What's the key to clean sounding fingered trills at lower pitch? I can lip-trill fine at the top of the stave and upwards, and the pitch snaps cleanly back and forth between the upper and lower notes. But fast fingered trills lower down sound messy, because if I concentrate on the upper note, and think about trilling downwards to the lower note, then the upper note is OK but the lower one is indistinct. Conversely, if I concentrate on the lower note and trill upwards, the upper note is indistinct. Do I have to pitch each note in the trill, like I would if I were lip-trilling? Or is there something else I have to do?