RECENT ADVANCES IN WAX RECORDING 163 



high frequency cut-off of the recorder. Fig. 3 shows a characteristic 

 obtained with such a laboratory model. This shows uniform per- 

 formance within ± 1 TU from 250 to 7,500 cycles and within 

 ± 4 TU from 30 to 8,000 cycles. Although its immediate practical 

 value might be limited by other portions of the system, this device 

 is of great interest in that it establishes beyond question the fact 

 that an extremely broad range of frequencies can be successfully 

 recorded in the "wax." 



The broad, flat characteristic obtained with electric recorders has 

 been made possible by so designing their elements that they constitute 

 correctly designed transmission systems. In such a transmission 

 system, whether it be an electrical recorder or a long telephone line, 

 a correct terminating impedance is required. The load imposed by 

 the "wax" is somewhat variable but fortunately is rather small. 

 It has been found desirable to make the other impedances in the 

 recorder relatively large so as to dominate the system and thus 

 minimize the effects of any changes in the impedance imposed on the 

 stylus by the "wax." The mechanical load used as a terminating 

 impedance and to control the device has consisted of a rod of gum 

 rubber 25 cm. long. Torsional vibrations are transmitted along this 

 rod. The rate of propagation is about 3,000 centimeters per second 

 so that its length is equivalent to an ideal electrical line of about 

 1,500 miles. The dissipation along this rubber rod is such that a 

 vibration is substantially dissipated by the time it has travelled down 

 the line and back. It thus constitutes substantially a pure mechanical 

 resistance, the magnitude of this resistance being approximately 2,500 

 mechanical abohms, referred to the stylus point as its point of 

 application. 



"Wax" Record ^ 



In recording the usual procedure is to use a disc from 1 in. to 2 in. 



thick and from 13 in. to 17 in. in diameter, composed of a metallic 



soap with small amounts of various addition agents to improve the 



texture. This is shaved to a highly polished surface on a lathe. 



This polished disc or so-called "wax" is placed in a recording machine. 



In Fig. 4 is shown what is essentially a high grade lathe arranged to 



rotate the "wax" in a horizontal plane at a very uniform speed in a 



definite relation to the film with which it is synchronized. The 



recorder with its cutting tool or stylus is moved relative to the surface 



of the disc, common phonograph procedure being to record from the 



outer edge of the disc towards the center, whereas in the Western 



^"Some Technical Aspects of the Vitaphone," by P. M. Rainey, presented at 

 the meeting of the Society of Motion Picture Engineers at Norfolk, Va., April, 

 1927. 



