diff --git a/trunk/src/protocol/srs_raw_avc.cpp b/trunk/src/protocol/srs_raw_avc.cpp
index f05af8221..a7c015596 100644
--- a/trunk/src/protocol/srs_raw_avc.cpp
+++ b/trunk/src/protocol/srs_raw_avc.cpp
@@ -447,6 +447,13 @@ srs_error_t SrsRawAacStream::mux_sequence_header(SrsRawAacStreamCodec* codec, st
     SrsAacObjectType audioObjectType = codec->aac_object;
     char channelConfiguration = codec->channel_configuration;
 
+    // Here we are generating AAC sequence header, the ASC structure,
+    // because we have already parsed the sampling rate from AAC codec,
+    // which is more precise than the sound_rate defined by RTMP.
+    //
+    // For example, AAC sampling_frequency_index is 3(48000HZ) or 4(44100HZ),
+    // the sound_rate is always 3(44100HZ), if we covert sound_rate to
+    // sampling_frequency_index, we may make mistake.
     char samplingFrequencyIndex = codec->sampling_frequency_index;
     if (samplingFrequencyIndex >= SrsAAcSampleRateNumbers) {
         samplingFrequencyIndex = 4; // Default to 44100
diff --git a/trunk/src/protocol/srs_raw_avc.hpp b/trunk/src/protocol/srs_raw_avc.hpp
index de81bf2ad..944fac560 100644
--- a/trunk/src/protocol/srs_raw_avc.hpp
+++ b/trunk/src/protocol/srs_raw_avc.hpp
@@ -73,12 +73,14 @@ public:
 // The header of adts sample.
 struct SrsRawAacStreamCodec
 {
+    // Codec level informations.
     int8_t protection_absent;
     SrsAacObjectType aac_object;
     int8_t sampling_frequency_index;
     int8_t channel_configuration;
     int16_t frame_length;
-    
+
+    // Format level, RTMP as such, informations.
     char sound_format;
     char sound_rate;
     char sound_size;