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Handle channel count parsing for AC-4 with discrete objects #2526

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Handle channel count parsing for AC-4 with discrete objects #2526

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a119b1f
Merge pull request #19 from androidx/main
ybai001 Jun 5, 2025
209c159
Handle channel count parsing for AC-4 with discrete objects
ybai001 Jun 11, 2025
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68 changes: 51 additions & 17 deletions libraries/extractor/src/main/java/androidx/media3/extractor/Ac4Util.java
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,7 @@
import androidx.media3.common.Format;
import androidx.media3.common.MimeTypes;
import androidx.media3.common.ParserException;
import androidx.media3.common.util.Log;
import androidx.media3.common.util.ParsableBitArray;
import androidx.media3.common.util.ParsableByteArray;
import androidx.media3.common.util.UnstableApi;
Expand Down Expand Up @@ -118,6 +119,8 @@ private SyncFrameInfo(
/** Maximum rate for an AC-4 audio stream, in bytes per second. */
public static final int MAX_RATE_BYTES_PER_SECOND = 2688 * 1000 / 8;

private static final String TAG = "Ac4Util";

/** The channel count of AC-4 stream. */
// TODO: Parse AC-4 stream channel count.
private static final int CHANNEL_COUNT_2 = 2;
Expand Down Expand Up @@ -415,28 +418,59 @@ public static Format parseAc4AnnexEFormat(
ac4Presentation.hasBackChannels,
ac4Presentation.topChannelPairs);
} else {
// The ETSI TS 103 190-2 V1.2.1 (2018-02) specification defines the parameter
// n_umx_objects_minus1 in Annex E (E.11.11) to specify the number of fullband objects. While
// the elementary stream specification (section 6.3.2.8.1 and 6.3.2.10.4) provides information
// about the presence of an LFE channel within the set of dynamic objects, this detail is not
// explicitly stated in the ISO Base Media File Format (Annex E). However, current
// implementation practices consistently include the LFE channel when creating an object-based
// substream. As a result, it has been decided that when interpreting the ISO Base Media File
// Format, the LFE channel should always be counted as part of the total channel count.
int lfeChannelCount = 1;
channelCount = ac4Presentation.numOfUmxObjects + lfeChannelCount;
// TODO: There is a bug in ETSI TS 103 190-2 V1.2.1 (2018-02), E.11.11
// For AC-4 level 4 stream, the intention is to set 19 to n_umx_objects_minus1 but it is
// equal to 15 based on current specification. Dolby has filed a bug report to ETSI.
// The following sentence should be deleted after ETSI specification error is fixed.
if (ac4Presentation.level == 4) {
channelCount = channelCount == 17 ? 21 : channelCount;
if (ac4Presentation.numOfUmxObjects > 0) {
// The ETSI TS 103 190-2 V1.2.1 (2018-02) specification defines the parameter
// n_umx_objects_minus1 in Annex E (E.11.11) to specify the number of fullband objects.
// While the elementary stream specification (section 6.3.2.8.1 and 6.3.2.10.4) provides
// information about the presence of an LFE channel within the set of dynamic objects, this
// detail is not explicitly stated in the ISO Base Media File Format (Annex E). However,
// current implementation practices consistently include the LFE channel when creating an
// object-based substream. As a result, it has been decided that when interpreting the ISO
// Base Media File Format, the LFE channel should always be counted as part of the total
// channel count.
int lfeChannelCount = 1;
channelCount = ac4Presentation.numOfUmxObjects + lfeChannelCount;
// TODO: There is a bug in ETSI TS 103 190-2 V1.2.1 (2018-02), E.11.11
// For AC-4 level 4 stream, the intention is to set 19 to n_umx_objects_minus1 but it is
// equal to 15 based on current specification. Dolby has filed a bug report to ETSI.
// The following sentence should be deleted after ETSI specification error is fixed.
if (ac4Presentation.level == 4) {
channelCount = channelCount == 17 ? 21 : channelCount;
}
} else {
// This presentation includes a substream with discrete objects. Due to limitations in the
// current AC-4 specification (ETSI TS 103 190-2 V1.2.1 (2018-02)), discrete object number
// information is not explicitly stated in the ISO Base Media File Format (Annex E). To
// prevent exceptions, "Maximum number of tracks if audio presentation includes object
// audio" in table 77 of ETSI TS 103 190-2 V1.2.1 (2018-02) is used as the channel count.
switch (ac4Presentation.level) {
case 0:
// This should not happen because level 0 is always channel coded.
channelCount = 2;
break;
case 1:
channelCount = 6;
break;
case 2:
channelCount = 8;
break;
case 3:
channelCount = 10;
break;
case 4:
channelCount = 12;
break;
default:
Log.w(TAG, "AC-4 level " + ac4Presentation.level + " has not been defined.");
channelCount = 2;
break;
}
}
}

if (channelCount <= 0) {
throw ParserException.createForUnsupportedContainerFeature(
"Can't determine channel count of presentation.");
"Cannot determine channel count of presentation.");
}

String codecString =
Expand Down