Math calculations about the speed of a TT..help...

In school I learned that the different parts of a spinning circular object are moving at different speeds depending on how far they are from the center. This is because for a point, lets say half way down a record, to rotate once back to its position it must travel a certain distance ( the diameter up to that point times pi - circumference ) and this distance is different then that of a point at the very edge of the album, or at the begining. The record rotates together though, reaching one revolution at the same time throughout so for this to be possible the different parts would have to travel different speeds. So how do records play at a certain speed ( thirty three and one third etc. ) when acoording to mathematics this is immpossible? How does this work? Also how is the album pressed to take this into consideration? I have just learned this recently so any thing anyone wants correct/ add is greatly appreciated! Thanks for your time!
weisera
6 responses
 03-08-2003 2:27amIt is because the master is cut the same way it is played. 03-08-2003 2:37amGreat answer Twl. Right to the point.KF 03-08-2003 3:04amIt is possible because it is 33.33 revolutions, not inches per second. So yes, at the inner grooves the information is compressed into a smaller space. This is why resolution suffers at the inner grooves. Mistracking is also greater due to the increased radius of curvature. As Tom so aptly points out, if you cut the record this way it will play back the same way. Interestingly, the compact disc varies its rotational speed so as to keep a like amount of information always being read. Compact discs also read from the innermost point to the outermost, the opposite of LPs. 03-08-2003 3:26amWeisera - You're confusing angular velocity with linear velocity. The angular velocity of the LP is 33 and a third revolutions per minute (RPM). This value applies to all points of the record (except the point at the geometric center.)The linear velocity, however, does vary with radial distance from the center of the record. The linear velocity of a point on the record is equal to the product of the point's distance from the center (effective radius) and the angular velocity of the record.Thus the linear velocity of a point 3 inches from the center of a record with an angular velocity of 33 and a third RPM would be 100 inches per minute.So what does this mean? All things being equal, the higher the linear velocity at the stylus, the better the sound. Thus an LP sounds its best toward the outside of the record, and its worst at toward the inner grooves.That's why some audiophile records are cut with large runout zones. They avoid using the inner third or so of the record to keep the sound quality up. Another strategy is to cut the record at 45 RPM, which increases the linear velocity at all points of the record by about 135% ((45/33.33)x100%), but cuts overall playing time by about 20%.In real life a cutting engineer has to balance fidelity across the record, required playing time on the side, bass response, and dynamic range. The latter two items affect groove width, and thus impact playing time. 03-08-2003 4:57amNice post, Ghostrider. Didn't you sit behind me in physics? :-) 03-08-2003 8:02amVery informative responses. Thanks for sharing your knowledge folks. Taught me something new today. Sean>