Why does decklinksrc need sync=FALSE on xvimagesink?

[Stirling Westrup]

We've been working to integrate input from a BlackMagic video capture card
into our video-wall application. The limited documentation we got with the
card indicated we should use decklinksrc and that we should set sync=false
on the output sink. Sure enough, this works:

gst-launch-1.0 decklinksrc mode=18 connection=1 ! videoconvert !
xvimagesink sync=false

However, if we set sync to true, then output drops immensely; we seem to be
getting just a few frames a second. The question is: why?  We'd really
rather not have to have a special exception for one input source to cause
us to set the sync flag differently on our output, but I can't figure out
why the decklinksrc needs it in the first place.

-- 
Stirling Westrup
Programmer, Entrepreneur.
https://www.linkedin.com/e/fpf/77228
http://www.linkedin.com/in/swestrup
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http://sourceforge.net/users/stirlingwestrup
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