<filename> is the device you're restoring data from when you supply the 'f' switch. The default device is /dev/rmt/0 on sun, /dev/sa0 on OS X, and /dev/rmt0 on linux so if you remove the 'f' parameter, the command would extract from the default device and would look like:

tar vx [filename]

If it's a different device

tar xvf [device] [filename]

Carl