[Mp4-tech] two questions on H.264 HRD

[Gary Sullivan]

I suppose that the issue of needing to feed the bitstream to the decoder
in some fashion (including selecting the SchedSelIdx or interpolated
schedule and getting the data into the decoder according to that
schedule somehow) is the reason for the concept of the HSS.  Although
the details of how to do that are out of scope, I think the standard is
pretty clear on what to do.  The "H" in HSS and HRD, of course, stands
for hypothetical, which is an important thing to keep in mind.
Best Regards,
Gary Sullivan
________________________________
	From: Nancy Johnson [mailto:nancyjohnson111 yahoo.com] 
	Sent: Friday, July 07, 2023 6:13 PM
	To: Gary Sullivan; mp4-tech lists.mpegif.org
	Subject: RE: [Mp4-tech] two questions on H.264 HRD
	Dr. Sullivan,
	Thanks for providing these valuable information. Regarding to
the value of SchedSelIdx, 
	1. I believe the HRD is a golden model which will be used to
test the conformance of bitstreams and real decoders.
	2. I believe the behavior of the HRD have been accurately
specified in the spec due to 1. However, if I were asked to write a
behavior model of the HRD by C or Verilog using the spec, I don't know
how the HRD can determine the value of
initial_cpb_removal_delay[SchedSelIdx] (at least it will influence the
coded picture removal time from the CPB and further influence the
decoded picture output time) because I can not find any information in
the spec for the HRD to know the value of SchedSelIdx it should use.
	Maybe the HRD can derive an interpolated
initial_cpb_removal_delay according to the actual bit rate it detected.
Maybe the HRD is assumed to know an exact or interpolated value of
SchedSelIdx from some system level info outside the bitstream. But this
is not mentioned in the spec.
	I wish my question was invalid just because I misunderstood
something or missed something.
	Thanks a lot.
	Best Regards,
	Nancy Johnson
	Gary Sullivan <garysull windows.microsoft.com> wrote:
		Regarding question 1 -- I think you would need to read
the HD DVD specs to find out, but personally I would not recommend
trying to sell a lot of units of an HD DVD player that can't be output
timing conforming at least the vast majority of the time for the vast
majority of movie content.
		Regarding question 2 -- It is important to distinguish
between the operation of a real decoder in a real system environment and
decoder or bitstream unit testing for conformance.  For output timing
conformance testing purposes, we assume that the HSS can force-feed the
decoder in a manner consistent with the standard somehow, but we really
don't care how.  It doesn't have to exclusively consist of the use of
information within the bitstream.  Also note that HRD decoder testing is
not necessarily performed by just selecting a value of SchedSelIdx --
there is also the possibility of "interpolated" scheduling.  There are
really two different kinds of constraints specified in the standard --
checks that a conforming bitstream must be able to fulfill and checks
that a conforming decoder must be able to fulfill.  It's important not
to confuse the two.  Also, when operating in a real system environment,
the system will ordinarily have some additional way of conveying the
bitstream and associated timing information to the decoder.  In the case
of HD DVD there are a significant amount of timing issues that are
controlled at the MPEG-2 systems level rather than needing to rely on
the information within the video elementary bitstream and I believe the
system-level information and operation should ordinarily be considered
more important to the actual operation of a real decoder in such an
application environment.
		Best Regards,
		Gary Sullivan
________________________________
			From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Nancy Johnson
			Sent: Thursday, July 06, 2023 4:38 PM
			To: mp4-tech lists.mpegif.org
			Subject: [Mp4-tech] two questions on H.264 HRD
			Dear H.264 experts,
			Could you please help me to figure out these 2
questions?
			1. For HD-DVD decoding application, is it
necessary for the H.264 decoder to claim output timing decoder
conformance? It seems that for this case the decoder can be fed the bit
stream in an "on demand" way.
			2. According to page 267 equation C-7 of H.264
03/2005, the removal time of the access unit from the CPB will be
influenced by initial_cpb_removal_delay[SchedSelIdx]. Although
initial_cpb_removal_delay[i] (i=0~cpb_cnt_minus1) are sent to HRD by
buffering period SEI message (page 277 section D.1.1 of H.264 03/2005),
I can not find a way in the H.264 stream definition for the HRD to know
the specific value of SchedSelIdx in equation C-7. If this is true, how
to make sure the value of SchedSelIdx in equation C-7 used by DUT are
the same as that used by HRD? (Is this necessary for the decoder to meet
output timing conformance?)
			Thanks a lot.
			Sincerely yours,
			Nancy Johnson
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