Dirk's Accordion Tuner / Stimmgerät explained in the German book 'Das PC Tonstudio' from Horst Zander. This book comes in seven parts. Part four (Der PC als elektroakustisches Meßgerät) explains Dirk's Accordion Tuner:

In the fourth volume of the PC Tonstudio, we get down to the nitty-gritty. What are ostensibly frequencies and signals can also be analyzed in detail. This volume offers profound insights into most of the relevant questions. Can voices and moods be identified, and if so – how can melodies be recognized? What methods of audio measurement technology exist? Which products can be used for these on a PC? How is PC-based measurement technology employed for the design and sound reinforcement of rooms, and where does audio analysis find concrete application?