Circuitry for filtering a distinct variable spectral component includes a band-rejection filter formed by a summing amplifier with a feedback loop and a bandpass-filter connected to one of its inputs. The summing amplifier is connected to form the difference between the unfiltered spectrum at one of its inputs and the spectrum component to be suppressed at another of its inputs. The summing amplifier delivers the filtered signal spectrum at its output. A control circuit locks the common center frequency of the bandpass and band-rejection filters to the frequency of the spectral component to be suppressed.
A second-order highpass difference filter constructed according to the present invention includes a difference amplifier and a feedback processing circuit. The difference amplifier includes an operational amplifier OP.sub.1, and four resistors R.sub.1, R.sub.2, R.sub.3 and R.sub.4. The feedback processing circuit is composed of two operational amplifiers OP.sub.2 and OP.sub.3, resistors R.sub.5 and R.sub.6, and two serial capacitors C.sub.1 and C.sub.2. The inverting and noninverting terminals of the operational amplifier OP.sub.3 are connected to the output of the difference amplifier and of the operational amplifier OP.sub.2, respectively. The output of the operational amplifier OP.sub.2 is also fed back to the inverting terminal of the operational amplifier OP.sub.2 via the resistor R.sub.5, and the noninverting terminal of the operational amplifier OP.sub.2 is grounded. The output terminal of the operational amplifier OP.sub.3 is connected to the inverting terminal of the operational amplifier OP.sub.2 via the two serial capacitors C.sub.1 and C.sub.2 and to the noninverting terminal of the operational amplifier OP.sub.1 via the resistor R.sub.4. The two serial capacitors C.sub.1 and C.sub.2 are connected therebetween to one terminal of the resistor R.sub.6 of which another terminal is grounded.
A second-order differential highpass filter constructed according to the present invention includes a difference amplifier and a feedback processing circuit. The difference amplifier includes an operational amplifier OP.sub.1, and four resistors R.sub.3, R.sub.4, R.sub.5 and R.sub.6, wherein R.sub.4 /R.sub.3 =R.sub.6 /R.sub.5. An input voltage V.sub.1 is fed to the inverting terminal (-) of the operational amplifier OP.sub.1 via the resistor R.sub.5. Another input voltage V.sub.2 is fed to the noninverting terminal (+) of the operational amplifier OP.sub.1 via the resistor R.sub.3. The output of the operational amplifier OP.sub.1 is fed back to the inverting terminal (-) of the operational amplifier OP.sub.1 via the resistor R.sub.6. The feedback processing circuit includes an operational amplifier OP.sub.2, two resistors R.sub.1 and R.sub.2, and two serial capacitors C.sub.2 and C.sub.1. The output of the operational amplifier OP.sub.2 is fed back to the inverting terminal (-) of the operational amplifier OP.sub.2 via the two serial capacitors C.sub.1 and C.sub.2, and is connected to the noninverting terminal of the operational amplifier OP.sub.1 via the resistor R.sub.4, and the noninverting (+) terminal of the operational amplifier OP.sub.2 is grounded. The inverting terminal of the operational amplifier OP.sub.2 is connected to the output terminal of the operational amplifier OP.sub.1 via the resistor R.sub.1. The two serial capacitors C.sub.1 and C.sub.2 are connected therebetween to one terminal of the resistor R.sub.2 of which another terminal is grounded.
A distortion measuring device for automatic distortion measuring is disclosed, having means in the form of tunable active band stop filters with associated control circuits for automatically tuning out the fundamental while transferring the harmonics to a measuring circuit. Further is disclosed a measuring circuit for such a device, in which the measuring of the harmonics is carried out as a measuring of the amplification in active filters provided with amplification control for obtaining constant output level. Such a measuring circuit is described combined with means for measuring the output level from the filters to enlarge the measuring range beyond the limit where the amplification control of the filters fail to maintain the constant value of the output level of the filters.
The use of electronically tunable filters permit an efficient extraction of characteristic speech features, such as pitch frequency and the position of spectral peaks, called formants, that are present in vowels and voiced consonants. A pair of electronically tunable active filters each controlled for CF and Q, are utilized to "lock in" on a particular formant and to follow its inflections. The control voltage supplied to the active filters directly corresponds to the formant position and gives an inexpensive and simple technique for the characterization of voiced phonemes.
A narrow-band rejection filter inserted in a received signal path is arranged so that its rejection frequency may be changed by a control signal; the narrow-band rejection filter is controlled so that its rejection frequency may substantially coincide with the center frequency of a jamming wave; thereafter, the phase difference between signals on the input and the output side of the narrow-band rejection filter is detected; and the abovesaid narrow-band rejection filter is controlled by a signal corresponding to the phase difference so that its rejection frequency may automatically coincide with the center frequency of the jamming wave.
