Revolutions in sound recording
Revolutions in sound recording

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Revolutions in sound recording

2.5 Making multiple copies

Berliner was aware that Edison had problems duplicating cylinders. Initially copies were made from a master cylinder using a mechanical engraving process. Unfortunately this method caused the master cylinder to wear out after making just a few copies, so performers had to be asked to record several masters to ensure enough cylinders could be duplicated. An improved recording system allowed multiple master cylinders to be made by feeding several recording phonographs from one horn, but the cylinder-copying process was still far from satisfactory.

It took Berliner six years to perfect disc duplication but it was time well spent, for the principles are still used today to manufacture compact discs. Box 2 compares the manufacturing processes for cylinders and discs.

Box 2: Cylinder and disc manufacture

A process to mass-produce cylinders was finally developed in 1901. Cylinders were moulded using a hard black wax medium that reduced playback wear. The process was known as ‘gold-moulded’ because a gold-coloured vapour was given off during the process. Sub-master moulds were created from the master cylinder, and the wax cylinders manufactured from these moulds. This meant that the artist had to record only a single master cylinder. About 150 cylinders a day could be produced from a single mould.

Disc manufacture using the Berliner process started with the creation of a master disc, known as a matrix. This was a wax-coated metal disc into which the artist cut the recording. Each master disc was inscribed with an identification number, known as the matrix number, which appears near the centre of every record near the label. The master matrix was then used to make moulding tools, known as matrix stampers, using an electroplating process that deposited a layer of metal onto the master. When separated from the master the stamper became a negative replica of the master. The stamper was fitted into a hydraulic press, along with an identification label and the disc material. Once perfected, up to three discs per second (180 per minute) could be pressed in this fully automated process. Early discs were stamped on just one side but eventually double-sided discs were developed that played on both sides.

Unfortunately Berliner's discs, one of which is illustrated in Figure 11, were of variable quality, tending to have flat spots in the groove that caused the pickup to skate across the disc surface (remember, the groove positioned the pickup). From 1897 a hard-wearing compound of shellac, slate powder and carbon black was used (shellac is a resin derived from the secretion of the Lac beetle, Coccus lacca, found in Malaysia). This improved the quality and lowered production costs. The abrasive nature of the slate dust sharpened the playing needle to ensure a continued good fit in the groove (so maintaining the high frequency response) but at the expense of users regularly having to replace worn-down needles – another source of income to record manufacturers! Shellac discs gave an acceptable surface noise and were easily mouldable at relatively low temperatures. At room temperatures shellac discs were brittle and shattered if dropped. Eventually, a relatively unbreakable plastic material called vinyl (short for polyvinyl chloride) was used to manufacture discs.

Figure 11: An example of Berliner's vulcanite disc

Activity 12

When a recording company makes a decision to issue a CD of a recording originally released on a 78 rpm disc, the remastering engineer prefers to get hold of the matrix and make the transfer from that rather than from a normal copy of the released disc. Why should this be the case?

Answer

Comment

Every time an analogue recording is copied there is additional noise added to the original sound, lowering the signal-to-noise ratio. Using the original matrix will ensure the best possible sound. Unfortunately access to the original matrix is not always possible and many reissues of original 78 recordings use normal mass-produced discs.

TA212_3

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