Patent GB-1384233-A (GB)
Title: Quality of electrical speech signals
IPC: G10L21/00
Filed: 1971-01-06
Abstract: 1384233 Equalizer BRITISH BROADCASTING CORP 21 March 1972 [6 Jan 1971] 697/71 Heading H4R A method of modifying an electrical speech signal, e.g. a signal received over a telephone line, comprises separating the signal into a plurality of channels, adjusting the gains of the signals in the respective channels and adding the signals thus provided to provide an output signal, the said gains being predominantly effective at different frequencies in the different channels respectively and being so adjusted that the amplitude-frequency spectrum of the output signal is related in a predetermined manner to a reference spectrum. The embodiments are based on the fact that the amplitude-frequency spectrum of the signal produced by a given person speaking over a given telephone line is substantially constant irrespective of what the person is saying. The amplitude-frequency spectrum is determined as follows: a signal is divided up into a number of frequency channels ,e.g. about 30; the amplitude of the signal at the output of each channel is sampled several times per second: and the successive outputs of each channel are summed over a period of time, e.g. between 10 and 60 seconds. It is found that the shape of the amplitude frequency spectrum so produced is constant for a given speaker and telephone line. (Figs. 1a-1d, not shown). Hence in order to alter the amplitude-frequency spectrum of a received signal to correspond with a reference spectrum it is only necessary to adjust a variable gain device in each channel. Fig. 2, shows a simplified circuit of one embodiment, in which an incoming signal is divided into three channels by band pass filters 12a-12c which have contiguous frequency bands. In a practical system there may be 11 contiguous third-octave filters with centre frequencies from 320 Hz to 3200 Hz when the circuit is used in conjunction with a telephone line. The output of each filter is fed to a respective attenuator 14a-14c whose output is fed to both an adding circuit 16 together with an input from each of the other filters as well as to a respective comparator 20a-20c. Each comparator integrates the output of the attenuator over a ten second period and compares the amplitude with that of the reference spectrum. The reference spectrum is obtained by determining the amplitude frequency spectrum of the particular speaker using high quality audio equipment and storing it. The comparator causes the gain of the associated attenuator to be adjusted in accordance with the difference between the integrated signal and the reference signal. Instead of having an automatic comparison of the spectra, it is possible to use a manual technique as the incoming amplitude-frequency spectrum tends to be substantially constant. To this end an oscilloscope is used to display the incoming amplitude-frequency spectrum and a transparent plate having the reference spectrum on it is placed over the oscilloscope screen. The gains of the attenuators are then manually adjusted until the spectra coincide. It is possible to insert a frequency notch into the filter characteristics to reduce any continuous tones present in the input signal. Fig. 4, shows one channel of a second embodiment which employs a variable depth notch filter, a typical frequency power characteristic of which is illustrated in Fig. 3 (not shown). The filter associated with each channel has a different notch frequency and its output is fed to an adder (not shown), as before and to a spectrum shaper 36. This shaper is a specially tailored filter having an amplitude versus frequency response which is the inverse of the required voice spectrum or of a related spectrum particular to the channel in question. If the output of the filter 33 is correctly equalized the output of the shaper 36 will have a flat spectrum. The output of the shaper is applied to a bandpass filter 37 and a band stop filter 38 and measuring circuits 39 and 40 determine the average amplitude of the signal in the region of the notch centre and in the remainder of the telephone band. The two average amplitude signals are subtracted in a comparator 41 whose output is used to control the variable depth notch filter 33. The invention may be used to improve the quality of telephone calls required to be broadcast.