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

[Shankar Manuel Aghito]

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).
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).
If you then remove entire NALUs from the bitstream the decoder will conceal for the lost NALUs. 
Note that if you loose entire pictures, you should enable the error concealment in the decoder configuration file. 
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)
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
Best regards, 
Manuel 
	-----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