Main characteristics of the micro furrow
The lightness Initially the micro furrow weighed 180 grams unlike the previous 78 laps which weighed 360 for the same size (30 centimeters). For economic issues related to production, the weight has dropped to 130 grams, however causing a loss of the overall quality of the substrate, i.e. a greater vulnerability to mechanical deformations including, distortion of the shape, and undulations, deterioration of the sound quality due to a increase in resonances. As for the 17 cm micro furrow, that is the 45 rpm, with the same recording duration, compared to the 78 rpm it has a weight of about 1/10.Sturdiness If the 78 rpm disc broke when it fell, the micro furrow, thanks to the material from which it is made, is unbreakable thanks to the small weight, at the price, however, of a much more delicate surface of the micro furrow compared to that of the 78 rpm, therefore more sensitive to scratches.The duration Initially with 78 laps the duration of a 30 cm disc, using both of its sides, rarely exceeded 9 minutes of recording. The arrival of the 33 laps micro furrow allowed to extend the execution time up to 30 minutes for each of the facades, with a total duration approximately 6 times greater than that of the 78 laps. The main feature that has allowed this remarkable improvement lies in the adoption of the technique called the "variable pitch" which consists in varying the space between the turns of the groove according to the sound modulations, instead of keeping them constant as happened previously, allowing in thus more efficient use of the disc surface. In practice, in the presence of a very strong modulation the furrow path will be wider and therefore the distance between the upper turns; otherwise, when the modulations are weaker the track will be more straight, it will therefore be possible to bring the coils closer together, gaining space on the surface of the disc. Loyalty Among the improvements undergone by the disc in search of ever greater loyalty, the use of a new type of material, polyvinyl acetone chloride, is certainly a decisive point. Its molecular structure allows the attenuation of background noise and the increase in sound dynamics, i.e. the difference between the weakest perceptible intensities before the background noise and the strongest tolerable intensities before the saturation distortion. At the same time this material allows the incision of a groove much thinner than the one incident on the old 78 rpm, a feature that combined with the new recording techniques has led to the growth of the sound spectrum or the range of sounds reproduced when reading the disc.Speed and diameter The best compromise between listening duration and sound quality is represented by the 33 rpm format (actually 33 rpm and 1/3 per minute) at 30 centimeters in diameter, which has become the current standard. The adoption of this model stems from the need to transport as much information on a constant-size support that is practical to use. For this purpose, the groove width has been decreased and the angular speed of the disc has been reduced, creating the possibility of having an overall duration close to 60 minutes of performance with good sound quality. However, what is gained in durability is lost in quality: a reduction in the rotation speed below 33 rpm would result in a decrease in the linear speed (ie the amount of surface "read" by the stylus at each revolution of the disc) such as to compromise the sound quality, especially when approaching the center of the disc. In fact, the stretch traveled by the stylus on the outer edge of a 30 centimeter disc is about 90 centimeters, while at the center of the disc only 35 centimeters, therefore maintaining a constant angular speed of 33 rpm and 1/3 per minute, the linear speed varies from 50 cm / sec. outside less than 20 cm / sec. in the center. This means that the same amount of information must be "housed" in less space, for this reason the diameter of the last loop must not be less than 10.6 centimeters otherwise the quality would have an unacceptable collapse.Stereophony Stereophony is a recording and listening technique, which allows the spatial localization of sound sources giving the listener the sensation of acoustic relief, that is, producing the directional effect, or stereophonic effect; simplifying, this procedure consisted of dividing the sound into two channels, the left and right channels. This solution involved a new method of engraving as well as the use of new heads for reproduction. In monophonic reproduction, the information content of the sulcus is located at its edges, which the head explores to extract its mechanical information which will then be translated into sound signals.As for stereo recording, however, having this two audio tracks (left channel and right channel), there was the need to enter double information in a single groove. The stereophonic engraving principle consists of a combination of the two existing types of engraving: 1. vertical incision (with variable depth) used before the arrival of the micro furrow2. horizontal (constant depth) incision used for the monophonic micro furrowIn practice, a sinuous groove is obtained as that of a monophonic disc, whose two sides, however, are incised in a different way: the internal side corresponds to the left channel and the external side to the right channel. Of course, at the beginning of the marketing of this new format there was the problem of compatibility, the points used for monaurals could not work (due to the vertical component) on stereophonic discs. The first solution to this problem was to adopt a new type of monophonic head, equipped with a stylus also suitable for reading stereo discs; he would not have enjoyed the stereophonic effect, but there was no risk of ruining his records.Discs compatible with both reproduction methods called "Universal Engraving", also known as "Stereo-Mono", "45-45", "Sincrostereo", etc. were introduced onto the market. Without going into detail, these new supports provided for the engraving of a shallower groove, in which the vertical modulation amplitudes that exceeded a certain threshold were voluntarily limited, in this way even the monophonic stylus did not risk damaging the disc or leaving the furrow during his reading.