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Paging Mr. Schroeder

Mr. Manfred Schroeder, you are wanted at the psychoacoustics desk…I was rummaging through old papers in prep for my annual taxing visit, when I can upon an old receipt. It was from Sound Concepts, a now defunct hifi company in Brookline Massachusetts. The item, purchased back on 09/22/81, was a low cost “schroderizer.” What the heck is a schroderizer? :: For the answer, we must step back a moment and discuss the concept of stereo…We all know about stereo reproduction — two distinct channels fed to two loudspeakers, placed in a roughly equilateral triangle relative to the listener’s head. OK, all well and good. Trouble with this approach, and one of the many reasons why headphone listening is subjectively so different from loudspeaker playback, it that each speaker “feeds” both ears. Assuming the listener is facing the speakers, the left speaker is heard first by the left ear then, it is later heard by the right ear. This produces a more or less ambiguous collection of unnatural sensations that the brain has to sort out. What if you could remove this left–into–right and right–into–left signal leakage? That’s just what a schroderizer does. :: The process, also know as cross–correlation cancellation, starts with two delay lines, one for each channel and ear. A delayed and EQ’d version of the “unwanted” signal is inverted or polarity flipped, then added to the opposite channel. The sum is then fed to the speaker. The amount of delay, adjustable in my Sound Concepts IR2100, is designed to approximate the transit time between left and right ears. If the inverted signal is heard at the same time as the acoustically delayed signal arrives at the “wrong” ear, they should cancel out, sort of. :: This is, as the Brits might say, all rather dodgy, with too many variables for it to be an exact solution to the problem. However, on a good day, depending on how a piece of music was engineered, it works quite well. The result is that the normal phantom image trapped between two loudspeakers becomes very much wider indeed, with sounds apparently arriving from hard left, hard right and everywhere in between. :: Thanks to the pioneering work of Monsieurs Schroder, Blauert, Jeklin and others, psychoacousticians have a fairly good idea of how we localize sounds. For all the complex spatialization algos built into modern integrated amplifiers, I miss my simple IR2100, if only because it was a cheap kludge that, with the right music, really knocked your socks off. :: All for now, time to tend my dinner…

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