Tracking Error

Radial ArmWhen the  master Disc is cut, the cutter head tracks in a dead straight line across the record from the outer edge to the centre of the record, using a radial arm, (as shown in Fig 1) so that the vibrations of the cutting stylus takes place in a plane which is precisely at right angles to the groove.  Ideally, during playback, the stylus should follow this same path, maintaining the same angle.  However, given that most pick-up arms are pivoted near one end, the stylus, rather than travelling in the ideal straight line over the record, travels along an arc of a circle, the centre of which being the pivot.  This difference in tracking causes distortion known as 'Tracking Error'.  If the radius (pivot to stylus) was equal to the distance from the pivot to the centre of the record, the tracking error would be zero at that centre point as shown in fig 2.

To minimise this difference in tracking, manufactures make geometrical changes to the pick-up arm by setting the stylus at an angle to the arm known as the 'offset angle'.  In the case of a straight tube arm, the changes are made by setting the headshell at an angle, with the other option being to shape the arm itself to the familiar 'J' and 'S' shapes. In addition to the offset angle of the arm, fine adjustments can be made to the cartridge within the headshell to ensure that the stylus is at right angles to the radius.

Click to enlarge In addition to the offset angle, consideration should be given to the distance in which the stylus extends beyond the centre of the record, known as the 'Overhang'.  The 'Overhang' adjustment is made by moving the arm's pivoting point close enough to the turntable, (or by moving the cartridge forward within the headshell) so that the sylus traces an arc beyond the centre of the record from the arm's pivoting point.  Fig 3 shows the overhang set, this coupled with the correct offset, the tracking error is set to zero at two points, with the other tracking error points between the two being negligibly small.

Most quality pick up arms come with a template and 'Alignment Protractor' for positioning the correct distance from the pick-up arm pivot to the centre of the turntable spindle and overhang setting.

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