Distortion reducer for companded analog systems

Claim 2

In an analog communication system comprising a transmitter, a transmission path of bandwidth B 2 , and a receiver, a method of recovering components of a signal m(t) of bandwidth B 1 which are cutoff in the transmission of such signal over said transmission path, where B 2 ≧B 1 , and comprising the steps of: compressing the signal at the transmitter; transmitting the signal over said transmission path; demodulating the transmitted signal at the receiver to produce a signal r(t); expanding and filtering the signal r(t) to produce an initial reconstructed signal m E (t); compressing said signal m E (t) to produce a signal C[m E (t)] with frequency components f c extending above the frequency bandwidth B 2 ; filtering said signal C[m E (t)] through a filter of bandwidth B 2 to produce a signal S F corresponding to C[m E (t)] but with the frequency components f c extending above B 2 being cutoff; subtracting S F from C[m E (t)] to produce the frequency components f c ; and adding frequency components f c to r(t) to produce a first order of correction of the received signal r(t).

Claim 3

A receiver for receiving by way of a transmission path having a first bandwidth an analog message signal from a transmitter in which the said analog message signal is generated by amplitude compressing an analog source message having a second bandwidth according to a predetermined characteristic in such a fashion that said analog message signal as transmitted has a third bandwidth exceeding said first bandwidth, whereby said analog message signal received by said receiver has thereby suffered a loss of information, said receiver comprising: receiver input terminals adapted for receiving said analog message signal; expansion means coupled to said receiver input terminals for expanding said analog message signal according to a second characteristic which is the inverse of said predetermined characteristic to form a regenerated source message, subject to said loss of information; compression means coupled to said expansion means for compressing said regenerated source message according to said predetermined characteristic for generating a first facsimile analog message signal which is an approximation of said analog message signal as transmitted; transmission channel simulating filter means coupled to said compression means for filtering said first facsimile analog message signal with said first bandwidth to produce a second facsimile analog message signal representative of said analog message signal as received at said input terminals; differencing means coupled to said compression means and to said transmission channel simulating filter means for taking the difference between said first and second facsimile analog message signals for generating a facsimile lost information signal; summing means coupled to said differencing means and to said receiver input terminals for adding together said analog message signal and said facsimile lost information signal to form a corrected analog message signal; and second expansion means coupled to said summing means for expanding said corrected analog message signal according to said second characteristic for forming a corrected regenerated source message signal in which approximate correction has been made for said lost information.

Claim 4

A receiver for receiving by way of a transmission path having a first bandwidth an analog message signal from a transmitter in which the said analog message signal is generated by amplitude compressing an analog source message having a second bandwidth according to a predetermined characteristic in such a fashion that said analog message signal as transmitted has a third bandwidth exceeding said first bandwidth, whereby said analog message signal received by said receiver has thereby suffered a loss of information, said receiver comprising: receiver input terminals adapted for receiving said analog message signal; expansion means coupled to said receiver input terminals for expanding said analog message signal according to a second characteristic which is the inverse of said predetermined characteristic to form a regenerated source message, subject to said loss of information; compression means coupled to said expansion means for compressing said regenerated source message according to said predetermined characteristic for generating a first facsimile analog message signal which is an approximation of said analog message signal as transmitted; transmission channel simulating filter means coupled to said compression means for filtering said first facsimile analog message signal with said first bandwidth to produce a second facsimile analog message signal representative of said analog message signal as received at said input terminals; differencing means coupled to said compression means and to said transmission channel simulating filter means for taking the difference between said first and second facsimile analog message signals for generating a facsimile lost information signal; summing means coupled to said differencing means and to said receiver input terminals for adding together said analog message signal and said facsimile lost information signal to form a corrected analog message signal; and second expansion means coupled to said summing means for expanding said corrected analog message signal according to said second characteristic for forming a corrected regenerated source message signal in which approximate correction has been made for said lost information.

Claim 5

In a communication system for transmitting a source signal m(t), said system comprising a transmitter, a transmission path of bandwidth B 2 and a receiver, in which the signal transmitted is compressed at baseband to produce signal C(m(t)) with the frequency components f L thereof extending beyond bandwidth B 2 being cutoff by said transmission path and in which the received baseband signal r(t) is expanded and filtered to produce a received signal m E (t) which is not a perfect replicar of said source signal m(t) because of said cutoff of signal components beyond the bandwidth B 2 , a method of substantially recovering said cutoff signal components f L at the receiver comprising the steps of: (a) compressing m E (t) to produce a signal C(m E (t)) having frequency components f c lying above B 2 , where f c ≅f L ; (b) filtering C(m E (t)) to produce a signal S F equal to C(m E (t)) less the frequency components f c ; (c) subtracting S F from C(m E (t)) to provide the components f c ; (d) adding components f c to r(t) to produce a signal C(m 1 (t)) which is a first order corrected approximation of C(m(t)); and (e) expanding and filtering C(m 1 (t)) to produce a first order corrected approximation signal m 1 (t) of m(t).

Claim 6

In a communication system for transmitting a source signal m(t), said system comprising a transmitter, a transmission path of bandwidth B 2 and a receiver, in which the signal transmitted is compressed at baseband to produce signal C(m(t)) with the frequency components f L thereof extending beyond bandwidth B 2 being cutoff by said transmission path and in which the received baseband signal r(t) is expanded and filtered to produce a received signal m E (t) which is not a perfect replicar of said source signal m(t) because of said cutoff of signal components beyond the bandwidth B 2 , a method of substantially recovering said cutoff signal components f L at the receiver comprising the steps of: (a) compressing m E (t) to produce a signal C(m E (t)) having frequency components f c lying above B 2 , where f c ≅f L ; (b) filtering C(m E (t)) to produce a signal S F equal to C(m E (t)) less the frequency components f c ; (c) subtracting S F from C(m E (t)) to provide the components f c ; (d) adding components f c to r(t) to produce a signal C(m 1 (t)) which is a first order corrected approximation of C(m(t)); and (e) expanding and filtering C(m 1 (t)) to produce a first order corrected approximation signal m 1 (t) of m(t).

Claim 7

A method as in claim 3 and further comprising the step of repeating steps (a) through (e) with the signal m 1 (t) to produce a signal m 2 (t) which is a second order corrected approximation of m(t).

Claim 8

In a communication system for transmitting a signal m(t) comprising a transmitter, a transmission path of bandwidth B 2 , a receiver, a compressor at the transmitter for compressing said signal to produce a signal C[m(t)] having signal components f L lying above bandwidth B 2 , and an expander for expanding the received compressed signal r(t) at said receiver to produce an initial estimated signal m E (t) which is an initial estimate of m(t) but with the signal components f L having been cutoff during transmission, a system for generating a first order corrected approximation of said signal m(t), comprising: means for compressing initial estimated signal m E (t) in the same manner as m(t) was compressed at the transmitter to produce a signal C(m E (t)) with signal components f c lying above bandwidth B 2 ; means for filtering C(m E (t)) through a filter having similar filter characteristics L as said transmission path to produce the signal (C(m E (t))-f c ); means for subtracting (C(m E (t))-f c ) from C(m E (t)) to produce f c ; means for adding signal components f c to said received compressed signal to produce reconstituted signal (r(t)+f c ); and means for expanding and filtering said reconstituted signal (r(t)+f c ) to produce a signal m 1 (t) which is a first order corrected approximation of said signal m(t).

Claim 9

In a communication system for transmitting a signal m(t) comprising a transmitter, a transmission path of bandwidth B 2 , a receiver, a compressor at the transmitter for compressing said signal to produce a signal C[m(t)] having signal components f L lying above bandwidth B 2 , and an expander for expanding the received compressed signal r(t) at said receiver to produce an initial estimated signal m E (t) which is an initial estimate of m(t) but with the signal components f L having been cutoff during transmission, a system for generating a first order corrected approximation of said signal m(t), comprising: means for compressing initial estimated signal m E (t) in the same manner as m(t) was compressed at the transmitter to produce a signal C(m E (t)) with signal components f c lying above bandwidth B 2 ; means for filtering C(m E (t)) through a filter having similar filter characteristics L as said transmission path to produce the signal (C(m E (t))-f c ); means for subtracting (C(m E (t))-f c ) from C(m E (t)) to produce f c ; means for adding signal components f c to said received compressed signal to produce reconstituted signal (r(t)+f c ); and means for expanding and filtering said reconstituted signal (r(t)+f c ) to produce a signal m 1 (t) which is a first order corrected approximation of said signal m(t).