The two-control interface to this envelope filter seems to have worked out pretty well. There's two controls - sensitivity and start point. Sensitivity pans - from minimum to maximum - between the 'traditional' up and down-drive filter sounds. At extreme setttings the control is at maximum sensitivity, whereas around the middle the filter doesn't respond to dynamics at all, providing a fixed-filter sound. Start point sets the 'start' of the filter sweep, whether that be upwards, downwards or static (if the sensitivity control is set to dead-centre or 12 o'clock).