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-i = input file name
-s = set frame size, qcif=176x144
-vcodec = force video codec
-r = frame-rate [default = 25]
-b = bit-rate [200 kb/s]
-acodec = force audio codec
-ab = audio bitrate in bits/s [64k]
-ac = no. of audio channels 
-ar = audio sampling frequency [44100 Hz]
-sameq = use same video quality as source (implies VBR)
-f = force format
-y = overwrite output files