Front and center

The ear brain mechanism we use to identify sounds and their locations is an extraordinary device. Imagine having to engineer something similar, with only two microphones to replace ears. The processing power required would be enormous. The simple act of measuring distance by comparing arrival times between both microphones is a decent enough challenge, but add to that spatial positioning, room boundaries, object recognition by tonal qualities, and rejection of unwanted noise, and the required processing increases by magnitudes. Yet our brains handle these tasks easily with only a few years training. Tricking our internal localization mechanisms into believing sound does not come directly from loudspeakers is more than just a clever parlor trick - it is something both possible and desirable in stereo systems - a requirement for proper imaging of music, and something that can be taught through a better understanding of what takes place and why. Today let's start with the simplest disappearing act of all time, the phantom center image. Before Alan Blumlein 'invented' stereo sound there was only mono. Sound came from one speaker with localization information that includes distance between the source of sound and the microphone, but not direction. Specific lateral spatial information is missing and Blumlein's discovery changed all that. The addition of a second loudspeaker added more than just another dimension; as two became three when the phantom center channel was born. Tomorrow we'll start to look at how this works.