 |
 |
----- Original Message ----- From: "Mike Richter" <[log in to unmask]> > > > Yes and no. Eccentric wow is periodic but also variable in amplitude. > > > The amplitude change depends on the pitch of the grooves, so may be > > > predictable or may be a function of the music content. > >You are thinking of the variable pitch cutting used for LPs? I don't > >think that was used for 78s. > I am simply saying that the rate of change of the magnitude of pitch > variation depends on the pitch of the recording. True, that was intended to > be well-behaved on 78s and was designed to vary only in microgroove, but > the statement holds. > >But would that affect the off-centre wow? The size of the bumps in a > >road is not affected by the slope of a hill. > It very definitely does. Eccentric wow is many times more audible near the > center of a disc than at the edge. > Eccentric wow arises because the off-center spindle makes the radius change > significantly during a single revolution. The percentage change of radius > for a given eccentricity is greater the shorter the radius. Thus, the > effective pitch change (or time shift) is greater at short radii than at long. > > > The frequency > > > of the time variation is exactly one cycle per rotation. > >And the rotation time could be fixed by playing all discs at a known > >rpm, set by a stroboscope. However, most restorers seem to prefer to try > >to match the playing rpm to the recording rpm, as closely as possible. > >So unless you know this exactly, you would need to adjust. > If there is no information on how the disc was played (e.g., one begins > with a tape made by an unknowable process), then the period is determined > just as phase is estimated: by listening to the end of the recording where > the amplitude is largest. If there is information on the speed of playback, > that can materially affect the accuracy of correction. > As someone suggested, picking up hum is a good way to get a reference - but > that won't help discs pre-1925 and is far from easy on any 78s. Something > about frequency response. :-) Now...this discussion may have already been put to rest...but reading the thread has be interested! First... the centre hole will be offset by a fixed distance along an axis which could be defined as (x) where (0) is the exact point of concentricity. Draw a line connecting the two points (exact centre and centre of hole) and measure between those two points. You now have (and I'll admit this is more complicated than I can comprehend this late at night) a point from which to establish a radius for the series of circles forming the spiral. Obviously, there would seem to be a method to create an equation, using fairly advanced analytic geometry (which I last studied four decades ago!) defining the speed variation created by the eccentricity (of the hole, not of the collectors involved!). It seems to me this would be dependent only on the distance between the two points involved...and possibly with the diameter of outside and inside circles which establish the spiral. Thus, the spiral, if perfectly centered, keeps the linear velocity at a linearly-increasing rate, exactly matching the linear velocity of the playing stylus...if not, the rate of variance will be dependent on the variation of the nominal "radius" induced by the variation in distance due to the mismatching of the two centres (I think calculus gets involved here, folks!). Now, somebody who still uses and/or remembers the detials of his/her/its math classes may take over... Steven C. Barr