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CLASS 381,	ELECTRICAL AUDIO SIGNAL PROCESSING SYSTEMS AND DEVICES
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SECTION I - CLASS DEFINITION

This class provides for: (a) wired one-way electrical transmission or processing systems for audio signals, (b) Stereophonic systems, which are not elsewhere classified, (c) instrument or process for converting an electrical audio information signal to or from humanly audible form.

SECTION II - LINES WITH OTHER CLASSES AND WITHIN THIS CLASS

A transducer for conversion of speech or similar sound (e.g., music) to or from a corresponding electrical signal, and the combination of such a transducer with acoustical structure is classified herein (see Subclass References to the Current Class, below). Note that transducer structure combined with circuitry, other than a lead or connector, is classified with the system, either in this class (381), or in an external class for processing an audio signal.

Excluded from this class are two-way wired electrical audio signal transmission systems which are classified elsewhere. Transmissions of pulse signals which do not represent audio signals is classified elsewhere. Multiplex Communication Systems (except stereophonic multiplex systems) are classified elsewhere. Modulated Carrier Wave Communication Systems and devices are classified elsewhere. Combinations of subject matter of this class with structure of a more comprehensive combination (e.g., Television Set) are classified with the combination. (See References to Other Classes, below.)

SECTION III - SUBCLASS REFERENCES TO THE CURRENT CLASS

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SECTION IV - REFERENCES TO OTHER CLASSES

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SUBCLASSES

1	BINAURAL AND STEREOPHONIC:
	This subclass is indented under the class definition.  Subject matter where there are two or more independent sound signals which are to be separately reproduced so as to create a sense of depth. (1) Note. "Sound signals" are electric signals representing sounds. (2) Note. Stereophonic or pseudo stereo- phonic systems are classified here, not in Class 370. SEE OR SEARCH CLASS: 360,  Dynamic Magnetic Information Storage or Retrieval,   for magnetically recording or reproducing stored stereophonic information. 369,  Dynamic Information Storage or Retrieval,   subclasses 85+ for recording or reproducing stored stereophonic information. 370,  Multiplex Communications,   appropriate subclasses for multiplex in general.

2	Broadcast or multiplex stereo:
	This subclass is indented under subclass 1.  Subject matter for the broadcasting or transmission of two or more independent sound signals which are to be reproduced so as to create a sense of depth. SEE OR SEARCH CLASS: 360,  Dynamic Magnetic Information Storage or Retrieval,   for magnetically recording or reproducing stored stereophonic information. 369,  Dynamic Information Storage or Retrieval,   subclasses 85+ for recording or reproducing stored stereophonic information. 370,  Multiplex Communications,   appropriate subclasses for multiplex in general. 455,  Telecommunications,   appropriate subclasses for broadcast, nonstereo radio communications.

3	FM final modulation:
	This subclass is indented under subclass 2.  Subject matter where stereophonic signals are broadcast by means of a radio carrier wave which is only frequency modulated. (1) Note. There may also be an amplitude modulated subcarrier. SEE OR SEARCH CLASS: 455,  Telecommunications,   subclasses 42+ , 93, 110+, and 205+ for frequency modulated carrier wave communications in general.

4	AM subcarrier:
	This subclass is indented under subclass 3.  Subject matter where a carrier wave is frequency modulated by a subcarrier which has been amplitude modulated by a signal.

5	Four discrete channels:
	This subclass is indented under subclass 4.  Subject matter where the independent sound signals are four in number. SEE OR SEARCH THIS CLASS, SUBCLASS: 19+, for quadrisonic sound systems in general.

6	Having transmitter:
	This subclass is indented under subclass 5.  Subject matter including means for generating and modulating a carrier wave by a stereophonic signal and usually including the coupling of the modulated carrier wave to the transmission medium. SEE OR SEARCH CLASS: 455,  Telecommunications,   subclasses 39 through 129for radio transmitters in general.

7	Switch-type detector or modulator:
	This subclass is indented under subclass 4.  Subject matter having a detector or modulator which extracts information from or adds information to an input waveform only at instants determined by a selector pulse.

8	Two diodes:
	This subclass is indented under subclass 7.  Subject matter in which the detector or demodulator is limited to containing, two diodes.

9	Four or more diodes:
	This subclass is indented under subclass 7.  Subject matter in which the detector or demodulator contains at least four diodes.

10	Channel separation control:
	This subclass is indented under subclass 4.  Subject matter where there is control over the electrical or acoustical difference between the independent sound signals.

11	Automatic switchover between mono and stereo modes:
	This subclass is indented under subclass 4.  Subject matter where a receiver is changed from a monaural receiver to a stereophonic receiver in response to the presence of a stereophonic signal at its input.

12	Stereo indicators (e.g., stereo presence):
	This subclass is indented under subclass 4.  Subject matter having an indicator which gives an indication in response to the presence of a stereophonic signal.

13	Antinoise:
	This subclass is indented under subclass 4.  Subject matter having provision for the reduction or elimination of an unwanted signal. SEE OR SEARCH CLASS: 455,  Telecommunications,   subclasses 63.1 through 65,222-224, 283-288, 296-312, and subclasses 501-506 for noise elimination in radio communications in general.

14	Having transmitter:
	This subclass is indented under subclass 4.  Subject matter including means for generating and modulating a carrier wave by a stereophonic signal and usually including the coupling of the modulated carrier wave to the transmission medium. SEE OR SEARCH CLASS: 455,  Telecommunications,   subclasses 39 through 129for radio transmitters in general.

15	AM or both AM and angle final modulation:
	This subclass is indented under subclass 2.  Subject matter having a carrier wave which is amplitude modulated and which may also be angle modulated.

16	Having transmitter:
	This subclass is indented under subclass 15.  Subject matter including means for generating and modulating a carrier wave by a stereophonic signal and usually including the coupling of the modulated carrier wave to the transmission medium. SEE OR SEARCH CLASS: 455,  Telecommunications,   subclasses 39 through 129for radio transmitter in general.

17	Pseudo stereophonic:
	This subclass is indented under subclass 1.  Subject matter having a device or arrangement which transforms single channel audio signals into a plurality of channels of audio information to create a sense of depth. SEE OR SEARCH THIS CLASS, SUBCLASS: 63+, for reverberators, which are often used to create a stereophonic effect.

18	Pseudo quadrasonic:
	This subclass is indented under subclass 17.  Subject matter where the plurality of channels are four in number.

19	Quadrasonic:
	This subclass is indented under subclass 1.  Subject matter in which four independent sound signals are to be reproduced.

20	Matrix:
	This subclass is indented under subclass 19.  Subject matter where the four independent sound signals are either (1) combined or coded to give one or more complex signals or (2) one or more complex signals are combined or decoded to recover the four independent sound signals.

21	4-2-4:
	This subclass is indented under subclass 20.  Subject matter where four quadrasonic signals are multiplexed into two more complex signals, then expanded back into four signals.

22	Variable decoder:
	This subclass is indented under subclass 21.  Subject matter where the two more complex signals are combined in variable ratios in order to recover the four independent sound signals.

23	With encoder:
	This subclass is indented under subclass 21.  Subject matter where the four independent sound signals are combined or coded to give two complex signals.

23.1	Hearing aid:
	This subclass is indented under subclass 1.  Subject matter having a small ambient sound reproducing system wherein two or mote output transducers transmit independent sound signals directly into the ear or skull for use by a hearing impaired individual to increase the sound level received by the ear (i.e., listening assistance. SEE OR SEARCH THIS CLASS, SUBCLASS: 68+, and the search notes therein for hearing aids, in general.

26	Stereo sound pickup device (microphone):
	This subclass is indented under subclass 1.  Subject matter including a single transducer system for converting sound waves into two or more independent sound signals.

27	Center channel:
	This subclass is indented under subclass 1.  Subject matter where a third sound signal is derived from combination of two independent sound signals.

28	Amplifier:
	This subclass is indented under subclass 1.  Subject matter including a device to increase the amplitude of the independent sound signals. SEE OR SEARCH CLASS: 330,  Amplifiers,   appropriate subclasses for an amplifier not having a specific source of input signals or a specific load.

54	HELIUM SPEECH:
	This subclass is indented under the class definition.  Subject matter where an output speech signal is of differing apparent predominant frequency but of the same duration as the input speech signal. SEE OR SEARCH THIS CLASS, SUBCLASS: 29+, for bandwidth compression or expansion of a speech signal.

55	AUDIO TRANSDUCER PROTECTION CIRCUITRY:
	This subclass is indented under the class definition.  Subject matter where there is an audio transducer and circuitry to protect the transducer from damage due to abnormal operating conditions. SEE OR SEARCH THIS CLASS, SUBCLASS: 111+, for systems having specific audio transducers. SEE OR SEARCH CLASS: 361,  Electricity: Electrical Systems and Devices,   subclasses 1+ for circuitry to protect other systems and devices in general from damage due to abnormal operating conditions.

56	MONITORING OF SOUND:
	This subclass is indented under the class definition.  Subject matter having a qualitative (yes-no) indication dependent upon a parameter of sound waves, e.g., loudness, quality, etc. SEE OR SEARCH THIS CLASS, SUBCLASS: 58+, for monitoring the operation of audio devices. 77+, for systems having a microphone at a location remote from a listening location (i.e., one-way intercoms). SEE OR SEARCH CLASS: 73,  Measuring and Testing,   subclasses 570+ for measuring and testing sound waves where a quantative (numerical) indication is given, especially 645+ for measuring an acoustic parameter. 367,  Communications, Electrical: Acoustic Wave Systems and Devices,   subclasses 135+ for acoustic wave receiver circuitry.

57	Amplification control responsive to ambient sound:
	This subclass is indented under subclass 56.  Subject matter where the ambient sound level, at a place where sound is being reproduced, is evaluated and the result used to control the volume of the reproduced sound.

58	MONITORING/MEASURING OF AUDIO DEVICES:
	This subclass is indented under the class definition.  Subject matter having an indicator, measuring device, or eavesdropping means. SEE OR SEARCH THIS CLASS, SUBCLASS: 56+, for monitoring of sound waves. SEE OR SEARCH CLASS: 340,  Communications: Electrical,   subclasses 500+ for condition responsive alarms. 714,  Error Detection/Correction and Fault Detection/Recovery,   subclasses 712+ for testing transmission facilities.

59	Loudspeaker operation:
	This subclass is indented under subclass 58.  Subject matter where an operating parameter of a sound reproducing transducer is measured or indicated.

60	Testing of hearing aids:
	This subclass is indented under subclass 58.  Subject matter in which at least part of a hearing aid is tested. (1) Note. A hearing aid is a small audio reproducing system for the hard of hearing. SEE OR SEARCH THIS CLASS, SUBCLASS: 68+, for hearing aids. SEE OR SEARCH CLASS: 73,  Measuring and Testing,   subclasses 570+ for measuring vibration including sound waves. 324,  Electricity: Measuring and Testing,   for electrical testing or measuring in general. 370,  Multiplex Communications,   subclasses 241+ for multiplex testing. 455,  Telecommunications,   subclasses 67.11 through 67.7for modulated carrier system testing, subclasses 115.1-115.4 for transmitter testing; and subclasses 226.1-226.4 for receiver testing.

61	SOUND EFFECTS:
	This subclass is indented under the class definition.  Subject matter where the audio signal is selectively distorted. SEE OR SEARCH CLASS: 369,  Dynamic Information Storage or Retrieval,   subclasses 174+ for signal modification in a recording system.

62	Tremolo or vibrato effects:
	This subclass is indented under subclass 61.  Subject matter having means to produce a rapid variation in amplitude or pitch of a signal. SEE OR SEARCH THIS CLASS, SUBCLASS: 63, for means to introduce echoes into a signal. SEE OR SEARCH CLASS: 84,  Music,   subclass 1.25 for producing vibrato in an electronic musical instrument. 181,  Acoustics,   subclass 143 for a moving sound producer or deflector which can produce tremolo effects.

63	Reverberators:
	This subclass is indented under subclass 61.  Subject matter having a device for producing echoes in the audio signal. SEE OR SEARCH CLASS: 84,  Music,   subclass 1.24 for reverberation production in electronic musical instruments, and digest 26 for musical reverberation. 181,  Acoustics,   subclasses 123+ for echo system. 333,  Wave Transmission Lines and Networks,   subclasses 138 through 164for delay lines. 360,  Dynamic Magnetic Information Storage or Retrieval,   subclass 7 for recording for momentary delay to produce echoes. 369,  Dynamic Information Storage or Retrieval,   subclass 60.01 for using a recording system to delay a signal.

64	Mechanical (e.g., reverberation chamber):
	This subclass is indented under subclass 63.  Subject matter where the echoes are produced by other than electrical means.

65	Helical spring:
	This subclass is indented under subclass 64.  Subject matter where the echoes are produced by reflecting a compressional sound wave along a helical spring.

66	DEREVEBERATORS:
	This subclass is indented under the class definition.  Subject matter for removing echoes from a sound signal.

67	STETHOSCOPES, ELECTRICAL:
	This subclass is indented under the class definition.  Subject matter forming an instrument used to convey sounds to the ear of a user. (1) Note. If the instrument is particularly adapted for use on or in a living body, classification is in Class 128, Surgery, but if the instrument can be used generally (e.g., to detect leaks or the internal sounds of machinery) classification is here. SEE OR SEARCH CLASS: 181,  Acoustics,   subclass 131 for mechanical stethoscopes. 600,  Surgery,   subclass 528 for specific structure, adapted to be placed on or in a living body, for detecting sounds emanating from the heart.

70	ARTIFICIAL LARYNX, ELECTRICAL:
	This subclass is indented under the class definition.  Subject matter for replacing or supplementing the larynx and involving electrical actuation. SEE OR SEARCH CLASS: 84,  Music,   subclass 375 for imitation trumpets, jew"s harps, and reed horns of variable pitch. 116,  Signals and Indicators,   subclasses 137+ for horns, whistles and compressional wave generators. 128,  Surgery,   subclasses 207.14+ for tubes to be inserted into the trachea or larynx. 181,  Acoustics,   pertinent subclasses for sound amplifying and transmitting means and methods. 446,  Amusement Devices: Toys,   subclasses 297+ for a toy having means to emit sound imitating a voice. 472,  Amusement Devices,   particularly subclass 64 for an illusion caused by sound imitation or effect. 623,  Prosthesis (i.e., Artificial Body Members), Parts Thereof, or Aids and Accessories Therefor,   subclasses 24+ for electrically actuated artificial body members other than a larynx and subclass 9 for nonelectrical artificial larynxes. 704,  Data Processing: Speech Signal Processing, Linguistics, Language Translation, and Audio Compression/Decompression,   subclasses 258+ for speech signal synthesis and subclass 271 for speech signal processing applied to a handicap aid.

71.1	ACOUSTICAL NOISE OR SOUND CANCELLATION:
	This subclass is indented under the class definition.  Subject matter, not elsewhere classifiable, comprising a noise cancellation system where audible noise is at least partially eliminated by generating and acoustically emitting an out-of-phase replica of an offending sound. SEE OR SEARCH THIS CLASS, SUBCLASS: 73.1, for sound or noise masking. 94.1+, for suppression of the effects of unwanted signals on desired signals. SEE OR SEARCH CLASS: 415,  Rotary Kinetic Fluid Motors or Pumps,   subclass 119 for noise elimination in fans, turbines, jet engines, etc.

71.11	Adaptive filter topology:
	This subclass is indented under subclass 71.8.  Subject matter having the functional interconnections between two or more transfer function models which make up an overall circuit or computational arrangement designed to generate a counterwave. SEE OR SEARCH THIS CLASS, SUBCLASS: 71.13, for the generation of the out-of-phase replica using analog circuitry or non-adaptive filter. SEE OR SEARCH CLASS: 708,  Electrical Computers: Arithmetic Processing and Calculating,   subclasses 322+ for adaptive digital filter structures, per se.

71.12	Algorithm or formula (e.g., LMS, Filtered-X, etc.):
	This subclass is indented under subclass 71.8.  Subject matter having a specific computational sequence or a mathematical formula used in computation of a counterwave.

71.13	Analog or nonadaptive:
	This subclass is indented under subclass 71.8.  Subject matter in which the out-of-phase replica is generated using either analog circuitry or an electrical circuit having a fixed transfer function. SEE OR SEARCH THIS CLASS, SUBCLASS: 71.11, for the generation of a counterwave using adaptive filter topology.

71.14	Tonal noise or particular frequency or band:
	This subclass is indented under subclass 71.8.  Subject matter in which the noise cancellation system is particularly designed to either eliminate noise having a high degree of underlying periodicity or process noise at a particular frequency or within a limited or particular frequency band.

71.2	Acoustic, nonairborne vibration sensing or counterwave emission:
	This subclass is indented under subclass 71.1.  Subject matter which senses vibration from or which imparts a cancelling wave directly to a solid or liquid medium or structure. (1) Note. In order to be properly classifiable here, the primary object should be attenuation of acoustic sounds. In most cases, this will mean that a sensor such as a microphone will be needed to detect sound propagated through air, such as the residual sound after the counterwave source"s output has been acoustically summed with the original noise. SEE OR SEARCH CLASS: 52,  Static Structures (e.g., Buildings),   subclass 167.1 + for means compensating for an earth-transmitted force, i.e., earthquake. 267,  Spring Devices,   subclasses 136+ for resilient shock or vibration absorbers. See especially subclasses 140.14 and 140.15 for subject matter including electronic or magnetic control, or subclass 140.11 for subject matter having an energy absorbing feature.

71.3	From appliance:
	This subclass is indented under subclass 71.1.  Subject matter wherein the noise cancellation system cancels noise emanating from a device such as an air conditioner, refrigerator, vacuum cleaner, etc.

71.4	Within cabin or compartment of vehicle:
	This subclass is indented under subclass 71.1.  Subject matter wherein the noise cancellation system cancels noise within a humanly occupied space of an aircraft, automobile, etc. SEE OR SEARCH THIS CLASS, SUBCLASS: 86, for electrical audio systems combined with vehicles.

71.5	Within duct:
	This subclass is indented under subclass 71.1.  Subject matter wherein the noise cancellation system cancels noise propagating in a tube, canal, pipe, or similar conduit, including automobile inlet and exhaust pipes.

71.6	Adjacent ear:
	This subclass is indented under subclass 71.1.  Subject matter wherein the noise cancellation system is associated with a structure in a manner designed to cancel noise in the immediate vicinity of a person"s ears. (1) Note. Structures included in this subclass are headrests, hearing protectors, etc. Hearing protectors based on active noise cancellation are classified herein. SEE OR SEARCH THIS CLASS, SUBCLASS: 72, for hearing protector without noise cancellation.

71.7	Particular transducer or enclosure structure:
	This subclass is indented under subclass 71.1.  Subject matter specifying details of an electro-acoustic energy converter or housing for use in an active noise cancellation system. SEE OR SEARCH THIS CLASS, SUBCLASS: 88+, for enclosure or housing. 150+, for electro-acoustic audio transducer.

71.8	Counterwave generation control path:
	This subclass is indented under subclass 71.1.  Subject matter having either a feedback or feedforward circuit or a sequence of computations to generate the out-of-phase replica of the offending noise.

71.9	Non-acoustically derived reference signal:
	This subclass is indented under subclass 71.8.  Subject matter in which a transducer used to obtain a signal for processing detects a form of energy other than sound vibration. (1) Note. Examples are spark timing, crankshaft position, manifold pressure of an engine, etc.

72	HEARING PROTECTORS, ELECTRICAL:
	This subclass is indented under the class definit