. RADIO TELEPHONE NOISE REDUCTION 485 



switched to the transmitting channel when the land subscriber talks. 

 This switching is done by voice-operated relays. ''• ^ The noise reducer 

 in addition to improving the intelligibility of the speech received 

 protects these voice-operated relays against false operation by the 

 received noise. 



Figure 6 shows the application of the noise reducer to such a control 

 terminal. Speech entering the terminal from the left goes through the 

 upper branch of the circuit, with volume regulating means and privacy 

 apparatus, to the radio transmitter. Speech received from the distant 

 terminal enters at the lower right from the radio receiver and proceeds 

 through the privacy apparatus, the noise reducer, receiving regulating 

 network and amplifier to the two-wire line. Outgoing speech operates 

 the transmitting path and disables the receiving path. Incoming 

 speech operates the receiving amplifier detector, which disables the 

 transmitting amplifier detector, thus preventing singing and reradia- 

 tion of received waves. 



Without the noise reducer the receiving relay may be operated by 

 noise in the receiving path and such operation to an excessive extent 

 will interfere with outgoing speech. To avoid this effect, it is custom- 

 ary to reduce its sensitivity so that noise may not operate it. This 

 results in the weaker speech parts also failing to operate the receiving 

 relay. This weak speech and noise returned to the transmitting path 

 through the land line connection may be strong enough to operate the 

 transmitting relays and thus cut off incoming speech. This is avoided 

 by reducing the volume to the land line. Therefore, any device which 

 reduces noise in the receiving path in the absence of speech effects an 

 improvement not only in the switching operation but also in the 

 received volume. By placing the noise reducer in the receiving path 

 false operation is diminished and volume increases of 5 to 15 db are 

 realized. The noise reducer is applied to the receiving side of the 

 terminal beyond the privacy apparatus so that it does not introduce 

 any distortion in the privacy portion of the circuit. It is placed 

 ahead of the receiving amplifier detector, thereby reducing noise 

 between words which might affect the operation of this relay apparatus. 



Summary 



The noise reducer, which is a voice controlled variolosser with 

 limited and controllable action, has been provided for use on short- 

 wave radio telephone circuits and has proved to be a valuable and 

 relatively inexpensive means of securing noise reduction. Improved 

 reception is obtained for many of the transmission conditions experi- 

 enced on such circuits. This results in better intelligibility to the 



