Everything you need to know about cartridges: Supplementary specifications
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SUPPLEMENTARY SPECIFICATIONS
The specifications discussed in the foregoing are the ones that appear most often in brochures and articles. In addition, however, more technical publications give supplementary specifications, and the most important of these are described below.
FIM
Frequency intermodulation is a form of distortion that occurs in a cartridge when it is required to reproduce two or more tones simultaneously. A high value of FIM will give the cartridge a raw, metallic sound. In a quality cartridge, frequency intermodulation distortion can be kept below 1% when the recommended tracking force is applied.
Vertical tracking angle
To achieve good tracking ability and channel separation, it is important that the record be played back at the same vertical angle as it was cut. Nearly all cartridge manufacturers today give their cartridges a vertical tracking angle of 20°. This is done on account of the proposal put forward to standardize the vertical cutting angle at 20°. The cartridge’s vertical tracking angle is defined as the angle that arises between the surface of the record and the straight line drawn from the diamond tip to the cantilever’s point of rotation inside the cartridge. Deviations from the correct vertical tracking angle will increase FIM distortion.
Equivalent stylus tip mass
The components that together comprise the equivalent stylus tip mass are the diamond, the cantilever and, depending on the design principle, the magnet, armature or coils, and the damping rubber bearing; although the contribution of the latter to the equivalent stylus tip mass is not constant, and depends on the frequency. At the lowest and medium range frequencies, it is the compliance that has greatest influence, whilst at high frequencies, it is the mass of the moving system that dominates.
It is important that the equivalent stylus tip mass must be as low as possible, in order to get the natural resonance of the system above the audible range where it can, to an extent, be kept under control by correct cartridge loading.
The weight of the cartridge
In order to be able to ascertain the natural resonance for a given cartridge and tonearm, it is necessary to know the weight of the cartridge. As all tonearms can only be balanced with cartridges having a weight within a specific range (for example, 5 - 11 grams or 2 - 8 grams), it is necessary to check whether a given cartridge matches a particular tonearm in this respect also, and, of course, vice versa.
As a rule, the best record reproduction is obtained with cartridges that have as low a weight as possible, and this should, therefore be taken into consideration when choosing a cartridge, tonearm and turntable.
Pinch effect
The gramophone record is cut with a triangular shaped cutting stylus, whereas the stylus that plays back the record has a different shape. The difference in shape between the cutting stylus and the cartridge stylus results in the so-called “pinch effect”, which is particularly noticeable in playback with a spherical diamond. As the groove does not have the same width throughout, the spherical diamond will be forced to make vertical movements while at the same time tracking the groove information. If the playback is in stereo, these vertical movements by the cartridge will result in unwanted signals in each channel that are out of phase. These unwanted signals will be heard as distortion. Pinch effect can be reduced by using a stylus with a shape that resembles the cutting stylus, for example elliptical or Fine Line styli.
Phase distortion
As a result of the difference in shape between the cutting stylus and the playback stylus, the tracking point in playback will move constantly, so that the two channels are not reproduced at precisely the same time, but with a low level of phase distortion, which will disturb the direction in the stereo image, making it difficult to pinpoint the individual instruments. This form of phase distortion is particularly pronounced when using a spherical diamond. Specially shaped styli like the Shibata and Fine Line give least distortion of this kind.
HF distortion
Distortion in the treble range occurs when the radius of the diamond is wider than the record groove’s radius of curvature. Even though the stylus may track well, it is unable to reach right out into the high frequency tips, and reproduction in this range may be distorted. As in the previous case, it is the spherical diamond which gives the highest distortion, while Fine Line and Shibata shapes give less distortion on account of their narrow profiles at the horizontal level.