media/base/audio_timestamp_helper.cc

   1 // Copyright (c) 2012 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 #include "media/base/audio_timestamp_helper.h"
   6
   7 #include "base/logging.h"
   8 #include "media/base/timestamp_constants.h"
   9
  10 namespace media {
  11
  12 AudioTimestampHelper::AudioTimestampHelper(int samples_per_second)
  13     : base_timestamp_(kNoTimestamp()),
  14       frame_count_(0) {
  15   DCHECK_GT(samples_per_second, 0);
  16   double fps = samples_per_second;
  17   microseconds_per_frame_ = base::Time::kMicrosecondsPerSecond / fps;
  18 }
  19
  20 void AudioTimestampHelper::SetBaseTimestamp(base::TimeDelta base_timestamp) {
  21   base_timestamp_ = base_timestamp;
  22   frame_count_ = 0;
  23 }
  24
  25 base::TimeDelta AudioTimestampHelper::base_timestamp() const {
  26   return base_timestamp_;
  27 }
  28
  29 void AudioTimestampHelper::AddFrames(int frame_count) {
  30   DCHECK_GE(frame_count, 0);
  31   DCHECK(base_timestamp_ != kNoTimestamp());
  32   frame_count_ += frame_count;
  33 }
  34
  35 base::TimeDelta AudioTimestampHelper::GetTimestamp() const {
  36   return ComputeTimestamp(frame_count_);
  37 }
  38
  39 base::TimeDelta AudioTimestampHelper::GetFrameDuration(int frame_count) const {
  40   DCHECK_GE(frame_count, 0);
  41   base::TimeDelta end_timestamp = ComputeTimestamp(frame_count_ + frame_count);
  42   return end_timestamp - GetTimestamp();
  43 }
  44
  45 int64 AudioTimestampHelper::GetFramesToTarget(base::TimeDelta target) const {
  46   DCHECK(base_timestamp_ != kNoTimestamp());
  47   DCHECK(target >= base_timestamp_);
  48
  49   int64 delta_in_us = (target - GetTimestamp()).InMicroseconds();
  50   if (delta_in_us == 0)
  51     return 0;
  52
  53   // Compute a timestamp relative to |base_timestamp_| since timestamps
  54   // created from |frame_count_| are computed relative to this base.
  55   // This ensures that the time to frame computation here is the proper inverse
  56   // of the frame to time computation in ComputeTimestamp().
  57   base::TimeDelta delta_from_base = target - base_timestamp_;
  58
  59   // Compute frame count for the time delta. This computation rounds to
  60   // the nearest whole number of frames.
  61   double threshold = microseconds_per_frame_ / 2;
  62   int64 target_frame_count =
  63       (delta_from_base.InMicroseconds() + threshold) / microseconds_per_frame_;
  64   return target_frame_count - frame_count_;
  65 }
  66
  67 base::TimeDelta AudioTimestampHelper::ComputeTimestamp(
  68     int64 frame_count) const {
  69   DCHECK_GE(frame_count, 0);
  70   DCHECK(base_timestamp_ != kNoTimestamp());
  71   double frames_us = microseconds_per_frame_ * frame_count;
  72   return base_timestamp_ + base::TimeDelta::FromMicroseconds(frames_us);
  73 }
  74
  75 }  // namespace media