5.2.1 Introduction

5.2.1.1 The first audio recordings made were mechanical recordings, and this approach remained almost the only viable method for capturing sound until developments in electrical circuitry began to create a market for magnetic recordings during and after the 1930s. Mechanical recordings are recognised by the presence of a continuous groove in the surface of the carrier into which the signal is encoded. The encoding of monophonic audio is achieved either by modulating the bottom of the groove up and down with respect to the surface (vertical or hill-and-dale recordings), or from side to side (lateral recordings). All cylinder recording are vertical recordings, as are Edison Diamond Discs, some early shellacs and discs recorded by Pathé up until about 1927, when they began to record laterally cut discs. For a time, some radio transcription discs were also vertically cut recordings, primarily in the US. Lateral cut recordings are the more common form, and most coarse groove recordings (sometimes called 78s), transcription, and instantaneous discs are lateral, as are monophonic Long Play (LP) microgroove records. Microgroove discs are discussed separately in section 5.3.

5.2.1.2 Mechanical sound recording formats are analogue, so called because groove wall is modulated in a continuous representation of the wave form of the original audio. Almost all of the mechanical recordings discussed are now obsolete,in that the industry which once created these artefacts no longer supports them.Early mechanical recordings were acoustic,as the sound waves acted directly on a lightweight diaphragm which drove the cutter directly into the recording surface. Later mechanical recordings were “electrical recordings” as they used a microphone and amplifier to drive an electrical cutting head. From 1925 onwards almost all recording studios began to make electrical recordings.

5.2.1.3 As the early mechanical recordings were all made when the industry was developing, there were few standards. Those that existed were poorly observed as the technology was constantly evolving, and many of the manufacturers would keep their latest techniques secret in order to gain a market advantage. One legacy of this period is the immense degree of variation in most aspects of their work, not least in the size and shape of the recorded groove (see 5.2.4), recording speed (5.2.5) and equalisation required (5.2.6). Consequently, there is a need for those working with the recordings to have specific knowledge about the historical and technical circumstances under which these recording were created. For obscure or non standard recordings, it is advisable to seek advice from specialists, and even for the more common types of recording, caution should be exercised.