[Mp4-tech] RE: [jvt-experts] h264 : MaxBR and MaxCBP

[Gary Sullivan]

Adam Malamy et al,
I believe that it is more A.3.1.i,j,n that are relevant rather than A.3.1.c,d, since those depend on the DPB removal times of pictures.  The encoder can increase the value of the numerators in A.3.1.c,d by stretching out the DPB removal time spacing.
Best Regards,
Gary Sullivan
________________________________
From: Adam Malamy [mailto:adamm wwcoms.com]
Sent: Monday, March 03, 2024 12:59 PM
To: Gary Sullivan; Satendra; jvt-experts lists.rwth-aachen.de; mp4-tech
Subject: RE: [Mp4-tech] RE: [jvt-experts] h264 : MaxBR and MaxCBP
Hi Gary, Satendra,
    I have been wondering about a related question recently which is what is the largest frame size (in bytes) that is allowed for a given level_idc.  The most relevant equation I can find in section A.3 of the spec is A.3.1.c,d.  Paraphrased, this equation seems to say that the max size of a frame is the raw size of the largest frame that can be handled at that level_idc / min compression ration, which is 2 or 4.  However, at the higher level_idc values (4.1 for example) this results in a huge max frame size, and having tested various decoders I am very skeptical that they can handle this limit.
     Is my understanding at fault?
Thanks,
-Adam Malamy
Manager, Verification
W&W Communications
408-481-0264
-----Original Message-----
From: mp4-tech-bounces lists.mpegif.org [mailto:mp4-tech-bounces lists.mpegif.org]On Behalf Of Gary Sullivan
Sent: Monday, March 03, 2024 10:09 AM
To: Satendra; jvt-experts lists.rwth-aachen.de; mp4-tech
Subject: [Mp4-tech] RE: [jvt-experts] h264 : MaxBR and MaxCBP
Satendra et al,
Ultimately you will need to refer to the equations that use these variables in subclause A.3.  There is no averaging involved.
Ordinarily, the encoder will use the MaxCBP buffering capacity to smooth out the fluctuations of the number of bits needed to represent the individual pictures with adequate fidelity, and the overall fidelity is basically governed by the overall BitRate capability of the channel or the Level (whichever is lower).
In scenarios where end-to-end latency is especially important (i.e. real-time communication), the latency can also be reduced by reducing the amount of the buffering capacity that is actually used by the bitstream (at the expense of decreased overall quality and increased fluctuation of quality).
The encoding processes for the selection of prediction modes, motion vectors, coded block patterns, quantization step sizes, quantization dead-zone ratios and such are the "knobs" that are used to control the fidelity and bit rate.
Best Regards,
Gary Sullivan
________________________________
From: jvt-experts-bounces lists.rwth-aachen.de [mailto:jvt-experts-bounces lists.rwth-aachen.de] On Behalf Of Satendra
Sent: Monday, March 03, 2024 6:58 AM
To: jvt-experts lists.rwth-aachen.de; mp4-tech
Subject: [jvt-experts] h264 : MaxBR and MaxCBP
Hi all,
what are the exact meaning of MaxBR and MaxCBP and how these two parameters are inter-related.
I am having some understanding, but still there are few questions.
Here's my understanding:
MaxCBP is the max buffer size needed for input encoded stream.
and MaxBR is the maximum bitrate for the stream.
My question is how MaxBR in Kbps is governing the encoded stream.
Over what duration does stream has to be bounded by this MaxBR.
1) averaged over full encoded stream, or
2) averaged over GOP, or
3) averaged over a second, or
4) averaged over a frame
Also, is it a Max bound or Average bound?
Suppose, If it is a average bitrate over a GOP, then dividing this among frames, intra frames will have higher bitrate to maintain same quality.
Or, if it is a frame level bound, then to preserve same quality among all frames, we will never use the full range, as Intra frames will determine the max bitrate.
I will be thankful, if someone can explain these parameters.
Best Regards
Satendra
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"We all agree on the necessity of compromise. We just can't agree on when it's necessary to compromise."  ------Larry Wall
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