Nick and Lewm,
I should have checked my post, as I meant to say that the arm in Nick's straight arm example "has zero skating forces at one point, which coincides with it being a null..." (not overhang)
The arm has underhang as measured from the turntable centre, and so experiences positive and negative skating forces, as the orientation of the arm varies as it pivots. There is just one point where both the arm and cantilever are tangent to the groove (as in a linear tracking arm) and so there is neither skating force nor tracking angle error. However, everywhere else the error becomes extreme and difficult to compensate for.
To reduce the error there has to be overhang, and as this is increased until, at two points (which will later be the nulls) the errors are equalised either side of a particular value.
The reason for cartridge offset is to minimise the error angle with regard to the stylus, and set the cantilever and stylus in line with the groove, thus constituting a null at two points, but this does not alter the angle between groove tangent, stylus and arm pivot which still varies across the record and is never zero.
And it is this angle which generates the skating force i.e. that between the groove tangent and the arm effective length, and only at the nulls is it the same as the value of the equalised tracking error angle and the cartridge offset angle.
I should have checked my post, as I meant to say that the arm in Nick's straight arm example "has zero skating forces at one point, which coincides with it being a null..." (not overhang)
The arm has underhang as measured from the turntable centre, and so experiences positive and negative skating forces, as the orientation of the arm varies as it pivots. There is just one point where both the arm and cantilever are tangent to the groove (as in a linear tracking arm) and so there is neither skating force nor tracking angle error. However, everywhere else the error becomes extreme and difficult to compensate for.
To reduce the error there has to be overhang, and as this is increased until, at two points (which will later be the nulls) the errors are equalised either side of a particular value.
The reason for cartridge offset is to minimise the error angle with regard to the stylus, and set the cantilever and stylus in line with the groove, thus constituting a null at two points, but this does not alter the angle between groove tangent, stylus and arm pivot which still varies across the record and is never zero.
And it is this angle which generates the skating force i.e. that between the groove tangent and the arm effective length, and only at the nulls is it the same as the value of the equalised tracking error angle and the cartridge offset angle.
