Centring

The centring of the grooves in relation to the pin of the turntable with which the record is played must, of course, correspond precisely to that with which the master lacquer was cut. See the following figure:

The recording radius is ri. In an ideal situation, the playback radius and the recording radius coincide perfectly. If they do not coincide:

• the playback stylus is nonetheless constrained in a circular trajectory, as it follows the groove;
• the centre of rotation of the groove, due to some possible issues that we will analyse below, may cyclically shift from the centre with respect to which the groove was cut;
• this causes a cyclical fluctuation of the reproduction radius rr and consequently of the peripheral velocity Vp;
• since the vibrations of the cutting stylus were recorded on a circular trajectory with a peripheral speed relative to the recording radius ri, which is slightly different from rr, and the speed of the playback turntable is constant and coincides with that of the recording, the frequency of the vibrations of the playback stylus is also cyclically altered, because it is not reproduced at the same peripheral speed at which it was recorded.

Especially at mid frequencies, this can produce a sound modulation that is easily perceptible to the human ear and extremely annoying. Naturally, it all depends on how severe the eccentricity is.

Let‘s take a numerical example, assuming that, for any reason, ri and rr have a maximum deviation of 1mm. Taking a random point, let‘s suppose that the original recording radius r is 7cm. This will cause the playback radius rr to fluctuate between a maximum of 7.05 and a minimum of 6.95cm.

To see the impact of eccentricity on the reproduction of recorded frequencies, we can split the path of the playback stylus in two uniform semicircular motions, knowing that it will follow the path with the minimum playback radius over half a turn of the disc, and that with the maximum radius over the other half turn, coinciding with the recording radius exactly at the midpoint of the complete turn.

Remembering that the peripheral speed is:

and that the angular velocity is, in this specific case:

by substituting we obtain:

The period T is not influenced by the radius and is therefore common to both peripheral playback speeds; in the case of playback at 33.33rpm it is:

We can therefore calculate the minimum and maximum playback peripheral speeds:

remembering that the groove had been written on a 7cm recording radius we also calculate the correct (recording) peripheral speed:

It can therefore be concluded that an eccentricity of 1mm around a recording radius of 7cm generates a difference of ±7.3‰ in the peripheral speed during playback. This means that, for example, if the groove contains a single sinusoidal waveform at 1kHz, throughout the entire rotation of the disc, instead of the fixed frequency of 1000Hz, a slight sweep would be heard that cyclically moves from 996Hz to 1004Hz.

To have a musical point of reference, we must consider that the shift of a semitone is approximately 5.9% of the frequency of that tone, therefore an eccentricity of 1mm when played at 33.33rpm is equivalent to just over 1/10 of a semitone.

The mechanisms normally provided to create and maintain the centring of the grooves before and during the pressing of the records are:

• the microscope centring of the stampers, before their preforming to adapt them to the shape of the bushes and moulds of the press;
• the locking of the stampers onto the press moulds by means of conical bushes, which tighten the centre of the stamper against the centre of the press mould. At the centre of these bushes, two pins with a diameter of 7.4mm slide, which, during the pressing process, keep pushing against each other, ensuring that the central hole of the record is formed. In some presses, the pins are designed to act like a small punch, both with sharp edges, one flat pin, and the other with a hollowed-out part inside. In other presses, one of the two bushes is blind, and the hole is created only by the flat pin. 

• locking systems for the moulds, so that, even when subjected to maximum force, they don‘t move. In some types of presses, these are sliding bushes that are inserted at the back of the mould using a handle, while in other types there are 4 fixed bushes at the corners of the frame on which the mould is mounted;

• equally important is the trimming turntable on which the record, just removed from the press, is placed. The record is held by vacuum while two knives trim the excess PVC, that is, the PVC that has come out of the two moulds following the pressing of the disc. This PVC can vary in amount, depending on the type of press. In manual presses, where the disc does not need to be automatically removed from the moulds, care is usually taken to ensure that the excess PVC is minimal, but in automatic presses it serves a specific function and therefore must be wide enough for the record to be carried off the press. In fact, the arm or the clamps that remove the record from the moulds have a specific surface to which the PVC must adhere in order to facilitate the transport of the record. Only a few types of automatic presses use vacuum for removal, and in that case, the same considerations apply as for manual presses.

In order to prevent the hole of the disc from being damaged during its placement in the trimming turntable, the centre pin of this turntable is deliberately thin for 3/4 of its length, and becomes conical only towards the end. This is to ensure that any small movements of the disc during its placement are progressively guided by the conical part.

It is also important that the centre hole of the record is not too hot, as the centrifugal force exerted by the rotation of the turntable and excessive friction generated by the trimming of the PVC on the outside could transfer their force right at the centre, if it is the weakest point, causing an inevitable deformation of the hole.