Signal conditioning circuit for compressing audio signals

A circuit for conditioning an input signal in accordance with a compression ratio, comprising:

• an input buffer having an output terminal, said input buffer responding to an input signal by producing a buffer signal at said output terminal that follows said input signal and also producing a rectified input signal;
• an averaging circuit that produces an averaged signal from said rectified input signal, said averaging circuit having a programmable input that selects an integration period for averaging said rectified input signal;
• an interface circuit that produces a gain signal that is a function of said averaged signal and said compression ratio;
• and
• an amplifier connected to amplify the buffer signal by an exponential function of said gain signal to produce a compressed output signal.

An analog circuit for conditioning an input signal in accordance with a compression ratio, comprising:

• an analog input buffer having primary and secondary output terminals, said input buffer responding to an input signal by a) producing a buffer signal at said primary output terminal that follows said input signal and b) directly sensing and rectifying the buffer signal at said primary output terminal to produce a rectified signal at said secondary output terminal that more accurately represents the buffer signal than the input signal;
• an analog averaging circuit that produces an averaged signal from said rectified signal;
• an analog interface circuit that produces a gain signal that is a function of said averaged signal and said compression ratio, said interface circuit providing fixed and selectable compression ratios when the input signal lies above and below a rotation point, respectively, and having a programmable input that selects said compression ratio when the input signal lies below said rotation point;
• and
• a voltage controlled amplifier (VCA) connected to amplify the buffer signal by an exponential function of said gain signal to produce a compressed output signal that is more accurate than it would have been if the input signal were directly sensed and rectified to produce the rectified signal.

A circuit for conditioning an AC input signal in accordance with a compression ratio, comprising:

• an input buffer having an output terminal, said input buffer responding to an input signal by producing a buffer signal at said output terminal that follows said input signal and also producing a rectified current signal;
• said input buffer comprising,
• a differential-to-single ended input stage having first and second inputs and an output, said input stage producing a drive signal at said output in response to said AC input signal being applied to said first input, said second input being connected to the buffer's output terminal to provide a feedback path;
• an output stage comprising first and second active devices that conduct source and sink currents, respectively, on alternate half cycles of said AC input signal in response to said drive signal to alternately source and sink current to said output terminal such that said buffer signal approximately follows said AC input signal;
• and
• a full-wave rectifier that senses said source and sink currents to produce said rectified current signal;
• an averaging circuit that produces an averaged signal from said rectified current signal;
• an interface circuit that produces a gain signal that is a function of said averaged signal and said compression ratio;
• and
• an amplifier connected to amplify the buffer signal by an exponential function of said gain signal to produce a compressed output signal.

A signal conditioning circuit for conditioning an input signal in accordance with a compression ratio, comprising:

• an input buffer having primary and secondary output terminals, said input buffer responding to an input signal by producing a buffer signal at said primary output terminal that follows said input signal, said input buffer comprising a full-wave rectifier that directly senses and rectifies the buffer signal at said primary output terminal to produce a rectified input signal at said secondary output terminal that more accurately represents the buffer signal than the input signal;
• an averaging circuit that produces an averaged signal from said rectified input signal;
• an interface circuit that produces a gain signal that is a function of said averaged signal and said compression ratio;
• a voltage controlled amplifier (VCA) connected to amplify the buffer signal by an exponential function of said gain signal to produce a compressed output signal that is more accurate than it would have been if the input signal were directly sensed and rectified to produce the rectified signal;
• and
• a single capacitor that is connected between the input buffer's primary output terminal and said VCA to simultaneously block the DC component of the buffer signal applied to a) the VCA from the buffer's primary output and b) the buffer's full-wave rectifier due to DC variations in the buffer and the VCA.

A circuit for conditioning an input signal in accordance with a compression ratio, said circuit having a characteristic compression curve that relates its compressed output signal to the input signal, comprising:

• an input buffer having an output terminal, said input buffer responding to an input signal by producing a buffer signal at said output terminal that follows said input signal and also producing a rectified input signal;
• an averaging circuit that produces an averaged signal from said rectified input signal;
• an interface circuit that produces a gain signal that is a function of said averaged signal and said compression ratio, said interface circuit comprising,
• a biasing circuit that establishes a rotation point on said characteristic compression curve;
• a comparator that compares said averaged signal to said rotation point;
• a compression circuit that establishes a nominal value for said compression ratio, and scales said averaged signal in accordance with said nominal value to produce said gain signal when said input signal is less than said rotation point;
• and
• a limiting circuit that increases said compression ratio to a limiting value and scales said averaged signal in accordance with said limiting value to reduce said gain signal when the input signal exceeds said rotation point;
• and
• an amplifier connected to amplify the buffer signal by an exponential function of said gain signal to produce a compressed output signal.

A circuit for conditioning an input signal in accordance with a compression ratio, comprising:

• a input buffer having primary and secondary output terminals, said input buffer responding to an input signal by a) producing a buffer signal at said primary output terminal that follows said input signal and b) directly sensing and rectifying the buffer signal at said primary output terminal to produce a rectified input signal at said secondary output terminal that more accurately represents the buffer signal than the input signal;
• an averaging circuit that produces an averaged signal from said rectified input signal, said averaging circuit comprising:
• a log converter for producing a log signal response to said rectified signal;
• a first multiplier for producing a log power signal as the log of the input signal raised to a known power factor by scaling said log signal by said known power factor;
• and
• a log filter for producing said averaged signal as a log mean-power signal by averaging said log power signal;
• an interface circuit that produces a gain signal that is a function of said log mean-power signal and said compression ratio, said interface circuit producing fixed and selectable compression ratios when the input signal lies above and below a rotation point, respectively, said interface circuit having a programmable input for selecting said compression ratio when the input signal lies below said rotation point;
• and
• a voltage controlled amplifier (VCA) connected to amplify the buffer signal by an exponential function of said gain signal to produce a compressed output signal that is more accurate than it would have been if the input signal were directly sensed and rectified to produce the rectified signal.

A circuit for conditioning an input signal in accordance with a compression ratio, comprising:

• an input buffer having an output terminal, said input buffer responding to an input signal by producing a buffer signal at said output terminal that follows said input signal and also producing a rectified input signal;
• an averaging circuit that produces an averaged signal from said rectified input signal,
• an interface circuit that produces a gain signal that is a function of said averaged signal and said compression ratio;
• and
• a voltage controlled amplifier (VCA) connected to amplify the buffer signal by an exponential function of said gain signal to produce a compressed differential output current, said VCA comprising,
• a gain core stage that produces a differential feedback current and said differential output current in response to a differential drive current and said gain signal;
• a transresistance stage having a dynamic impedance that varies in accordance with said gain signal such that the loop gain of said VCA is approximately constant, said buffered input signal being injected with said differential feedback current to produce a differential voltage across said dynamic impedance;
• and
• a transconductance stage that low pass filters the differential voltage and supplies said differential drive current.

An analog circuit for conditioning an input signal in accordance with a compression ratio, comprising:

• an analog input buffer having primary and secondary output terminals, said input buffer responding to an input signal by a) producing a buffer signal at said primary output terminal that follows said input signal and b) directly sensing and rectifying the buffer signal at said primary output terminal to produce a rectified input signal at said secondary output terminal that more accurately represents the buffer signal than the input signal;
• an analog averaging circuit that produces an averaged signal in response to said rectified signal;
• an analog interface circuit that produces a gain signal that is a function of said averaged signal and said compression ratio, said interface circuit comprising a compression circuit that has a programmable input for selecting a nominal value for said compression ratio, a limiting circuit that increases the value of said compression ratio when said input signal exceeds a rotation point, and a downward expansion circuit that reduces the value of said compression ratio when said input signal is less than a break point;
• and
• a voltage controlled amplifier (VCA) connected to amplify the buffer signal by an exponential function of said gain signal to produce a compressed output signal that is more accurate than it would have been if the input signal were directly sensed and rectified to produce the rectified signal.

A circuit for conditioning an input signal in accordance with a compression ratio, said circuit having a characteristic compression curve that relates its compressed output signal to the input signal, comprising:

• an input buffer having an output terminal, said input buffer responding to an input signal by producing a buffer signal at said output terminal;
• an averaging circuit that produces an averaged signal in response to said input signal;
• an interface circuit that produces a gain signal that is a function of said averaged signal and said compression ratio, said interface circuit comprising:
• an input terminal for receiving said averaged signal,
• a compression circuit that establishes a nominal value for said compression ratio,
• a limiting circuit that includes:
• a first biasing circuit that establishes a rotation point on said characteristic compression curve, and
• a first comparator that compares said averaged signal to said rotation point and increases the value of said compression ratio when said averaged signal exceeds the rotation point;
• a downward expansion circuit that includes
• a second bias circuit that establishes a break point that lies below said rotation point on said characteristic curve;
• and
• a second comparator that compares said averaged signal to said break point and reduces the value of said compression ratio when said averaged signal is less than said break point;
• and
• an amplifier connected to amplify the buffered signal by an exponential function of said gain signal to produce a compressed output signal.

A circuit in accordance with 35, said limiting circuit comprising:

• a sensing circuit that senses when said averaged signal exceeds said high reference voltage and causes a limiting current to flow through said first input resistor, and that produces a control signal;
• a limiting circuit that supplies said limiting current in response to said control signal;
• and
• a mirror circuit that mirrors a portion of said limiting current to reduce said gain signal and said compression ratio.