CARRIER NATURE OF SPEECH 



499 



modulatory control is exerted on the carrier wave in part as the carrier 

 is generated and in part as it is transmitted after generation. 



Relevant Carrier Theory 



The heart of the speech-synthesizing circuit of Fig. 2 is the part in 

 which the group of waves making up the message modulate the com- 

 ponent waves of the carrier. In any one of these modulations, there 

 is the simple carrier process blocked out in Fig. 3. Here a message ^ 



r SOURCE OF "I 



[in formation] 



fcONTROLl 



[circuit J 



MESSAGE- 

 MODULATED 

 CARRIER 



INFORMATION 

 AT DESIRED 

 FREQUENCY 



r SOURCE OF "1 



[frequency rangeJ 

 Fig. 3 — The elements of a carrier sender. 



containing the information modulates a carrier determining the fre- 

 quency range so that the end product in the form of the message- 

 modulated carrier contains the information of the message translated 

 to frequencies in the neighborhood of the carrier. In this way the 

 carrier sound stream of speech is imprinted with the message. 



The prerequisites of the carrier system sender are, as indicated in 

 Fig. 3, first, a carrier wave source; second, a message wave source; and 

 third, a modulating circuit of variable impedance by which the message 

 controls the carrier. The carrier wave is for the simplest case a single 

 sine wave function of time characterized by an amplitude, a frequency 

 and a phase. The message wave as a rule is more complex but may be 

 analyzed as the sum of component sine waves each of which is char- 

 acterized by its own amplitude, frequency, and phase. In most carrier 

 circuits the frequency range of the message is below that of the carrier. 

 This is true of speech production. 



The function of the modulating circuit is supplying a means for the 

 message wave to modify a characteristic of the carrier. If the carrier 

 wave amplitude is modified by the message wave amplitude the process 

 is known as amplitude modulation; if the carrier wave frequency is so 

 modified the process is called frequency modulation while if the carrier 

 wave phase is so modified the process is called phase modulation. No 

 distinction is made as to whether the modification occurs during or 



* The word "message" has been substituted for the usual carrier term "signal" 

 to avoid confusion since the input signal is commonly speech whereas here the output 

 wave is speech. "Message" seems particularly appropriate with its suggestion of 

 code as in telegraph. 



