Patent GB-983194-A (GB)
Title: Apparatus for digitally encoding sounds
IPC: G10L13/04
Filed: 1962-12-10
Abstract: 983,194. Speech analysis and synthesis; transistor amplifying and switching circuits; automatic gain control. INTERNATIONAL BUSINESS MACHINES CORPORATION. Dec. 10, 1962 [Dec. 21, 1961], No. 46518/62. Headings H3T and H4R. Apparatus for digitally encoding sounds comprises means for separating the sound into frequency bands, and repeatedly recording the presence or absence of components within these bands in a buffer store together with information regarding the intensity of the original sound, and then repeatedly sampling the buffer store for entering all the information into a further store. Coder, Fig. 1-Sound signals submitted to A.G.C. in amplifier 102 are then fed to preemphasizers 105 . . . 108 for the sibilant, high, medium and low ranges, after which they are separated by filters 109 . . . 118 and fed to biased integrators 119 . . . 128 which produce outputs provided the band in question contains signals above a prescribed level. Simultaneously the amplitude level is fed to an intensity digitizer which emits the intensity value in binary form over leads 143, 144. All the information is then fed to a buffer store 129 being clocked in by a pulse source operative at 10 msec. intervals, which simultaneously reads out the buffer to a magnetic drum store 146. The information is then read out to punch cards which may then be collated and sorted to provide suitable messages which are then stored at 151. Decoder, Fig. 2.-The message stored on disc 201 (151 in Fig. 1) is read out into buffer stores 206, the first ten of which apply enabling potential to corresponding oscillators AO 1  . . . AO 10 , and the last two provide amplitude control signals to circuit 217. Circuit 217 also comprises " squelch " means for quenching existing oscillations when a new character is fed in to the buffer store. The circuit 217 comprises transistors caused, at the appropriate times, to enable a shunting transistor in the case of the quenching control and shunting transistors in series with appropriate resistors in the case of volume control, Fig. 10 (not shown). Pressure of the highest frequency signal (#1, B1) enables or-gate 224 to cause a hiss source to superimpose its output upon the oscillators. Presence of the higher range of frequencies (#1, B 2 , B 3 , B 4 ) also enables the hiss to be connected up by or gates 225, 224, provided the signal is unvoiced. If however the signal is voiced, as indicated by the presence of low frequency bits (#2, B 3 , B 4 ), the gate 225 is inhibited to prevent enablement of the hiss source. The presence of low frequency bits also connects up a sawtooth generator 222 which modulates the outputs of the oscillators. The oscillator outputs are fed to a device 227, operable in accordance with the number of oscillators in use (as indicated by the superimposed D.C. outputs of active oscillators), to reduce the overall A.C. output in accordance with the number of oscillations present. The A.C. output from the device 227 is then fed to the loud-speaker 220. Circuits.-Automatic gain control, Fig. 3. Amplified signals reach the emitter follower 304 feeding a grounded base amplifier 305 which in turn feeds an output amplifier 306 whose input impedance is shunted by transistor 313. Positive output peaks are applied to integrator transistor 311 via diode 309 and transistors 307, 310 and negative output peaks are applied via 312 and 308 so as to affect the integrator 311 in the same manner. The output of the integrator controls the impedance of 313. Pre-emphasis amplifier, Fig. 3b, shows an amplifier arranged to emphasize frequencies in a range determined by capacitors 324, 325, 326. Frequency band selector, Fig. 4.-This comprises a two-stage transistor amplifier having opposite type transistors and having a negative feedback circuit comprising a parallel-T network. High-pass frequency selector, Fig. 5, has a transistor 503 fed by two signals in antiphase from the input, via transistor 501 and tuned circuit 504, 505. The latter passes low frequencies so that these cancel out in transistor 503. High frequencies however only pass via the transistor 501 and are thus reproduced. A biased integrator, Fig. 6.-Pulses exceeding a threshold value charge capacitor 601 incrementally until transistor 606 conducts sufficiently to remove the bias on 608 which then also conducts, the feedback via 602 then overriding that of 601 to cause both transistors to conduct strongly. The circuit restores when incoming pulses drop below the threshold level. Sawtooth generator, Fig. 12.-Negative applied to its base causes 1201 to conduct, whereupon 1202 conducts and via condenser 1207 cuts off 1203. This causes 1205 to begin to conduct but the displacement current through 1206 does not affect 1204 which is already conducting. Whilst 1206 is charging the base of 1203 is rising and when 1203 begins to conduct again the emitter of 1203 is at + 2 volts. The drop of collector potential of 1203 lowers the base potential of 1204 which cuts off. The emitter of 1203 rises to cut the latter off and the collector of 1202 rises so raising the emitter of 1203 to + 2 volts. The base and collector of 1203 and the emitters of 1203, 1205 are now all at + 2 volts and the collector of 1202 at + 6 volts. Condenser 1206 now discharges in a tight degenerative loop via 1204, 1202, 1203 and 1205, and timing control is effected by resistor 1209. Figs. 11a . . . 11g depict the potentials at the gate input, the base of 1202, the collector of 1202, the base of 1203, the collector 1203, the emitter of 1202 and the emitter of 1205 respectively.