344 BELL SYSTEM TECHNICAL JOURNAL 



tubes themselves. Since the transatlantic radio-telephone circuit is 

 so operated that the strength of the voice waves, or "electrical vol- 

 ume," is constant at the output of the radio receiver,^^ the maximum 

 allowable noise at this point in the circuit is likewise constant. Good 

 engineering practice specifies that the continuous "tube noise" should 

 be more than 40 TU below the signal or less than 0.01 microwatt for 

 the specified receiver output of 100 microwatts when using any gain 

 up to the maximum of 84 TU. (With uniformly distributed noise 

 over the voice-frequency band, this is equivalent to about 400 noise 

 units.^^) 



4. The Type of Telephone Transmission to be Received. The 

 "single-sideband, suppressed-carrier " type of telephone transmission, 

 invented by John R. Carson,^^ has long been used in the Bell System 

 in carrier systems on wire circuits.^^ Since the advantages of single 

 sideband in radio transmission have been described by Hartley ,^^ 

 and in the radio transmitter by Heising,^^ we shall only briefly review 

 the benefits arising from its use. 



Transmission of two sidebands with the carrier suppressed repre- 

 sents an improvement over the "carrier and two-sideband" method 

 ordinarily used in "broadcasting" since all of the transmitter power 

 may be concentrated in the intelligence-bearing frequencies. By 

 transmitting only one sideband, further advantages are gained since 

 the frequency space occupied is slightly more than halved for the same 

 grade of circuit, the distortion at the output of the receiver is decreased, 

 and practical simplifications may be made at the transmitting and 

 receiving stations.-^ If the radio transmitter radiates equal power in 

 each of the above-mentioned suppressed carrier transmission schemes 

 and if the radio receiver accepts only the intelligence-bearing fre- 



J. B. Johnson, "The Schottky Effect in Low Frequency Circuits," Phys. Rev., 26, 

 71; July, 1925. 



21 S. B. Wright and H. C. Silent, "The New York-London Telephone Circuit," 

 Bell System Tech. Jour., 6, 736; October, 1927. 



^2 The noise unit is an arbitrary unit used in the Bell System for coinparison of 

 any noise with a certain arbitrary source of noise known as a noise standard. The 

 output of the noise standard may be attenuated to produce the same interfering 

 effect on speech as the noise being measured. See W. H. Harden, "Practices in 

 Telephone Transmission Maintenance Work," Bell System Tech. Jour., 4, 26; Jan. 

 1925, for details of making such comparisons. 



23 U. S. Patents Nos. 1,343,306 (1920); 1,343,307 (1920); 1,449,382 (1923), to 

 J. R. Carson. 



2* E. H. Colpitts and O. B. Blackwell, "Carrier Current Telephony and Teleg- 

 raphy," rmwj. ^. 7. £.£., -/O, 205; 1921. 



2^ R. V. L. Hartley, "Relation of Carrier and Sideband in Radio Transmission," 

 Proc. I. R. E., 11, 34; Feb., 1923. 



^^ R. A. Heising, "Production of Single Sideband for Transatlantic Radio Tele- 

 phony," Proc. I. R. E., 13, 291; June, 1925. 



2' J. R. Carson, "Signal-to-Static Interference Ratio in Radio Telephony," 

 Proc. I. R. E., 11, 271; June, 1923. 



