Use passthrough decoder for (E)AC3 formats

media/formats/ac3/ac3_util.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "media/formats/ac3/ac3_util.h"
+
+#include "base/logging.h"
+#include "base/macros.h"
+#include "media/base/bit_reader.h"
+
+namespace media {
+
+namespace {
+
+// The size in byte of a (E-)AC3 synchronization frame header.
+const int kHeaderSizeInByte = 8;
+// The number of new samples per (E-)AC3 audio block.
+const int kAudioSamplesPerAudioBlock = 256;
+// Each synchronization frame has 6 blocks that provide 256 new audio samples.
+const int kAudioSamplePerAc3SyncFrame = 6 * kAudioSamplesPerAudioBlock;
+// Number of audio blocks per E-AC3 synchronization frame, indexed by
+// numblkscod.
+const int kBlocksPerSyncFrame[] = {1, 2, 3, 6};
+// Sample rates, indexed by fscod.
+const int kSampleRate[] = {48000, 44100, 32000};
+// Nominal bitrates in kbps, indexed by frmsizecod / 2.
+const int kBitrate[] = {32,  40,  48,  56,  64,  80,  96,  112, 128, 160,
+                        192, 224, 256, 320, 384, 448, 512, 576, 640};
+// 16-bit words per synchronization frame, indexed by frmsizecod.
+const int kSyncFrameSizeInWordsFor44kHz[] = {
+    69,  70,  87,  88,  104, 105, 121,  122,  139,  140,  174,  175, 208,
+    209, 243, 244, 278, 279, 348, 349,  417,  418,  487,  488,  557, 558,
+    696, 697, 835, 836, 975, 976, 1114, 1115, 1253, 1254, 1393, 1394};
+
+// Utility for unpacking (E-)AC3 header. Note that all fields are encoded.
+class Ac3Header {
+ public:
+  Ac3Header(const uint8_t* data, int size);
+
+  const uint32_t eac3_frame_size_code() { return eac3_frame_size_code_; }
+
+  const uint32_t sample_rate_code() { return sample_rate_code_; }
+
+  const uint32_t eac3_number_of_audio_block_code() {
+    DCHECK(sample_rate_code_ != 3);
+    return eac3_number_of_audio_block_code_;
+  }
+
+  const uint32_t ac3_frame_size_code() { return ac3_frame_size_code_; }
+
+ private:
+  // bit[5:15] for E-AC3
+  uint32_t eac3_frame_size_code_;
+  // bit[16:17] for (E-)AC3
+  uint32_t sample_rate_code_;
+  // bit[18:23] for AC3
+  uint32_t ac3_frame_size_code_;
+  // bit[18:19] for E-AC3
+  uint32_t eac3_number_of_audio_block_code_;
+};
+
+Ac3Header::Ac3Header(const uint8_t* data, int size) {
+  DCHECK_GE(size, kHeaderSizeInByte);
+
+  BitReader reader(data, size);
+  uint16_t sync_word;
+  reader.ReadBits(16, &sync_word);
+  DCHECK(sync_word == 0x0B77);
+
+  reader.SkipBits(5);
+  reader.ReadBits(11, &eac3_frame_size_code_);
+  reader.ReadBits(2, &sample_rate_code_);
+  reader.ReadBits(6, &ac3_frame_size_code_);
+  eac3_number_of_audio_block_code_ = ac3_frame_size_code_ >> 4;
+}
+
+// Search for next synchronization word, wihch is 0x0B-0x77.
+const uint8_t* FindNextSyncWord(const uint8_t* const begin,
+                                const uint8_t* const end) {
+  DCHECK(begin);
+  DCHECK(end);
+  DCHECK_LE(begin, end);
+
+  const uint8_t* current = begin;
+
+  while (current < end - 1) {
+    if (current[0] == 0x0B && current[1] == 0x77) {
+      if (current != begin)
+        DVLOG(2) << __FUNCTION__ << " skip " << current - begin << " bytes.";
+
+      return current;
+    } else if (current[1] != 0x0B) {
+      current += 2;
+    } else {
+      ++current;
+    }
+  }
+
+  return nullptr;
+}
+
+// Returns the number of audio samples represented by the given E-AC3
+// synchronization frame.
+int ParseEac3SyncFrameSampleCount(Ac3Header& header) {
+  unsigned blocks =
+      header.sample_rate_code() == 0x03
+          ? 6
+          : kBlocksPerSyncFrame[header.eac3_number_of_audio_block_code()];
+  return kAudioSamplesPerAudioBlock * blocks;
+}
+
+// Returns the size in bytes of the given E-AC3 synchronization frame.
+int ParseEac3SyncFrameSize(Ac3Header& header) {
+  return 2 * (header.eac3_frame_size_code() + 1);
+}
+
+// Returns the number of audio samples in an AC3 synchronization frame.
+int GetAc3SyncFrameSampleCount() {
+  return kAudioSamplePerAc3SyncFrame;
+}
+
+// Returns the size in bytes of the given AC3 synchronization frame.
+int ParseAc3SyncFrameSize(Ac3Header& header) {
+  if (header.sample_rate_code() >= arraysize(kSampleRate) ||
+      Ac3Header.ac3_frame_size_code() >=
+          arraysize(kSyncFrameSizeInWordsFor44kHz)) {
+    DVLOG(2) << __FUNCTION__ << " Invalid frame header."
+             << " fscod:" << Ac3Header.sample_rate_code()
+             << " frmsizecod:" << Ac3Header.ac3_frame_size_code();
+    return -1;
+  }
+
+  int sample_rate = kSampleRate[Ac3Header.sample_rate_code()];
+  if (sample_rate == 44100) {
+    return 2 * kSyncFrameSizeInWordsFor44kHz[Ac3Header.ac3_frame_size_code()];

[chcunningham, 2017/05/05 18:57:18]

Can you document where the 2, 6, and 4 (LHS of return multiplications) come
from? I assume this is in some table defined in the spec, so just reference that
spec version and table number 

[AndyWu, 2017/05/06 01:46:48]

Done.

+  }
+
+  int bitrate = kBitrate[Ac3Header.ac3_frame_size_code() / 2];
+  if (sample_rate == 32000) {
+    return 6 * bitrate;
+  }
+
+  // sample_rate == 48000
+  return 4 * bitrate;
+}
+
+// Returns the total number of audio samples in the given buffer, which contains
+// several complete (E-)AC3 syncframes.
+int ParseTotalSampleCount(const uint8_t* data, size_t size, bool is_eac3) {
+  DCHECK(data);
+
+  if (size < kHeaderSizeInByte) {
+    return 0;
+  }
+
+  const uint8_t* const end = data + size;
+  const uint8_t* current = FindNextSyncWord(data, end);
+  int total_sample_count = 0;
+
+  while (current && end - current > kHeaderSizeInByte) {
+    Ac3Header header(current, end - current);
+
+    int frame_size = is_eac3 ? ParseEac3SyncFrameSize(header)
+                             : ParseAc3SyncFrameSize(header);
+    int sample_count = is_eac3 ? ParseEac3SyncFrameSampleCount(header)
+                               : GetAc3SyncFrameSampleCount();
+
+    if (frame_size > 0 && sample_count > 0) {
+      current += frame_size;
+      if (current > end) {
+        DVLOG(2) << __FUNCTION__ << " Incomplete frame, missing "
+                 << current - end << " bytes.";
+        break;
+      }
+
+      total_sample_count += sample_count;
+    } else {
+      DVLOG(2)
+          << __FUNCTION__
+          << " Invalid frame, skip 2 bytes to find next synchronization word.";
+      current += 2;
+    }
+
+    current = FindNextSyncWord(current, end);
+  }
+
+  return total_sample_count;
+}
+
+}  // namespace anonymous
+
+// static
+int Ac3Util::ParseTotalAc3SampleCount(const uint8_t* data, size_t size) {
+  return ParseTotalSampleCount(data, size, false);
+}
+
+// static
+int Ac3Util::ParseTotalEac3SampleCount(const uint8_t* data, size_t size) {
+  return ParseTotalSampleCount(data, size, true);
+}
+
+}  // namespace media