Patent GB-1072998-A (GB)
Title: Improvements relating to speech analysis systems
Inventor: BANDAT KURT
IPC: G10L21/04
Filed: 1965-09-27
Abstract: 1,072,998. Vocoders. INTERNATIONAL BUSINESS MACHINES CORPORATION. Sept. 27, 1965 [Oct. 16, 1964], No. 40964/65. Heading H4R. In a vocoder the aggregate function signals and the excitation function signal are modified to give the effect of recorded speech replayed at an incorrect speed. Fig. 1 shows an embodiment in which outputs of the spectrum analysis channels, defining the aggregate function, are sampled sequentially by switch AS and the resulting signals fed to the analogue to digital converter ADW the parallel binary outputs of which are rendered serial by the switch PSW1 so that the signal at point (a) comprises multi. plexed binary coded information, each time slot in a frame containing, in a binary code, a value representing the energy in a corresponding analysis channel. The signal is then fed to a series chain of delay lines DL1 to DL4 each of which has a delay of one time slot. In normal circumstances, with switch S on position N the coded information is delayed by two time slots in lines DL1 and DL3 before being fed to an output terminal SC, the framing of this signal corresponding to that used at the receiver. By operating switch S the delay DL1 may be cut out or the delay DL2 inserted causing the signal at terminal SC to be advanced or retarded by one time slot so that, at the receiver, the reconstituted aggregate function, having been demultiplexed by a switching device whose framing remains constant, will be shifted up or down in frequency by the frequency difference between adjacent analysis channel centres. With a change in frequency of the aggregate function a change in shape may be desirable, e.g. a sharpening or broadening of peaks in the energy distribution; this is provided by adding to, in adder A2, or subtracting from, in subtracter SB, the signal appearing on the output of delay line DL3 a signal derived by adding together in adder A1 proportions, select edby multipliers M1 and M2, of the signals in the adjacent analysis channels, i.e. signals before the one time slot delay DL3 and after the one time slot delay DL4. The signals in the outputs of the adder A2 and subtracter SB are multiplied in 1 multipliers Fl and F2 by the factors - 1+2n 1 and - respectively to provide outputs 1-2n SCB and SCS which may be used instead of the output SC, the output used being selected on a subjective basis. At the same time the excitation function requires modification to provide a change in pitch. In a system, such as described in Specification 991,994, in which successive pitch pulses reset a binary counter Z and then gate the output of a pulse generator TG into the counter so that the state of the counter before resetting may be transmitted as a binary code corresponding to the instantaneous pitch frequency, the pitch frequency may be varied by taking the counter output, rendered in serial form by switch PSW2, and multiplying .the binary signal by an appropriate factor in a multiplier M3. Alternatively the frequency of the pulse generator TG may be varied by the appropriate amount . to give the required change in pitch. With the change in frequency scale produced as described above a corresponding change in time scale or articulation rate may be produced by the apparatus of Fig. 4. In this embodiment, for use in a system in which pitch pulses govern the sampling rate in the aggregate function transmission, the individual pitch pulses are fed to the circuit DS which drives stepping switches S1 and S2 which have the pitch pulses applied on their fixed contacts. With switch S5 on the normal position N the pitch pulses feed directly to drive the multiplexing switch PSW3. With switch S5 on position T one pulse in every six is eliminated while with switch S5 on position H every sixth pulse is repeated by the delay line DLY2, so that the articulation rate is changed by suppressing or by repeating every sixth sample, a store Sp being provided to buffer the transmitted signals. An alternative arrangement for modifying the aggregate function is shown in Fig. 5 in which the aggregate function signal passes through three unit time slot delays SP1, SP2, and SP3 and the shift in frequency and modification of shape is provided by the suitable manipulation of switches S3 and S4 connecting the multipliers M1<SP>1</SP>, M2<SP>1</SP>, and M3<SP>1</SP>, and the inputs and outputs of the lines. Again the pitch may be varied by multiplier M4 and the articulation rate changed in EL before the signals are stored for transmission in SP.