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diff --git a/src/common_audio/vad/vad_sp.h b/src/common_audio/vad/vad_sp.h
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+++ b/src/common_audio/vad/vad_sp.h
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+/*
+ *  Copyright (c) 2011 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.
+ */
+
+
+// This file includes specific signal processing tools used in vad_core.c.
+
+#ifndef WEBRTC_COMMON_AUDIO_VAD_VAD_SP_H_
+#define WEBRTC_COMMON_AUDIO_VAD_VAD_SP_H_
+
+#include "typedefs.h"
+#include "vad_core.h"
+
+// Downsamples the signal by a factor 2, eg. 32->16 or 16->8.
+//
+// Inputs:
+//      - signal_in     : Input signal.
+//      - in_length     : Length of input signal in samples.
+//
+// Input & Output:
+//      - filter_state  : Current filter states of the two all-pass filters. The
+//                        |filter_state| is updated after all samples have been
+//                        processed.
+//
+// Output:
+//      - signal_out    : Downsampled signal (of length |in_length| / 2).
+void WebRtcVad_Downsampling(int16_t* signal_in,
+                            int16_t* signal_out,
+                            int32_t* filter_state,
+                            int in_length);
+
+// Updates and returns the smoothed feature minimum. As minimum we use the
+// median of the five smallest feature values in a 100 frames long window.
+//
+// Inputs:
+//      - feature_value : New feature value to update with.
+//      - channel       : Channel number.
+//
+// Input & Output:
+//      - handle        : State information of the VAD.
+//
+// Returns:
+//                      : Smoothed minimum value for a moving window.
+int16_t WebRtcVad_FindMinimum(VadInstT* handle,
+                              int16_t feature_value,
+                              int channel);
+
+#endif  // WEBRTC_COMMON_AUDIO_VAD_VAD_SP_H_