Gstreamer timestamps bad values, buffer has no offset or duration

[Letty]

I am using Gstreamer to record two live cameras at the same time with Python.
I want to see if the cameras are streaming in synch, so I access the buffer
timestamps (pts and dts), offset and duration and save them all to file. I
am confused by the fact that for each camera the timestamps seem to not be
regular at all. I am capturing at 30fps, so I am expecting the difference
between consecutive timestamps to be around 33ms, but what I see instead
varies between 10ms to 50ms. Moreover the two timestamps pts and dts are the
same, and the offset and duration have no valid value.

Here is my input pipeline

def gstreamer_in(cam_id):
    gst_in =  ('nvarguscamerasrc sensor-id={} ! ' 
    'video/x-raw(memory:NVMM), height=1080, width=1920,
framerate=(fraction)30/1, format=NV12 ! ' 
    'nvvidconv ! video/x-raw, height=1080, width=1920, format=BGRx ! '
    'videoconvert ! video/x-raw, format=BGR ! '
    'appsink emit-signals=true sync=false').format(cam_id)
    return gst_in

and here are a few of the values that I am getting

N frame, dts,pts, offset, duration
0, 0:00:03.527208860, 0:00:03.527208860, CLOCK_TIME_NONE, CLOCK_TIME_NONE
1, 0:00:03.606427245, 0:00:03.606427245, CLOCK_TIME_NONE, CLOCK_TIME_NONE
2, 0:00:03.643383157, 0:00:03.643383157, CLOCK_TIME_NONE, CLOCK_TIME_NONE
3, 0:00:03.682110536, 0:00:03.682110536, CLOCK_TIME_NONE, CLOCK_TIME_NONE
4, 0:00:03.730528664, 0:00:03.730528664, CLOCK_TIME_NONE, CLOCK_TIME_NONE
5, 0:00:03.771878491, 0:00:03.771878491, CLOCK_TIME_NONE, CLOCK_TIME_NONE

I am a beginner in Gstreamer, any help about understanding what's going on
would be greatly appreciated.




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