ABSTILiCTS OF TECHNICAL ARTICLES 137 



obtained by use of a system having a volume range of 65 decibels and a 

 frequency range from 60 to 8000 cycles per second. Substantially complete 

 fidelity for the transmission of speech is obtained by a system having a 

 frequency range from 100 to 7000 cycles per second and a volume range of 

 40 decibels. 



Preliminary experiments comparing a single-channel system and a two- 

 channel stereophonic (auditory perspective) system showed that stereo- 

 phonic transmission with an upper frequency limit of 5000 cycles per second 

 was preferred to single-channel transmission with an upper limit of 15,000 

 cycles per second. A definite improvement was obtained in the stereo- 

 phonic system by using three channels instead of two. 



A New Direct Crystal-Controlled Oscillator for Ultra-Short-Wave Fre- 

 quencies.^ W. P. Mason and I. E. Fair. An ultra-high-frequency crystal 

 oscillator is described which utilizes a mechanical harmonic of an AT or BT 

 crystal. With the oscillator frequencies as high as 197 megacycles, hormon- 

 ics as high as the 23rd have been excited. Taking the second electrical 

 harmonic of the oscillator, frequencies as high as 300 megacycles, or 1 meter 

 have been obtained. Since a mechanical harmonic is used, the crystal can be 

 of a practical size to handle and adjust. The harmonic vibration of the AT 

 and BT crystals have as low a temperature coefficient as the fundamental 

 mode, and temperature coefficients of less than two parts per miUion per 

 degree centigrade are easily obtained. Stability curves for this type of 

 oscillator are shown and the results indicate that at 120 megacycles stabilities 

 in the same order of magnitude as for ordinary crystal oscillators can be 

 obtained. Without temperature or voltage control it appears likely that 

 the frequency should remain constant to ± 0.0025 per cent. 



Some measurements have been made of the properties of harmonic crystals 

 at high frequencies. It was found that the () of a crystal is independent of 

 the frequency but in general increases with harmonic order. The ratio of 

 capacitances ^ of a crystal increases as the square of the harmonic order. It 

 is shown that in order to obtain a positive reactance in the crystal Q > 2r. 

 This relation will only be satisfied for harmonics of AT crystals less than the 

 7th. As a result oscillator circuits such as the Pierce circuit cannot be used 

 to drive crystals at high harmonic frequencies. A discussion of oscillator 

 circuits is given and it is shown that a capacitance-bridge oscillator circuit 

 with the crystal in one arm is the best type to use for high-frequency har- 

 monic crystals. 



War Activities of the Bell Telephone System.^ Keith S. McHugh. The 

 scope of the Bell System's nation-wide service is, even in peace, difficult to 



3 Proc. I. R. E., October 1942. 



^ Bell Telephone Magazine, November 1942. 



