58 BELL SYSTEM TECHNICAL JOURNAL 



ends. The one at the sending end converts electrical energy into sound 

 which is transmitted through the pipe and the one at the receiving end 

 converts the sound waves back into electrical energy. The pipes are 

 quite suitable because they have approximately the same delay at all 

 frequencies. Various devices are required to reduce the reflections 

 which occur at the junction of the pipe and the receiver and to equalize 

 the attenuation. 



Using devices of this nature, experimenters have found it possible 

 to talk fairly conveniently over circuits representing time intervals as 

 great as 0.7 second in each direction. So great delays would be con- 

 sidered undesirable for commercial use, however. Delays of about a 

 third of this, in general, are considered about the maximum which is 

 satisfactory. 



Delay Distortion 



In designing circuits which are electrically long, care must be exer- 

 cised to insure that the transmission times for all frequencies in the 

 transmission range are sufficiently alike to avoid objectionable tran- 

 sient phenomena. These eff"ects may occur in one-way circuits as 

 well as in two-way circuits and are not related to echo effects. 



The appearance of these transients to the listener depends on whether 

 the excess delay is at low frequencies or at high frequencies. It is 

 rather difficult to describe the characteristic sound of a circuit with 

 low-frequency delay. A high-frequency delay, if it is in an extreme 

 form, sounds as though a high-pitched reed, such as a harmonica reed, 

 was being plucked whenever there is a sudden transition in the voice 

 sounds being transmitted over the circuit. 



The characteristic effects of transients are conveniently described by 

 the aid of oscillograms of spurts of alternating current taken before 

 and after being sent over circuits having various delay characteristics. 

 To begin with, it must be recalled that when any wave shape is applied 

 to a circuit, the transmitted wave in the circuit can be expressed as the 

 sum of the series of sinusoidal waves whose frequencies range from very 

 low to very high values. 



In the case where a sinusoidal wave of frequency F is suddenly ap- 

 plied to the sending end of the line, the effect may therefore be ex- 

 plained as due to an infinity of sinusoidal waves so proportioned and 

 phased as to add up to zero, up to the instant of application of the wave, 

 and to equal the steady-state value of the wave at that instant. Of 

 these waves, the most important have frequencies close to F. They 

 are propagated over the line individually with a velocity correspond- 

 ing to the frequency. If the velocity of the line is the same for all 



