[M4IF Technotes] video packet length

[ramakrishna_kakarala agilent.com]

Gary,
The maximum video packet length is described in Table A.1 of
http://www.m4if.org/resources/profiles/index.php
Footnote 6 says this applies only if data_partioning is used.
Appending E.1.1 of the 14496-2 standard describes fixed
interval synchronization as "This method requires that VOP
start codes and resynch. markers (i.e, the start of a
video packet) appear only at legal fixed interval locations
within the bistream....when fixed interval synch. is used
the decoder is only required to search for a VOP start code
at the beginning of each fixed interval."  I guess my questions
really are: how does the decoder know that fixed interval
synch. is used and how does it know the interval?
Best regards
Ram
---------------------------------
Ramakrishna Kakarala  
Agilent Technologies  
5301 Stevens Creek Blvd, MS 51L-GX
Santa Clara, CA 95052-8059
ramakrishna_kakarala   agilent.com 
Tel (408) 553-2349
Fax (408) 553-3794
> -----Original Message-----
> From: Gary Sullivan [mailto:garysull   windows.microsoft.com]
> Sent: Wednesday, June 18, 2023 11:45 AM
> To: ramakrishna_kakarala   agilent.com; technotes   lists.m4if.org
> Subject: RE: [M4IF Technotes] video packet length
> 
> 
> 
> I believe there is no signal of video packet length in an 
> MPEG-4 Visual
> bitstream.  The decoder can (and shall) determine the length by
> examining the values of the bits in the stream to see whether the
> contents match macroblock data or bit-stuffing/resync/start-code.
> 
> Although you say you know it is specified, I'm not aware of any
> specified maximum length (other than the rather absurd limit computed
> from the VBV fullness and VBV buffer capacity or computed from the
> remaining number of macroblocks in a picture).
> 
> Fixed-interval synchronization is also not specified.  It is something
> an encoder can choose to do or not to do, as far as the video spec is
> concerned.  What need is there to signal this to a decoder?
> 
> Best Regards,
> 
> Gary Sullivan
> 
> +> -----Original Message-----
> +> From: technotes-admin   lists.m4if.org 
> +> [mailto:technotes-admin   lists.m4if.org] On Behalf Of 
> +> ramakrishna_kakarala   agilent.com
> +> Sent: Wednesday, June 18, 2023 10:03 AM
> +> To: technotes   lists.m4if.org
> +> Subject: [M4IF Technotes] video packet length
> +> 
> +> 
> +> Hi,
> +> 
> +> How is video packet length signalled to the
> +> decoder? I don't see a field for it in the 
> +> video bitstream syntax.  I know the *maximum* length 
> +> is specified (if data partitioning is used)
> +> but I don't know how the actual value is
> +> signalled.  Is it signalled in the systems
> +> layer?
> +> 
> +> If the packet length is not signalled,
> +> then how does fixed interval synchronization
> +> work? 
> +> 
> +> thanks for your help
> +> Ram
> +> ---------------------------------
> +> Ramakrishna Kakarala  
> +> Agilent Technologies  
> +> 5301 Stevens Creek Blvd, MS 51L-GX
> +> Santa Clara, CA 95052-8059
> +> ramakrishna_kakarala   agilent.com 
> +> Tel (408) 553-2349
> +> Fax (408) 553-3794> 
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