Despite the very good performance obtainable from a tonearm conceived and realised according to the offset and overhang principle, it remains true that some lateral tracking error must be tolerated across the entire travel of the tonearm; except at the two null points.
This fact alone has ensured that some equipment designers still hanker after a radial tonearm which tracks across the record in exactly the same way as the original cutting head, thereby reducing lateral tracking error to zero.
In truth, because tracking error may be reduced to negligible proportions using the standard offset and overhang tonearm, the real advantages of a radial tracking arm is not in the elimination of horizontal tracking error as is often claimed. Rather it is in the elimination of side-thrust and the ability substantially to reduce the length of the tonearm so that its resonant modes play a diminished role.
Two ways to tango
There are two main design philosophies by which such a radial, linear-tracking tonearm may be realised which we might call active and passive.
Active
Active linear tracking tonearms are driven across the radius of the record via a motor. The very famous Technics SL-10 (illustrated above) incorporated an active tonearm of this type. The short tonearm travels on a parallel pair of bars which run across the back of the turntable. The tonearm is driven by a cord, propelled by a small electric motor. In this way, a very compact design may be realised. The footprint of the Technics SL-10 was only slightly bigger than an LP record sleeve
In all linear tracking designs, free vertical movement of the cartridge is permitted (as in a conventional tonearm) to allow for record warps. Downforce control is provided either using gravity as in a conventional tonearm or, in the case of Technics, electrodynamically so that records could be played with the player on its side - quite a party trick!
Because the groove pitch of modern records varies, it is not acceptable to have the motor advance the tonearm at a constant speed. Instead, the motor must be controlled by a sensor input to a servo control system. In the case of the SL-10, the tonearm is allowed to pivot very slightly, the degree of angular offset being sensed by an optical sensor which provides a control voltage to the servo amplifier which supplies the drive current to the tracking motor. Should the motor advance the arm too slowly, the tonearm will start to pivot towards the centre of the record which causes the optical sensor to produce an increased voltage output. This increased voltage, suitably processed by the servo amplifier, is fed to the motor to make it gather speed until the equilibrium condition is obtained such that the arm pivot is perfectly aligned with the playing groove.
In the situation which is the reverse of the above, so that the arm is advanced too quickly, the optical sensor produces a decreased voltage output which, suitably processed, is fed to the motor to make it run slower until the equilibrium condition is obtained.
Other manufactures used variations on this theme; for example, with the tonearm driven via a leadscrew rather than with a cord.
Passive designs
Passive linear tracking systems rely on the reduction of friction of the radial tracking mechanism such that the engagement of the stylus in the groove provides enough motive power to pull the tonearm across the surface of the disc in sympathy with the groove spiral as it does with a standard, pivoted tonearm.
However, whereas the reduction of friction in a rotating bearing is relatively easily accomplished (usually with a high-finish needle running a miniature ball-race), the elimination of friction in a sliding bearing is a more complicated engineering proposition. Modern, passive linear tonearms of this type often rely on relatively exotic technologies. For example, using a small electric pump to supply compressed air into the gap of the sliding bearing. (One such tonerm due to the Japanese company CS Port is illustrated.)
Other approaches...
A few radial (linear-tracking) designs do not fall easily into the active or passive categories.
In the 1980s, Harmon Kardon developed a radial tracking arm in which a turning drive-shaft runs the whole radius of the disc driven by an electric motor (right). Upon this drive-shaft runs an angled runner wheel which drives the tonearm. Momentary errors of tracking alter the bias forces on the angled runner and produce correcting forces to bring the arm back into alignment with the playing groove: a very ingenious mechanical design¹.
And a very early design of radial tonearm developed by RCA² also used a turning, threadless drive-shaft upon which ran the sled carrying the cartridge. RCA had done a great deal of work in discovering and developing a turning, high-finish rotating "runner" as an almost frictionless sliding bearing. Both these designs might be termed threadless leadscrew designs.
Finally, we should mention a development of the standard, pivoted tonearm in which a second tube is used to rotate the head of the tonearm as it passes across the record surface; thereby maintaining the cartridge at a perfect tangent to the groove spiral at all radii. These designs seem to combine the simplicity of a pivoted arm with a linear-tracking arrangement. The idea seems to have originated with Burne Jones Ltd. (left), a British manufacturer in the late 1950s and is periodically rediscovered, most famously in the True Tangent tonearm fitted on the Garrard Zero 100 range of turntables in the 1970s.
The problem with the Burne Jones concept is that, of the four principal requirements of a tonearm: low tracking-error; low inertia, low friction and low audio-range resonances, only the first is prioritised. Very considerable engineering excellence must be pressed into service in such a design if the other three requirements are not to be compromised. For example, in place of the one pivot bearing of the conventional tonearm, four are required in the Burne Jones design. Modern implementations of the Burne Jones approach are thereby very expensive.
No side-thrust
In the case of a tonearm designed on the offset and overhang principle, as the groove passes the stylus it provides a forward drag which is tangential the record groove at the point of contact. The line of action of this drag is displaced from the pivot of the tonearm to the extent of the linear offset. Thus, the forward force at the stylus produces a clockwise moment about the pivot which tends to press the stylus against the inner groove wall³. This is called side-thrust and the effect must be properly balanced by an opposing balancing outward force arranged by the tonearm designer. This balancing force is sometimes arranged by means of a small extra weight , or it is arranged via a small coil spring or a magnetic assembly. Read more about the origins of side-thrust and its amelioration.
Because the linear-tracking tonearm is perfectly straight and aligned with the groove, the line of action of the drag at the stylus is along the axis of the tonearm itself and no side-thrust is generated. This is a significant advantage of the radial tonearm because, when applying a balancing force to a conventional arm via an external device, great care must be taken to ensure that the device does not add friction of its own and impede the free movement of the arm. It is quite feasible for more harm than good to occur particularly at low playing weights. The radial tonearm completely sidesteps these issues.
We want to be able to play 12 inch (30cm) records, so, a conventional tonearm must be, at least, 7 inches (18cm) long with 9 inches being standard to allow for gimbal bearings, adjustments an so on. But, the longer the tonearm, the greater its moment of inertia and this constrains the performance of the dynamic system at low and sub-audible frequencies. The lighter the tonearm can be made, the more the dynamic system may be made independent of the tonearm inertia. Because the radial tonearm tracks across the record it is not constrained by the dimension of the disc and may be made very short and light compared with a conventional, pivoted arm.
The electrical output from an electrodynamic cartridge is proportional to the difference in velocity between the stylus and the cartridge body. A massive tonearm design absorbs the energy imparted to the cartridge from the modulated groove, only a tiny fraction of which is converted into electrical energy.
Once a lightweight tonearm is chosen made of stiff, lightweight materials, the pickup cartridge cannot be considered a rest mass. In other words, the stylus will be moving relative to a cartridge which itself is not still. Therefore the movements of the tonearm will be tranduced every bit as reliably as the movement of the stylus. Consequently, the tonearm itself must be designed considering its vibrational properties because it too is contributing to the reproduced sound.
Since the tonearm consists basically of a longitudinal member, the resonances are chiefly the well-known longitudinal, torsional, and flexural resonances which are characteristic of a tube.
Noel Keywood4 has studied the resonant behaviour of tonearms and explains,
We attach a Bruel & Kjaer accelerometer Type 4517 to the headshell, above the cartridge and just forward of the headshell screws [and] use a vertically cut sweep on JVC TRS-1007 test disc that runs from 20Hz up to 20kHz.
Our measurement of vibration in the arm shows how severe this problem is, and what form it takes..... In practice arms vibrate in various complex ways.... Arms tubes flex (first order bending mode) at around 200Hz [and] also suffer high frequency effects that our measurements suggest are headshell related. In a resonant arm a 0.3g peak at 200Hz is common and it has a velocity of 0.2cm/sec. The mechanical velocity on the disc at 200Hz is 1.38 cms/sec. This puts the headshell resonance -17dB below groove excitation, so it is no small effect.
It's interesting to listen to an A/B comparison of the effect of the tonearm. In the following audio clip, the first half includes the effects of the tonearm, simulated using high-Q filters and added to6 the original signal in the proportions reported by Noel Keywood4. The second half is the original audio untreated. The differences are subtle but clearly audible. Counterintuitively, the tonearm effects are not on tonal balance, but seem to add a certain (unwanted) ambience, so that the original file sounds slightly "drier".
The challenge in tonearm design for the modern designer is therefore to develop a dynamical system which in which the arm inertia is low but one in which the tonearm tube won't "ring" and influence the reproduced sound. One option is to add damping, but this adds to the mass.
The horns of this dilemma are made less sharp if the length dimension the tonearm can be reduced. In a normal, pivoted arm, this is not an option. But, a radial arm may be made very short and thus may be damped without adding sufficient mass to spoil the low-frequency cartridge-arm resonance.
Oddly enough, a fair proportion of practical, commercial radial tracking tonearms employ long tonearm tubes and thereby sidestep this worthwhile advantage.
References and notes
1. Linear-track turntables. Popular Science June 1980
2. The Radial Tone Arm - an Unconventional Phonograph Pickup Suspension. Rovst, H.E., Presented at the Seventh Annual Convention of the Audio Engineering Society, October 12-15, 1955. JOURNAL OF THE AUDIO ENGINEERING SOCIETY JULY 1956, VOLUME 4, NUMBER 3
3. Dynamic Side Thrust in Pickups CRABBE, H. J. F. WIRELESS WORLD, MAY 1960
6. Actually, the extra, simulated signal is subtracted from the original because we assume the vibration of the arm in in-phase with the acoustic stimulation.
Active linear tracking tonearms are driven across the radius of the record via a motor. The very famous Technics SL-10 (illustrated above) incorporated an active tonearm of this type. The short tonearm travels on a parallel pair of bars which run across the back of the turntable. The tonearm is driven by a cord, propelled by a small electric motor. In this way, a very compact design may be realised. The footprint of the Technics SL-10 was only slightly bigger than an LP record sleeve
Noel Keywood4 has studied the resonant behaviour of tonearms and explains,
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