media/base/audio_discard_helper.h

   1 // Copyright 2014 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_DISCARD_HELPER_H_
   6 #define MEDIA_BASE_AUDIO_DISCARD_HELPER_H_
   7
   8 #include "base/memory/ref_counted.h"
   9 #include "base/time/time.h"
  10 #include "media/base/audio_timestamp_helper.h"
  11 #include "media/base/decoder_buffer.h"
  12 #include "media/base/media_export.h"
  13 #include "media/base/timestamp_constants.h"
  14
  15 namespace media {
  16
  17 class AudioBuffer;
  18
  19 // Helper class for managing timestamps and discard events around decoding.
  20 class MEDIA_EXPORT AudioDiscardHelper {
  21  public:
  22   // |sample_rate| is the sample rate of decoded data which will be handed into
  23   // the ProcessBuffers() call.
  24   //
  25   // |decoder_delay| is the number of frames a decoder will output before data
  26   // corresponding to the first encoded buffer is output.  Callers only need to
  27   // specify this if the decoder inserts frames which have no corresponding
  28   // encoded buffer.
  29   //
  30   // For example, most MP3 decoders will output 529 junk frames before the data
  31   // corresponding to the first encoded buffer is output.  These frames are not
  32   // represented in the encoded data stream and instead are an artifact of how
  33   // most MP3 decoders work.  See http://lame.sourceforge.net/tech-FAQ.txt
  34   AudioDiscardHelper(int sample_rate, size_t decoder_delay);
  35   ~AudioDiscardHelper();
  36
  37   // Converts a TimeDelta to a frame count based on the constructed sample rate.
  38   // |duration| must be positive.
  39   size_t TimeDeltaToFrames(base::TimeDelta duration) const;
  40
  41   // Resets internal state and indicates that |initial_discard| of upcoming
  42   // frames should be discarded.
  43   void Reset(size_t initial_discard);
  44
  45   // Applies discard padding from the encoded buffer along with any initial
  46   // discards.  |decoded_buffer| may be NULL, if not the timestamp and duration
  47   // will be set after discards are applied.  Returns true if |decoded_buffer|
  48   // exists after processing discard events.  Returns false if |decoded_buffer|
  49   // was NULL, is completely discarded, or a processing error occurs.
  50   //
  51   // If AudioDiscardHelper is not initialized() the timestamp of the first
  52   // |encoded_buffer| will be used as the basis for all future timestamps set on
  53   // |decoded_buffer|s.  If the first buffer has a negative timestamp it will be
  54   // clamped to zero.
  55   bool ProcessBuffers(const scoped_refptr<DecoderBuffer>& encoded_buffer,
  56                       const scoped_refptr<AudioBuffer>& decoded_buffer);
  57
  58   // Whether any buffers have been processed.
  59   bool initialized() const {
  60     return timestamp_helper_.base_timestamp() != kNoTimestamp();
  61   }
  62
  63  private:
  64   // The sample rate of the decoded audio samples.  Used by TimeDeltaToFrames()
  65   // and the timestamp helper.
  66   const int sample_rate_;
  67
  68   // Some codecs output extra samples during the first decode.  In order to trim
  69   // DiscardPadding correctly the helper must know the offset into the decoded
  70   // buffers at which real samples start.
  71   const size_t decoder_delay_;
  72
  73   // Used to regenerate sample accurate timestamps for decoded buffers.  The
  74   // timestamp of the first encoded buffer seen by ProcessBuffers() is used as
  75   // the base timestamp.
  76   AudioTimestampHelper timestamp_helper_;
  77
  78   // The number of frames to discard from the front of the next buffer.  Can be
  79   // set by Reset() and added to by a front DiscardPadding larger than its
  80   // associated buffer.
  81   size_t discard_frames_;
  82
  83   // The last encoded buffer timestamp seen by ProcessBuffers() or kNoTimestamp
  84   // if no buffers have been seen thus far.  Used to issue warnings for buffer
  85   // sequences with non-monotonic timestamps.
  86   base::TimeDelta last_input_timestamp_;
  87
  88   // Certain codecs require two encoded buffers before they'll output the first
  89   // decoded buffer.  In this case DiscardPadding must be carried over from the
  90   // previous encoded buffer.  Enabled automatically if an encoded buffer is
  91   // given to ProcessBuffers() with a NULL decoded buffer.
  92   bool delayed_discard_;
  93   DecoderBuffer::DiscardPadding delayed_discard_padding_;
  94
  95   // When |decoder_delay_| > 0, the number of frames which should be discarded
  96   // from the next buffer.  The index at which to start discarding is calculated
  97   // by subtracting |delayed_end_discard_| from |decoder_delay_|.
  98   size_t delayed_end_discard_;
  99
 100   DISALLOW_IMPLICIT_CONSTRUCTORS(AudioDiscardHelper);
 101 };
 102
 103 }  // namespace media
 104
 105 #endif  // MEDIA_BASE_AUDIO_DISCARD_HELPER_H_