The earliest machines produced distortion during the recording process which German engineers significantly reduced during the Nazi Germany era by applying a "bias" signal to the tape. In 1939, one machine was found to make consistently better recordings than other ostensibly identical models, and when it was taken apart a minor flaw was noticed. Instead of DC, it was introducing an AC bias signal to the tape , and this was quickly adapted to new models using a high-frequency AC bias that has remained a part of audio tape recording to this day. The quality was so greatly improved that recordings surpassed the quality of most radio transmitters, and such recordings were used by Adolf Hitler to make broadcasts that appeared to be live while he was safely away in another city.