Patent GB-743150-A (GB)
Title: Improvements in or relating to apparatus for generating pulse code modulation electric signals
IPC: H03M1/00
Filed: 1953-08-14
Abstract: 743,150. Pulse code modulation circuits. GENERAL ELECTRIC CO., Ltd., ELSON, D. W., and IMM, R. C. Aug. 31, 1954 [Aug. 14, 1953], No. 22623/53. Class 40 (5). In a binary coder in which the voltage of an integrator circuit is periodically compared with a reference voltage to determine the next code digit and the polarity of a pulse to be supplied to the integrator circuit, the reference voltage is varied periodically or continuously by supplying to a capacity, through a resistance, a voltage dependent upon the voltage to be compared. The time constant of said resistance-capacity combination is of such a value that the reference voltage is dependent upon the mean level of the voltage to be compared therewith. In the embodiment described, the signal to be coded is applied from terminal 1 and cathode-follower stage 2 to the storage condenser 6 through a periodically closed sampling switch 3. The voltage across condenser 6 is fed to a comparator 11, where it is compared with the reference voltage five times to give five code digits and supply a pulse of positive or negative polarity to condenser 6 depending on the result of the comparison, as described in Specification 664,401. These pulses are derived from a damped wave generator 12 brought into operation at the beginning of each coding cycle. Diodes 13, 14 produce from the wave, at points 15, 16, positive and negative pulses, the amplitude of each pulse being arranged to be one half that of the preceding pulse of the same train. These two pulse trains are fed through cathodefollower stages 17, 18 to a switch comprising diodes 19, 20 biased by the voltage on line 21 derived from the comparator 11 so as to permit a pulse of one train or the other to be fed to the condenser 6 through the cathode-follower stage 22. The comparator 11 comprises a triggerpair of triodes 23, 24 having a common cathode resistance 25 and separate anode resistances 36, 37, the anode of valve 23 being coupled to the grid of valve 24 through a condenser 39. The voltage across condenser 6 is fed to the grid of valve 23 through a cathode-follower stage 9. The reference voltage on the grid of valve 24 is developed across a condenser 35. If the voltage across condenser 6 is greater than the reference voltage, triode 23 is conducting and 24 is cut off, while if it is less 23 is cut off and 24 is conducting. At each comparison instant a negative pulse is applied to the grid of valve 23 through transformer 30 to set the circuit before the comparison proper occurs. The anode voltage of valve 24 is amplified in pentode 57 to supply the bias voltage on line 21, and provides from terminal 58 the pulse code modulation signal. The reference voltage on condenser 35 is arranged to follow any change in the mean level of the signal fed to the slicer 11 by connecting a resistor 43 between the condenser and the cathode of stage 9 through an electronic switch 44, which is opened by a pulse just after each sample of the signal has been supplied to the storage condenser 6. The mean value of the voltage fed to the grid of the comparator valve 23 from the stage 9 is arranged to be slightly less than the reference voltage by deriving it from a point of lower potential than the cathode on the cathode resistors 49, 50. A small compensating voltage is supplied to the cathode-follower stages 17, 18 from the anode resistance 37 of valve 24 through circuit  61 to remove coding errors which would be caused by the finite impedances of diodes 18, 19. A clamping diode 34 prevents the potential of the grid of valve 24 from rising above the reference voltage on condenser 35. A condenser 53 may be connected in the grid circuit of valve 23, a diode 54 in parallel with a resistance 55 being shunted across this condenser and the transformer 30, to provide a D.C. clamp and compensate for any drift occurring in diode 34 the two diodes forming a single double-diode valve.