Transcription Disc - Disc Types

Disc Types

Transcription discs are of two basic types: pressings and instantaneous discs.

Pressings were created in the same way as ordinary records. A master recording was cut into a blank wax or acetate disc. This was electroplated to produce a metal stamper from which a number of identical discs were pressed in shellac or vinyl in a record press. Although the earliest transcription discs were pressed in shellac, in the mid-1930s quieter vinyl compounds were substituted. These discs were used to distribute syndicated programming to individual radio stations. Their use for this purpose persisted long after the advent of magnetic tape recording because it was cheaper to cut and plate a master disc and press 100 identical high-quality discs than to make 100 equally high-quality tape dubs.

Instantaneous discs are so called because they can be played immediately after recording without any further processing, unlike the delicate wax master discs which had to be plated and replicated as pressings before they could be played non-destructively. By late 1929, instantaneous recordings were being made by indenting, as opposed to engraving, a groove into the surface of a bare aluminum disc. The sound quality of these discs was inadequate for broadcast purposes, but they were made for sponsors and performers who wanted to have recordings of their broadcasts, a luxury which was impractically expensive to provide by the wax mastering, plating and pressing procedure. Only a very few pre-1930 live broadcasts were deemed important enough to preserve as pressings, and many of the bare aluminum discs perished in the scrap metal drives of World War II, so that these early years of radio are mostly known today by the syndicated programs on pressed discs, typically recorded in a small studio without an audience, rather than by recordings of live network and local broadcasts.

In late 1934, a new type of instantaneous disc was commercially introduced. It consisted of an aluminum core disc coated with black cellulose nitrate lacquer, although for reasons which are unclear it soon came to be called an "acetate" disc by radio professionals. Later, during World War II, when aluminum was a critical war material, glass core discs were used. A recording lathe and chisel-like cutting stylus like those used to record in wax would be used to engrave the groove into this lacquer surface instead. Given a top-quality blank disc, cutting stylus, lathe, electronics and recording engineer, the result was a virtually noiseless broadcast-quality recording which could be played several times before the effects of wear started to become apparent. The new medium was soon applied to a number of purposes by local stations, but not by the networks, which had a policy against broadcasting prerecorded material and mainly used the discs for archiving "reference recordings" of their broadcasts.

Standard 16 inch transcription discs of the 1930s and 1940s usually held about 15 minutes of audio on each side, but this was occasionally pushed to as much as 20 minutes. Unlike ordinary records, some were recorded inside out, with the start of the recording near the label and the end near the edge of the disc. The label usually noted whether the disc was "outside start" or "inside start". If there was no such notation, an outside start was assumed. Beginning in the mid-1950s, some transcription discs started employing the "microgroove" groove dimensions used by the 12 and 10 inch 33 1/3 rpm vinyl LP records introduced for home use in 1948. This allowed 30 minutes to fit comfortably on each side of a 16 inch disc. These later discs can be played with an ordinary modern stylus or a vintage "LP" stylus. The earlier discs used a larger groove, nearer in size to the groove of a typical 78 rpm shellac record. Using a "78" stylus to play these "standard groove" discs usually produces much better results, and also insures against the groove damage that can be caused by the point of a too-small stylus skating around in the groove and scoring its surface. Some specialist audio transfer engineers keep a series of custom-ground styli of intermediate sizes and briefly test-play the disc with each in order to find the one that produces the best possible results.