2.1.1 Recording principle

Mechanical carriers constitute the oldest, commonly used type of carrier used for recording and reproducing audio. The first true recording system was the cylinder phonograph, invented by Thomas A. Edison in 18774, improved and marketed from 1888 onwards. Originally intended as an office device for dictation purposes, it became popular for scholarly recording of language and ethnic music from the 1890s until the 1950s. Cylinders were also used by the phonographic industries for pre-recorded music. This format, however, was less successful as a commercial product than the gramophone disc and, though it was still used for recording, replicated carriers vanished from the market in the late 1920s. Mechanical disc formats governed the pre-recorded music market from the early 20th century until the 1980s, when they were superseded by the Compact Disc.

In recording a mechanical carrier, the sound, which is a function of the variation of air pressure, is transformed into movements of a cutting stylus and engraved into the surface of a rotating medium. This was originally done by purely mechanical methods: the sound was captured by a horn and moved a membrane at the closed end of the horn. The membrane was connected directly or by levers to a cutting stylus, which engraved the movement of the membrane into the surface of a rotating wax cylinder or disc. The reproduction of the sound reverses the process: a stylus is moved by the modulated groove and drives a membrane, the vibrations of which are amplified by the horn.

By the mid-1920s this acousto-mechanical process was superseded by a magneto-electrical system in which the sound is transformed by a microphone into an electrical signal that moves an electrically driven cutting stylus. The reproduction was also improved by electrical pick-up systems, the amplified signals of which are converted to mechanical movement by a membrane in a loudspeaker or headphones. Recently optical, contact-less replay of mechanical carriers has been developed which, however, for various reasons, has not achieved wider acceptance. (For signal retrieval from mechanical carriers see IASA-TC 04, sections 5.2 and 5.3.)

4. This first “tinfoil phonograph” of 1877–78, which recorded by indenting a tinfoil sheet wrapped temporarily around a cylinder, is typically distinguished from the later “cylinder phonograph” which recorded by cutting a groove in a permanently cylindrical carrier. Cylinders

With cylinders, the groove is cut in a helix across the surface. The modulation of the sound signal is engraved vertically (“hill and dale”).

Figure 1: The recording and reproduction principle of cylinders.

There are instantaneous and replicated cylinders. Cylinder replication was possible either by a copying process from masters, which allowed a limited number of slave cylinders to be made. Another process was replication from a galvanoplastic negative, a copper tube, which carried the “inverted” groove on the surface of its inner side. These negatives were used to make wax casts or to create celluloid (=cellulose nitrate) positive tubes produced under high pressure steam. The celluloid tube was then stabilized by inserting a core made of plaster or other materials.

Figure 2: Replicated cylinders: wax (left), celluloid (centre) and wax “Pathé”.


Figure 3: Self recorded cylinders: wax, affected by mould (left) and “Edison Concert” (right).

The various wax compositions used for wax cylinders are chemically fairly stable, if properly stored. Wax, however, is highly susceptible to fungus growths and, as many cylinders were inadequately stored in their earlier lives, fungal infection (mycosis) is commonly found. Fungi aggressively attack the surface of the cylinders and seem to aim for wax as their primary nutrition source. Additionally, the digestion process is associated with the secretion of acids and enzymes, further damaging the material of the cylinders. Complete removal is not possible. Prevention of further fungus growth is, therefore, of utmost importance. Chemical breakdown can also occur under the same conditions that promote fungus growth. Typically this takes the form of “efflorescence,” which can be mistaken for mould, but appears to involve the actual separation of constituent materials in the metallic soap composition.

Celluloid cylinders suffer from brittleness of their cellulose nitrate surface, but catastrophic deterioration as occurs in nitrate films has not been experienced. Mechanically, all wax cylinders and the plaster cores of celluloid cylinders are extremely fragile. Coarse groove discs (gramophone discs)

Emile Berliner invented the gramophone in 1887. The groove is arranged as a spiral on the surface of a disc. Generally, the modulation of the grooves is lateral, as opposed to vertical for cylinders. Only a few disc formats (Pathé, Edison) have vertically cut grooves. The big advantage of the disc shape, apart from greater ease of storage, is that galvanoplastic negatives can be easily made and used for replication by pressing. As the number of pressings is limited, the first metal negative (“father”) serves only as a master for a metal positive (“mother”), which is used to produce an unlimited numbers of metal stampers (“sons”), which are used for the pressing tools for the replicated discs. This method, established at the beginning of the 20th century, is still being used for micro groove discs (“vinyls”), and for the production of replicated CDs, DVDs and BDs.

Figure 4: Coarse and micro groove discs recording principle. Replicated coarse groove discs. The majority of coarse groove discs — the so-called shellac records — consist of a mixture of mineral powders bonded together by binders, originally containing shellac resin. These materials are chemically generally very stable if kept under fairly dry conditions. They are, however, fragile: when dropped, they break. Apart from shellacs, other record types existed in much smaller quantities and used different materials. These were often of lesser stability, for example, Edison Diamond discs, which are extremely susceptible to moisture.5 Instantaneous discs were recording media that were widespread in radio stations, before the advent of magnetic tape. The discs were used to record and replay signals without the need for galvanoplastic processing and pressing. Their surfaces are soft enough to permit the cutting of the groove, but hard enough to permit a number of replays. Most of these discs are unique recordings. If not recognisable by their distinct appearance, almost all instantaneous discs can be identified by their handwritten or typed labels.

There are homogeneous discs made from one single material component such as aluminium, zinc, PVC, or gelatine, as well as laminated discs, which are composed of a substrate and a surface coating made from different materials that is engraved with the recording. Lacquer discs. The most widespread type of instantaneous discs is laminated: the lacquer or “acetate” discs. A lacquer coating consisting mainly of cellulose nitrate and usually plasticized with castor oil or camphor carries the information. The substrate that supports the information layer of the discs is generally of metal (e.g. aluminium or zinc); some are of glass, cardboard, or paper.

Lacquer discs can be easily identified as the base material can usually be seen between the outer lacquer layers, either within the centre hole or at the disc edge (IASA-TC 04,

Cellulose nitrate decomposes continuously over time by reacting with water vapour or oxygen. This process produces acids that act as a catalyst for these hydrolytic reactions. Elevated temperature and humidity levels will further accelerate these reactions. Gradual degradation, along with loss of plasticisers, causes progressive embrittlement and shrinking of the lacquer coating. As the lacquer is bonded to a substrate that cannot shrink, internal stresses ultimately result in a cracking and flaking off of the lacquer coating, leading to the loss of the sound carrying layer. The mechanical instability of cardboard/paper bases often results in uneven or cracked surfaces while the fragility of glass bases often results in broken discs.

Figure 5: Metal based lacquer disc in the process of deterioration 1990–2001.


Figure 6: Cardboard based lacquer disc in the process of deterioration. (Stig-Lennard Molneryd)

Internal stresses are difficult to detect. Lacquer discs should not, therefore, be exposed to mechanical or thermal stresses. As their further life expectancy is unpredictable, the recordings on such discs should be immediately transferred to digital files before they are lost. Other instantaneous discs. In addition to lacquer discs, all other instantaneous discs, irrespective of their particular composition, should also be considered to be at great risk.

5. For details of early coarse groove discs see St-Laurent 1996. Microgroove discs (“LPs”, “vinyls”)

From the late 1940s onward a new material was used for replicated pressed discs: a co-polymer of polyvinyl chloride (PVC) and polyvinyl acetate (PVA) was introduced for two different new formats. The RCA record company launched a seven-inch (= 17 cm) disc that runs at 45 rpm for three minutes per side — in terms of duration, a continuation of the old shellac disc format. Columbia started the 10-inch (= 25 cm) LP, later enlarged to 12 inch (= 30 cm), both of which run at 331/3 rpm. Playing times are 15 and 25 minutes per side, respectively. This new material with its almost amorphous structure allowed much finer mechanical signal representation, which made narrower grooves, lower speeds and, therefore, longer playing times possible. The amorphous structure of the plastic also produced considerably less surface noise than shellac discs. PVC/PVA co-polymer, colloquially termed “vinyl”, is chemically very stable. Except for a few very early discs, an average vinyl disc is chemically in good shape. The material is comparatively soft, however, and therefore vulnerable to damage by scratching or abrasion.

In the early days of microgroove disc, small numbers were produced by injection moulding using styrene. These discs can be identified by their light weight and their relatively matte surface in relation to the shiny surfaces of “vinyls”. In reproduction, they have a higher surface hiss level than vinyl records. No systematic stability problems have been observed with this type of LP.