A Recording Transmission Measuring System 

 For Telephone Circuit Testing 



By F. H. BEST 



A number of types of measurement are made on telephone circuits to 

 determine their transmission performance, these measurements being made 

 with manually operated devices. This paper describes a transmission 

 measuring system which automatically records the results of many of these 

 measurements. 



THE making of transmission measurements on telephone circuits 

 is essentially a delicate operation. However, with the aid of 

 vacuum tubes and, more lately, copper-oxide rectifiers, devices have 

 been developed for measuring the various important transmission 

 characteristics of telephone circuits, including transmission losses and 

 gains for single frequencies, speech volume and noise, all of these 

 measurements being made with meters as are measurements of the 

 performance of electric power systems. 



There has now been developed an experimental model of a system 

 not only for indicating but also for recording the results of transmission 

 measurements on telephone circuits. It was developed particularly 

 for the purpose of automatically plotting curves of transmission loss 

 versus frequency, this characteristic of telephone circuits being a very 

 important index of the ability of the circuit to transmit speech clearly. 

 It is, however, also suitable for making various records of performance 

 as a function of time, including transmission loss, speech volume and 

 noise. 



The essential elements of the automatic recording system are shown 

 in Fig. 1 as they are used in making a transmission-frequency run on 

 a telephone circuit. At one end of the circuit is an adjustable fre- 

 quency oscillator which generates testing power, a sending panel for 

 supplying this power to the circuit and adjusting it to the proper value 

 and a synchronous motor for varying the oscillator frequency. At 

 the other end is a receiving panel which amplifies the weak received 

 testing power and converts it to direct current which causes the pointer 

 of the recording meter to move. The meter is calibrated to record 

 the transmission efficiency of the circuit directly in decibels. The 

 heavily outlined parts are those used for recording work only, the re- 



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