Handbook for Sound Engineers

Analog Disc Playback 1025

variable pitch common to all present-day recordings, the

speed with which the tonearm moves across the record

varies and at times may even be zero. Because of this

variation, the mechanism that generates the antiskating

force should be able to generate a uniform force at all

times, regardless of the motion of the tonearm. Anti-

skating force can be generated by using springs,

magnets, weights with pulleys, electrical devices, and

mechanical linkages and weights, Fig. 27-14. Any

method to apply the clockwise bias in a horizontal plane

to the tonearm to counteract the skating force produces

positive results; however, compensation may not be

accurate for all types of systems.

The effectiveness of the antiskating force mechanism

depends to a high degree on the dynamic behavior of

the tonearm. If the tonearm is not dynamically balanced

(and most of them are not), any tilt of the turntable may

result in a change of skating force, endangering the

tracking ability of the pickup. As was previously

mentioned, the dynamic balancing of the tonearm

implies that the pivot point of the tonearm is also the

center of mass. In most modern tonearms this center of

weight is shifted toward the cartridge end in order to

produce tracking force, Fig. 27-15. In a dynamically

balanced tonearm, tracking force is produced by using

either a spring or a permanent or electromagnet (sole-

noid). A properly dynamically balanced tonearm could

play a record with the turntable being in any position

and is completely insensitive to jarring of the turntable

or floor vibrations.

Vertical Tracking Angle. An important adjustment of
the tonearm is in positioning the cartridge over the sur-
face of the disc. Cartridges are mounted in tonearms so
that the mounting surface of the cartridge is parallel to
the record surface, Fig. 27-16A. Sometimes tilting the
cartridge fore or aft results in lower tracking distortion.
Some cartridges are designed to produce the lowest dis-
tortion when playing vertical modulation that was
recorded at the vertical cutting angle of 25°, Fig.
27-16B. At the same time most of today’s records are
cut with the vertical angle of 10°–15°. So in order to
reduce the distortion during playback, matching the two
angles by moving or tilting the cartridge backward a
few degrees may help reduce tracing distortion.

Tonearm Resonance Damping. A Shure Brothers, Inc.
study revealed that the warp frequencies of LP records
lie in the region from one revolution (0.5 Hz) peaking at
3 Hz and tapering down at 7–8 Hz. Because the audible
range of frequencies starts at around 20 Hz, tonearm res-
onance placed between the warp frequency region and
the audible region will allow minimum distortion of the

signal due to tonearm bounce. As a result of this

research, improvements were made in the tonearms by

applying vertical damping to the tonearm. The vertical

tonearm motion control was attacked by Discwasher,

Inc., by designing a special damping mechanism named

Disctracker, which attached to the cartridge. Shure

Brothers introduced their stabilizer brush that attached

to the cartridge similar to the brushes invented and used

Figure 27-14. Different methods of generating antiskating

force.

C. Magnets.

A. Mechanical linkages and weights.

B. Weights and pulleys.

D. Springs.

Wire

Weight

Wire

Pulley

Weight

Arm shaft

Non magnetic

material

Taper magnetic

sheet

Magnet

Adjustment

knob

Spring