How did a German tape recorder and Bing Crosby change how music is made?

By making recordings that sounded live and could be cut, fixed and done again. Magnetic tape records sound as a pattern of magnetism: a head polarizes the magnetic material in the moving tape in proportion to the audio signal, and a playback head turns the changing field back into an electrical signal for a loudspeaker. Once a German machine learned to do this faithfully, tape gave artists the power to record and re-record with minimal loss in quality and to edit and rearrange recordings with ease.

The idea began around 1930 in Germany, as paper tape with oxide lacquered on it. The German company AEG made the first practical tape recorder, the K1, shown in Berlin in 1935. Early results were poor: when the London Philharmonic heard a 1936 recording of their concert, they were greatly disappointed by the distortion and noise. The fix was found by accident in 1941. A Magnetophon that produced recordings of extraordinary quality was sent in for repair, and engineers found it had an oscillating bias amplifier. Adding an inaudible high-frequency tone, they realised, gave a striking improvement. The new reproduction was so good that even people well acquainted with the industry could not tell it from live play.

Then came a very American twist. Live music was the standard for radio, and the networks did not allow disc recording in many programs because the sound was comparatively poor. Bing Crosby disliked the regimentation of live broadcasts and preferred the relaxed studio, where the best parts of a performance could be kept. An engineer named John Mullin had brought back two Magnetophons and fifty reels of tape. Crosby's season premiere on 1 October 1947 was the first magnetic tape broadcast in America, and he invested $50,000 in the company Ampex. His shows were edited by tape-splicing into a pace and flow wholly new to radio.