diff --git a/media/java/android/media/MediaFormat.java b/media/java/android/media/MediaFormat.java
index 169502b..a2eb8d9 100644
--- a/media/java/android/media/MediaFormat.java
+++ b/media/java/android/media/MediaFormat.java
@@ -26,7 +26,7 @@
  *
  * The format of the media data is specified as string/value pairs.
  *
- * Keys common to all formats:
+ * Keys common to all formats, <b>all keys not marked optional are mandatory</b>:
  *
  * <table>
  * <tr><th>Name</th><th>Value Type</th><th>Description</th></tr>
@@ -52,7 +52,7 @@
  * <tr><td>{@link #KEY_SAMPLE_RATE}</td><td>Integer</td><td></td></tr>
  * <tr><td>{@link #KEY_IS_ADTS}</td><td>Integer</td><td>optional, if <em>decoding</em> AAC audio content, setting this key to 1 indicates that each audio frame is prefixed by the ADTS header.</td></tr>
  * <tr><td>{@link #KEY_AAC_PROFILE}</td><td>Integer</td><td><b>encoder-only</b>, optional, if content is AAC audio, specifies the desired profile.</td></tr>
- * <tr><td>{@link #KEY_CHANNEL_MASK}</td><td>Integer</td><td>A mask of audio channel assignments</td></tr>
+ * <tr><td>{@link #KEY_CHANNEL_MASK}</td><td>Integer</td><td>optional, a mask of audio channel assignments</td></tr>
  * <tr><td>{@link #KEY_FLAC_COMPRESSION_LEVEL}</td><td>Integer</td><td><b>encoder-only</b>, optional, if content is FLAC audio, specifies the desired compression level.</td></tr>
  * </table>
  *