An optical type disc is recorded with information signals on each sloping surface of a spiral or concentric V-shaped grooves formed on a recording surface thereof, wherein the information signals are recorded as rows of geometrical depressions or projections on each sloping surface of the V-shaped groove...http://www.google.com.tw/patents/US4674070?utm_source=gb-gplus-share專利 US4674070 - Optical type disc with V-shaped groove and optical type reproducing apparatus therefor
