[mythtv] [mythtv-commits] Ticket #9410: Positioning problems with BBC HD and BBC ONE HD recordings

[John P Poet]

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7.4.1.2.4 Detection of the first VCL NAL unit of a primary coded picture
This subclause specifies constraints on VCL NAL unit syntax that are
sufficient to enable the detection of the first VCL
NAL unit of each primary coded picture for coded video sequences that
conform to one or more of the profiles specified
in Annex A that are decoded using the decoding process specified in clauses 2-9.
Any coded slice NAL unit or coded slice data partition A NAL unit of
the primary coded picture of the current access
unit shall be different from any coded slice NAL unit or coded slice
data partition A NAL unit of the primary coded
picture of the previous access unit in one or more of the following ways.
–
frame_num differs in value. The value of frame_num used to test this
condition is the value of frame_num that
appears in the syntax of the slice header, regardless of whether that
value is inferred to have been equal to 0 for
subsequent use in the decoding process due to the presence of
memory_management_control_operation equal to 5.
.
.
.
etc
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When looking at one of the samples on this ticket, I am seeing a frame
number sequence like:

32, 33, 25, 33, 34, 35, 25, 35, 36, 37, 25, 37, 38, 25, 38, 39, 25,
39, 40, 41, 25, 41, 25, 41, 42, 43, 25, 43, 44, 25, 44, 45, 25, 45,
46, 47, 25, 47, 25, 48, 49, 25, 49, 50, 51, 25, 51, 52, 53, 25, 53,
54, 25, 54, 55, 25, 55, 56, 57, 25, 57, 25, 57, ...

So, my question is, where are those (bogus?) frame_nums of 25 coming
from?  Is BBC really putting those in there, or are we miss-parsing
the slice header?  If we are miss-parsing the slice header, is it
because we are consuming too many bits, or too few bits?

We use ffmpeg's ff_find_start_code routine to fine the beginning of
the next NAL UNIT.  Is it miss-identifying the start of the NAL UNIT?

Or, am I jumping at shadows, and that is all perfectly legit somehow?


John
-- 
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?