1 // Copyright 2013 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 #ifndef MEDIA_BASE_AUDIO_HASH_H_ 6 #define MEDIA_BASE_AUDIO_HASH_H_ 7 8 #include <string> 9 10 #include "base/basictypes.h" 11 #include "base/strings/string_piece.h" 12 #include "media/base/media_export.h" 13 14 namespace media { 15 16 class AudioBus; 17 18 // Computes a running hash for a series of AudioBus objects. The hash is the 19 // sum of each sample bucketed based on the frame index, channel number, and 20 // current hash count. The hash was designed with two properties in mind: 21 // 22 // 1. Uniform error distribution across the input sample. 23 // 2. Resilience to error below a certain threshold. 24 // 25 // The first is achieved by using a simple summing approach and moving position 26 // weighting into the bucket choice. The second is handled during conversion to 27 // string by rounding out values to only two decimal places. 28 // 29 // Using only two decimal places allows for roughly -40 dBFS of error. For 30 // reference, SincResampler produces an RMS error of around -15 dBFS. See 31 // http://en.wikipedia.org/wiki/DBFS and http://crbug.com/168204 for more info. 32 class MEDIA_EXPORT AudioHash { 33 public: 34 AudioHash(); 35 ~AudioHash(); 36 37 // Update current hash with the contents of the provided AudioBus. 38 void Update(const AudioBus* audio_bus, int frames); 39 40 // Return a string representation of the current hash. 41 std::string ToString() const; 42 43 private: 44 // Storage for the audio hash. The number of buckets controls the importance 45 // of position in the hash. A higher number reduces the chance of false 46 // positives related to incorrect sample position. Value chosen by dice roll. 47 enum { kHashBuckets = 6 }; 48 float audio_hash_[kHashBuckets]; 49 50 // The total number of samples processed per channel. Uses a uint32 instead 51 // of size_t so overflows on 64-bit and 32-bit machines are equivalent. 52 uint32 sample_count_; 53 54 DISALLOW_COPY_AND_ASSIGN(AudioHash); 55 }; 56 57 } // namespace media 58 59 #endif // MEDIA_BASE_AUDIO_HASH_H_ 60
