Tone Control Device

The invention relates to an improvement of a tone control device such as a graphic equalizer.

In a conventional tone control device, as shown in FIG. 1, the outputs of bandpass filters 1.sub.1 through 1.sub.n having corresponding transfer functions B.sub.n (s) are applied to the input and output amplifiers 3 and 4 through variable resistors 2.sub.1 through 2.sub.n, respectively, which have grounded center taps so that the gains at a plurality of particular frequencies are boosted or cut. This conventional tone control device is disadvantageous in that the quality of signals is lowered as the gain is varied due to the non-parallel arrangement of the filters.

In another prior art arrangement, a tone control device was obtained by forming a circuit which had the following transfer function T(s): ##EQU1## where s is the Laplace operator, .omega..sub.0 is the center angular frequency, Q is the sharpness, k.sub.1 is the attenuation factor at the angular frequency .omega..sub.0, and k.sub.2 is the amplification factor at the angular frequency .omega..sub.0. With this transfer function, if the values k.sub.1 and k.sub.2 are varied with the values .omega..sub.0 and Q held constant, then a flat frequency characteristic can be obtained with k.sub.1 =k.sub.2 =1, a boosted frequency characteristic with k.sub.1 =1 and k.sub.2 >1, and a cut frequency characteristic with k.sub.1 >1 and k.sub.2 =1, all as shown in FIG. 4.

A prior art circuit arrangement which was intended to cover the audio frequency band by combining circuits having transfer functions T.sub.1 (s) through T.sub.n (s) having different center frequencies is shown in FIG. 5. However, this arrangement suffers from noise problems and an increased distortion factor.

Accordingly, an object of this invention is to provide a tone control device in which the bandpass filters are connected in parallel with one another and the frequency characteristics of the device can be varied without lowering the quality of signals.

An object of this invention is also to provide a tone control device in which band-stop filters are connected in parallel and tone control in plural channels is effected while the device is free from noise and has a low distortion factor.

These, as well as other objects of the invention, are met in accordance with the invention by a tone control device having a plurality of bandpass filters each of which has a different center frequency and each of which has valuable characteristics to provide for tone control, a signal sum circuit for summing output signals of the bandpass filters and an input signal, an output signal from the sum circuit being coupled to inputs of the bandpass filter, and a signal sum and different circuit, outputs of the bandpass filters being coupled to inverting input terminals of the sum and different circuit and the output signal from the sum circuit being coupled to a non-inverting input terminal of the sum and different circuit, in output signals from the sum and different circuit forming the output signal of the device. The output signals of the bandpass filters and the input signal are coupled to inverting input terminals of the sum circuit. Thus, it may be appreciated that the term sum circuit as used herein is a generic expression covering both addition and subtracting circuits.

Yet further in accordance with the objects of the invention, there may be provided a tone control device including characteristic circuits n in number corresponding to n channels which are parallel-connected between input and output signal sum circuits provided on input and output sides of the device. Each of the characteristic circuits includes a first amplifier having an amplification factor of 1/n, a band-stop filter which receives as an input an output of the input sum circuits and with an output connected to an input of the output sum circuit through the first amplifier, a second amplifier having the same amplification factor of 1/n which receives as an input a signal representing a difference between an output of the signal sum circuit on the input side and an output of the band-stop filter with an output signal from the second amplifier being coupled to an input of the output signal sum circuit, a third amplifier which receives as an input the signal representing the difference with an output of the third amplifier being coupled to an input of the output signal summer, and a fourth amplifier which receives as an input the signal representing the difference with an output of the third amplifier being coupled to an input of the input signal summer. The inputs of the input and output sum circuits are, in a preferred embodiment, all inverting inputs.

FIG. 1 is a circuit diagram showing a conventional tone control device;

FIG. 2 is a circuit diagram illustrating a preferred example of a tone control device according to the invention;

FIG. 3 is a graphic representation indicating the frequency response characteristics of the tone control device shown in FIG. 2;

FIG. 4 is a graphical representation indicating frequency characteristics for one channel of a graphic equalizer;

FIG. 5 is a block diagram showing cascade-connected circuits having different transfer functions;

FIG. 6 is a block diagram showing an example of a tone control device according to the invention; and

FIG. 7 is a graphical representation indicating various frequency characteristics of the device shown in FIG. 6.

A preferred example of a tone control device according to this invention will be described with reference to FIG. 2. In the example shown in FIG. 2, the input terminals of n bandpass filters 1.sub.1 through 1.sub.n are connected to the output terminal of a signal sum circuit 5 while the output terminals of the bandpass filters are connected to inverting input terminals of a sum and difference circuit 6 and to the inverting input terminals of the sum circuit 5. The output terminal of the sum circuit 5 is connected to the non-inverting input terminal of the sum and difference circuit 6.

The transfer function B.sub.n (s) of the bandpass filter 1.sub.1 is expressed by the following equation (2): ##EQU2## where .omega..sub.n is the central angular frequency, X.sub.n is the gain, and s is the complex frequency.

Accordingly, the transfer function T(s) between the signal input terminal 7 and the signal output terminal 8 in FIG. 2 can be represented by the following equation (3): ##EQU3##

Simulation of the equation (3) according the following Tables 1, 2 and 3 is as indicated in FIG. 3. More specifically, reference character a in FIG. 3 indicates the simulation according to Table 1, reference character b indicates the simulation according to Table 2, and reference character c indicates the simulation according to Table 3.

As is apparent from the above description and FIG. 3, according to the invention, boosting and cutting can be carried out without lowering the quality of the signal. Thus, the tone control device according to the invention can readily be employed as a graphic equalizer or the like.

Another example of a tone control device according to the invention will be described with reference to FIG. 6. As shown in FIG. 6, signal sum circuits 11 and 12 each having opposite-phase input terminals are provided at the input and output sides of the device, respectively. Characteristic circuits of three channels are interposed between the sum circuits 11 and 12. With respect to a first channel, a band-stop filter 13.sub.1 which receives the output of the sum circuit 11 and has a transfer function E.sub.1 (s) is connected to an amplifier 14.sub.12 whose amplification factor is 1/3 with the output of the amplifier 14.sub.12 connected to the sum circuit 12. The output of the sum circuit 11 and the output of the band-stop filter 13.sub.1 are compared and subtracted at the subtraction point 15.sub.1 with the result applied through an amplifier 14.sub.11 having an amplification factor 1/3 to the sum circuit 12. In addition, the comparison and subtraction result from point 15.sub.1 is applied through an amplifier 16.sub.1 having an amplification factor k.sub.1 to the sum circuit 12 and through an amplifier 17.sub.1 having an amplification factor D.sub.1 to the sum circuit 11.

In the second and third channels, reference characters 13.sub.2 and 13.sub.3 designate band-stop filters having transfer functions E.sub.2 (s) and E.sub.3 (s), respectively; 14.sub.21, 14.sub.22, 14.sub.31 and 14.sub.32, amplifiers each having an amplification factor 1/3; 15.sub.2 and 15.sub.3, comparison and subtraction points; 16.sub.2 and 16.sub.3, amplifiers having amplification factors k.sub.2 and k.sub.3, respectively; and 17.sub.2 and 17.sub.3, amplifiers having amplification factors D.sub.2 and D.sub.3, respectively. These elements are connected in a similar manner as in the first channel.

The transfer function E.sub.n (s) (where n=1 through 3) can be represented by the following equation (4): ##EQU4## where s is the Laplace operator, Q.sub.n is the sharpness, and .omega..sub.n is the center angular frequency.

The center frequencies and the sharpnesses of the channels are determined by the characteristics of the band-stop filters 13.sub.1 through 13.sub.3. In addition, the degrees of boost and cut at the center frequencies of the channels are determined by the amplification factors of the amplifiers 16.sub.1 through 16.sub.3 and 17.sub.1 through 17.sub.3.

Assuming a status variable x is applied to the output of the sum circuit 11, then signals at various points are as illustrated. Accordingly, the following equation (5) is established: ##EQU5##

The transfer function T(s) of the entire circuit shown in FIG. 6 may then be written as: ##EQU6##

FIG. 7 shows various frequency characteristics of the circuit arrangement shown in FIG. 6. In FIG. 7, the center frequency f.sub.1 of the band-stop filter 13.sub.1 is 500 Hz, that of the band-stop filter 13.sub.2 is 1 KHz, and that of the band-stop filter 13.sub.3 is 2 KHz, with Q being 3 for each band-stop filter. The gains of curves a through f at the center frequencies of the filter 13.sub.1 through 13.sub.3 are as listed in the following Table 1:

As is apparent from the above description, according to the invention, the center frequency, the sharpness Q and the gain in each channel can be varied independently. Thus, this embodiment of a tone control device according to the invention is also quite suitable as a graphic equalizer. No interference is caused between the channels with regard to frequency characteristics and therefore the tone control device is free from problems such as noise and an increased distortion factor.

This embodiment of the invention has been described with reference to an example in which three channels are employed. However, it is obvious that the technical concept of the invention is applicable to a tone control device having n channels, n being larger than three. In this case, the amplification factor of each of the amplifiers 14.sub.11, 14.sub.12, . . . 14.sub.n1 and 14.sub.n2 is set to 1/n.

Thus, the intended object has been sufficiently achieved by the provision of the tone control device according to the invention.