Abstract: An instantaneous compressor and an instantaneous expandor both of which use the logarithmic characteristics of diodes to provide compression and expansion, respectively, of uncompressed analog PAM signals and compressed analog PAM signals. Both the compressor and expandor also include circuitry having a temperature dependent gain characteristic. The signal at the compressor output is a temperature independent compressed analog signal which follows the compressor's logarithmic encoding characteristic. The signal at the output of the expandor is a temperature independent uncompressed analog signal which is substantially identical to the analog signal at the compressor input.

Abstract: Circuit for automatic dynamic compression an/or expansion. The circuit has adjusting member (10) for varying the propagation constant in a useful signal path and a further similar adjusting member (30) for varying the propagation constant in a branch path which serves for deriving a control voltage which is applied to the control inputs (11,31) of the adjusting members (10,30) in the useful signal path and in the branch path. Further the circuit has a lowpass circuit (20 . . . 26) which is connected to the adjusting member (10) located in the useful signal path. The circuit can be changed over to a different compander characteristic when the adjusting member (10) located in the useful signal path is connected up as a controllable highpass (10,15). The limiting frequency is of the order of magnitude of the limiting frequency of the lowpass circuit (20 . . . 26).

Abstract: The signal-to-noise ratio of a signal to be transmitted through a communication channel having an upper and lower limit in magnitude is sampled digitally and a common corrective factor for the deviations about the mean value of the signal is determined for amplifying the low portions of the signal without exceeding the limits. By a reverse process at the receiving end of the communication channel, the signal is returned to its original amplitude, thus reducing any noise superposed in the transmission.The invention is applicable to a video channel where the sampled signal is an audio signal. In such case, the deviations of the samples about the mean of the digital information of the audio signal are matched with the mean of the video channel between the black level and the maximum white level.

Abstract: A compressor comprises a variable gain path and a feedback path. The variable gain path includes a modulator which, in response to an input signal and a feedback signal, outputs a train of pulses whose widths and heights are modulated. Also in the variable gain path, a demodulator converts the pulse train into a compressor output. Feedback is derived from the variable gain path.

Abstract: A circuit for the selective automatic dynamic compression and expansion of a signal having frequency components within a given frequency spectrum. It comprises amplifier means having at least one input coupled to the input terminal of the circuit and a summing circuit having its output coupled to the output terminal of the circuit. A plurality of frequency selective channels, each capable of transmitting signals having frequencies within a predetermined portion of the frequency spectrum are coupled between the output of the amplifier means and the input of the summing circuit. One of the frequency selective channels is adapted for transmitting signals in the high frequency range of the frequency spectrum and consists of only a passive filter and an associated adjusting device. The remaining frequency selective channels each include a passive filter, an associated adjusting device and an impedance converter.

Abstract: A frequency region split type noise reducing apparatus, comprising a compressor and an expander; the compressor (or the expander) including a plurality of frequency region splitting filters for splitting the frequency range of the signal into a plurality of frequency regions, a plurality of level sensors for detecting the levels of the signals in the respective frequency regions split by the plurality of filters, a plurality of voltage controlled amlifiers responsive to the level detected outputs of the corresponding level sensors for linearly compressing (or expanding) in terms of a logarithmic scale the dynamic range of the signal, and an adder for adding the outputs as compressed by the voltage controlled amplifiers for providing a composite signal, and the expander (or the compressor) including an amplifier and a switch circuit for connecting the compressor (or expander) to the amplifier to form a negative feedback circuit, whereby the expander (or the compressor) is implemented by a negative feedback amp

Abstract: In a circuit for automatic dynamic compression or expansion the control voltage generator generates a control voltage which is linear against the logarithm of the input amplitude. This characteristic is achieved by a transfer member consisting of several transistors which are subject to different reference voltages and the series resistances of which are staggered.

Abstract: In a preferred embodiment of the invention the audio signal is bandwidth limited before being differentiated to provide either a positive or negative signal (depending on the slope of the audio signal) to a comparator which provides a corresponding logical one or zero at its output. A digital logic circuit connected to the comparator output is clocked to provide a digital output signal transition from a logical one to a zero or vice versa if its input has changed from a logical one to a zero or vice versa at least once during the clock period. The clocked digital output of the logic circuit is provided as an input to a computer where it is stored. The digital data is subsequently read out of the computer and provided to a filter and integrator to provide an audio signal replica of the filtered input audio signal.

Abstract: A compander system wherein, for compatibility with systems of constant amplification, dynamic compression and expansion are effected only within a limited level range.

Abstract: A syllabic compandor using a pilot in the signal frequency band, with a compression ratio between approximately 2 and 20, including a frequency and delay equalizer in the negative-feedback loss-control circuits of the compressor and expandor, to permit high compression and expansion ratios without positive feedback at any frequency or signal level, the expansion ratio materially less than the compression ratio in some cases, and sampling at more than the Nyquist frequency for the signal band to remove any unwanted periodic wave such as the pilot wave, lying in the signal frequency band, from the output, without altering phase and amplitude of other waves in the signal frequency band.

Abstract: This invention discloses new and unique detectors suitable for controlling any device needing precise conversion of AC voltage to an equivalent DC voltage. The detector may also be used as an excellent non-linear low pass filter. More particularly, the detector is suitable for controlling meter movements and the variable gain amplifiers which are used in compressors, expanders, compandors, limiters and other noise reduction systems, The detectors control gain such that unnecessary gain changes are eliminated, overshoot transient distortion is minimized, distortion due to ripple is minimized, and pumping is eliminated. As described herein, the detector uses two interacting memories to provide a output signal from the detector which can be used to drive a variable gain amplifier for achieving optimum characteristics of a compressor, expandor or compandor.

Abstract: A syllabic compander uses logarithmic chains in both the compressor and expander. The logarithmic chains are each constructed of a logarithmic amplifier, a scaler, and an antilogarithmic amplifier or their equivalents. The output of a logarithmic chain is equal to the input raised to a power determined by the scale factor of the scaler. A logarithmic chain is used with a multiplier and an average value circuit to implement equations of the formu.sub.o =Ku.sub.i U.sub.awhere u.sub.i is the input to the compressor or expander, u.sub.o is the output of the compressor or expander, U is the rms (or pseudo-rms) value of u.sub.i or u.sub.o averaged over the time of one syllable, K is a constant, and a is the scale factor which is a function of the compression or expansion rate. Since the scale factor a may be easily adjusted, the compander permits flexibility in changing the compression and expansion rates.

Abstract: The invention relates to a compander system. In such a compander system an adjusting member in the useful signal path is controlled by a control voltage which is generated in a branch path connected to the useful signal path and including at least one further adjusting member. The control voltage serves at the same time to control the adjusting members in the branch path in a counter-regulating sense. According to the present invention the branch path is connected to the input of the useful signal path during compression and to the output of the useful signal path during expansion.

Abstract: A noise reduction system having an encoder which compresses the dynamic range of an analog audio signal, a transmission medium coupled to the encoder, and a decoder coupled to the transmission medium for expanding and thereby restoring the dynamic range of the compressed analog audio signal. The encoder includes a voltage controlled amplifier (VCA) receiving the audio signal, a high-pass filter also coupled to the analog audio signal, a circuit for obtaining a difference between the outputs of the VCA and the high-pass filter, weighting and level sensing circuits coupled to the encoder output for sensing the level of the compressed analog audio signal, and a limiter circuit coupled to the output of the weighting and level sensing circuits and producing a control voltage applied to the VCA control input in such a manner that the ratio of the gain of the VCA and the gain of the high-pass filter at the pass band thereof does not become less than one.

Abstract: This disclosure relates to an electronic circuit which has a controllable impedance. This type of circuit is well suited for application in noise suppression circuits or in signal compression/expansion circuits. The circuit has two stages. The first stage is a voltage to current converter. The second stage is a variable gain current amplifier. The output of the second stage is connected by a feedback loop to an input of the first stage. The circuit has a variable impedance which can be controlled by an external signal. The imput impedance of the circuit is linearly related to the applied control signal.

Abstract: A circuit arrangement for automatic dynamic compression and/or expansion having an auxiliary circuit provided in the useful signal path or in the negative feedback path of an amplifier. The auxiliary circuit has a first and a second connection point. On the one hand there are arranged between the first and the second connection point a first damping or amplifying member, an electronically controllable damping member and a second damping or amplifying member connected in series. On the other hand means for limiting the voltage or current are arranged between the first and second connection point.

Abstract: An amplitude modulated transmitter includes an audio amplifier which, via a radio frequency (RF) power supply modulator, modulates an RF amplifier and carrier. Fluctuations in the direct current DC bias to the RF amp, such as are common in mobile applications, vary the modulation index of the transmitted signal.To minimize DC bias affects on the modulation index, the invention contemplates an audio compressor which senses both audio signal level and DC bias level and, in response thereto, controls audio signal compression to maintain the desired percent modulation. Compression is effected by changing the bias on a semiconductor device which, in turn, attenuates audio signals at the input of a fixed gain audio amplifier. A feedback path around the audio amplifier senses both DC voltage and audio signal level and produces an appropriate control signal to bias the semiconductor device, thereby altering audio compression.

Abstract: Electrical circuitry for varying the relative distinctiveness between the lead and rhythm audio signals produced by an electrical guitar, or the like, according to the strength with which the musician picks the guitar strings. The rhythm signal is compressed in response to how hard the player strums or picks the guitar, while the lead signal is expanded as a direct function of rhythm signal compression. An electrical signal is generated from the rhythm signal for controlling the extent of compression and expansion of the audio signals.

Abstract: Complementary circuits for pre-emphasis and de-emphasis of video luminance signals, the circuits permitting an improved signal-to-noise ratio in a video signal transfer channel. The video signal has separate frequency- and amplitude-sensitive portions as well as a linear gain portion, thereby providing a fixed gain up to a predetermined lower frequency, a fixed response between this frequency and a variable upper frequency breakpoint, and an amplitude-dependent gain above the upper frequency breakpoint. De-emphasis is performed by a circuit also having separate frequency- and amplitude-sensitive portions and a linear gain portion, and which may be readily matched to the pre-emphasis circuit.

Abstract: A compander system is described, in which the `compression` and `expansion` modes are established by the selective use of a compression or expansion circuit as such or as a negative feed-back branch for a back-coupled amplifier.

Abstract: A noise reduction system including an encoder for compressing the dynamic range of an audio input signal in accordance with the amplitude thereof, a transmission medium coupled to the encoder, and a decoder having a transfer function inverse to that of the decoder. The encoder includes a first controlled amplifier circuit in which, when the input audio signal is in low- and middle- levels, sufficient emphasis is made to the signal at high frequencies, and when it is in high-level, the emphasis quantity at high frequencies is restrictive. The decoder includes a voltage-current converting circuit for converting voltage of the reproduced audio signal into current and a second controlled amplifier circuit amplifying the converted current and having a transfer function inverse to that of the first controlled amplifier circuit.

Abstract: A noise reduction system is provided with an encoder for compressing the dynamic range of an audio input signal in accordance with the amplitude of the input signal and recording the compressed one into a tape-recorder, and a decoder for expanding the dynamic range of a reproduced audio signal from the tape-recorder to the original one in accordance with the amplitude of the reproduced audio signal. The encoder includes a first controlled amplifier circuit in which, when the input audio signal is low- and middle-levels, sufficient emphasis is made to the signal at high-frequencies, and when it is in high-level, the emphasis quantity at high-frequencies is restrictive. The decoder includes a second controlled amplifier circuit having a transfer function related to the character of the first controlled amplifier circuit substantially in inverse-function manner.

Abstract: A circuit for optional dynamic compression or expansion, with one amplifier located in the useful signal path and controllable in amplification, and an auxiliary branch including one amplifier controllable in amplification and a control voltage generator, to the input of which a useful signal path is connected and the output voltage of which serves to control the controllable amplification in the useful signal path and in the auxiliary branch, and wherein as adjusting members for controlling the amplification of the controllable amplifiers there serve field effect transistors to the control electrodes of which respective direct control voltages are applied. In parallel with the source-sink path of one field effect transistor serving as adjusting member for an amplifier located in the useful signal path there is connected a voltage divider the tapping of which is connected via a separating amplifier to the control electrode of the field effect transistor.

Abstract: An information signal is compressively encoded for application to a limited amplitude transmission channel, such as a cinemagraphic film, by generating a gain control signal which represents the information signal level, delaying the information signal until the gain control signal has been substantially generated, and using the gain control signal to control encoding of the delayed information signal. The encoded information signal may be transmitted by itself with a similar delay step employed to decode it after reception, or the gain control signal may be transmitted along with the information signal and employed in the decoding operation, thereby avoiding the need for a delay step in the decoding process. In the latter case the gain control signal may be encoded along with the information signal, and employed in a feedback control loop for the encoding process.The invention also includes apparatus designed to perform the disclosed method.

Abstract: A noise reduction system is provided with an encoder for compressing the dynamic range of an audio input signal in accordance with the amplitude of the input signal and recording the compressed one into a tape recorder, and a decoder for expanding the dynamic range of a reproduced audio signal from the tape recorder to the original one in accordance with the amplitude of the reproduced audio signal. The encoder includes a first controlled amplifier circuit in which, when the input audio signal is in low- and middle-levels, sufficient emphasis is made to the signal at high frequencies, and when it is in high-level, the emphasis quantity at high frequencies is restrictive. The decoder includes a second controlled amplifier circuit having a transfer function related to the character of the first controlled amplifier circuit substantially in inverse-function manner.

Abstract: An expander circuit for expanding stereo signals comprises left and right channel variable gain stages whose gain varies in proportion to the amplitude of the control signals fed to the control inputs thereof; and left and right channel expansion control signal providing means for receiving respectively at least portions of the left and right channel signals and feeding expansion control signals to the control inputs of the variable gain stages. These control signals have amplitudes directly proportional to the amplitudes of the signals fed to the inputs of the expansion control signal providing means. The improvement in the circuit is the addition of a signal coupling network for coupling a portion of the left-right channel stereo image producing signal components to the input of the associated expansion control signal providing means and a much smaller porportion thereof to the expansion control signal providing means associated with the other channel.

Abstract: The auxilliary signal path in a compander system includes, in series, a first amplifier 7, a weighting function amplifier 8, and a detection circuit 9. The output of the latter controls the gain of a voltage-controlled variable gain circuit 11 also connected to the first amplifier output, and the integrated output of the variable gain circuit is fed back to the input of the first amplifier via a subtraction circuit 13 also supplied with the auxilliary signal path input. Alternatively, the first amplifier may comprise an operational amplifier, in which case the integrated signal is directly coupled to one of its inputs.

Abstract: An adaptive signal weighting system for use in electrical signal transmission along a signal path comprises a variable filter disposed in the signal path having electrical transmission characteristics capable of varying in response to a first control signal the relationship between the spectral region of dominant energy in a transmitted signal relative to the other spectral regions of the transmitted signal. Frequency discrimination means senses the spectral region of dominant signal energy in the transmitted signal and provides the first control signal as a function of frequency of the dominant signal energy. Gain control means disposed in the signal path and coupled in series with the filter means provides a gain variable on the transmitted signal responsively to a second control signal. This second control signal is logarithmically related to the amplitude of the transmitted signal over at least a portion of the frequency range of the signal.

Abstract: The voice processing system for radio telephone consists of a compressor in the transmitter and an expander in the receiver. The compressor divides the input voice signal into syllabic groups and applies a gain to each syllabic group depending on the peak amplitude of the signal of the respective group, resulting in a reduction or compression of the dynamic range of the voice. The expander on the other hand increases or expands the dynamic range of the signal restoring the voice to its original form. The compressor and expander are synchronized such that the amount of expansion or instantaneous gain in the receiver is exactly the reciprocal of the amount of compression in the transmitter, i.e. when the gain of the compressor is N, the gain of the expander is 1/N. In this way the total system gain is always unity and is therefore transparent to voice and other analog signals passing through it.

Abstract: Compandor apparatus including a compressor comprised of an operational amplifier having a plurality of gain controlling feedback paths that are respectively responsive to different portions of the input frequency spectrum, and an expandor comprised of an operational amplifier having a plurality of gain controlling circuits that are respectively responsive to different portions of the input frequency spectrum.

Abstract: An amplitude modulated transmitter includes an audio amplifier which, via a radio frequency (RF) power supply modulator, modulates an RF amplifier and carrier. Fluctuations in the direct current DC bias to the RF amp, such as are common in mobile applications, vary the modulation index of the transmitted signal.To minimize DC bias affects on the modulation index, the invention contemplates an audio compressor which senses both audio signal level and DC bias level and, in response thereto, controls audio signal compression to maintain the desired percent modulation. Compression is effected by changing the bias on a semiconductor device which, in turn, attenuates audio signals at the input of a fixed gain audio amplifier. A feedback path around the audio amplifier senses both DC voltage and audio signal level and produces an appropriate control signal to bias the semiconductor device, thereby altering audio compression.

Abstract: A complementary distortion circuit for reducing the level of IM products in the output of a TWT amplifier. One embodiment of the invention includes a 3 db, 90.degree. coupler to divide the input signal into two components of equal amplitude but differing phase with an IM generator positioned in the path of each component. The IM generator may comprise non-linear phase modulators or non-linear amplitude modulators. The two components are then re-combined in a second, 3 db, 90.degree. coupler. The non-linear phase-modulators may comprise varactors and the non-linear amplitude modulators may comprise varistors. The unit is intended to operate in the microwave region.

Abstract: A transmitter has a dynamic compressor provided with a dynamic control device and a compression rectifier fed by the signals to be transmitted, whose output signal controls the dynamic control device via a threshold device and also comprises a signal source which supplies a pilot signal, the signal to be transmitted and the pilot signal being fed to the input of the dynamic control device.

Abstract: An adaptive signal weighting filter for use in a compander performs a frequency response weighting function so that during the encoding process the spectral region of dominant signal energy of the program signal is preemphasized with respect to other spectral regions and during the decoding process the corresponding spectral regions of the encoded signal are deemphasized with respect to one another in a complementary manner.

Abstract: An input signal supplied to a signal measuring circuit is either amplified or attenuated as necessary to establish the magnitude of the input signal within the defined dynamic range of the measuring circuit and the output signal developed by the measuring circuit is subsequently readjusted through amplification or attenuation to develop an output signal which corresponds to the magnitude of the initial input signal.

Abstract: A compression and expansion system comprises a control circuit including a variable impedance element, whose impedance is varied in accordance with a control signal applied thereto. The system has a frequency-response characteristic which varies in accordance with variation of the impedance of the variable impedance element. A control signal is formed in accordance with the level of an input signal and is applied to the variable impedance element of the control circuit thereby to vary the impedance thereof. A switching means forms passing paths in a manner such that, at the time of compressing, the input signal is supplied by way of the control circuit to a negative feedback amplifier. At the time of expanding, the input signal is supplied to the negative feedback amplifier, and, at the same time, the output of the negative feedback amplifier is fed back by way of the control circuit to the input of said negative feedback amplifier.

Abstract: A current variable shunt impedance with a non-linear control characteristic intended to functionally imitate and replace a field effect transistor responsive to a control voltage applied to its gate electrode having a source-drain path connected as a variable impedance shunt resistor in the variable frequency high pass filter section of the commercially popular Dolby B noise reduction system. The invention allows a large part of the Dolby B encode decode circuitry to be embodied in low-cost integrated circuit form while maintaining performance characteristics which closely match existing standards. These standards were previously established using noise reduction systems fabricated from discrete circuit components. The complementary nature of the encode and decode operations used to process a signal makes such matching of performance characteristics to the established standards critical.

Abstract: A compression amplifier processes both the audio modulating signal to the transmitter, and the audio output from the receiver, in a conventional amplitude modulated transceiver. Unique switching circuitry provides isolation between transmitter and receiver inputs to, and outputs from, the compression amplifier.

Abstract: Signal expansion/compression apparatus wherein an input signal is compressed by an exponential factor and then subsequently may be expanded by the reciprocal of that exponential factor, wherein neither the signal compression circuits nor the signal expansion circuits need be provided with exponential amplifiers or logarithmic function generators. The signal compressio circuit includes a plurality of serially-connected voltage controlled amplifiers, a first of such amplifiers receiving the input signal to be compressed and a predetermined one of such amplifiers providing the compressed output signal. A control signal generator is connected to the last of the voltage controlled amplifiers to produce a control voltage which is a function of the output signal of that last amplifier and a reference voltage. This control voltage is supplied to all of the voltage controlled amplifiers so as to determine the respective gains thereof.

Abstract: The disclosure relates to band compression devices. The input signals are sampled and stored in at least two memories. In a sequence of n samples, two samples are taken in pairs, compared and the greatest of them is transferred to an output memory from which it can be read for a period equal to or less than the duration of the sequence. When using a non-shifting mode, the output memory is reset at the end of each sequence of n samples. A shifting mode is used in a device comprising a plurality of identical stages connected in series, where each new sample is transmitted to the next stage only if it is greater than the previous stored sample in each stage.

Abstract: A hearing aid is arranged to restrict the dynamic range of the audio input signal to be compatible with the range which is characteristic of the hearing defect of the user. By means of phase-shift modulation for example, the input signal is converted to a single sideband at HF which is then peak-clipped to a predetermined value. The effective degree of clipping is determined by the AGC level of an input amplifier which is controlled by the user. The clipped signal is restored to the audio band for reproduction at a preset level.

Abstract: The invention relates to transmission systems with dynamic compansion for the coded transmission of speech signals. By the use of a limiting device in the dynamic control circuit of the receiver the intelligibility is improved. The limiting device is such that the dynamic range of a transmitted signal is limited so that no relevant information is lost in the period the human ear needs to adapt itself to rapid sound variations.

Abstract: In a circuit for the automatic dynamic compression or expansion of a signal between its input and its output, the circuit presenting a useful signal path connected between its input and its output and connecting a first variable gain member which is electronically controllable for varying the transmission factor between the circuit input and output, the circuit also presenting a branch path connected at one point to the useful signal path and containing a second variable gain member which is electronically controllable for varying the transmission factor exhibited by the branch path, and a control voltage generator for generating a control voltage applied to control the variable gain members for effecting a counterregulation of the transmission factor of the branch path, relative to the transmission factor variation imparted to the useful signal path, there is further provided a combining circuit which receives the output signal from the second variable gain member and a signal proportional to a signal at a p

Abstract: A single channel carrier system is disclosed in which carrier techniques are used to add a second private talking path on a normal telephone subscriber loop. A central office unit modulates the second subscriber's signal to an inaudible range while the remote subscriber's unit demodulates this signal to recover the voice signal. The same process is used in the opposite direction utilizing a different carrier frequency.Each unit includes a carrier receiver having automatic gain control circuits and a variable attenuation circuit. The automatic gain control circuit includes a direct current clamp to minimize excursions of the AGC signal in the absence of a received carrier. The variable attenuation circuit uses a diode bridge shunting the signal path and controlled by the AGC signal. A linear carrier oscillator is implemented by providing a constant current source of oscillation energy.

Abstract: This invention relates to real time control either in compression or expansion of the amplitude of electrical waves and more specifically to a real time expandor or compressor of the amplitude of sound waves. In general a source of waves which may be the electrical equivalent of a sound including continuous discourse is applied to a plurality of first filters whose output is connected to respective power circuits, that is, circuits that produce at its output a wave whose amplitude is a power, either greater or less than one, of the amplitude of the wave applied to its input. The outputs of the power circuits are applied to respective second filters which are similar to the filters connected to the input of the power circuits and the output of the second filters is connected to a utilization device. Amplifiers may be connected where needed.

Abstract: A transmitting and reproducing system includes an encoder for selectively combining at least four signals for transmission along two channels. At least three of the signals are combined in a predetermined amplitude and phase relationship to form a first unmodulated signal and at least one of the three signals is used to vestigially modulate a carrier. The unmodulated signal and the modulated carrier are combined for transmission along one of the channels. A different grouping of three of the signals are combined in a different predetermined amplitude and phase relationship to form a second unmodulated signal, and at least one signal different from the first modulating signal is used as a second modulating signal for vestigial modulation of the carrier. The second unmodulated signal and the carrier modulated by the different signal are combined for transmission on the second channel.

Abstract: In a signal compressor wherein an amplified and smoothed control signal is employed to control a variable gain or variable impedance device, a switch is provided to connect a positive feedback loop across the control signal amplifier, thereby to place the compressor in an oscillator mode for providing an alignment or calibration signal of stable amplitude.

Abstract: A switchable compression-expansion device, in which a first and a second impedance are included in the emitter circuits of two emitter followers. The impedances are also included in two isolated signal circuits which are coupled via a current mirror circuit. A selection means maintains one of the two emitter followers in the non-conducting state. The current which is produced in the associated impedance by the conducting emitter follower is coupled to the other impedance by the current mirror arrangement. In the other condition of the selection circuit the reverse occurs so that two complementary transfer functions are realized.

Abstract: In a floating electrical output circuit according to co-pending application Ser. No. 395,608, now U.S. Pat. No. 3,909,737, issued Sept. 30, 1975, two load terminals are driven from amplifying means having differential current outputs. A differential amplifier connected across the load terminals provides a feedback signal which is compared with the input signal to provide a control signal for the amplifying means.In the present invention the amplifying means are simplified to a single differential current amplifier.

Abstract: The invention concerns a dynamic range compressor type encoder or expander type decoder, in which a main signal component in a main path is boosted or bucked by a further signal component derived from a point in the main path by a further path having the characteristics of so restricting the further signal component that the boosting or bucking action is only appreciable below a low level threshold. In the present invention the further signal component is a difference signal formed between a direct signal derived from a point in the main path and a delayed version of either the same signal or of another signal derived from another point in the main path. At the frequency equal to the reciprocal of the delay, and at harmonics of this frequency, the direct and delayed signals cancel. The compressor or expander action, and hence noise reduction action, takes place only at intervening frequencies.