approach may give higher fidelity, because the "difference" signal is usually of low power, and is thus less affected by the intrinsic distortion of "hill and dale"-style recording. Additionally, surface noise tends to be picked up in a greater capacity in the vertical channel, therefore a mono record played on a stereo system can be in worse shape than the same record in stereo and still be enjoyable. (See Gramophone record for more on lateral and vertical recording.) Although this system was conceived by Alan Blumlein of EMI in 1931 and was patented in the U.K. the same year, it was not reduced to practice by the inventor as was a requirement for patenting in the U.S. and elsewhere at that time. (Blumlein was killed in a plane crash while testing radar equipment during WW-II, and he, therefore, never reduced the system to actual practice through both a recording and'' a reproducing means.) EMI cut the first stereo test discs using this system in 1933, but it was not applied commercially until a quarter of a century later, and by another company entirely (Westrex division of Litton Industries Inc, as the successor to Western Electric Company), and dubbed StereoDisk. Stereo sound provides a more natural listening experience, since the spatial location of the source of a sound is (at least in part) reproduced. In the 1960s, it was common practice to generate stereo versions of music from monophonic master tapes, which were normally marked "electronically reprocessed" or "electronically enhanced" stereo on track listings. These were generated by a variety of processing techniques to try to separate out various elements; this left noticeable and unsatisfactory artifacts in the sound, typically sounding "phasey". However, as multichannel recording became increasingly available, it has become progressively easier to master or remaster more plausible stereo recordings out of the archived multitrack master tapes. Compact disc The Red Book CD specification includes two channels by default, and so a mono recording on CD either has one empty channel, or else the same signal being relayed to both channels simultaneously. Common usage In common usage, a "stereo" is a two-channel sound reproduction system, and a "stereo recording" is a two-channel recording. This is cause for much confusion, since five (or more)-channel home theater systems are not popularly described as "stereo". Most multichannel recordings are stereo recordings only in the sense that they're stereo "mixes" consisting of a