Actually, you don't even need the AC3 Filter, even though it is pretty good. I used to use it but just recently uninstalled it after reading that you can do realtime encoding to AC3 with the most recent versions of FFDShow.
For analog output of 5.1, you need the following filters enabled:
AC3
Uncompressed
The following filters need to be in this order:
Mixer
LFE
Volume
Mixer tell it to mix the channels as 3/2+LFE
LFE enable the cross over and set it to about 120
Volume enable this, and set to Normalize, and View current volume level. Good for debugging purposes.
Now if you want to go to a complete digital route, you'll need one of the most current versions of FFDShow as it includes the ability to do Real Time encoding to AC3 output.
Codecs in FFDshow need to be setup as the following:
MP3/MP2 disabled
AC3 - SPID/F
DTS - SPID/F or enable this to the default handler if your digital receiver doesn't handle DTS.
Uncompressed - all supported types
Filters in the following order:
Resample - 48000 (it is a good idea to resample to the correct AC3 Supported audio level).
Mixer - 3/2+LFE
LFE - 120
Volume - Normalize, View current volume levels
Output - set this to AC3
Now whenever an a video is played, if it has an MPEG 2 or MP3 stream, the uncompressed audio will be converted to a 3/2+LFE stream, and when OUTPUTed will be encoded to an AC3 digital stream sent out through the SPID/F digital output.
AC3Filter has similar support for realtime encoding, and doesn't produce very good results (lots of pops and hisses). The AC3 encoding seems to be pretty good for FFDShow Audio.