * erl.average;
    metrics->erl.average = (int) dtmp;
  } else {
    metrics->erl.average = kOffsetLevel;
  }

  metrics->erl.max = (int) erl.max;

  if (erl.min < (kOffsetLevel * (-1))) {
    metrics->erl.min = (int) erl.min;
  } else {
    metrics->erl.min = kOffsetLevel;
  }

  // ERLE
  metrics->erle.instant = (int) erle.instant;

  if ((erle.himean > kOffsetLevel) && (erle.average > kOffsetLevel)) {
    // Use a mix between regular average and upper part average.
    dtmp = kUpWeight * erle.himean + (1 - kUpWeight) * erle.average;
    metrics->erle.average = (int) dtmp;
  } else {
    metrics->erle.average = kOffsetLevel;
  }

  metrics->erle.max = (int) erle.max;

  if (erle.min < (kOffsetLevel * (-1))) {
    metrics->erle.min = (int) erle.min;
  } else {
    metrics->erle.min = kOffsetLevel;
  }

  // RERL
  if ((metrics->erl.average > kOffsetLevel)
      && (metrics->erle.average > kOffsetLevel)) {
    stmp = metrics->erl.average + metrics->erle.average;
  } else {
    stmp = kOffsetLevel;
  }
  metrics->rerl.average = stmp;

  // No other statistics needed, but returned for completeness.
  metrics->rerl.instant = stmp;
  metrics->rerl.max = stmp;
  metrics->rerl.min = stmp;

  // A_NLP
  metrics->aNlp.instant = (int) a_nlp.instant;

  if ((a_nlp.himean > kOffsetLevel) && (a_nlp.average > kOffsetLevel)) {
    // Use a mix between regular average and upper part average.
    dtmp = kUpWeight * a_nlp.himean + (1 - kUpWeight) * a_nlp.average;
    metrics->aNlp.average = (int) dtmp;
  } else {
    metrics->aNlp.average = kOffsetLevel;
  }

  metrics->aNlp.max = (int) a_nlp.max;

  if (a_nlp.min < (kOffsetLevel * (-1))) {
    metrics->aNlp.min = (int) a_nlp.min;
  } else {
    metrics->aNlp.min = kOffsetLevel;
  }

  return 0;
}

int WebRtcAec_GetDelayMetrics(void* handle, int* median, int* std) {
  aecpc_t* self = handle;

  if (handle == NULL) {
    return -1;
  }
  if (median == NULL) {
    self->lastError = AEC_NULL_POINTER_ERROR;
    return -1;
  }
  if (std == NULL) {
    self->lastError = AEC_NULL_POINTER_ERROR;
    return -1;
  }
  if (self->initFlag != initCheck) {
    self->lastError = AEC_UNINITIALIZED_ERROR;
    return -1;
  }
  if (WebRtcAec_GetDelayMetricsCore(self->aec, median, std) == -1) {
    // Logging disabled.
    self->lastError = AEC_UNSUPPORTED_FUNCTION_ERROR;
    return -1;
  }

  return 0;
}

int32_t WebRtcAec_get_error_code(void *aecInst)
{
    aecpc_t *aecpc = aecInst;

    if (aecpc == NULL) {
        return -1;
    }

    return aecpc->lastError;
}

AecCore* WebRtcAec_aec_core(void* handle) {
  if (!handle) {
    return NULL;
  }
  return ((aecpc_t*) handle)->aec;
}

static int EstBufDelay(aecpc_t* aecpc) {
  int nSampSndCard = aecpc->msInSndCardBuf * sampMsNb * aecpc->rate_factor;
  int current_delay = nSampSndCard - WebRtcAec_system_delay(aecpc->aec);
  int delay_difference = 0;

  // Before we proceed with the delay estimate filtering we:
  // 1) Compensate for the frame that will be read.
  // 2) Compensate for drift resampling.
  // 3) Compensate for non-causality if needed, since the estimated delay can't
  //    be negative.

  // 1) Compensating for the frame(s) that will be read/processed.
  current_delay += FRAME_LEN * aecpc->rate_factor;

  // 2) Account for resampling frame delay.
  if (aecpc->skewMode == kAecTrue && aecpc->resample == kAecTrue) {
    current_delay -= kResamplingDelay;
  }

  // 3) Compensate for non-causality, if needed, by flushing one block.
  if (current_delay < PART_LEN) {
    current_delay += WebRtcAec_MoveFarReadPtr(aecpc->aec, 1) * PART_LEN;
  }

  aecpc->filtDelay = WEBRTC_SPL_MAX(0, (short) (0.8 * aecpc->filtDelay +
          0.2 * current_delay));

  delay_difference = aecpc->filtDelay - aecpc->knownDelay;
  if (delay_difference > 224) {
    if (aecpc->lastDelayDiff < 96) {
      aecpc->timeForDelayChange = 0;
    } else {
      aecpc->timeForDelayChange++;
    }
  } else if (delay_difference < 96 && aecpc->knownDelay > 0) {
    if (aecpc->lastDelayDiff > 224) {
      aecpc->timeForDelayChange = 0;
    } else {
      aecpc->timeForDelayChange++;
    }
  } else {
    aecpc->timeForDelayChange = 0;
  }
  aecpc->lastDelayDiff = delay_difference;

  if (aecpc->timeForDelayChange > 25) {
    aecpc->knownDelay = WEBRTC_SPL_MAX((int) aecpc->filtDelay - 160, 0);
  }

  return 0;
}