Circuit for Suppressing Amplitude Peaks Caused by Stop Consonants in an Electroacoustic Transmission System

The present invention relates in general to sound transmitting equipment, and in particular to a new and useful arrangement for suppressing amplitude peaks in an electroacoustic transmission system, which peaks are caused by spoken stop consonants.

In voice transmission, microphones are generally troubled by those consonants which are formed by a so-called explosion in the articulation, i.e. a sudden release of impounded breath. Such consonants, called stop consonants or stops, comprise the sounds B,D,T and quite particularly P. Upon exposing a microphone to sound from a distance of about 1 cm up to 12 cm in front of the mouth and without providing a satisfactory sound pickup protection, the microphone will always tend to reproduce the stop consonants in a distorted and overaccentuated way. This phenomenon is termed "pop noise", with the pop signal itself being produced predominantly by low-frequency oscillations up to 300 Hz.

Attempts have repeatedly been made to design a microphone wind guard so as to make the pop noise substantially inaudible. Such microphones protected against wind and blow are disclosed in U.S. Pat. No. 4,263,484, German AS No. 17 62 944 and German OS No. 15 62 210. However, these disclosures do not teach how, and to what extent, the wind guard influences the pop of the microphone. Particularly, only mechanical acoustic remedies are provided.

It is known from speech and word formation analyses that while forming a stop, such as P or T, a pause of about 100 ms precedes the following explosive start of the consonant, which pause is a period of quiescence. Details in this regard may be found in the periodical Acustica, Vol. 43, pages 167 to 173, and in the 11th ICA Report, Paris 1983, pages 205 to 208.

The present invention is directed to the elimination of so-called "pops" while avoiding mechanical acoustic elements as far as possible, and to an electronic arrangement for this purpose, being connected between the microphone and the microphone amplifier and permitting at relatively low costs an inconspicuous, noiseless suppression of such noises.

The invention starts from the consideration that in general, the pop noise frequencies do not exceed 500 Hz and that, consequently, such noise can be suppressed by means of a high-pass filter which remains connected in the transmission path only for the duration of the peak values of the pop signal. To control the effect of the high-pass filter, the appearance of the pause of about 100 ms immediately before the formation of the stop consonant is utilized, and the high-pass filter is connected just during this period of time. More particularly, according to the inventive design, a high-pass following the microphone is associated with an electronic switch which is controllable by a voltage derived from the microphone through an electronic train and has a self-locking time of about 10 to 30 ms during which the electronic switch provides an electrical connection making the high-pass filter operative.

The invention is advantageous primarily in that the pop noises are cut out of the transmission path without producing any other noise instead. The invention can by no means be compared to amplitude clippers which reduce the dynamics of the transmission and may also lead to distortions and, mainly, become effective only after the amplitude peaks have already occurred. In contradistinction thereto, the inventive high-pass filter is connected into the path prior to the occurrence of the stop, namely within the preceding period of quiescence, and remains effective for about 10 ms to 30 ms, depending on the adjustment, during the time in which the large amplitudes caused by the stop are produced. After this time, the high-pass filter is automatically and smoothly switched off, since then the pop signal has already faded out to an extent requiring no further filtering. In contradistinction thereto, none of the measures hitherto attempted with acoustic filters were absolutely effective, since both the position of the microphone relative to the mouth and the individual formation of the stop consonant by the speaker or singer determine a stronger or weaker effect of the acoustic filter.

In an advantageous embodiment of the invention it is provided that a control voltage derived from the microphone for the electronic switch is directed through an electronic train which is effective as a detector and comprises a first amplifier, a high-pass filter, a second amplifier, a signal rectifier, and a comparator. The electronic switch is connected to the comparator by its control input, and the comparator is supplied with a reference voltage.

The purpose of the detector is to recognize the no-signal pauses and, after a waiting time of 7 ms, to actuate the electronic switch and thus connect the high-pass filter into the transmission path between the microphone and the microphone amplifier. If then an audio signal occurs, and due to the recognition of this signal by the detector, the high-pass filter is smoothly switched off by the controlled electronic switch after a locking time of 10 to 30 ms.

It is advantageous for a quite satisfactory function of the inventive arrangement to provide a cutoff frequency of about 500 Hz for both the high-pass filter connected in the transmission path and the high-pass filter forming a part of the electronic train to the electronic switch. The detector formed by the train leading to the electronic switch must be protected against overdriving audio frequency amplitudes, which may be done very simply by means of a high-pass filter comprising two passive elements. Since the high amplitude peaks known as pop noise occur in the lowest frequency range, it is advisable to provide the limit or cutoff frequencies of the two high-pass filters either equal to each other, or at least in the region of 500 Hz.

The inventive design may be such that the high-pass filter connected in the transmission path comprises a half section which is formed by a coupling capacitor and an inductance and connected between the microphone and the active section of the electronic switch formed by a field effect transistor, with the junction between the coupling capacitor and the inductance being applied to the input of a following amplifier for amplifying the microphone signal. The half section forming the high-pass filter is the simplest arrangement of at least two passive component parts, with one of them, namely the capacitor, being used for two functions, i.e. also as a coupling or separating capacitor between the microphone and the microphone amplifier.

Accordingly an object of the present invention is to provide an arrangement for suppressing amplitude peaks caused by stop consonants in an electroacoustic transmission system having a microphone at its input, comprising a high-pass filter following the microphone in a transmission path, an electronic switch connected to the high-pass filter which is controllable by a voltage derived from the microphone to activate the high-pass filter, the voltage being derived from a train of electronic elements which detect a characteristic cause before the stop consonant peak, the electronic switch having a self-lock time of about 10 to 30 ms during which the high-pass filter is effective.

A further object of the invention is to provide an arrangement for suppressing amplitude peaks caused by stop consonants which is simple in design rugged in construction and economical to manufacture.

A still further object of the invention is to provide a method of suppressing amplitude peaks which includes activating a high-pass filter after a characteristic delay before the stop consonant is detected, the high-pass filter being connected to an output of the microphone and being activated by a switch which is self-locking for a selected time period and which is activated by a detection of the characteristic pause.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.

Further details of the invention may be learned from the following description referring to the drawings in which:

FIG. 1 shows an oscillogram of the word or word segment "oper" (which means opera in German);

FIG. 2 shows the switching on and off operation of a high-pass filter in time, in association with the word "oper" and the supression of the pop signal occurring therein;

FIG. 3 is a block diagram of the inventive arrangement;

FIG. 4 shows a circuit embodying the invention; and

FIG. 5 is a block diagram of another inventive arrangement.

The oscillogram of FIG. 1 shows that the vocal "o" is followed by a pause of about 100 ms, and that only thereafter the stop consonant "p" occurs. This is followed by the "er" sound to complete the word "oper". The period of quiescence is needed for building up the stop and it has an almost identical duration for all stops in any known or imaginable word. To produce the excess pressure in the mouth necessary to form the stop, the lips must be closed immediately before, which produces the pause quite automatically. The explosive rise of the amplitude of the stop P is determining for the pop noise produced by the microphone. FIG. 2 illustrates the switching on and off of a high-pass filter in association with the oscillogram of the voiced word "oper", including the filtered stop P. The filter cperates in a way such that, for example, 30 ms after the very start of the stop consonant, the filter is switched off. To keep intact the acoustic impression the spoken or sung word produces on the listener, and also make the switching off of the filter inaudible, it is advisable to select a switch-off time between 10 ms and 30 ms at most. The annoying pop noise remains cut out and cannot be heard.

The block diagram of FIG. 3 is to explain the function of the high-pass filter. Be it assumed that an electrical audio frequency signal analog to the picked up acoustic signal is available at the output of microphone M. In the transmission path between microphone M and microphone amplifier V.sub.M, a high-pass filter 1 is provided which is switched on or off depending on the audio frequency signal, whereupon, if switched on, filter 1 remains in this state for a period of time of 10 to 30 ms. Filter 1 is switched on or off by means of a train of electronic component parts comprising a first amplifier stage 2, a high-pass filter 3, a second amplifier stage 4, a signal rectifier 5, and a comparator 6 to which a time delay switching stage 7 is connected. Amplifier stages 2 and 4 correspondingly amplify the audio frequency signal present at the microphone output. Frequencies below about 500 Hz contained in this signal are suppressed in high-pass filter 3, and do not pass to amplifier stage 4. In signal rectifier 5, the audio frequency signal is rectified and supplied as a d.c. voltage signal to comparator 6. As soon as this d.c. voltage exceeds a constant reference voltage U supplied to the comparator, the following integrator 7 having a time constant .tau.=10 ms to 30 ms becomes effective. After the expiration of a time period between 10 ms and 30 ms, high-pass filter 1 is switched off by stage 7 through a field effect transistor acting as a switching transistor, so that now even frequencies below 500 Hz of the audio frequency signal again reach the amplifier. Should no audio frequency signal be applied, high-pass filter 1 switches on after about 7 ms.

The inventive circuitry corresponding to the block diagram in FIG. 3 may be learned from FIG. 4. Integrated circuits IC.sub.1 and IC.sub.2 correspond to amplifier stages 2 and 4 of FIG. 3, with stage 2 having a gain of 40 db and stage 4 a gain of 26 db. The RC combination comprising a resistor R.sub.3 and a capacitor C.sub.3 corresponds to a high-pass filter 3 having a cutoff frequency of about 500 Hz. Signal rectifier 5 is formed in the embodiment of FIG. 4 by a diode D.sub.3, and a low-pass filter R.sub.4 and C.sub.5. Comparator 6 is formed by an integrator circuit IC.sub.3 and compares the positive voltage adjusted at a control resistor R.sub.8 and applied to the inverting input of circuit IC.sub.3, with the rectified audio frequency signal voltage from rectifier D.sub.3, R.sub.4, C.sub.5 applied to the non-inverting input of circuit IC.sub.3. As soon as the voltage of the rectifier exceeds the voltage from control resistor R.sub.8, a positive voltage appears at the output of integrated circuit IC.sub.3. In such an instance, diode D.sub.4 is operated in the reverse direction and the RC combination of a control resistor R.sub.11 and a capacitor C.sub.9 becomes effective. This means that within the time period of 10 to 30 ms and in accordance with the resistance adjusted at control resistor R.sub.11, a positive voltage is applied to the inverting input of an integrated circuit IC.sub.4 operating as an integrator, which positive voltage depends on the time constant of the integrator and produces a zero potential at the output thereof. Field effect transistor T.sub.1 effective as a switching transistor is thereby blocked and makes inductance L.sub.1 ineffective, which means that the high-pass filter formed by inductance L.sub.1 and coupling capacitor C.sub.8 is inoperative.

During the pause for building up a stop consonant, no audio frequency voltage is present, and then the circuit operation is such that field effect transistor T.sub.1 makes the high-pass filter formed by inductance L.sub.1 and coupling capacitor C.sub.8 effective. Both the switching on and switching off of the high-pass filter is "smooth" i.e. without an audible click, if the integration constant of the integrator is correctly selected. The switching on of the high-pass filter within 7 ms after an interruption of an audio frequency signal, is determined by the time constant of the RC section formed by resistor R.sub.4 and capacitor C.sub.5. The audio frequency voltage (AF) to be supplied to the following amplifier is present at the junction between L.sub.1 and C.sub.8. The other end of coupling capacitor C.sub.8 is applied to the microphone output. The high-pass filter shown in this embodiment is the most simple form of such a filter, of course. More complicated forms may be provided without departing from the scope of the invention. This is only a question of costs and the requirements on the properties of the filter.

FIG. 5 is a block diagram of another embodiment, very similar to that of FIG. 3. The high-pass filter 1 connected in the transmission path between microphone M and microphone amplifier V.sub.M is utilized for two purposes, namely as a filter in the transmission path, for suppressing the amplitude peaks at the start of stop consonants, and as a high-pass filter (designated 3 in FIG. 3) and forming a part of the electronic train effective as a detector, to clip the amplitude of all signals below about 500 Hz. Otherwise, the circuit arrangement according to FIG. 3 is maintained and need not be described in this connection.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.