36 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



digit pulses are sent on a time-division basis. It is not necessary to do this, 

 however, since the digit pulses may be sent over separate wire or frequency- 

 division carrier circuits. In the experimental setup used in these studies 

 each of the five digits is transmitted over a separate wire circuit. The total 

 bandwidth required in the transmission medium is essentially the same for 

 both methods of transmission. A single one-way television circuit for five 

 digits would require from 50 to 100 megacycles bandwidth in a microwave 

 system. The actual required band would depend upon the state of the art 

 and the complication permitted in the repeater equipment. 



From many points of view the transmission medium is the most important 

 part of the system. In non-regenerative systems, for example in the carrier 

 system used in present day coaxial cable transmission, most of the distortion 

 and noise that appears in the final output is the additive resultant of a large 

 number of small contributions arising in the individual repeater links that 

 make up the complete transmission medium. It is easy to see that, for this 

 method of transmission, each repeater link must be much better than the 

 overall system. For a signal that is sampled and quantized in amplitude, 

 however, it is possible to generate a new signal at each repeater which is 

 essentially perfect. In the absence of noise the quantized signal would have 

 one of the permitted amplitudes at the sampling time. A small amount of 

 noise will change this situation so that the ampUtude will not be exactly 

 the correct value at the sampling time. As long as the noise or other 

 disturbance is not too great, it is possible to requantize the signal and 

 to transmit the correct amplitude at the sampling times. This process 

 which is known as regeneration can be used for any type of signal 

 that has been sampled and quantized in amplitude. For a system using 

 binary pulses where only two levels are present, the regenerative process is 

 technically possible. Regenerative repeaters would transmit new pulses, 

 which would be accurately timed and properly shaped. As long as the noise 

 is kept below a threshold value, the noise would not accumulate from link 

 to link and the final decoded signal would be of the same quality as one ob- 

 tained from a monitor located at the transmitter. 



This means that the quality of the final output of the system depends upon 

 the size of the time and amplitude quanta used in the PCM system. In other 

 words, the final quality depends upon the sampling rate and the number of 

 digits used and not upon the length of the system. 



The last two columns of Fig. 1 show how the digit pulses can be decoded 

 to produce the output signal. The decoder produces the weighted equivalents 

 of the digit pulses which are then added for each code group. Each of these 

 summation pulses represents one of the input samples in a quantized form. 

 These summation pulses are then passed through an appropriate low-pass 

 filter to the output of the system. 



