One of our readers sent me a link to a filter design site: T-Filter. It specialized in the calculation of FIR filters. I had a quick look at it and it looks nice. The filter calculation was very fast. But then I noticed that I was only calculating a filter for 2000 Hz sampling frequency. This is not really a real-world audio calculation. Then I tried 48 kHz sample rate. This also work, but the default passband ripple of 5 db is way too much for HiFi audio applications. Reducing the ripple to 0.5 db often led to empty filter. But the software is still in beta, therefore it may work in the future.