dvbscale[=aspect]
Set up optimal scaling for DVB cards, scaling the x axis in
hardware and calculating the y axis scaling in software to keep
aspect. Only useful together with expand and scale.
<aspect>
Control aspect ratio, calculate as DVB_HEIGHT*ASPECTRA-
TIO (default: 576*4/3=768), set it to 576*(16/9)=1024
for a 16:9 TV.
EXAMPLE:
-vf dvbscale,scale=-1:0,expand=-1:576:-1:-1:1,lavc
FIXME: Explain what this does.