Compressing and/or expanding system comprising fixed and variable frequency characteristic changing circuits

Claim 2

A compressing circuit comprising: a first circuit means comprising a first frequency characteristic changing circuit means having a specific fixed frequency response characteristic which reinforces the level of an input signal in a predetermined frequency band, and a second frequency characteristic changing circuit means including a control element the resistance of which is varied in accordance with a voltage applied thereto and having a frequency response characteristic which is variable between a characteristic complementary with said frequency response characteristic of said first frequency characteristic changing circuit means and a flat characteristic; an amplifier means connected in series with said first circuit; a second circuit means provided in a negative feedback loop connected between the output side and the input side of said amplifier and comprising a third frequency characteristic changing circuit means with substantially the same circuit construction as said first frequency characteristic changing circuit means and a fourth frequency characteristic changing circuit means with substantially the same circuit construction as said second frequency characteristic changing circuit means; a control circuit means for generating, in response to the input signal level, a control signal voltage for application to said control element in said second frequency characteristic changing circuit means and for controlling the characteristic of said second frequency characteristic changing circuit means, and means whereby the second frequency characteristic changing circuit means causes the input signal to change in a manner which approaches the complementary characteristic when said input signal has a higher level and in a manner which approaches the flat response when said input signal has a lower level; and a voltage supply circuit means for supplying a constant voltage to a control element means in said fourth frequency characteristic changing circuit means to make a frequency response characteristic of said fourth frequency characteristic changing circuit means a fixed characteristic.

Claim 3

The compressing circuit according to claim 1, wherein said fourth frequency characteristic changing circuit means, applied with said constant voltage, manifests a characteristic complementary with the frequency response characteristic of said third frequency characteristic changing circuit means and said second circuit means manifests a resultant flat frequency response characteristic.

Claim 4

The compressing circuit according to claim 1, wherein said control circuit means, applied with an output signal from said amplifier means as an input signal to said control circuit means, produces a control signal voltage in response to the level of said input signal.

Claim 5

The compressing circuit according to claim 1, wherein said control circuit means comprises a filter means for filtering a signal in a predetermined frequency band out of an input signal and a level detector means for detecting the level of said signal filtered by said filter means and for generating said control signal voltage.

Claim 6

An expanding circuit for expanding a transmitted signal which has been compressed by the compressing circuit according to claim 1 comprising: a third circuit means comprising a fifth frequency characteristic changing circuit means having a specific fixed frequency response characteristic in a predetermined frequency range and a sixth frequency characteristic changing circuit means including a control element, the resistance of which is varied in response to a voltage applied thereto; a second amplifier means connected in series with said third circuit means; a fourth circuit means provided in a negative feedback loop connected between the output side and the input side of said second amplifier means and comprising a seventh frequency characteristic changing circuit means with substantially the same circuit construction as said fifth frequency characteristic changing circuit means and an eighth frequency characteristic changing circuit means with substantially the same circuit construction as said first frequency characteristic changing circuit means, which is capable of varying its frequency response between a characteristic which is complementary with a frequency response characteristic of said seventh frequency characteristic changing circuit means and a flat characteristic; a second voltage supply circuit means which supplies a constant voltage to a control element in said sixth frequency characteristic changing circuit means and makes the frequency response characteristic of said fifth frequency characteristic changing circuit means a fixed characteristic; and a second control circuit means for generating, in response to the level of a signal transmitted from said compressing circuit, a control signal voltage for application to a control element contained in said eighth frequency characteristic changing circuit means, and means whereby said control signal voltage causes said eighth frequency characteristic changing circuit means to cause the input signal to approach the complementary characteristic when said input signal has a higher level and to approach the flat characteristic when said input signal has a lower level.

Claim 7

The expanding circuit according to claim 5, wherein said fifth frequency characteristic changing circuit means has substantially the same circuit construction as said first frequency characteristic changing circuit means, which said sixth frequency characteristic changing circuit means has substantially the same circuit construction as said second frequency characteristic changing circuit means.

Claim 8

The expanding circuit according to claim 6, wherein said seventh frequency characteristic changing circuit means has a specific fixed frequency response characteristic which reinforces the level of an input signal to said circuit In a predetermined frequency band, said fourth circuit means manifests, in itself, a characteristic which approaches a flat characteristic when the level of an input signal fed to said fourth circuit means is a relatively higher level, and which reinforces the level of said input signal in said predetermined frequency band when said input signal is a relatively lower level, and said amplifier means and its associated negative feedback loop including said fourth circuit means attenuates said input signal with a characteristic which is opposite to the characteristic of said fourth circuit.

Claim 9

The expanding circuit according to claim 5, wherein said sixth frequency characteristic changing circuit means, applied with said constant voltage, manifests a characteristic which is complementary with a frequency response characteristic of said fifth frequency characteristic changing circuit means, and said third circuit means has a resultant flat frequency response characteristic.

Claim 10

The expanding circuit according to claim 5, wherein said second control circuit means, applied with an input signal to said second amplifier means as an input signal to said second control circuit means, produces a control signal voltage in response to said signal level.

Claim 11

The expanding circuit according to claim 5, wherein said second control circuit means comprises a second filter means for filtering a signal in a predetermined frequency band out of said input signal and a second level detector means for detecting the level of a signal filtered by said filter means and producing said control signal voltage.

Claim 12

A compressing and expanding circuit comprising: a first circuit comprising a first frequency characteristic changing circuit means having a specific fixed frequency response characteristic which reinforces the level of an input signal in a predetermined frequency band, and a second frequency characteristic changing circuit means including a control element the resistance of which varies in response to a voltage applied thereto and having a frequency response characteristic which is variable between a characteristic complementary with the frequency response characteristic of said first frequency characteristic changing circuit means and a flat characteristic; an amplifier means connected in series with said first circuit; a second circuit means provided in a negative feedback loop connected between the output side and the input side of said amplifier means and comprising a third frequency characteristic changing circuit means having substantially the same circuit construction as said first frequency characteristic changing circuit means, and a fourth frequency characteristic changing circuit means having substantially the same circuit construction as said second frequency characteristic changing circuit means; a control circuit means for producing a control signal voltage in response to the level of the input signal; a voltage supply circuit means for supplying a constant bias voltage to each of the control elements in said second and fourth frequency characteristic changing circuit means; first switching means for switching over an input to said control circuit means respectively to an output signal from said amplifier means or an input signal to said first circuit means in accordance with the compression or expansion mode; and second switching means for switching over the output control signal voltage from said control circuit means as a supply voltage to said second frequency characteristic changing circuit means or to said fourth frequency characteristic changing circuit means in accordance with the compression or expansion mode.

Claim 13

A compressing and expanding circuit comprising: a first circuit comprising a first frequency characteristic changing circuit means having a specific fixed frequency response characteristic which reinforces the level of an input signal in a predetermined frequency band, and a second frequency characteristic changing circuit means including a control element the resistance of which varies in response to a voltage applied thereto and having a frequency response characteristic which is variable between a characteristic complementary with the frequency response characteristic of said first frequency characteristic changing circuit means and a flat characteristic; an amplifier means connected in series with said first circuit; a second circuit means provided in a negative feedback loop connected between the output side and the input side of said amplifier means and comprising a third frequency characteristic changing circuit means having substantially the same circuit construction as said first frequency characteristic changing circuit means, and a fourth frequency characteristic changing circuit means having substantially the same circuit construction as said second frequency characteristic changing circuit means; a control circuit means for producing a control signal voltage in response to the level of the input signal; a voltage supply circuit means for supplying a constant bias voltage to each of the control elements in said second and fourth frequency characteristic changing circuit means; first switching means for switching over an input to said control circuit means respectively to an output signal from said amplifier means or an input signal to said first circuit means in accordance with the compression or expansion mode; and second switching means for switching over the output control signal voltage from said control circuit means as a supply voltage to said second frequency characteristic changing circuit means or to said fourth frequency characteristic changing circuit means in accordance with the compression or expansion mode.