VirtualDub has an audio filter called "format convert"; however, it doesn't tell me how to use it. It only has an integer field called Precision, which defaults to some number depending on the incoming stream's sampling rate and bit depth. When I feed it a 16-bit 16kHz mono stream, it defaults to "3". How do I use this filter?

If you want to know why I am asking this question:

I want to convert a 48kHz 16-bit stereo PCM stream to a 16kHz 4-bit mono ADPCM stream that I can later insert into a YM2608+ADPCM (NEC SpeakBoard: 6FM+3SSG+2Rhythm+ADPCM) chiptune. Wavosaur cannot go below 8-bit depth, and it doesn't handle ADPCM WAV files.