Manuel Tiglio wrote:
On Jul 31, 2017, at 11:29 AM, Nicolas George <geo...@nsup.org>
wrote:
Le tridi 13 thermidor, an CCXXV, Manuel Tiglio a écrit :
1.23. For VOD content the peak bit rate SHOULD be no more than
200% of the average bit rate.
Without a proper definition of "peak bit rate", this sentence is
meaningless.
Are you saying that Apple’s authoring requirements for HLS are
meaningless?
A bit rate is a quantity of information divided by a time. Since it
is discrete, there is no way of doing calculus, and therefore there
is no notion of instantaneous bit rate. Thus, without saying the
period of time on which the bit rate is computed, it does not mean
anything.
When doing streaming you typically send packets of 6-10 secs, so in
that interval the peak (i.e. maximum value) bitrate does not have to
exceed 10% (110% constrained VBR) or 100% (200% constrained VBR) of
the average bitrate in that interval.
But the exact value of that length of time is irrelevant.
The fact that it is a discrete series is also irrelevant, you can
compute discrete bitrates in the same way that you compute finite
differences (for example).
This is really really standard for anyone working on streaming. What
is not standard is that ffmpeg has such large fluctuations between
the peak and average bitrate
You are encoding with libx264 not ffmpeg, it's just passing params.
What are you using to measure these "large fluctuations" and over what
timescale?
I see in the guide you linked that chunks are required to start with an
I frame - these for the same quality are usually far bigger than b or p
frames, so if you measure over small time there is bound to be a high
variation.
FWIW the player buffer size constraint AIUI is meant to absorb these
sort of things when fed at a constant/limited rate = it's not a
parameter that means the bitstream will at a small scale be constant
rate, it means the player using that buffer size at a certain "feed"
rate will not under/overflow when it meets short term fluctuations. If
you set it too small in some effort to make the encoder avoid these you
will likely end up with I frame pulsing.
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