Sick Of Talk

Since the playback stylus tracking angle has been standardized at 15 degrees from vertical with the stylus pointing into the disc rotation, the cutting head must be angled so that it produces a groove that is oriented in the same way. This is complicated by the fact that the lacquer springs back somewhat after being cut. As a result, cutting heads are operated at a greater angle of about 18 degrees in order to produce a 15- degree groove. This standardization of reproducer tracking angles is necessary because the nature of the recording stylus assembly causes vertical information to be cut in an arc, rather than strictly vertical. As the stylus moves in this arc, the groove angle becomes wider than 90- degrees. If the playback stylus tracking angle is not set to compensate for this increased groove angle, second-harmonic distortion would be introduced into the program.
    As shown in Fig. 12-14A, with no vertical information, the recording stylus is in position 1, and a 90-degree groove angle is cut. When the cutter receives vertical information, the stylus cuts deeper into the disc. It does not move straight down, rather the support arm pivots as shown by stylus position 2. This produces a groove angle greater than 90-degrees. Fig. 12-14B shows that since the playback stylus cutting angle is set to a (greater than 15 degree angle to compensate for spring back) so that the groove walls will present a 90-degree angle to the playback stylus. Fig.12-14C shows the vertical tracking angle error with the stylus perpendicular to the disc. Peaks of the stylus motion occur before the peaks of the groove motion, and dips in the stylus motion occur after dips in the groove.

    Other potential sources of signal degradation such as tracing distortion, the pinch effect, reaching the groove excursion limit and mis- tracking can also occur during playback of the finished record. Tracing distortion results from the difference in the shape of the recording and the playback styli. The recording stylus cutting edge comes to a sharp point while the playback stylus has a rounded point. Because of this, the playback stylus point of contact with the groove varies depending on the instantaneous amplitude of the groove modulation., and the path traced in playback is not the same as that recorded (Fig. 12-15). The point of groove wall contact for the play stylus wanders as the modulation is traced, producing an output corresponding to the dotted path rather than the modulation. The distortion increases as the signal level increases and as the wavelength of the recorded signal decreases and approaches the dimensions of the stylus tip. Thus, tracing distortion increases as the signal frequency increases and as groove diameter decreases, because both of these factors cause the wavelength of a signal to decrease (wavelength equals groove velocity divided by signal frequency, and groove velocity decreases as groove diameter decreases).


Tracing distortion refers to the inability of the playback stylus to follow groove modulations in the vertical plane. A similar problem in the lateral plane is called pinch effect. Due to the triangular shape of the cutting stylus, the width of the groove measured perpendicular to the line cut by the stylus tip does not remain constant (Fig. 12-16A). The playback stylus is pinched where the groove narrows, and therefore it rides higher in the groove, As the groove widens, the stylus rides lower in the groove (Fig. 12-16B). This motion adds a vertical component to the signal output that was not present in the input signal. The pinch effect is greater for high level and high frequency signals, for these cause abrupt changes in groove direction and therefore causes the width of the groove to become narrower.

    The groove excursion limit is reached when the radius of curvature of the groove modulation is equal to the tip radius of the playback stylus, preventing it from following all of the modulation. As shown in Fig. 12-17, the radius of curvature of the groove modulation is equal to the stylus tip radius, producing 50% second-harmonic distortion. Since the radius of curvature decreases both with increase in level as well as with increase in frequency, a limit exists on the level of high frequency information that can be accurately reproduced.
    All three of these types of distortion would be reduced if the playback stylus point were very small. Distortion would be eliminated altogether if the playback stylus was shaped like a recording stylus. A stylus with a small tip radius, however, would ride on the bottom of the groove, producing noise and other types of distortion. However, a cutter shaped stylus would tend to cut into the disc and erase high frequency information.

    A compromise solution is the use of bi-radial elliptical stylus rather than the standard conical stylus. The conical stylus has a round tip with a radius of 0.6 mil. The elliptical stylus has an edge radius of 0.2 mil and a radius at right angles to the groove of 0.7 mil. The small edge radius follows groove excursions more accurately than the conical stylus, while the larger radius prevents the stylus from hitting the bottom of the groove. Distortion is reduced somewhat, at the cost of increased wear on the disc.
    The Shibata stylus, designed for playing discrete quadraphonic discs, reduces wear by contacting the groove walls in a line rather than at only two points as do the conical and elliptical styli. The edge of the Shibata stylus is narrower than the elliptical stylus and therefore traces even higher recorded velocities. Fig.12-18 shows the cross sectional areas of the conical, elliptical and Shibata styli.


A more effective means of reducing tracing, pinch effect and groove excursion limit distortion is through the use of a “tracing simulator” such as Neumann’s Model TS-66. All three of the discussed types of distortion are predominately of the second harmonic variety. Pre-distorting the signal to be cut on the disc in an inverse manner to the distortion generated by the playback stylus (Fig.12-19) can reduce their effect. The tracing simulator generates a voltage corresponding to the second harmonic of the program signal and adds it to the program out of phase with the distortion created in playback. This second harmonic component is cancelled by the tracing, pinch effect, and groove excursion limit generated during playback, resulting in a disc with substantially less distortion and greater high frequency level capability.

Since the amount of distortion produced in the playback is a function of groove velocity, which is in turn determined by the groove diameter for any particular turntable speed, a series of micro-switches are connected in the lathe bed to sense the position of the cutter head. The micro-switches enable the drive signal to be pre-distorted the proper amount for that groove diameter.
    The tracing simulator can only optimize distortion for one tip radius at a time; it is usually adjusted for 0.6 mil that is the international standard for the radius of a conical stylus. The elliptical and Shibata styli with their narrow side radii would not completely cancel the second-harmonic distortion added to the signal by the tracing simulator. Therefore, these styli would not receive the full benefits of the distortion reduction.
    Mistracking results from loss of stylus contact with the groove walls during playback, due to record velocities in excess of that which the stylus can follow. The recorded velocity is measured in centimeters per second (cm/sec) and is determined by computing the distance that the playback stylus must move, laterally or vertically, per second from the unmodulated groove position to accurately track the groove modulation. Since recorded velocity is constantly changing, the value used is instantaneous peak velocity.
    Mistracking often sounds like a buzz or crackling on heavily modulated passages, or sibilance on vocals. Mistracking can cause instruments which should have sharp, clear sounds, such as bells, to produce a dull thud at the beginning of each note. Increasing the tracking force (pressure of playback stylus on the groove) reduces mistracking. The maximum recorded velocity, listed by frequency, which can be accurately tracked at a certain tracking force is a function of its design and specifies the trackability of a stylus/cartridge assembly. The better the trackability, the lower the tracking force needed to prevent mistracking.
    The pressure of the stylus results in some indention of the groove modulations. As the radius of curvature of the recorded signal increases, the stylus indents the record surfce more, resulting in less output. This playback loss more severe as frequency rises and as groove diameter decreases, because both of these increase the radius of curvature. Typical finished records have playback losses of 3 to 4 dB at 15- kHz at minimum diameter, relative to the master tapes. Stiffer record compounds are indented less and therefore have lower losses.
    Most phonograph cartridges are designed using either magnetic or piezoelectric principles. The magnetic cartridge coverts stylus motion into electrical current flow through use of a permanent magnets and coils. Designs are available using fixed magnets with fixed coils in which the stylus motion varies the magnetic coupling to the coils. In all cases, the coils are angled 90 degrees apart from each other and are oriented so that signals in the outer wall of the groove produce current flow in the left channel coil. The coils are phased so that lateral stylus motion produces in-phase outputs from both coils, while vertical stylus motion produces outputs that are out of phase.
    The piezoelectric or crystal phonograph cartridge generates an output voltage proportional to the varying pressure applied to a crystal by the stylus motion. Crystal cartridges are much less expensive to manufacture than magnetic ones, they also produce higher outputs levels so that the extra stage of pre-amplification required by magnetic cartridges is not needed. The magnetic cartridge, however, is quieter and operates at lower tracking forces, creating less wear on the grooves. Crystal cartridges are mainly used on lower end record players.