Clarify APV sample format (#72)

* clarification of APV sample format

Signed-off-by: Kwang Pyo Choi <kp5.choi@samsung.com>

* removed 'sample_size' loop

Signed-off-by: Kwang Pyo Choi <kp5.choi@samsung.com>

* clarification more

Signed-off-by: Kwang Pyo Choi <kp5.choi@samsung.com>

---------

Signed-off-by: Kwang Pyo Choi <kp5.choi@samsung.com>

Author: kpchoi

readme/apv_isobmff.md

@@ -159,7 +159,20 @@ the full_range_flag field in the frame header of the frames whose value of the p
 ## APV Sample Description
 
 ###	Format of sample
-When APV coded bitstream is encapsulated in a track with APVSamspleEntry, each sample shall contain one and only one access unit of APV coded data
+When APV coded bitstream is encapsulated in a track with APVSampleEntry, each sample shall contain a size of access unit and an access unit of APV coded data.
+
+~~~~
+aligned(8) class APVSample{
+    unsigned int(32) au_length;
+    bit(au_length * 8) access_unit;
+}
+~~~~
+
++ au_length
+> indicates the size of an access unit measured in bytes. The size does not include the length field itself.
+
++ access_unit
+> contains a single access unit. It includes both the 4-characters signature and the variable length encapsulated PBU streams.
 
 ###	Sync sample
 Every samples of APV bitstream shall be sync samples.