[M4IF Technotes] Stuffing Bit Pattern for Start Codes

[Nitin Gupta--DSP, Noida]

Hi Neelkanth & all,
Q :- I parsed 4 bits to reach the byte aligned position, shall I now look
for 8 bit stuffing if the start code is not present?
To this the answer in all probability is NO .. u need not look for any more
stuffing bits once u have reached a byte aligned
position .. so if u don't find the start marker (it can be a
user_data_start_code , vop_start_code or it can be the resync marker
as resync markers are also byte aligned in case of video packets being
present in the vop ) , then the most probable condition
is that there is an error present in the bitstream. 
This can be concluded from the fact that the standard never says at any
point of time that u need to go to the next byte aligned
position in case u don't find a marker after u have reached a particular
byte aligned position. Also with the experience of decoding
more than 100 mpeg4 conformance streams, i never confronted such a case.
I replied to this query as there is no reply from anyone for some time. Plz
correct me if i m wrong.
Hope my reply helps u.
Thanx & Regards,
Nitin Gupta
HCL Technologies, Noida,
India.
-----Original Message-----
From: Neelakanth [mailto:neel2265   yahoo.com]
Sent: Friday, April 26, 2024 12:25 PM
To: technotes   lists.m4if.org
Subject: [M4IF Technotes] Stuffing Bit Pattern for Start Codes
Hi, 
Please refer to 5.2.4 (Definition of next_start_code() function) of ISO/IEC
14496-2 (MPEG-4 Video Standard). In this function stuffing bit are parsed to
go to next byte aligned position. The stuffing bits can be upto 8 ( 8 if
position is already byte-aligned ). I want to know that once we have reached
the byte aligned position then also shall we look for stuffing bits? For
example I parsed 4 bits to reach the byte aligned position, shall I now look
for 8 bit stuffing if the start code is not present?
Thanks and Regards
Neelakanth
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