Use passthrough decoder for (E)AC3 formats

media/base/ac3_util.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.

[DaleCurtis, 2016/12/14 20:00:03]

Probably want to use a BitReader here instead. SyncFrame instead of syncframe.
Also some of the constant names are weird case and abbreviation, don't mix the
spec names in with the constants.

[AndyWu, 2016/12/15 22:20:02]

Done, thanks. I still keep the weird abbreviations in local variables to match
the spec. Let me know if you think I should rename them, too.

+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "media/base/ac3_util.h"
+
+#include "base/logging.h"
+#include "base/macros.h"
+
+namespace media {
+
+namespace {
+
+// The size in byte of a (E-)AC3 syncframe header.
+const int kHeaderSizeInByte = 8;
+// The number of new samples per (E-)AC3 audio block.
+const int kAudioSamplesPerAudioBlock = 256;
+// Each syncframe has 6 blocks that provide 256 new audio samples. See ETSI TS
+// 102 366 4.1.
+const int kAc3SyncframeAudioSampleCount = 6 * kAudioSamplesPerAudioBlock;
+// Number of audio blocks per E-AC3 syncframe, indexed by numblkscod.
+const int kBlocksPerSyncframeByNumblkscod[] = {1, 2, 3, 6};
+// Sample rates, indexed by fscod.
+const int kSampleRateByFscod[] = {48000, 44100, 32000};
+// Nominal bitrates in kbps, indexed by frmsizecod / 2. (See ETSI TS 102 366
+// table 4.13.)
+const int kBitrateByHalfFrmsizecod[] = {32,  40,  48,  56,  64,  80,  96,
+                                        112, 128, 160, 192, 224, 256, 320,
+                                        384, 448, 512, 576, 640};
+// 16-bit words per syncframe, indexed by frmsizecod / 2. (See ETSI TS 102 366
+// table 4.13.)
+const int kSyncframeSizeWordsByHalfFrmsizecod441[] = {
+    69,  87,  104, 121, 139, 174, 208,  243,  278, 348,
+    417, 487, 557, 696, 835, 975, 1114, 1253, 1393};
+
+// Search for next syncword, wihch is 0x0B-0x77.
+const uint8_t* FindNextSync(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) {

[DaleCurtis, 2016/12/14 20:00:03]

You can scan two bytes at a time by check current[1] if it's 0x0B and skipping
two ahead otherwise.

[AndyWu, 2016/12/15 22:20:02]

Done.

+    if (current[0] == 0x0B && current[1] == 0x77) {
+      if (current != begin)
+        DVLOG(2) << __FUNCTION__ << " skip " << current - begin << " bytes.";
+
+      return current;
+    }
+    ++current;
+  }
+
+  return nullptr;
+}
+
+// Returns the number of audio samples represented by the given E-AC3 syncframe.
+int ParseEac3SyncframeSampleCount(const uint8_t* const data,
+                                  const uint8_t* const end) {
+  DCHECK(data);
+  DCHECK(end);
+  DCHECK_GE(end - data, kHeaderSizeInByte);
+
+  return kAudioSamplesPerAudioBlock *
+         (((data[4] & 0xC0) >> 6) == 0x03
+              ? 6
+              : kBlocksPerSyncframeByNumblkscod[(data[4] & 0x30) >> 4]);
+}
+
+// Returns the size in bytes of the given E-AC3 syncframe.
+int ParseEac3SyncframeSize(const uint8_t* const data,
+                           const uint8_t* const end) {
+  DCHECK(data);
+  DCHECK(end);
+  DCHECK_GE(end - data, kHeaderSizeInByte);
+
+  return 2 * (((data[2] & 0x07) << 8) + (data[3] & 0xFF) + 1);
+}
+
+// Returns the number of audio samples in an AC3 syncframe.
+int GetAc3SyncframeSampleCount() {
+  return kAc3SyncframeAudioSampleCount;
+}
+
+// Returns the size in bytes of the given AC3 syncframe.
+int ParseAc3SyncframeSize(const uint8_t* const data, const uint8_t* const end) {
+  DCHECK(data);
+  DCHECK(end);
+  DCHECK_GE(end - data, kHeaderSizeInByte);
+
+  unsigned fscod = (data[4] & 0xC0) >> 6;
+  unsigned frmsizecod = data[4] & 0x3F;
+  unsigned half_frmsizecod = frmsizecod / 2;
+
+  if (fscod >= arraysize(kSampleRateByFscod) ||
+      half_frmsizecod >= arraysize(kSyncframeSizeWordsByHalfFrmsizecod441)) {
+    DVLOG(2) << __FUNCTION__ << " Invalid frame header."
+             << " fscod:" << fscod << " half_frmsizecod:" << half_frmsizecod;
+    return -1;
+  }
+
+  int sample_rate = kSampleRateByFscod[fscod];
+  if (sample_rate == 44100) {
+    return 2 * (kSyncframeSizeWordsByHalfFrmsizecod441[half_frmsizecod] +
+                (frmsizecod % 2));
+  }
+
+  int bitrate = kBitrateByHalfFrmsizecod[half_frmsizecod];

[DaleCurtis, 2016/12/14 20:00:03]

Don't use else blocks with return.

[AndyWu, 2016/12/15 22:20:03]

Done.

+  if (sample_rate == 32000) {
+    return 6 * bitrate;
+  } else {  // 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 = FindNextSync(data, end);
+  int total_sample_count = 0;
+
+  while (current && end - current > kHeaderSizeInByte) {
+    int frame_size = is_eac3 ? ParseEac3SyncframeSize(current, end)
+                             : ParseAc3SyncframeSize(current, end);
+    int sample_count = is_eac3 ? ParseEac3SyncframeSampleCount(current, end)
+                               : 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 syncword.";
+      current += 2;
+    }
+
+    current = FindNextSync(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