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DirectShowSource("asffilename.grf", fps=29.97)
If you don't need the audio from the ASF, then you can just use AviSynth without Graphedit, as that's only really needed to preserve audio sync. Of course, using Graphedit isn't always a fool-proof method, and it may not preserve audio sync since you need to specify a framerate. One 99% foolproof method of maintaining audio sync is to decode the ASF or WMV files to AVI using TMPGEnc instead.
Similarly, using Graphedit+AviSynth is the recommended way to serve in H.264-encoded MKV, MP4, or MOV files into VDub. Although in that case the fps function isn't needed.