# US 4,340,780 A · Self-correcting audio equalizer > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 4,340,780 A - Document ID: US4340780A - Status: Granted - Kind: A - Application number: 06/128,245 - Filed: 1980-03-07 - Granted: 1982-07-20 - Earliest claimed: 1980-03-07 - Assignees: Transcale AB - Inventors: Lars Odlen - Web page: https://patentgrep.com/patent/US4340780A ## CPC - H03G 5/165: {Equalizers; Volume or gain control in limited frequency bands} - H03G: CONTROL OF AMPLIFICATION ## Abstract A self-correcting audio equalizer is disclosed for use in a high fidelity sound reproduction system. The equalizer (46) responds to the audio signal to provide an equalized audio signal to a sound reproducing device (28) for generating a corresponding acoustic signal. The equalizer includes circuitry (48- 58) for dynamically measuring the differences between the frequency versus amplitude characteristics of the audio and acoustic signals. Another circuit (60- 68) automatically adjusts the frequency versus amplitude characteristics of the equalized audio signal so that the measured differences are reduced. The adjustment of the equalizer thus takes place automatically and substantially continuously during normal operation of the system. ## Claims ### Claim 1 An audio equalizer for use in a system including an audio signal source for providing a first audio signal, and sound reproducing means responsive to a second audio signal for reproducing sounds in accordance therewith, said audio equalizer being responsive to said first audio signal for providing said second audio signal to said sound reproducing means, comprising: means responsive to said reproduced sounds and to said first audio signal for determining the difference between the frequency versus amplitude characteristics thereof, and means for automatically adjusting the frequency versus amplitude characteristics of said first audio signal in accordance with said determined difference, with the resulting adjusted audio signal being provided to said sound reproducing means as said second audio signal. ### Claim 2 (depends on 1) An audio equalizer as set forth in claim 1, wherein said difference determining means comprises means for determining the time averaged said difference. ### Claim 3 (depends on 1) An audio equalizer as set forth in claim 1, wherein said difference determining means comprises means for determining the time averaged said difference and having a time constant on the order of 0.5 minutes. ### Claim 4 (depends on 1) An audio equalizer as set forth in claim 1, and further comprising means controllable by an operator for disabling said automatic adjustment means to thereby maintain the frequency versus amplitude characteristics of said first audio signal as they were just prior to said disablement. ### Claim 5 (depends on 1) An audio equalizer as set forth in claim 1, wherein said difference determining means comprises means responsive to said reproduced sounds to provide a third audio signal in accordance therewith, and means for determining the difference between said frequency versus amplitude characteristics of said reproduced sounds and said first audio signal by determining the difference between the frequency versus amplitude characteristics of said first and third audio signals. ### Claim 6 (depends on 5) An audio equalizer as set forth in claim 5, wherein said means for determining the difference between the frequency versus amplitude characteristics of said first and second audio signals comprises means for separating each of said first and second audio signals into a plurality of frequency components, means for comparing the level of each of said frequency components of said first audio signal with the level of the like frequency component of said second audio signal, means for automatically adjusting the amplitudes of each of said frequency components of said first audio signal in accordance with the results of the corresponding said comparison, and means for combining the resulting amplitude adjusted frequency components to thereby derive said second audio signal. ### Claim 7 (depends on 6) An audio equalizer as set forth in claim 6, wherein said means for providing said third audio signal has a frequency response characteristic which is not flat, whereby the frequency versus amplitude characteristic of said third audio signal is not identical to the frequency versus amplitude characteristic of said reproduced sounds, and wherein said means for comparing includes means for adjusting said comparisons in accordance with said frequency response characteristic. ### Claim 8 (depends on 6) An audio equalizer as set forth in claim 6, wherein said means for comparing comprises a plurality of level detector means, each associated with a corresponding one of said frequency components of said first and third audio signals, for each providing a signal whose DC level varies with the level of the corresponding frequency component, a plurality of difference amplifier means each responsive to the two DC signals provided by two level detector means which respond to corresponding frequency components of said first and third audio signals to provide a difference signal which varies with the difference between said two DC signals, and wherein said means for automatically adjusting comprises means for adjusting the amplitude of each said frequency components of said first audio signal in accordance with the corresponding said difference signal. ### Claim 9 (depends on 8) An audio equalizer as set forth in claim 8, wherein said means for adjusting the amplitudes of each said frequency components comprises a plurality of heat sensitive means, each associated with a corresponding one of said frequency components, and each providing an amplitude adjustment of the corresponding component in dependence upon the temperature of that heat sensitive means, and means for controlling the temperatures of each of said heat sensitive means in accordance with the corresponding said difference signal. ### Claim 10 An audio equalizer for use in an audio system producing audio frequency sounds in a listening environment, comprising: first input means adapted to be responsive to a first audio signal to be equalized, output means for providing an equalized audio signal to be used in producing said audio frequency sounds, second input means responsive to a second audio signal derived from sounds produced in said listening environment by said audio system, and means for, during normal use of said audio system, automatically determining the differences between the frequency versus amplitude characteristics of said first and second audio signals and adjusting the frequency versus amplitude characteristic of said first audio signal in accordance with said determined difference so as to thereby provide said equalized audio signal. ### Claim 11 (depends on 10) An audio equalizer as set forth in claim 10, wherein said automatic determining and adjusting means comprises microcomputer means programmed to adjust said frequency versus amplitude characteristics of said first audio signal in accordance with said determined difference. ### Claim 12 (depends on 11) An audio equalizer as set forth in claim 11, wherein said microcomputer means comprises means programmed to adjust said characteristics in accordance with the time averaged said difference. ### Claim 13 A method of automatically equalizing an audio signal during normal operation of a high fidelity sound reproduction system comprising the steps of: providing a first audio signal, generating an acoustic signal from an equalized said first audio signal, deriving a second audio signal from said acoustic signal, determining, during normal operation, the amplitude versus frequency characteristics of both said first and second audio signals, and automatically modifying the frequency versus amplitude characteristic of said first audio signal in accordance with the difference between said determined amplitude versus frequency characteristics of said first and second audio signals to thus provide said equalized first audio signal for use in generating said acoustic signal. ### Claim 14 A self-correcting audio equalizer for use in a system wherein an audio signal source provides a first audio signal which is to be reproduced as an acoustic signal by sound reproducing means, comprising: means for regenerating a second audio signal from the acoustic signal provided by said sound reproducing means, means for separating each of said first and second audio signals into a plurality of frequency components, means for comparing the level of each of said frequency components of said first audio signal with the level of the like frequency component of said second audio signal, means for automatically adjusting the amplitudes of each of said frequency components of said first audio signal in accordance with the results of the corresponding said comparison, and means for combining said amplitude adjusted frequency components to thereby derive an equalized said first audio signal for driving said sound reproducing means. ### Claim 15 (depends on 14) A self-correcting audio equalizer as set forth in claim 14, wherein said means for comparing comprises a plurality of level comparator means, each corresponding to a respective one of said frequency components of said first audio signal and responsive to said corresponding frequency component and the like frequency component of said second audio signal for providing an output signal whose level varies in accordance with the difference between the level of said components. ### Claim 16 (depends on 15) A self-correcting audio equalizer as set forth in claim 15, wherein said automatic adjusting means comprises a plurality of controlled attenuator means, each responsive to a corresponding frequency component of said first audio signal for adjusting the amplitude thereof in accordance with the output signal of the corresponding said level comparator means. ### Claim 17 (depends on 14) A self-correcting audio equalizer as set forth in claim 14, wherein said regenerating means has a frequency response characteristic which is not flat whereby the frequency versus amplitude characteristics of said second audio signal and acoustic signals are different, and wherein said comparing means includes means for compensating said comparison for said non-flat frequency response characteristic.