Ensuring consistent framerate with videorate

[Nimi Wariboko Jr.]

Hi,

I'm developing an application with gstreamer 1.16.2 (Ubuntu 20 LTS) and I
have a pipeline the starts with NAL units fed into an AppSrc, then to
h264parse, then nvv4l2decoder, and then videorate to a final AppSink with
`sync=false`. On the video rate element I've set:

     videorate.set_property("drop-only", true);
     videorate.set_property("max-rate", 15);
     videorate.set_property("skip-to-first", true);
     videorate.set_property("average-period", 3 * 1_000_000_000);

And what I expect to see is a consistent rate of frames at 15fps. In my
test bed, for streams that are 25fps I get 15fps as expected. However I
have one camera that is 20fps and for some reason:

1. I am seeing an FPS that varies from 18-19fps.
2. There is considerable lag in the stream from real time. After a couple
hours the videorate stream is behind the original stream.

The properties in the documentation aren't clear how these pieces work and
most examples online exclusively use the capsfilter. Any reason why I could
be seeing this behavior?

Thanks
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