572 BELL SYSTEM TECHNICAL JOURNAL 



quency band must also be considered. This is necessary because, 

 when transmission over long distance lines is involved, this delay 

 tends to be different for different parts of the frequency band and the 

 distortion produced is a function of the frequency-delay character- 

 istics. Tests have indicated that the high frequencies, say those in 

 the range 5,000 to 8,000 cycles, should not suffer delay in transmission 

 over the line more than 5 to 10 milli-seconds greater than the delay 

 suffered by frequencies in the neighborhood of 1,000 cycles. However, 

 at the low end of the scale more delay may be tolerated: for example, 

 50 cycle waves may be delayed as much as 75 milli-seconds more than 

 those in the neighborhood of 1,000 cycles without noticeable deteri- 

 oration in quality. 



Requirements must also be imposed as to "linearity" of the trans- 

 mission, that is, constancy of efficiency with different current strengths. 

 If the transmission departs too much from "linearity" several dis- 

 agreeable effects may be produced: (1) Spurious frequencies which 

 are by-products of the true frequencies will become large enough to 

 be annoying, (2) strong sounds will not be reproduced as well as weak 

 sounds, and (3) when weak sounds are transmitted along with strong 

 sounds the strong sounds will tend to obliterate the weak sounds. 



In the design of this program transmission circuit the criterion 

 was adopted that transmission put over the circuit at the maximum 

 prescribed volume level must not sound appreciably different than 

 transmission put over the circuit at a considerably lower level, at 

 which lower level the non-linear distortion is negligible. 



Volume Range 



A favorably-seated listener to a high-grade orchestra is treated to 

 a wide range of volumes. Opinions differ as to just how wide a 

 volume range can be appreciated by such a listener, but it seems 

 certain that it is at least 60 db, corresponding to a power range of one 

 million to one. The human ear can hear volume ranges in excess of 

 100 db, corresponding to a power range of ten billion to one. For 

 loudspeaker reproduction it has been found that a room must be 

 particularly quiet in order to be able to appreciate a volume range of 

 60 db. Rooms in three-quarters of the usual residences are probably 

 too noisy for a volume range as great as this to be appreciated. A 

 40 db volume range, corresponding to a power range of 10,000 to 1, 

 can be appreciated in most rooms where radio listening is done and 

 is quite satisfactory for most musical selections. 



From the standpoint of design, the maximum volume of a wire 

 program transmission system is limited by the requirement that the 



