IL33684A - Audio signal processor - Google Patents

Description

AUDIO SIGNAL PROCESSOR This invention relates to a system for controlling the deviation and audio frequenqy response of a modulation system.

Deviatipn control of phase and frequency mod ulated transmitters is important not only be ause it is required by the Federal Communications Commission but also > because it serves to prevent audio distortion, loss of intelllglb ility and decrease in the signal-to-noise ratio in the receiver which may be due to excess modulation deviation. Circuits have been developed for controlling the deviation by process ing the modulating signal . While these circuits have worked well in existing systems, they have not been des igned so that they can be readily formed as part of an integrated circuit . For example, they ma inc^de inductances which cannot at present be made part of an integrated circuit or they may include capacitance whose values are so large that it is extremely difficult to Include them in an integrated circuit structure . Further, the limiters used in some deviation control circuits using only solid state components have included a limiting transistor biased between cutoff and saturation.

While a transistor can be readily cutpff sharply, it is not easily biased to a definite saturation point so that the clipped waveform developed by a transistor in saturati n is not sharply defined . This gives rise tp an un^ymmetricaJt clipped waveform having excessive distortion. Also, ^Large numbers of components -are required tp temperature compensate the transis tors used in such a circuit .

The present invention provides a system for controlling the deviation and audip frequency response pf a modulation system in which a carrier wave signal frequenpy or including input circuit means for receiving the modulating signal, differentiatin means coupled to said input circuit means for producing therefrom a dif erentiated mod lating signal means for limiting the di ferentiated modulating sigr-nal including a pair of transistore having common electrodes coupled together, phase splitting means coupled to said differentiating means for developing a pair of dif erentiated modulating signals differing in phase by approximately 180% circuit means coupling said phase splitting means to said limiting means for applying one of said dif erentiated modulating signals to the input electrode of one of said transistors and the other of said differentiated modulating signals to the input electrode of the otii er one of said transistors , constant current means coupled in series with said common electrodes of said limiting transistors and to a first refer-* ence potential, said pair of differentiated modulating signals acting to bias each of said limitin transistors alternately between conduction and cutoff and said constant current mea,ns acting to limit the current through said conducting one of said current limiting transistors to a value less than saturation.

In practicing a preferred embodiment of this invention a voice signal which is to be used to modulate a carrier wave is di ferentiated and the differentiated modulating signal is applied to a phase splitting circuit:. The phase splitting circuit develops a pair of differentiated modulating signals, differing in phase by approximately 180°. A limiter is formed by a pair of transistors having common emitter electrodes coupled together. The pair of dif erentiated modulating signals are applied to the separate bases of the pair of V nately between conduction and cutoff, A constant current circuit is coupled in aeries with the common electrodes to maintain the current low through the limi ep at a p ed er^ mined value to limit the current through the conducting one of current limiting ran isto s to a value less than saturation, An output is taken from the collector of one of the limiting transistors. By this means the current through the limiting transistors alternates between zero a.nd a. prec¾eter_min§¾ JLey§.l less than saturation and the output waveform is s arply limits ed, symmetrical and has low distortion, The circuit a.lso may include semiconductor bias circuits which act to stabilize the circuit with changes in am-r bient temperature „ The circuit is readily adaptable to be formed as part of an integrated circuit .

The invention is illustrated }.n .drawings of which: . Figure 1 is a partial blpck diagram and partial schematic of a transmitter incorporating the circuit of the deviation control circuit of this invention; and Figure 2 is a drawing of ar] integrated circuit chip incorporating the modulating processing circuit of Figure 1.

Referring to Figure 1 modulating signals from microphone 10 are coupled to the signal modulating processing circuit 12 through capacitor 13. The output of modulation processing circuit 12 is coupled to oscillator 14 where it acts to vary the deviation of oscillator 14 in accordance with the modulation signal. The output pf oscillator 14 is amplified , and tripled in frequency in each of first and second trlplers 15 and 16. The output signal from second trlpler 16 is further amplified in driver 17 and final amplifier 19. The output of final amplifier 19 Is coupled to antenna 24 for radia The modulating signal applied to processing circuit 12 Is different iated by capa citor 13 and res istor 24 to provide pre^emphasis and is amplified by transistors 22 and 23. Transistor 25 is connected as a diode and ac ts to temperature compensate the amplifying transistors 22 and 23 . The output signal from transistor 23 is coupled from emit ter 26 through capacitor 28 to base 32 of transistor 31 and bas e 35 of transistor 34 i Further pre -emphasis or differentiation may be provided by capacitor 28 and the input impedance of trans Is ■* tors 31 and 34. Resistors 37 and 38 together with the input resistance of transistor 34 act to attenuate the signal applied to base 35. Emit ters 40 and 41 of transis tors 32 and 3 are c oupled together through resistors 42 and 43 tp form an emitter coupled amplifier . The Junction of res istors 42 a,nd 43 is coupled to a reference potential through a constant current source cons isting of trans istor 45 and resistor 46. A bias current for base 47 of transistor 45 is applied from a bias cir cuit cons isting of resistors 49 and 50 and transistor 5·1· connected as a diode . Diode 51 has the same base-to-remitter voltage drop characteristics with temperature as does transis tor 45 s o that the changes in voltage drop of diode 51 with temperature acts to compensate for the changes in base 47 to emitter 53 voltage drop in transistor 4J5 thus maintaining the current flow through the trans istor 45 at a relatively conr s tant level .

The cpnduction through transistors 31 and 3 is de termined by the difference between the input s ignal appea ring on bases 32 and 35. Since the signal appearing on base 35 is attenuated with respe ct to the signal appearing on base 32 the transistor having the greatest conduction alternates so that identical signals 180° out -of-phase with each other. Thus transistors 31 and 3 act as a phase splitter for the circuit. Transistor 70 connected as a diode together w th resistors 71 and 72 form a bias network fo base 32 of transistor 31 and base 35 of transistor 3 . Diode 70 ¾cts to temperature compensate bias network so that the bias voltage applied to transistors 31 and 3 is changed to compensate for changes in ambient temperature.

The output signal from collectors 55 and 56 of ran^-sistors 31 and 34 are coupled to bases 68 and 61 respectively of. transistors 65 and 60. Transistors 60 and 65 form a limiter-which produces a highly symmetrical clipped and limited waveform. The signal applied to bases 61 and 68 Is always sufficiently strong to cutoff one of the two transistors and bias the other transistor to conduction. Emitters 62. and 66 are coupled together and in series with transistor 74 and resistor 75. Base 76 of transistor 74 is coupled to the bias network which maintains the flow of current through transistor 74 at a constant value. Diode connected transistor 51 acts to temperature compensate constant current transistor 74 in the same manner as it acts to temperature compensate transistor 45.

The current flowing through transistors 60 and 65 is held constant at a value determined by transistor 74 . Since one of the two transistors 60 and 65 is always cutoff the entire amount of this current flows through the other of the two transistors. For example transistor 65 alternates between conduct-ion at a predetermine current level and non-conduction. The predetermined current level is chosen so that the transistor is operating at a value of conduction less than saturation. This produces a highly symmetrical putput waveform on collector tfoe transistor1 conduction curve as it approaches and enters saturation does not affect the output signal and distortion is maintained at a low value , The output signal from collector 67 of transistor 65 is coupled to oscillator 1 through filter 82 and ransls tors 86 and 87 , Filte 82 is a splatter filter req r by the Federal Communica ion commission and acts to remove harmonics from the output signal, In Figure 2 there is shown an integrated circuit chip incorporating the audio signal processor 12 of figure 1 , Fo -tions of the integrated circuit chip which have the same func?= tion as the circui elements of Figure 1 have the same reference numbers , Th microphone Input is made to terminal 21 of the integrated circuit chip and output is taken from terminal 93 , Capacitor 28 and filter 82 are discrete components connected to terminals 27 and 29 and terminals 83 and 84 respectively . Power is supplied to terminals 91 and 92 , Terminal 90 provides a test point . ' .

Claims (9)

1. A system for controlling the deviation and audio frequency response of a modulation system in which a carrier wave signal is frequency or phase modulated by a modulating signal, the control system including input circuit means for receiving the modulating signal, differentiating means coupled to said input circuit means for producing therefrom a differentiated modulating signal means for limiting the differentiated modulating signal including a pair of transistors having common electrodes , coupled together, phase splitting means coupled to said differentiating means for developing a pair of differentiated modulating signals differing in phase by approximately l80e, circuit means coupling said phase splitting means to said limiting means for applying one of said differentiated modulating signals to the input electrode of one of said transistors and the other of said differentiated modulating signals to the Input electrode of the other one of said transistors, constant current means coupled in series with said common electrodes of said limiting transistors and to a first reference potential, said pair of dlffeientiated modulating signals acting to bias each of said limiting transistors alternately between conduction and cutoff and said constant current means acting to limit the current through said conducting one of said current Uniting transistors to a value less than saturation.

2. The control system of claim 1 wherein said constant current means includes a transistor having a first electrode coupled to said common electrodes, a second electrode coupled to said first reference potential and a control eleiio-trode, and first bias circuit means coupled to said control electrode for applying a bias potential thereto for establishing the value of current through said first and second electrodes of said constant current means transistor.

3. The control system of claim 2 wherein said first electrode of said constant current means transistor is a collector electrode, said second electrode of said current means transistor is an emitter electrode and said control electrode of said constant current neans transistor is a base electrode, said first bias circuit means . including bias diode means having first and second electrodes connected in series with voltage divider resistor means between said first reference potential and a second reference potential and with. said, first electrode of said first bias diode being coupled to said base electrode of said constant current means transistor, the voltage drop of said bias diode means changing with temperature in the same manneras the base-to-emitter voltage dro of said constant current means transistor to compensate said constant current means transistor, for changes in temperature .

4. The control system of claim 1, 2 or 3, wherein said phase splitting means includes first and second phase splitting transistors each having base, emitter and collector electrodes,, said emitter electrodes of said phase splitting transistors being coupled together to form an emitter coupled amplifier, said collector electrodes of said phase splittin transistors being individually coupled to separate one of said input electrodes of said pair of limiting means transistors, said base electrodes of said first phase splitting transistor being coupled to said differentiating means, and attenuation means coupling said base electrode of said second phase splitting transistor to said differentiating means to attenuate signals applied to said second phase splitting transistor, whereby the differentiated modulating signals applied to said base electrode of said second phase shifting transistor have a smaller magnitude than the differentiated modulating signals applied to said base electrode of said first phase splitting transistor.

5. The control system of claims 2 and 4, wherein said phase splitting means further includes a current regulat ing transistor coupling said emitter electrodes of said phase splitting transistor to said first reference potential, and said first bias circuit includes bias diode means, said ' current regulating transistor including a base electrode coupled to said first electrode of said bi¾s diode means, said bias diode means providing temperature compensation for said current regulating transistor, second bias circuit means Including bias diode means coupled to said base electrodes of said first and second phase splitting transistors to provide bias current therefore, the voltage drop of said bias diode means of said second bias circuit means changing with temperature in the same manner as the base-to-emitter voltage drops of said first and second phase splitting transistors to compensate said phase splitting means for changes in temperature.

6. The control system of any one of the preceding claims, wherein said input circuit means Includes amplifying means coupled to said differentiating means for amplifying the modulating signal.

7. The control system of any one of the preceding claims, including output circuit means having filter means coupled to one of said first and second output electrodes of said limiting means transistors.

8. The control system of claim 1, wherein said differentiating means including a capacitor portion and an impedance portion, and said input circuit means, said impedance portion of said differentiating means, said phase splitting means, said circuit means, said limiting means and said constant current means being formed as a monolithic integrated circuit.

9. The control a/stem of claim 8, wherein said constant current means Includes a transistor formed as a portion of saLd monolithic integrated circuit and having a first electrode coupled in series with said common electrodes, a second electrode coupled to said first reference potential and a control electrode, first bias circuit means formed as a portion of said integrated circuit and coupled to said control electrode for applying a bias potential thereto for establishing the value of current through said first and second electrodes of said constant current means transistor. A system for controlling the deviation and the audio frequency response of a modulation system constructed and adapted to operate substantially as described herein with particular reference to the embodiments illustrated in the accompanying drawings .