Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03G—CONTROL OF AMPLIFICATION
  • H03G3/00—Gain control in amplifiers or frequency changers
  • H03G3/20—Automatic control
  • H03G3/30—Automatic control in amplifiers having semiconductor devices
  • H03G3/34—Muting amplifier when no signal is present
  • H03G3/345—Muting during a short period of time when noise pulses are detected, i.e. blanking
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; ELECTRIC HEARING AIDS; PUBLIC ADDRESS SYSTEMS
  • H04R25/00—Electric hearing aids
  • H04R25/50—Customised settings for obtaining desired overall acoustical characteristics
  • H04R25/502—Customised settings for obtaining desired overall acoustical characteristics using analog signal processing

Definitions

• the present invention relates to a method and a device for the damping/elimination of transients, preferably in hearing aids intended for people with impaired hearing or for noice protection devices, whereby the derivative of the incoming sound signal is calculated.
• Transient sounds consist of comparatively short ( ⁇ 80 ms) sounds caused by one or several correlated sources with intially very high amplitudes and with several transients.
• the opposite thereto, i.e. a high dBA-level but an insignificant reduction of hearing will be found in environments like sawmills or airplane cockpits where the noice is of the character of "white noice" ( ca used toy many uncorrelated sources), which does not contain transients or high peak factors.
• the peak factor mentioned above means the maximum instantaneous value divided by the dBA value.
• the dBA value is the weighted mean energy level at various frequencies.
• the weighing factors are determined by the equal loudness curve related to 40 dB SPL (Sound Pressure Level) and 1000 Hz.
• the human ear is not designed to tolerate the transient sounds caused by the industrialized world of today. People subjected to a serious reduction in the hearing are usually being prescribed some kind of hearing aid.
• a reduction in hearing involves an increased "hearing threshold".
• the "uncomfportability level” remains at the same level as at a rvormal ear. and in certain cases it may even be at a lower level. This reduces the dynamics (see fig. 5).
• the shaded area D represents the amplification need for comparatively weak noices.
• the arrow A shows the dynamics of a normally hearing ear and the arrow B shows the dynamics at a reduction in hearing where the uncomfortability level C also has been slightly lowered.
• the mean amplification requirement may for instance be 20 dB. This means that, in such a hearing aid, also loud sounds are amplified with 20 dB, which causes serious inconvenience and involves a substantial risk for further damages to the hearing.
• the compressors are frequency selective and need a certain integration time (attack time) > 0.5 ms, which causes that the high amplitudes at the beginning of the transient to pass through the hearing aid at a considerably higher amplification than what is acceptable. Furthermore, at low sound levels, the compressor hearing aid produces more noice than a conventional hearing aid.
• the electronics of the output stage will be overloaded at high instantaneous levels, which gives raise to distorsion and also causes resonances in the electronics which extend and amplify transient sounds.
• transients are characterized by very high derivatives
• the derivative is that quality in the time plane which mirrors what can not be shown by the amplitude spectrum in the frequency plane without the aid of a phace spectram, which in turn is impossible to measure in practice.
• Studies have shown that the discrimination between speech and transient sounds will increase by B-10 dB by derivating the sound signals. By integrating the half or full wave rectified derivative of the sound signal. a further increased ability of discrimination between speech and transient sounds is obtained.
• Hearing aids with various forms of transient elimination are previously known.
• one such hearing aid is described in the Danish Patent Specification 124439.
• This publication discloses a hearing aid which comprises apart from a microphone an amplifier and an earphone, an electronic circuit for the differentiation of the incoming signal.
• This hearing aid has several drawbacks. Among other things, at the detection of a transient it will completely interrupt the sound signal to the ear phone, which will cause new transients to be formed in the "flanks". Echo effects will occur when the ear phone is reconnected due to the fact that the transients causing the interruption have a considerable reverberence (>100 ms ). The total lack of sound during the interrupt periods are experienced as uncomfortable.
• This hearing aid has to be regarded as a primitive design and would not be usable in practice.
• the object of the present invention is to provide a transient eliminator which eliminates the above-mentioned drawbacks and which is reliable in operation and simple in use.
• the transient eliminator should, in a manner comfortable to the user, eliminate/reduce damaging/uncomfortable transient sounds but still permit the passage of non-damaging sounds e.g. normal conversation.
• One method of attaining this is that the mean derivative of the sound signal controls the reduction/elimination of damaging/uncomfortable transients occuring in the sound.
• An apparatus for attaining the above-mentioned is characterized in that a converter is provided in the signal processing unit and adapted to convert the mean derivative/differential delivered by the differentiator into a control signal which is adapted to proportionally control a damper in such a way, that damaging/uncomfortable transient sounds occuring in the audio signal supplied to the microphone will be reduced/eliminated.
• Fig. 1 is a block diagram of a transient eliminator according to the invention
• Fig. 2a-c illustrate in more detail various alternatives of a convertor
• fig. 2d is a circuit diagram showing an exmple of an "ideal diod”.
• Fig. 3 shows an analogue damper
• Fig. 4a-c show signal diagrams
• Fig. 5 is a diagram illustrating the frequency dependency of hearing in combination with a reduction of hearing.
• the transient eliminator according to the invention illustrated diagraromatically in fig. 1 comprises a microphone 1 which converts detected sounds into electrical signals, a signal processing unit 2 for a processing of incoming electrical signals. and an ear phone or loadspeaker 3 for converting the processed electrical signals into sound.
• the signal processing unit 2 consists of several circuit parts, most of which are in the form integrated circuits in complementary MOS-technology. This design will guarantee a low power consumption which gives comparatively long periods of operation between changes of battery. Furthermore, this technology is very simple to integrate into small dimensions which is very important in connection with hearing aids. These have to be miniaturized of estetical and practical reasons.
• the microphone 1 is connected to a first amplifier 4, the output signal of which is supplied, on the one hand, to a delay circuit 5 and on the other hand to a differentiator 6.
• the delay circuit 5, which is made in so called CCD-technology (Charge Coupled Devices), will "clock" the information supplied to the input, to the output in relation to the frequency of the internal oscilliator (not shown).
• the delay circuit 5 To the output of the delay circuit 5 there is connected a damper 5 the damping of which is controlled by a convertor B.
• the delay circuit 5 is adapted to delay the incoming signal to the damper 7, so that a maximum damping may be obtained at the highest passages of the transients.
• the differentiator 6 is adapted to derivate the incoming sound signal and supply the resulting signal to the converter 8 .
• a low pass filter (LP-filter) 9 the turn over frequency of which may be set to for instance 10 kHz
• LP-filter low pass filter
• signal control unit 10 by means of which the processed signal may be optionally adjusted with respect to tone and volume.
• An output emplifier 11 is provided between the signal processing unit 10 and the loadspeaker 3.
• the convertor 8 shown as a block in fig. 1 is shown in more detail in fig. 2a.
• the convertor comprises a diode 13a connected in succession with a first resistor 14.
• the diode 13a will rectify the incoming signal and, through its forward voltage drop, will provide a starting threshold for the analogue operation range of the damper 7.
• a capacitor 15 and a resistor 16 are connected in parallel and connected to chassi ground.
• the resistor 14 When the diod 13a is forwardly polorized, the resistor 14, the resistance of the diode 13a and the capacitor 15 will form an RC-circuit, the time constant of which will be designated hereinafter as T on
• the resistor 16 and the capacitor 15 will form an RC-circuit with a time constant T off when the diode 13a is reversely polorized.
• the convertor 8 also includes an externally adjustable control circuit 18, by means of which the damper 7 may be disconnected wholly or partially. In certain environments, where disturbing transients are very unusual, e.g. in a lecture hall, it may be of advantage to reduce the influence of the damper 7.
• Fig. 2b and 2c show alternative embodiments, wherein an A/D-convertor 19 is provided in front of the control circuit 18, said A/D-convertor 19 being intended to convert the analogue signal into a digital signal.
• This digital signal is used for controlling a digitally controlled damper which may be utilized instead of the previously mentioned analogue damper 7.
• the advantages of using a digitally controlled damper is that it is easily made logarithmic, is easy to integrate and does not necessitate any critical adjustment.
• Fig. 2c illustrates a convertor 8 provided with an "ideal diode” 13b and a "true” integrator.
• An ideal diode 13b may consist of for instance among other things an operational amplifier and two diodes (see fig. 2d). Due to the fact that such an "ideal diode” has a low impedance output, the time constant T on and T off will be approximately equal. This embodiment will not provide the advantages treshold level which will be obtained by using only a common diode.
• a damper 7 according to the invention and of analogue type is shown more closely in fig. 3. It comprises and amplifier 20, a resistor 21 and a field effect transistor 22.
• the converter 8 converts the derivative/differential to a control signal which is adapted for the purpose and which controls the damper 7 which will proportionally reduce/eliminate the sound signal/transient.
• the control signal is comprised of the "mean derivative " of the audio signal.
• Bv the mean derivative is ment that the derivative is full way or half rectified and supplied to an integrating network.
• the time constants in the integrating network T on an T off do not need to be equal.
• Tonis approximately equal to 1-4 ms and T off is considerably larger, although less than 200 ms (se the broken line B in fig. 4b).
• fig. 2a, 2b Tonis approximately equal to 1-4 ms and T off is considerably larger, although less than 200 ms (se the broken line B in fig. 4b).
• T on T off which is considered as "true” integration.
• Ton at "attack” which is equal to the value of the resistance of the diode 13a + the value of the resistor 16 multiplied by the value of the capacitor 15 .
• Ton at "attack" which is equal to the value of the resistance of the diode 13a + the value of the resistor 16 multiplied by the value of the capacitor 15 .
• T on is 1-4 ms where at a good discrimination has been obtained between speech and damaging transients.
• large values of T on means that the delay time in the delay circuit 5 has to be increased.
• the "release time”, i.e. the diminishing of the damping, which is determined by the time constant T off the value of the resistor 16 multiplied by the value of the capacitor 15, should be made considerably larger than T on since otherwise the control signal will contain much ripple.
• the damping at the end of a transient should be slightly larger than the envelope of the derivative in order to give a comfortable sound picture.
• the damping may preferably be at its maximum, for instance 30 dB.
• Fig. 4a-c show various diagrams where the vertical axis show signal amplitude and the horisontal axis show time.
• the curve illustrated in fig. 4a shows a typical example of a derivated speech signal where characteristic transients occur at regular intervals. These transients have a comparatively low amplitude but may still "trigger" a transient eliminator the detection of which occurs only through threshold detection, i.e. even a common level of speech may cause such a hearing aid to interrupt the sound to the ear.
• the risk of an ordinary conversation activating the transient eliminator according to the present invention is very small, since it is the mean value of the derivative signal which controls the damping of the signal.
• Fig. 4b illustrates more closely how a transient signal is detected according to the present invention.
• the broken line B shows the potiential across a capacitor 15 which increases with time upon detection of a transient with several transient periods. It is hereby important that the time constant T on is considerably smaller than the time constant T off so that a sufficiently rapid rise time is obtained.
• Fig. 4c shows a composite diagram with three horizontal time axes. This simplifies the comparison of time between the various signals shown.
• the upper curve diagrammatically illustrates the shape of a transient signal as it might look after having passed through the amplifier 4 in the signal processing unit 2.
• the middle time axis shows the same signal after time delay in the time delay circuit 5, i.e. the signal supplied to the damper 7.
• the lower most time axes shows the control signal from the convertor 8 to the damper 7, i.e. the mean derivative of the incoming signal. This clearly shows how the mean derivative of the sound signal will be rapidly increased so that maximum damping will be obtained in the damper 7 when the transient passes.
• the damping signal is allowed to diminish only slowly.

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• Health & Medical Sciences (AREA)
• General Health & Medical Sciences (AREA)
• Neurosurgery (AREA)
• Otolaryngology (AREA)
• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Acoustics & Sound (AREA)
• Signal Processing (AREA)
• Circuit For Audible Band Transducer (AREA)
• Tone Control, Compression And Expansion, Limiting Amplitude (AREA)
• Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)
• Soundproofing, Sound Blocking, And Sound Damping (AREA)

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• 1982
  • 1982-02-08 SE SE8200702A patent/SE436533B/sv not_active IP Right Cessation
• 1983
  • 1983-02-08 WO PCT/SE1983/000041 patent/WO1983002862A1/en not_active Ceased
  • 1983-02-08 EP EP19830900671 patent/EP0101469A1/de not_active Withdrawn

Non-Patent Citations (1)

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