[FFmpeg-devel] UDP constant bitrate feature (cbr)

Zach Swena zcybercomputing at gmail.com
Wed Nov 18 20:28:03 CET 2015

Are you referring to this seperate applciation?


While using that makes sense for Pavel's application, why should FFmpeg
produce a problematic UDP stream in the first place?  For applications like
I need that involve encoding the stream in the first place, why should I
have to use a seperate program to make the output stream complient?  FFmpeg
should keep track of the number of bytes between pcr and smoothly output
the stream regardless of what the input is.  I would argue that this very
much is a bug, not a feature.  Just because an external applciation can fix
the problem in some instances doesn't mean it isn't a problem in the first


On Wed, Nov 18, 2015 at 11:14 AM, Zach Swena <zcybercomputing at gmail.com>

> > Only for strict CBR streams which FFmpeg cannot know a priori. That's
> > why the only correct way is to index the stream as it's received and
> > play out at a smooth rate like multicat does.
> I am not sure what you are referencing with regards to multicat.  Is this
> an output option?  Smooth output streams are needed in other applications
> then just relaying a stream.  Why can't I find the term multicat in any of
> the code or documentation?
> Zach
> On Wed, Nov 18, 2015 at 10:37 AM, Zach Swena <zcybercomputing at gmail.com>
> wrote:
>> Yea, how it is supposed to happen isn't complicated.  To be of any help,
>> I have to wrap my mind around how FFmpeg currently times the output
>> stream.  That mechanism currently does not produce as stable of a
>> transmission rate as multiplexer and decoder hardware requires.  Since
>> there is a bounty available for this, it would be nice if someone more
>> qualified could tackle this problem.  Until someone steps up, I will be
>> poking at it from my end.  I am used to reading c++ intead of c code, so my
>> reading is still a little slow.  Fixing this problem will open up new
>> possibilities on how I can use FFmpeg.
>> Zach
>> On Wed, Nov 18, 2015 at 9:03 AM, Michael Niedermayer <
>> michael at niedermayer.cc> wrote:
>>> On Tue, Nov 17, 2015 at 03:06:27PM -0800, Zach Swena wrote:
>>> > Hi Pavel,
>>> >
>>> > I can confirm that there is a problem with the UDP packet engine in
>>> > FFmpeg.  FFmpeg has excessive jitter in it's UDP streaming output to
>>> the
>>> > point where hardware decoders can't handle it.  My solution was to
>>> buffer
>>> > to disk and have my own program read and send the datagrams via a very
>>> > tight event loop.  While increasing the PCR period is not a bad thing,
>>> > FFmpeg really should stream at a more consistant rate.  Can someone
>>> explain
>>> > the theory behind how the UDP rate control is currently implemented in
>>> > FFmpeg?  PCR sounds like a good way to tell when to send a packet,
>>> except
>>> > not every packet contains one.  If FFmpeg uses PCR to tell how long to
>>> wait
>>> > to send a packet, then do the packets in between go at line speed?  I
>>> plan
>>> > on taking a look at the code that does this, but I would really
>>> appreciate
>>> > it if someone who knows the code could explain the theory as I usually
>>> deal
>>> > with a slightly different dev setup.
>>> i suggest you read the mpeg specs, they detail when things should be
>>> sent down to each byte IIRC
>>> also IIRC its not that complicated, more like timestamp + bytepos/rate
>>> [...]
>>> --
>>> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>>> I know you won't believe me, but the highest form of Human Excellence is
>>> to question oneself and others. -- Socrates
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