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MediaFormat AAC DRC documentation updates

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Better doc for Target Ref level.
Clarify behavior of -1 for Target Ref level

Bug: 148385721
Test: atest DecoderTestXheAac DecoderTestAacDrc
Change-Id: I8ef83abc73d72552c4ea8e63839cc6efb4d22a45
This commit is contained in:
Jean-Michel Trivi
2020-02-17 10:35:14 -08:00
parent 9fdd67b839
commit fc3b8a8df3
1 changed files with 18 additions and 14 deletions
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32
media/java/android/media/MediaFormat.java
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@@ -650,16 +650,20 @@ public final class MediaFormat {
public static final String KEY_AAC_MAX_OUTPUT_CHANNEL_COUNT = "aac-max-output-channel_count";
/**
* A key describing a gain to be applied so that the output loudness matches the
* Target Reference Level. This is typically used to normalize loudness across program items.
* The gain is derived as the difference between the Target Reference Level and the
* Program Reference Level. The latter can be given in the bitstream and indicates the actual
* loudness value of the program item.
* A key describing the Target Reference Level (Target Loudness).
* <p>For normalizing loudness across program items, a gain is applied to the audio output so
* that the output loudness matches the Target Reference Level. The gain is derived as the
* difference between the Target Reference Level and the Program Reference Level (Program
* Loudness). The latter can be given in the bitstream and indicates the actual loudness value
* of the program item.</p>
* <p>The Target Reference Level controls loudness normalization for both MPEG-4 DRC and
* MPEG-D DRC.
* <p>The value is given as an integer value between
* 40 and 127, and is calculated as -4 * Target Reference Level in LKFS.
* Therefore, it represents the range of -10 to -31.75 LKFS.
* <p>For MPEG-4 DRC, a value of -1 switches off loudness normalization and DRC processing.</p>
* <p>For MPEG-D DRC, a value of -1 switches off loudness normalization only. For DRC processing
* options of MPEG-D DRC, see {@link #KEY_AAC_DRC_EFFECT_TYPE}</p>
* <p>The default value on mobile devices is 64 (-16 LKFS).
* <p>This key is only used during decoding.
*/
@@ -680,7 +684,7 @@ public final class MediaFormat {
* <tr><th>6</th><th>General compression</th></tr>
* </table>
* <p>The value -1 (Off) disables DRC processing, while loudness normalization may still be
* active and dependent on KEY_AAC_DRC_TARGET_REFERENCE_LEVEL.<br>
* active and dependent on {@link #KEY_AAC_DRC_TARGET_REFERENCE_LEVEL}.<br>
* The value 0 (None) automatically enables DRC processing if necessary to prevent signal
* clipping<br>
* The value 6 (General compression) can be used for enabling MPEG-D DRC without particular
@@ -697,8 +701,8 @@ public final class MediaFormat {
* 0 and 127, which is calculated as -4 * Encoded Target Level in LKFS.
* If the Encoded Target Level is unknown, the value can be set to -1.
* <p>The default value is -1 (unknown).
* <p>The value is ignored when heavy compression is used (see
* {@link #KEY_AAC_DRC_HEAVY_COMPRESSION}).
* <p>The value is ignored when heavy compression (see {@link #KEY_AAC_DRC_HEAVY_COMPRESSION})
* or MPEG-D DRC is used.
* <p>This key is only used during decoding.
*/
public static final String KEY_AAC_ENCODED_TARGET_LEVEL = "aac-encoded-target-level";
@@ -739,17 +743,17 @@ public final class MediaFormat {
public static final String KEY_AAC_DRC_ATTENUATION_FACTOR = "aac-drc-cut-level";
/**
* A key describing the selection of the heavy compression profile for DRC.
* Two separate DRC gain sequences can be transmitted in one bitstream: MPEG-4 DRC light
* compression, and DVB-specific heavy compression. When selecting the application of the heavy
* compression, one of the sequences is selected:
* A key describing the selection of the heavy compression profile for MPEG-4 DRC.
* <p>Two separate DRC gain sequences can be transmitted in one bitstream: light compression
* and heavy compression. When selecting the application of the heavy compression, one of
* the sequences is selected:
* <ul>
* <li>0 enables light compression,</li>
* <li>1 enables heavy compression instead.
* </ul>
* Note that only light compression offers the features of scaling of DRC gains
* Note that heavy compression doesn't offer the features of scaling of DRC gains
* (see {@link #KEY_AAC_DRC_BOOST_FACTOR} and {@link #KEY_AAC_DRC_ATTENUATION_FACTOR} for the
* boost and attenuation factors, and frequency-selective (multiband) DRC.
* boost and attenuation factors), and frequency-selective (multiband) DRC.
* Light compression usually contains clipping prevention for stereo downmixing while heavy
* compression, if additionally provided in the bitstream, is usually stronger, and contains
* clipping prevention for stereo and mono downmixing.