US 4,376,916 AGranted

Signal compression and expansion system

AssigneeCBS Inc.

InventorJohn B. Glaberson

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Abstract

An improved system for compressing or expanding the dynamic range of an electrical input signal of the type which includes a variable gain device for controlling the gain of the input signal and a circuit for generating a control signal for the variable gain device including a rectifier for producing responsively to the input signal a DC signal which substantially follows dynamic variations of the input signal. The improvement resides in the provision in the control signal generating circuit of a network of at least three signal paths having a common input terminal connected to receive the DC signal and a common output terminal connected to the variable gain device for coupling a control signal thereto, which signal paths have differing time constants and are automatically selectable as a function of the rate and amount of change in amplitude of the DC signal.

Claims25

Dependent claims

In a circuit for the automatic compression or expansion of a signal between its input and its output, the circuit having a useful signal path connected between its input and its output and containing an electronically controllable variable gain device for varying the gain between the circuit input and output, and control signal generating means for generating responsively to the input signal to the useful signal path a control signal and for applying the control signal to the variable gain device for causing the gain between the input and the output of the useful signal path to have a value dependent on the value of the control signal, wherein said control signal generating means comprises:

rectifier means responsive to said input signal for producing a rectified signal having a level proportional to the amplitude of the input signal,
a relatively fast-acting short time constant smoothing filter connected to receive said rectified signal for filtering ripple therefrom and producing a direct voltage output the level of which substantially follows dynamic variations of the applied rectified signal, and
a network of signal paths having first and second primary branches having a common input terminal connected to receive the direct voltage from said smoothing filter and a common output terminal connected to said variable gain device for applying a control signal thereto,
said first primary branch including relatively fast-acting high pass filter means and a first unidirectional conducting device exhibiting a predetermined forward voltage drop for transmitting to the common output terminal only signals exhibiting a large, fast increase in amplitude sufficient to exceed the said forward voltage drop to cause said first unidirectional conducting device to conduct, and
said second primary branch including first and at least second signal paths,
said first signal path comprising relatively long time constant low pass filter means for transmitting to the common output terminal any direct voltage signals exhibiting small, slow changes in amplitude, and
said at least second signal path comprising a second unidirectional conducting device exhibiting a predetermined forward voltage drop connected in series with second low pass filter means, said second unidirectional conducting device being poled to transmit to the common output terminal only signals exhibiting a relatively slow increase in amplitude sufficient to exceed the forward voltage drop of the second unidirectional conducting device to cause it to conduct.

4 more elements

An electronic circuit for compressing or expanding the dynamic range of an electrical input signal comprising a variable gain stage for amplifying the input signal, and control signal generating means including rectifier means responsive to the input signal to produce a rectified signal having a level proportional to the amplitude of the input signal and which substantially follows dynamic variations of the input signal, said variable gain stage including means for controlling the gain thereof as a function of the rectified signal level, the circuit characterized in that the control signal generating means comprises:

a network of signal paths having a common input terminal for receiving the said rectified signal and a common output terminal connected to the variable gain stage for coupling a control signal thereto, the said network comprising
a first signal path including a first unidirectional conducting device exhibiting a predetermined forward voltage drop for transmitting to the common output terminal only signals exhibiting a fast, large increase in amplitude sufficient to exceed the said forward voltage drop to cause said first unidirectional conducting device to conduct, and means for terminating conduction through the said first signal path after a relatively short time period,
a second signal path consisting of a first relatively long time constant low pass filter means for transmitting to the common output terminal any signal exhibiting small, slow changes in amplitude, and
a third signal path comprising a second unidirectional conducting device exhibiting a predetermined forward voltage drop connected in series with second low pass filter means, the second unidirectional conducting device being poled to transmit to the common output terminal only signals exhibiting a relatively slow increase in amplitude sufficient to exceed the forward voltage drop of the second unidirectional conducting device to cause it to conduct.

1 more element

A circuit for compressing or expanding the dynamic range of an input signal comprising a variable gain stage for amplifying the input signal, and control signal generating means including rectifier means responsive to the input signal to produce a rectified signal having a level proportional to the amplitude of the input signal and which substantially follows dynamic variations of the input signal, said variable gain stage including means for controlling the gain thereof as a function of the rectified signal level, the circuit characterized in that the control signal generating means includes

a network of at least three signal paths having differing time constants, said network having a common input terminal for receiving said rectified signal and a common output terminal connected to the variable gain stage for coupling a control signal thereto,
all except one of said signal paths each including means for causing the respective path to be conductive only in response to said rectified signal exhibiting a change in amplitude sufficient to exceed a predetermined voltage, and
the said one of said signal paths being conductive responsively to any differential in voltage between said common input and output terminals.

Dependencies are inferred from the claim text.