diff --git a/include/media/AudioRecord.h b/include/media/AudioRecord.h
index 521557d..b0bc2d0 100644
--- a/include/media/AudioRecord.h
+++ b/include/media/AudioRecord.h
@@ -145,7 +145,7 @@
      * Parameters:
      *
      * inputSource:        Select the audio input to record from (e.g. AUDIO_SOURCE_DEFAULT).
-     * sampleRate:         Data sink sampling rate in Hz.
+     * sampleRate:         Data sink sampling rate in Hz.  Zero means to use the source sample rate.
      * format:             Audio format (e.g AUDIO_FORMAT_PCM_16_BIT for signed
      *                     16 bits per sample).
      * channelMask:        Channel mask, such that audio_is_input_channel(channelMask) is true.
@@ -258,6 +258,7 @@
             bool        stopped() const;
 
     /* Return the sink sample rate for this record track in Hz.
+     * If specified as zero in constructor or set(), this will be the source sample rate.
      * Unlike AudioTrack, the sample rate is const after initialization, so doesn't need a lock.
      */
             uint32_t    getSampleRate() const   { return mSampleRate; }
diff --git a/include/media/AudioTrack.h b/include/media/AudioTrack.h
index a4b8571..e0fb603 100644
--- a/include/media/AudioTrack.h
+++ b/include/media/AudioTrack.h
@@ -167,7 +167,10 @@
      *
      * streamType:         Select the type of audio stream this track is attached to
      *                     (e.g. AUDIO_STREAM_MUSIC).
-     * sampleRate:         Data source sampling rate in Hz.
+     * sampleRate:         Data source sampling rate in Hz.  Zero means to use the sink sample rate.
+     *                     A non-zero value must be specified if AUDIO_OUTPUT_FLAG_DIRECT is set.
+     *                     0 will not work with current policy implementation for direct output
+     *                     selection where an exact match is needed for sampling rate.
      * format:             Audio format. For mixed tracks, any PCM format supported by server is OK.
      *                     For direct and offloaded tracks, the possible format(s) depends on the
      *                     output sink.
@@ -395,11 +398,14 @@
             status_t    setAuxEffectSendLevel(float level);
             void        getAuxEffectSendLevel(float* level) const;
 
-    /* Set source sample rate for this track in Hz, mostly used for games' sound effects
+    /* Set source sample rate for this track in Hz, mostly used for games' sound effects.
+     * Zero is not permitted.
      */
             status_t    setSampleRate(uint32_t sampleRate);
 
-    /* Return current source sample rate in Hz */
+    /* Return current source sample rate in Hz.
+     * If specified as zero in constructor or set(), this will be the sink sample rate.
+     */
             uint32_t    getSampleRate() const;
 
     /* Return the original source sample rate in Hz. This corresponds to the sample rate