short empty in the begin of aac data

Dear Experts,

Platform:DM8148 custome board/DVR RDK3.5

We encoded pcm audio to aac (adts).
Compare the raw pcm and encoded aac as show in below figure.
audio: the encoded aac waveform
test: the input raw pcm

There are around 12 msec empty (mute) in the begin of aac data, is this resonable ??

This will cause AV unsync.


HB