580 



BELL SYSTEM TECHNICAL JOURNAL 



ing stations WARC and WCAU. It proved to be very difficult to get 

 accurate and consistent data from such field observations without a 

 very extensive series of tests because the fortuitous variations in the 

 transmission medium continually altered the test conditions. These 

 were especially troublesome since the frequency difference between the 

 carriers is but one of the two independent variables of primary impor- 

 tance which affect the quality of the program received at any given 

 point, the other being the ratio of held strength received from the two 

 stations at the point in question. 



It was therefore necessary to set up in the laboratory apparatus 

 which would simulate as closely as possible the conditions existing in 

 the field but with all the variables under definite control Two identical 

 miniature transmitters were modulated by the same program. The 

 modulated carriers were then attenuated through independent trans- 

 mission paths and received by a high-quality detector. The layout of 

 the apparatus is shown schematically in the block diagram of Fig. 1. 



fc 



PHONO-|_ 

 GRAPH 



EQUALIZER 



AND 

 AMPLIFIER 



-yy<f-' 



TRANSMITTER 



^'^)Krv-y 



fc+Af 



TRANSMITTER 



-A^yc^ — 



DETECTOR 

 AMPLIFIER 



ho=^ 



LOUDSPEAKER 



Fig. 1 — Block diagram of apparatus set-up for determinations of quality impairment 

 with different degrees of isochronism and field strength ratios. 



It will be seen that with this equipment the signal strength received at 

 the detector from either station may be varied independently so that 

 any desired signal strength ratio may be obtained. The frequency 

 difference, A/, was fixed directly by the adjustment of the carrier 

 frequencies of the two transmitters to the required degree of isochron- 

 ism. These transmitters, operating at a frequency of approximately 

 50 kc. were quite stable and capable of accurate adjustment. 



The over-all audio-frequency transmission characteristic of the whole 

 system was even better than is available in the better commercial 

 radio receivers. The observers were engineers well acquainted with 

 the effects to be expected and whose judgment was extremely critical. 

 Tests were made with material consisting of both musical and talking 



