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BELL SYSTEM TECHNICAL JOURNAL 



Experimental Results 



An experimental system was set up as shown in Fig. 8. The pulse code 

 modulator, radio transmitter, and antenna comprised the transmitting 

 terminal; while an antenna, radio receiver and pulse code demodulator were 

 used for the receiving terminal. A short air-path separated the terminals. 

 The transmitter used a pulsed magnetron oscillator and the receiver em- 

 ployed a broad-band superheterodyne circuit. The results obtained with 

 this system were similar to those obtained by connecting the pulse code 



RADIO 

 TRANSMITTER 



RADIO 

 RECEIVER 



PULSE CODE 

 MODULATOR 



PULSE CODE 

 DEMODULATOR 



AUDIO 

 INPUT 



A 

 -O O— 



AUDIO 

 OUTPUT 



Fig. 8— Block diagram of PCM system. 



modulator and demodulator together without the radio equipment. In 

 fact, unless a large amount of attenuation was inserted in the path the 

 presence of the radio circuit could not be detected. 



It was possible to adjust the PCM transmitter so that different numbers 

 of digits could be produced. A brief study was made of the number of 

 digits required. It was found that, with regulated volume, a minimum 

 of three or four digits was necessary for good intelligibility for speech though, 

 surprisingly enough, a degree of intelligibility was obtained with a single 

 one. With six digits both speech and music were of good quality when 

 regulated volume was used. Even with six digits, however, it was possible 

 to detect the difference between PCM and direct transmission in A-B tests. 

 This could be done most easily by a comparison of the noise in the two 

 systems. If unregulated volume were used several more digits would proba- 

 bly be desirable for high quality transmission. 



In listening to the speech transmitted over the PCM system one obtained 

 the impression that the particular sound patterns of a syllable or a word 



