Persuading the normally in-head stereo image experienced with headphones to migrate to a more natural, external position is a trick that has been attempted, with varying degrees of success, for decades. Setting aside binaural recording -which itself often fails to create an externalised image - initial attempts focused on recreating the acoustic crosstalk of stereo loudspeakers (where the left ear hears some of the right channel and vice-versa) using external circuitry. In an AES paper of 1961, the famous Ben Bauer of CBS Laboratories described a passive circuit for this, and active filters would later allow the same to be achieved more elegantly. But it was with the arrival of digital signal processing that exciting new possibilities opened up. What DSP brings to the party, apart from the ability to perform crosstalk filtering far more precisely, is the option to apply room simulation - although even with this, extemalisation remains hard to achieve.
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