Page 20 - 2016Fall

Analog Sound Recording and Reproduction
In the acoustic period, the cylinder phonograph cutterhead was a soundbox with the diaphragm parallel to the axis of the cylinder with an engraving cutter driven from the cen- ter of the diaphragm. When electrical amplification entered the field, the cutterheads or vibrators for driving the cutting stylus were mechanically very much like the moving parts of loudspeakers because they had a high efficiency. This was good for laterally modulated records that required a large amplitude. The electric cutterhead of Maxfield and Harri- son was such an advance in the art that it was licensed to the largest record companies. They, in turn, experimented with developing alternative solutions to avoid this patent li- cense. Edward W. Kellogg (1927) from the General Electric Co. gave a very complete overview of all the electromagnetic possibilities for constructing pickups (for getting the signal off the record) and cutterheads (for recording), and most were tried in practice.
Alan D. Blumlein (1934), first at Columbia and then at EMI in the United Kingdom, developed a moving-coil cutterhead that had a one-turn secondary coil as the driving coil, being supplied with modulation from a transformer primary on the same magnetic circuit. This principle had been used 20 years earlier by Reginald Fessenden (1914) for a subaque- ous transducer. As piezoelectric salts and efficient moisture sealants were developed, several types of cutterhead were developed, giving a capacitive load with a high electrical im- pedance to the vacuum tube amplifiers, thereby avoiding an expensive component, the output transformer.
For vertical modulation that required much less amplitude, systems were developed, which were not only linearized by means of negative feedback from the actual movement of the stylus holder but were also given a uniform high mechanical output impedance.
Instantaneous Recordings
From its inception, the cylinder phonograph with its strong motor was a machine for recording and reproduction, whereas the disc phonograph was only for reproduction and only for suitable and mechanically strong records. There was a drive to avoid the complex manufacturing process when needing to make a permanent disc record. Equip- ment for moving a cutterhead slowly inward while cut- ting a spiral groove was well-known (a typical example is shown in Figure 7), but a suitable recording medium was required. Around 1930, several types of lacquer-coated flat discs were developed that could be recorded by an electri- cal cutterhead. The electrical motor supplied with ordinary
18 | Acoustics Today | Fall 2016
Figure 7. Typical recorder with radial feedscrew for instantaneous discs. The rotating turntable turns the feedscrew (24) via a gear, and the cutterhead (30) moves gradually inward while cutting a groove in the blank disc. A separate arm carries a pickup (55) for reproducing the record. Redacted from Goldman (1939).
disc phonographs was not strong enough to enable cutting; therefore the motors had to be stronger. The result was the lacquer record that could be played as soon as it had been recorded without being worn out in the process as the wax did (Brock-Nannestad, 2012).
The Post-World War II Transfer to Fine-Groove Recordings for Private Consumption and Hi-Fi
From the beginning of the 1930s, the quality of commercial recording was so uniform that very long works like sympho- nies and operas were recorded full length in sets of 12-in. (30-cm-diameter) 78 rpm records, perhaps 10 or more to a bound booklike album. Although the individual record sides were coupled in ingenious ways to enable a whole stack to be put on a disc phonograph with a record changer, the interruption every 5 min distracted from the illusion of a live performance. For continuity, broadcasting stations used huge 16-in. (40-cm-diameter) discs at 331⁄3 rpm or, as wealthy customers did, two turntables to make a smooth transition.
With the development of ever lighter pickups and low-noise PVC-based record materials, the thought of a commercial long-playing (LP) record materialized. The CBS subsidiary Columbia Records provided this in 1948 with up to 20 min per 12-in. side at 331⁄3 rpm. Quite independently in 1949, RCA introduced a system that used 7-in. (17-cm-diameter)