US 3691311 A
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United States Patent Wilson 51 Sept. 12, 1972 TELEPHONE USER SET 7  Inventor: Donald R. Wilson, Santa Cruz, Calif.
 Assignee: Pacific Plantronics, Inc., Santa Cruz, Calif.
 Filed: Dec. 10, 1970  Appl. No.: 96,685
Related U.S. Application Data  Continuation of Ser. No. 714,396, Feb. 27,
 U.S. Cl. ..179/8l B, 179/1 A  lnt. Cl. ..H04m U611  Field ofSearch ..179/l A,81B
 References Cited UNITED STATES PATENTS Ulin ..l79/8l B 3,330,912 7/1967 Koseki ..l79/8l B 3,046,354 7/1962 Clemency ..l79/8l B 3,177,294 4/1965 Schalkwijk ..l79/81 B Primary Examiner-William C. Cooper Assistant Examiner-William A. l-lalvestine Attorney-Flehr, l-lohbach, Test, Albritton & Herbert [5 7] ABSTRACT Telephone user set including amplifier apparatus operating with an electroacoustic transducer provides signal output on a pair of lines which also supply the operating power. One embodiment operates at high gain in response to voice input signal and operates with low gain in the absence of voice input signal for providing suppression of background noise.
8 Claims, 4 Drawing Figures PATENTED EP 12 m2 3 691. 311
SHEET 1 UF 2 PATENTEU 1 2 i973 3.691, 311
SHEET 2 [IF 2 OUTPUT -20 DBM 1000 e 72 00 04 e0 02 00 100 2 DB $00010 PRESSURE LEVEL INPUT REF .0002 DYNES/CM Flgure 3 /0UTPUT AT [1/ 0 TERMINALS 01,05 -4 0 /CARBON MICROPHONE (PRIOR ART) INPUT cunasmxmumu mas) INVENTOR. J 4 DONALD R. WILSON BY a-c. SWSLK ATTORNEY TELEPHONE USER SET CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation of my copending application Ser. No. 714,396, filed Feb. 27, 1968, entitled Signal Conditioner for Acoustic Transducer Apparatus, now abandoned.
BACKGROUND OF THE INVENTION Carbon granule microphones of the type which are widely used in telephone equipment possess a number of disadvantages. They produce harmonic distortion, have a high internal noise level, lose sensitivity with decrease of operating current level, have a tendency to lose sensitivity with packing of the carbon granules, and the basic sensitivity of the device decreases with physical size. The carbon granule microphone does possess two desirable featuresthat of low cost and it provides a measure of background noise suppression because of the relative insensitivity of the carbon granules to low level sound. However, because of the disadvantages of the carbon granule microphone, research has been conducted to find a replacement. Dynamic or magnetic type transducers have found some use in carbon granule replacement apparatus because of the fidelity, low internal noise, and high stability of the sensitivity of these types of transducers. In the prior art, microphone apparatus has been fabricated with these types of transducers using relatively simple amplifiers. These prior art apparatus have the disadvantages that they lose sensitivity with decrease in operating current and that the substantially uniform sound sensitivity over a large dynamic range of sound levels enables background noise to produce undesirable levels of output signal compared with the levels of output signal from carbon granule microphones due to background noise.
SUMMARY OF THE INVENTION Accordingly, the telephone user set and signal conditioner apparatus of the present invention eliminates the problems of the prior art and also maintains field compatibility with like apparatus and with carbon granule microphones when operated in parallel to share current from a single current source in a telecommunications system. This field-compatibility is satisfied by the present signal conditioner apparatus which functions with either polarity of terminal voltage bias; operates on as little as five milliamperes of bias current and with no more than about 2.3 volts of terminal voltage at the lower current limit of operation; operate over a thirteen to one change in bias current and a-two to one change in temlinal voltage; while maintaining substantially constant operating characteristics. Further, the
apparatus is capable of miniaturization so that it will not be obtrusive about the person of the user.
DESCRIPTION OF THE DRAWING FIG. 4 is a graph showing the output of the apparatus of FIG. 1 compared with the output of a typical carbon granule transducer for a given input sound level as a function of supply current.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the schematic diagram of FIG. l, there is shown a microphone transducer 11 connected to a preamplifier 12. The output of preamplifier 12 is applied through the controllable attenuator 13 to power amplifier 14 and is also applied to the circuitry 16, 17, 18, 20 and 21 which controls the signal transmission characteristics of attenuator l3. Signal applied to the input of power amplifier 14 is amplified to a level which is compatible with telephone communications standards (about 1 milliwatt) at a compatible impedance level of about 50 ohms. The amplified signal is delivered to the output terminals 81 and 83 through the polarity-steering network 15 which assures that the dc. voltage appearing across output terminals 81 and 83 is delivered to the various stages with proper polarity for biasing the active elements. Thus, operating bias voltages at terminals A, B and C for the active elements are derived from the voltages appearing on a pair of communication system conductors to which output terminals 81 and 83 may be connected.
The input impedance of the power amplifier 14 relative to the output impedance of attenuator 13 provides compensation to provide constant amplification of the present invention despite a wide range of supplycurrent values. Thus, since the input impedance of the power amplifier 14 which includes transistors 32 and 33 tends to increase with decreasing supply current in a known manner, the output impedance of the attenuator 13 is selected to be higher than the normal input impedance of power amplifier 14 operating at midto high-range values of supply current. Thus, as the bias voltage on lines 82 and 84 approaches low range values of about 1.4 volts, the input impedance of attenuator 13, thereby increasing the power conversion or transfer efficiency from attenuator 13 to amplifier 14. This effect tends to compensate for amplification falloff in amplifiers 12 and 14 due to decreases in current gain of the transistors 30, 31, 32 and 33 in a known manner with decreasing current through them. Similarly, the value of the driving impedance (primarily resistor to the input of detector amplifier 16 is also chosen higher than the input impedance of the detector amplifier 16 so that as this input impedance increases with decreasing supply current, the closer impedance match increases the power conversion or transfer efficiency which, in turn, compensates for reduction in the amplification of preamplifier 12 and amplifier 16. The overall transmission characteristics of the present invention for a given level of voice signal as a function of supply current, as shown in FIG. 4, thus remains substantially constant over greater than a 13 to 1 range of supply current values.
In the operation of the switching of the amplification of the apparatus, the controlled attenuator element 18 includes a common-emitter transistor stage 36 which is normally biased to a conducting state through coupling resistor 74 and the collector circuit of the preceding common emitter stage 35. Thus, any signal produced by the microphone transducer 11 due to background noise in the absence of voice signal is amplified by amplifier 12 but is greatly attenuated by the attenuator 13 which is connected to and controlled by the conductivity of element 18. For low-level sound signals, say, below a sound pressure level of about 80 to 84 db on a scale which is reference to db sound pressure of 0.0002 dynes/cm, the amplification of these low-level sound signals is substantially linear but at a low gain factor, as shown in FIG. 3. At higher sound levels, say, voice signals received by the microphone transducer 11, at a level above 80 to 84 db on the sound pressure scale, the associated electrical signals from the microphone transducer 11 are amplified by amplifier l2 and are applied to attenuator l3 and through attenuator 21 to the detector amplifier 16. The detector amplifier 16 comprises transistor 34 which is biased to conduction close to collector saturation by resistor 71. This condition of conduction near saturation provides low input impedance which increases as the supply voltage decreases. Detector amplifier 16 also comprises transistor 35 which is direct coupled between transistors 34 and 36 and which is biased substantially to nonconduction for input sound signals at the microphone transducer 11 which are below a threshold value, say 80 to 84 db on the scale previously described. As the signal applied to the detector amplifier 16 increases due to increasing sound level, transistor 34 becomes nonconductive and transistor 35 begins to conduct on half cycles of alternating voice signals. This causes the capacitor 75 in the time-delay network 17 to discharge and removes the forward-biasing signal at the base-emitter junction of silicon transistor 36 after a few milliseconds of delay. This renders the transistor 36 substantially nonconductive and thereby reduces the voice signal attenuation provided by attenuator 13 after a brief delay. Transistor 36 is operated with zero d.c. bias on the collector, and capacitor 76, which has relatively small reactance at voice signal frequencies, blocks the dc. return through the circuitry of attenuator 13.
Thus, for sound signals above the selected level, the present apparatus provides substantially linear signal amplification at a higher gain factor, as shown in FIG. 2. A typical difference between the high and low gain factors on highand low-level input signals may be selected at about db for an input sound pressure level of about 80 to 84 db, as shown in FIG. 2, so that the over-all characteristics closely match the change in sensitivity of a carbon-granule microphone to highand low-level sound signals. Of course, since the transition between the high and low gain factors in the present apparatus is clean and abrupt, the present apparatus provides the desired background noise rejection without the high signal distortion commonly associated with the carbon-granule type of microphone. Resistor 77 determines the 10 db value of the amplification change. A higher value of amplification change, for example 20 db, provides even larger background noise reduction but tends to make the operation of the apparatus more critical with respect to mispositioning of the acoustic transducer near the speakers mouth.
The signal level at the input of detector amplifier 16 which renders transistor 36 less conductive is determined by the gain of preamplifier 12 and by the bias currents of transistors 34 and 35 and thus this level may be selected by altering the values of resistors 46, 70, 71 and 72. The time required for the bias on transistor 36 to change sufficiently to alter the transmission characteristics at attenuator 13 when operating on a highlevel voice signal (attack time) is primarily determined by capacitor 75, resistor 74 and the transistor 35. Voice sounds associated with p and tspeech consonants are essential for proper speech transmission and thus the attack-time delay cannot exceed approximately 10 milliseconds to preclude the altering of these speech sounds. Sound pulses of shorter duration than the selected attack time, say 5 milliseconds, determined by delay network 17 are thus attenuated by a value equal to the magnitude of the switched gain. Similarly, the delay network 17 maintains transistor 36 in the nonconductive state during syllable pauses in high-level voice signals so that the transmission characteristics through the attenuator 13 remain substantially unchanged during such pauses. This pause-delay time is conveniently longer (typically a few hundred milliseconds) than the attack or build-up time because the charging path for capacitor 75 is through the high resistance 73 whereas the discharge path is through the low resistance 74 and the conducting transistor 35.
Feedback resistor 57 connected between the emitter of transistor 33 and the base of transistor 32 tends to stabilize the gain of the power amplifier 14 and also adjusts the output impedance level to about 50 ohms for compatible operation with other communications equipment.
The signal output from the power amplifier l4 appears on lines 82 and 84 which are also the current supply lines for the overall circuit. The bridge arrangement 15 of germanium diodes 37-40 couples the signal output on lines 82 and 84 to the output terminals 81 and 83 and also provides unidirectional biasing signal on lines 82 and 84 from signal power normally available on communication conductors to which the output terminals 81, 83 may be connected with random polarity. Germanium diodes 37-40 are used in the bridge arrangement because the low forward-conduction voltage drop of about 0.3 volts for germanium assures that adequate biasing voltage for the transistors in the circuit will be available on lines 82 and 84 under conditions where the voltage on the communication lines connected to the output terminals may vary from about 5 volts to as low as about 2.3 volts. The series inductor 69 and shunt capacitor 68 comprise a transient-suppressor filter for protecting the components of the circuit from damage due to line transients.
The unidirectional biasing signals for the active elements of the circuit are derived from the output lines 82 and 84 through the resistors 61, 63, 65 and filter capacitors 62, 64 and from the voltage drop across the forward-biased serially connected diodes 66 and 67.
1. In a telephone user set, a microphone circuit for use in telephone circuits in which power is supplied to said set by the telephone lines to which it is connected power being variable between higher and lower levels, including a microphone having a substantially linear electrical signal output in response to sound vibrations applied thereto, a preamplifier stage having an output and having an input connected to receive an electrical signal from said microphone, a power amplifier stage having an input and an output, a signal attenuator having a step shiftable value and serving to connect the output of said preamplifier to the input of said power amplifier, said attenuator including means responsive to an increase in the signal level from said preamplifier to decrease the amount of attenuation applied to said signal, said power amplifier exhibiting a characteristic input impedance and said preamplifier and attenuator exhibiting a characteristic output impedance, said output impedance being being selected to have a value higher than said input impedance for input power values to said set at the higher level of those supplied on said telephone lines to thereby achieve an impedance mismatch for an applied power level at said higher range, said power amplifier being of a type in which the input impedance shifts to higher values as the power supplied to said set drops to lower levels so that said impedance mismatch is reduced and the operating characteristics of the set remains substantially constant over wide variations in input power to said set, and a means for connecting the power inputs of said amplifier and preamplifier stages to said telephone lines to receive operating power therefrom and for serving to transmit electrical signals to said lines from the signal output of said power amplifier.
2. A telephone user set as in claim 1 in which said attenuator includes an electronically controllable circuit branch for step shifting the value of said attenuator from one value to another, a detector circuit for sampling the electrical signal from said preamplifier and for developing a control signal for application to said branch circuit to change the attenuation thereof to a lower value whenever the output of said preamplifier exceeds a predetermined amount so that low level background signals are attenuated and high level speech or intelligence signals are passed with reduced attenuation.
3. A telephone user set as in claim 2 in which said branch circuit includes a transistor connected to form a selective conductive path in series in said branch circuit and having a control electrode connected to the output of said detector circuit for operating said transistor.
4. Apparatus as in claim 2 wherein said signal attenuator includes a transistor having zero d.c. 6135 on the collector electrode thereof with respect to the emitter electrode thereof; and said detector circuit includes a timing circuit including resistance and reactance connected to apply control signal to the base electrode of said transistor for altering the conductivity thereof after a selected time delay.
5. Apparatus as in claim 4 wherein said detector circuit means also includes an amplifier stage which produces an output signal having increasing average dc value for ac signal of increasing amplitude applied to the input thereof.
6. Apparatus as in claim 4 wherein said detector circuit includes a transistor at the input thereof, means including said circuit means for biasing said transistor close to collector saturation conduction; and the input impedance of said transistor is lower than the output impedance of an attenuator connected to the input of said transistor, and said input impedance increases to a value approaching said selected attenuator Outfit! imere o pedance with decreasing bias signal applied from said circuit means.
7. A telephone user set as in claim 1 in which said means for connecting the power input of said amplifier and said preamplifier to said telephone lines is a current steering network.
8. In a telephone user set, a microphone circuit for use in telephone circuits in which power is supplied to said set by the telephone lines to which it is connected said power being variable between higher and lower levels, including a microphone having a substantially linear electrical signal output in response to sound vibrations applied thereto, a preamplifier stage having an output and having an input connected to receive an electrical signal from said microphone a power amplifier stage having an input and an output, an attenuator connecting the output of said preamplifier to the input of said power amplifier, said power amplifier exhibiting a characteristic input impedance and said preamplifier and attenuator exhibiting a characteristic output impedance, said output impedance being selected to have a value higher than said input impedance for input power values to said set at the higher levels of those supplied on said telephone lines to thereby achieve an impedance mismatch for an applied power level at said higher range, said power amplifier being of a type in which the input impedance shifts to higher values as the power supplied to said set drops to lower levels so that said impedance mismatch is reduced and the operating characteristics of the set remains substantially constant over wide variations in input power to said set, and a current steering network connecting the power input of said amplifier and preamplifier stages to said telephone lines such that telephone line power of either polarity is connected to the proper polarity of said power input, said current steering network also serving to transmit electrical signals to said lines from the signal output of said power amplifier.