Stereo sound recording on tape was perfected in 1943 by German audio engineers working for the AEG corporation. Around 250 stereo tape recordings were made during this period (of which only three have survived), but the technology remained a closely guarded secret within Germany until the end of World War II. After the war, American audio engineer John T. Mullin and the Ampex corporation pioneered the commercial development of tape recording in the USA, and the technology was rapidly taken up by radio and the music industry due to its superior sound fidelity and because tape – being a linear recording medium – could be easily edited, by physically cutting and splicing the tape, to remove unwanted elements and create a 'perfect' recording. 2-track tape recording was rapidly adopted for modern music in the 1950s because it enabled signals from two or more separate microphones to be recorded simultaneously, enabling stereophonic recordings to be made and edited conveniently, which in turn facilitated the rapid expansion of the consumer high-fidelity ("HiFi") market. Stereo (either true binaural two-microphone stereo or multimixed) quickly became the norm for commercial classical recordings and radio broadcasts, although many pop music and jazz recordings continued to be issued in mono sound until the late 1960s.