[Mp4-tech] Dropping NAL units on bit errors: Best approach?

[Shankar Manuel Aghito]

Hi Luqman,
I've look at the standard and I can see what you say, it seems that that
zero_byte before start_code_prefix_one_3bytes should be present only in
SPS, PPS and in the first NALU of each access unit. I don't know why (I
guess that it is because we are missing something in understanding the
standard), but I am quite sure that the reference software put 4 bytes
0x00000001 in the beginning of each NALU, also for multiple NALUs per
picture, i.e. when there are NALUs that are not the first in the access
unit. Try my matlab script, posted in "RE: [Mp4-tech] H264 Corrupted
Bitstreams" a couple of days ago.
Could someone please explain us this "mystery"?
Also you could try with JM11. I've read of problems with JM10.2 related
to use of multiple slices/picture.
If you still have problems, send encoder/decoder output and also
configuration files
---
About the error concealment parameter in the decoder configuration file.
2 (motion copy) often is the best in my experience. But you will only
see differences when you remove a whole picture, i.e. all the NALUs of
one picture. If you loose only some parts of a picture, different
concealment algorithms are used, but you cannot control that in the
config file.
---
You don't need to have Log2MaxFNumMinus4 soo high! With
Log2MaxFNumMinus4=12 then you can encode 2^(12+4) pictures before the
counter restarts!!!
Regards,
Manuel
-----Original Message-----
From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Luqman
Sent: 18. oktober 2006 17:59
To: mp4-tech lists.mpegif.org
Subject: Re: [Mp4-tech] Dropping NAL units on bit errors: Best approach?
hi Manuel Shankar,
thanks for your reply. 
> Shankar Manuel Aghito wanted us to know:

>The first approach is quite simple and works fine for me.
>You should set the encoder and the decoder to use Annex B output format
(there are parameters in the configuration files).
Now, I have implemented NALU dropping code directly from annexb file.
>
>
>What I am doing is searching for 0x00000001 in the bitstream. That will

>indicate the start of each NAL unit.
>I actually did not studied carefully all the details in Annex B, but in
my experience this always works (the number of NALUs always correspond
to the expected number).
Not every NALU has 0x00 0x00 0x00 0x01 as start code prefix. Some NALUs
can also have 0x00 0x00 0x01 as start code prefix. Only NALUs with SPS,
PPS and first NALU of an access unit contaings a zero byte leading the 3
byte start code prefix.
(page 270: TU-T Rec. H.264 (03/2005) - Prepublished version)
>
>If you then remove entire NALUs from the bitstream the decoder will 
>conceal for the lost NALUs.

That is something I am having problem with.  Decoder gives different
kinds of warnings and error messages when different NALUs are dropped. 
I am not sure whether I should ignore them or try to improve my code to
remove them.
>
>Note that if you loose entire pictures, you should enable the error 
>concealment in the decoder configuration file.

I have tried 0,1 and 2 in decoder configuration file for error
concealment but without any visible differencec.
>
>You should never remove the first two NALUs which contain Sequence and 
>Picture parameter set.
>
>Normally I also set Log2MaxFNumMinus4 such that MaxFNum >= 
>number_of_frames_to_encode, otherwise sometimes the decoder crashes (I 
>think it happens when you remove NALUs in two consecutive frames, 
>corresponding to the positions where the counter restarts, but if you 
>set this parameter high enough, this will never happen)
>

In encoder.cfg I set Log2MaxFNumMinus4 = 12 which is the maximum allowed
value.
>I am not normally using B frames but I think that if you use B frames 
>you might also need to take care of Log2MaxPOCLsbMinus4

I am using B frames either.
I will attach output of encoder and decoder for different dropped NALUs.
Hope you can give me some hints on where to look for possible solutions.
Regards,
Luqman 
>	-----Original Message----- 
>	From: mp4-tech-bounces lists.mpegif.org on behalf of Luqman 
>	Sent: Fri 13/10/2023 12:52 
>	To: mp4-tech lists.mpegif.org 
>	Cc: 
>	Subject: [Mp4-tech] Dropping NAL units on bit errors: Best
approach?
>	
>	
>
>	hello,
>	
>	I like to simulate video over UMTS link and then drop all NAL
units
>	containg bit error after the transport.
>	
>	One possibility would be to read the bitstream file directly,
look for
>	start_code_prefixes, i.e. 0x000000, to find NAL unit boundaries,
and
>	copy everything except for errored NAL units in a new file.
>	
>	Second approach would involve editing code in decoder directly.
I am
>	working with reference decoder software JM10.2.
>	
>	Since the standard is very complex and I am just a novice on
this topic,
>	I was wondering if the first approach wouldn't be more
appropriate for my
>	purpose. With my limited experience with h264 working on encoded
stream
>	file seems to be more straightforward and universal solution.
>	
>	I would appreciate any comments on this.
>	
>	Regards,
>	
>	--
>	Luqman
>	
>
-- 
Luqman