ffmpeg sdi unsync

if you found out that ffmpeg encoding result from SDI input will become audio / video unsync after some while, you may try:

1. changing the SDI card mode. or try  temporarily fixing fps by assigning input fps in the cmd: such as ffmpeg -r 29 -i $input .
2. compile ffmpeg latest version, by following the instruction below.

REF: https://trac.ffmpeg.org/wiki/CompilationGuide