Fello 1.0 is a software using the Cello’s sound as its sole controlling-data input. It has been developed by Florian Grote (Pure Data-Programming) and Andreas Otto (Cello) at STEIM in May 2005. The basic idea is to have an improvisation environment which can be characterised rather as an instrument than an effect-device. By means of pitch-detection and envelope-following, Fello 1.0 tries to follow the instrumentalist’s expression, loading the sound with the deficiency of perfection in the process of frequency analysis. The results range from stuttering melody fragments to feedback cascades, rendering both the instrumentalist and the computer attentive listeners.