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diff --git a/third_party/libwebrtc/modules/audio_processing/test/py_quality_assessment/quality_assessment/input_mixer.py b/third_party/libwebrtc/modules/audio_processing/test/py_quality_assessment/quality_assessment/input_mixer.py
new file mode 100644
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+++ b/third_party/libwebrtc/modules/audio_processing/test/py_quality_assessment/quality_assessment/input_mixer.py
@@ -0,0 +1,97 @@
+# Copyright (c) 2017 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.
+"""Input mixer module.
+"""
+
+import logging
+import os
+
+from . import exceptions
+from . import signal_processing
+
+
+class ApmInputMixer(object):
+    """Class to mix a set of audio segments down to the APM input."""
+
+    _HARD_CLIPPING_LOG_MSG = 'hard clipping detected in the  mixed signal'
+
+    def __init__(self):
+        pass
+
+    @classmethod
+    def HardClippingLogMessage(cls):
+        """Returns the log message used when hard clipping is detected in the mix.
+
+    This method is mainly intended to be used by the unit tests.
+    """
+        return cls._HARD_CLIPPING_LOG_MSG
+
+    @classmethod
+    def Mix(cls, output_path, capture_input_filepath, echo_filepath):
+        """Mixes capture and echo.
+
+    Creates the overall capture input for APM by mixing the "echo-free" capture
+    signal with the echo signal (e.g., echo simulated via the
+    echo_path_simulation module).
+
+    The echo signal cannot be shorter than the capture signal and the generated
+    mix will have the same duration of the capture signal. The latter property
+    is enforced in order to let the input of APM and the reference signal
+    created by TestDataGenerator have the same length (required for the
+    evaluation step).
+
+    Hard-clipping may occur in the mix; a warning is raised when this happens.
+
+    If `echo_filepath` is None, nothing is done and `capture_input_filepath` is
+    returned.
+
+    Args:
+      speech: AudioSegment instance.
+      echo_path: AudioSegment instance or None.
+
+    Returns:
+      Path to the mix audio track file.
+    """
+        if echo_filepath is None:
+            return capture_input_filepath
+
+        # Build the mix output file name as a function of the echo file name.
+        # This ensures that if the internal parameters of the echo path simulator
+        # change, no erroneous cache hit occurs.
+        echo_file_name, _ = os.path.splitext(os.path.split(echo_filepath)[1])
+        capture_input_file_name, _ = os.path.splitext(
+            os.path.split(capture_input_filepath)[1])
+        mix_filepath = os.path.join(
+            output_path,
+            'mix_capture_{}_{}.wav'.format(capture_input_file_name,
+                                           echo_file_name))
+
+        # Create the mix if not done yet.
+        mix = None
+        if not os.path.exists(mix_filepath):
+            echo_free_capture = signal_processing.SignalProcessingUtils.LoadWav(
+                capture_input_filepath)
+            echo = signal_processing.SignalProcessingUtils.LoadWav(
+                echo_filepath)
+
+            if signal_processing.SignalProcessingUtils.CountSamples(echo) < (
+                    signal_processing.SignalProcessingUtils.CountSamples(
+                        echo_free_capture)):
+                raise exceptions.InputMixerException(
+                    'echo cannot be shorter than capture')
+
+            mix = echo_free_capture.overlay(echo)
+            signal_processing.SignalProcessingUtils.SaveWav(mix_filepath, mix)
+
+        # Check if hard clipping occurs.
+        if mix is None:
+            mix = signal_processing.SignalProcessingUtils.LoadWav(mix_filepath)
+        if signal_processing.SignalProcessingUtils.DetectHardClipping(mix):
+            logging.warning(cls._HARD_CLIPPING_LOG_MSG)
+
+        return mix_filepath