Synchronization Issue

[Patrick Doyle]

Hi Jan,
Thank you very much.  At first glance, I'm not sure that will solve my
problem, as I don't have access to Gstreamer on the second machine.
But perhaps I can lift some concepts from that.

But, in order to lift some concepts, I need to make sure I understand
the basic concepts first.

It seems that GstNetClientClock provides a GstClock I can reference in
my application, but how would I associate that with the GstClock of my
pipeline?  Would I make that (some) the pipeline clock?  Or should the
pipeline clock be driven by the camera clock?  How would I associate
timestamps based on the camera clock with timestamps provided by
GstNetClientClock?

It's that sort of basic understanding that I am lacking at this point.

--wpd


On Thu, Jan 29, 2015 at 9:22 AM, Jan Schmidt <thaytan at noraisin.net> wrote:
> On 30/01/15 00:22, Patrick Doyle wrote:
>>
>> Hello fellow Gstreamer travelers...
>>
>> I have a system in which I collect and analyze videos on one computer
>> (let's call it A) and send the analysis reports to another computer
>> (B) via the network.  Now B needs to know the time at which Computer A
>> saw something.  But computer B's clock is completely independent of
>> that of Computer A.
>>
>> It strikes me that this is an already solved problem, and that
>> Gstreamer must have some infrastructure, and some techniques for
>> dealing with this sort of thing.  It seems very similar (in my mind)
>> to the situation where video frames are captured with a camera at 30
>> frames/sec, according to one crystal oscillator on the camera, while
>> audo is captured at 44100 samples/sec, according to the oscillator on
>> the audio card, and both must be combined to be transmitted over the
>> air via a (digital) broadcast television channel, whose (FCC
>> regulated) timing must be derived from yet a third, independent clock.
>>
>> So how is this handled in Gstreamer?
>
>
> What you want is the GStreamer network time provider and network clock. That
> will let you synchronise a common clock across different machines.
>
> http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer-libs/html/gstreamer-net.html
>
> Also, take a look at gstreamer/tests/examples/netclock I recently added to
> git master, along with some improvements for stability and accuracy of the
> clock implementation.
>
> Regards,
> Jan.
>
>>
>> I have read
>> http://gstreamer.freedesktop.org/data/doc/gstreamer/head/manual/html/chapter-clocks.html,
>> which states that every Gstreamer pipeline maintains a GstClock
>> object, and (a few pages later) that when "...the pipeline goes to the
>> PLAYING state, it will go over all elements in the pipeline from sink
>> to source and ask each element if they can provide a clock. The last
>> element that can provide a clock will be used as the clock provider in
>> the pipeline."
>>
>> All of that makes sense intellectually, but how does it work practically?
>>
>> In my case, I have a GigE Vision camera, to which I interface using
>> Emmanual Pacaud's excellent Aravis Gstreamer plugin.  That plugin is
>> derived from a GstPushSrc object.  Can/Should I assume that this
>> object provides the clock for the pipeline?  I can't find evidence of
>> that in gstpushsrc.c or gstbasesrc.c, but perhaps I'm not looking hard
>> enough.  What should I look for?
>>
>> As a side node, I'm curious why GstBaseSrc thinks that the aravissrc
>> is a  "pseudo-live" element instead of a "live" element.  What's the
>> difference between the two and why would I care?  (Right now, I only
>> care because I found it to be surprising when the GstBaseSrc element
>> computed a latency figure for the aravissrc object).
>>
>> Where am I going with all of this?  I don't know yet, but I thought I
>> would start by gaining a better understanding of what Gstreamer is
>> doing with clocks and such.
>>
>> Back to my original problem, if I could synchronize the clocks on the
>> two computers with NTP or somesuch, and I could report the difference
>> between the CPU clock and the videostream clock as measured on
>> computer A, then I should be able to relate that to time on computer
>> B.
>>
>> If I knew that the timestamps reported in the video pipeline were
>> those produced by the camera, how could could compute delta between
>> the camera clock and the CPU clock as early in the pipeline as
>> possible?  These clocks will drift with time, so I will need to track
>> this delta (or perhaps a low pass filtered version of it).
>>
>> Any help or tips would be greatly appreciated.
>>
>> Thanks for reading this far.
>>
>> --wpd
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