COMMON FREQUENCY BROADCASTING 



;81 



programs and, while the effects are more noticeable with musical pro- 

 grams due to the presence of sustained tones, the difference was not 

 marked. The observers compared the quality of the program received 

 from the two stations with varying held strength ratios and degrees of 

 carrier isochronism with that received from one of the stations trans- 

 mitting alone. The change from the test condition to the reference 

 condition could be made at will and the gains of the various circuit ele- 

 ments were adjusted so that the apparent program level was the same 

 under the two conditions. Each test covered a considerable period of 

 time and the curves shown in Fig. 2 mark the field strength ratios at 



Fig. 2 — Quality impairment vs. the frequency difference/i— /o and field strength ratio 

 £i/£2. The curves mark points where the quality impairment was just perceptible. 



which the observers could not distinguish between the test and refer- 

 ence conditions. The data shown are therefore believed to be dis- 

 tinctly conservative and to represent a criterion much more severe 

 than any which w^ill be encountered in commercial operation. These 

 results are also in agreement with the experimental data that were 

 obtained from our field tests and also check closely the data obtained 

 by the engineers of the British Broadcasting Company in similar field 

 tests in England. 



It will be noted that when the frequency difference is very small, 

 closely approaching isochronism, unimpaired reception is assured pro- 

 vided the field strength ratio is at least 10 db but that as soon as the 

 frequency difference is at all appreciable the required field strength 

 ratio for ordinary programs rises sharply to about 20 db and is approx- 

 imately constant within the range from 1 to 10 cycles per second. 



