406 BELL SYSTEM TECHNICAL JOURNAL 



Referring to Figs. 4 and 5, tlie "delayed control pulse" Curve 1 is the 

 principal timing pulse for the transmitting coder. It is used to sample the 

 audio wave and to start the step and timing-pip generators. Two sets of 

 timing-pips are produced; one, ("urvc 2, is used to generate the reference 

 step voltage while the other, Curve 7, is used for timing the digit pulses. 

 The reference step voltage, Curve 3, is used in the comparing circuit and in 

 the subtraction circuit. Curve 4 gives the output of the subtraction circuit, 

 while Curve 5 is the voltage on the storage condenser. The next plot gives 

 Curves 3 and 5 superimposed; the shaded area on this plot corresponds to 

 the time during which a pedestal voltage is generated. The pedestal voltage 

 is given by Curve 6, and the output of the pedestal modulator is given by 

 Curve 8. This last curve is a plot of the two code groups corresponding to 

 the two PAM pulses being coded . 



In studying these wave forms it will be noted that the delayed control 

 pulse, the two sets of timing-pips and the reference step voltage curves are 

 the same for each code group. On the other hand the storage condenser 

 voltage, the pedestal voltage, the group of code pulses, and the group of 

 pulses from the subtraction circuit are different for each code group. 



It will be recalled that a pedestal voltage is produced during the time that 

 the condenser voltage exceeds the reference step voltage. The leading edge 

 of each pedestal pulse is generated by the falling part of the reference step 

 voltage. The trailing edge of each pedestal pulse is produced by the falling 

 part of the condenser voltage. This drop in condenser voltage is the result 

 of the operation of the subtraction circuit. The output of the subtraction 

 circuit depends upon the delayed digit pulse which has just been passed by 

 the pedestal pulse. Its magnitude depends upon the reference voltage step 

 that applies to the particular digit being transmitted. The function of the 

 delay in the feedback path is to allow the outgoing digit pulse to be com- 

 pleted before the pedestal is terminated. 



It is seen that the pedestal voltage contains the same information as the 

 transmitted code groups. Under ideal conditions the use of auxiliary 

 timing pulses would not be required. However, in a practical circuit the 

 leading edge of the pedestal varies, both as to relative timing and as to rate 

 of rise. Under these conditions the auxiliary timing-pips permit accurate 

 timing of the outgoing PCM pulses, as well as constant pulse shape for the 

 input to the subtraction circuit. 



Summarizing the foregoing it is seen that in the coder under discussion 

 a comparison is made for each digit between a reference voltage and the 

 voltage across a storage condenser. Initially the voltage across this con- 

 denser represents the magnitude of the PAM pulse being coded. After 

 each digit I he voltage remaining on the condenser represents the magnitude 

 of the f)riginal PAM j)ulse remaining to l)e coded. A pedestal voltage is 



