CA1175489A - Receiver for am stereo signals having a circuit for reducing distortion due to overmodulation - Google Patents
Receiver for am stereo signals having a circuit for reducing distortion due to overmodulationInfo
- Publication number
- CA1175489A CA1175489A CA000391918A CA391918A CA1175489A CA 1175489 A CA1175489 A CA 1175489A CA 000391918 A CA000391918 A CA 000391918A CA 391918 A CA391918 A CA 391918A CA 1175489 A CA1175489 A CA 1175489A
- Authority
- CA
- Canada
- Prior art keywords
- signal
- overmodulation
- phase
- carrier
- switch
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/06—Receivers
- H04B1/10—Means associated with receiver for limiting or suppressing noise or interference
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04H—BROADCAST COMMUNICATION
- H04H20/00—Arrangements for broadcast or for distribution combined with broadcast
- H04H20/44—Arrangements characterised by circuits or components specially adapted for broadcast
- H04H20/46—Arrangements characterised by circuits or components specially adapted for broadcast specially adapted for broadcast systems covered by groups H04H20/53-H04H20/95
- H04H20/47—Arrangements characterised by circuits or components specially adapted for broadcast specially adapted for broadcast systems covered by groups H04H20/53-H04H20/95 specially adapted for stereophonic broadcast systems
- H04H20/49—Arrangements characterised by circuits or components specially adapted for broadcast specially adapted for broadcast systems covered by groups H04H20/53-H04H20/95 specially adapted for stereophonic broadcast systems for AM stereophonic broadcast systems
Abstract
PHD 80180 11 ABSTRACT: Receivers for AM-signals the carrier frequency or carrier phase being modulated are subject to serious distortions of the phase-mdoulated signal in the event of overmodulation of the carrier. According to the inven-tion, said distortions are suppressed by an overmodula-tion detector which becomes active when the carrier drops out or at sudden amplitude, frequency or phase transi-ents of the signal, therewith controlling a switch in the signal path of the frequency or phase-modulated signal such, that the signal path is interrupted for the duration of the overmodulation.
Description
I~75489 The invention relates to a receiver for receiv~
ing ~M-signals the carrier frequency or carrier phase being modulated which receiver comprises a signal channel having a frequency or phase demodulator. A receiver of this type is particularly suitable for the reception of medium-wave stereo signals, the carrier being amplitude-modulated by the sum signal and phase-modulated by the difference signal. Such a receiver is described in the U.S. Patent 4,408,097 which issued on October 4, 1983 and is assigned to U.S. Philips Corporation.
With a receiver of the above-described type amplitude overmodulation may result in significant dis-tortions on reception. In the event of overmodulation the amplitude of the signal which is amplitude-modulated on the carrier or the so-called envelope, is larger than or equal to the amplitude of the carrier or, put differ-ently: the amplitude modulation factor is larger than or equal to 1 (or 100%). Such distortions are particularly noticeable in the (difference) signal channel in which the phase demodulator is comprised, while they are not very disturbing in the other (sum) signal channel in which the amplitude demodulator is comprised, particularly when the overmodulation is moderate.
The invention has for its object to reduce in a receiver of the type defined in the preamble the dis-tortions in the output signal of the signal channel in which the frequency or phase demodulator, respectivelyr is included at the occurrence of overmodulation.
~ccording to the invention, a receiver is there-fore characterized by an electronic switch for blockingand releasing the last-mentioned signal channel, said electronic switch being controlled by an overmodulation detector which is energized by an overmodulation of the 3~-~ 175489 carrier by the AM signal.
An overmodulation results in sudden frequency and phase transients, which manifest themselves at the output of the frequency or phase demodulator, respec-tively, as interference pulses.
An overmodulation may alternatively occur for example if the carrier drops out. The envelope of a sig-nal with overmodulation passes through zero, the carrier phase then changing over 180. In practice the transmit-ter signal is, however, equal to zero for the duration ofan overmodulation. On the one hand, these effects are caused by the distortions mentioned in the preamble; on the other hand they also represent the criteria by which the overmodulation detector may be energized.
The switch must then be arranged such that the signal path to the overmodulation detector is not inter-rupted by the blocking of the signal channel. If possi-ble, the delay of the signal in the signal channel must be such that in the event of an overmodulation the switch blocks the signal channel when the distortion resulting from the overmodulation reaches the switch or just prior thereto.
It should be noted that the previously mentioned U.S. Patent 4,408,097 also shows a switch for blocking and releasing the signal channel. However, said switch serves only as a mono-stereo switch, the control of which does not follow a rapid change in the receiving conditions.
A further embodiment of a receiver in accordance with the invention is characterized in that the input sig-nal of the overmodulation detector is derived from thesignal path before the demodulator and that the overmodu-lation detector is of such a form that it is activated at the disappearance of the carrier.
In this further embodiment use is made of the above-mentioned fact that in practice the carrier drops out during an overmodulation. If then the switch is pro-vided in that part of the signal channel behind the demo-~ 17~89 PHD 80180 3 21.5.81 dulator, the delay of the signal in the demodulator it-self is generally of a sufficient duration -to ensure that the signal distorted by the overmodulation does not reach the switch until after it has already been opened.
It is, however, also possible to connect the ove~nodulation detector to the output of the frequency and phase demodulator, respectively.
A still further embodiment of the receiver in accordance with the invention is characterized in that the overmodulation detector comprises a threshold value switch to which the input signal is applied via a recti-fier circuit, the time constant of which is small compared to the period of the signal which is amplitude-modulated on the carrier, but large compared to the period of the carrier.
The rectifier circuit produces a signal which has only one polarity, the time constant ensuring that the output signal thereof and the input signal of the threshold value switch, respectively can indeed follow the envelope, but not the carrier signal. The threshold value switch must then be adjusted such that it generates a control signal for blocking the signal channel.
A further embodiment of such a receiver in ac-cordance with the invention is characterized in that the demodulator comprises a FM-demodulator and a subsequent integrator and that the switch is connected between the FM-demodulator and the integrator.
This further embodiment is based on the recog-nition of the fact that at a frequency-demodulator which is not accurately adjusted to the intermediate frequency a voltage shift is produced which is integrated by the integrator, so that the output voltage thereof may attain a maximum value already in the event of a single overmo-dulation carrier drop out. The switch provided between the FM-demodulator and the integrator prevents such a voltage shift, so that the voltage at the output of the integrator remains constant for the duration of the over-PHD 80180 4 21.5.81 modulation.
Preferably, the switch is then arranged in a signal short-circuiting path and controlled such by the overmodulation detector that it 8hort-circuits the inte-grator input in the event of overmodulation. When arrang-ing the switch serially in the signal path the integrator output voltage might charge during the overmodulation as a result of the leakage currents which inevitably flow through the electronic switch.
The invention will now be further explained by way of non-limitative example with reference to the ac-companying drawing.
Figure 1 shows a block-schematic circuit diagram of a receiver in accordance with the invention and Figure 2 shows an embodiment in which a P~L loop isiprovided as a phase demodulator.
Figure 1 shows a portion of the block schematic circuit diagram of a medium wave receiver which is suit-able for receiving a stereo signal, the sum signal being 2D amplitude-modulated on the carrier and the difference sig nal being phase-modulated on the carrier. The input signal is applied by an aerial 1 to the input of the radio-fre-quency stage 2, which is provided in known rnanner with an input stage (filter stage), a tunable oscillator and a mixer stage and which produces an output signal in the intermediate frequency range, for example at 455 K~Iz.
The output of the radio-frequency s-tage is connected to an intermediate frequency arnplifier 3 in which the signal is selected and amplified.
The output signal of the intermediate-frequency amplifier 3 is applied to an amplitude-demodulator 4, whose output ~ignal corresponds to the sum signal L + R.
In addition, the output signal of the intermediate-fre-quency amplifier 3 is applied to a limiter stage 5~ which produces an output signal the amplitude of which is con-stant in a wide range of the input voltage and therefore independent of the amplitude of the input voltage. In ~ 17548g this limiter stage 5 the superimposed (L + R) amplitude modulation is removed from the input signal of the limiter stage. The output signal of the limiter stage 5 is applied to a FM-demodulator 6 and via a decoupling capac-itor 1 to an integrator comprising an operational ampli-fier 9 the inverting input of which is connected to the capacitor 7 vla a resistor 8 and to the amplifier output v the capacitor lO. The non-inverting input of the operational amplifier 9 is connected to a reference volt-age UR.
The frequency demodulator 6 forms a phase demod-ulator in conjunction with the integrator 8...10. The output signal of said phase demodulator corresponds nor-mally to the difference signal L-R. This difference sig-nal, eventually after phase reversal, is combined with the output signal of the amplitude demodulator 4 in a demat-rixing circuit, not shown, at the output of which the sig-nals L and R are separately available. Up to this point the circuit is already described in the previously men-tioned U.S. Patent 4,408,097.
In the event of overmodulation by the (sum) sig-nal (L + R), which modulates the amplitude of the carrier, the output voltage of the intermediate frequency amplifier 3 is zero or almost zero. Consequently, the input voltage of the limiter stage 5 has a constant value, of zero or almost zero as well as the output voltage thereof. If the FM demodulator 6 is not accurately tuned to the inter-mediate frequency of 455 KHz, its output voltage deviates in that case from the temporary average value of the out-put signal of the limiter 5 prior to the appearance of theovermodulation. As a result thereof there is produced at the output of the FM-demodulator 6 a step-wise voltage change which reaches the input of the integrator 8...10 via the capacitor 7 and is integrated by said integrator.
The output signal of the integrator 8...10 increases lin-early and may assume values which exceed the amplitude of the normal modulation, particularly if the frequency of the sum signal caused by the overmodulation is rela-`'` ~ 17548g PHD 80180 6 21.5.S1 tively low and the overmodulation consequently continues for a comparatively long period of time, or if the over-modulation occurs during the several consecutive signal periods.
The distortions resulting therefrom are sup-pressed by means of an electronic switch in the form of a field effect transistor 11. During the overmodulation the source-drain path of said field effect transistor con-nects the non-inverting input of the operational amplifier 9 to the junction of the elements 7 and 8. During normal reception the field effect transistor 11 is cutoff.
During the overmodulation the integrator input is short-circuited thereby, so that the output voltage of the integrator remains constant for the duration of the overmodulation, that is to say for the period of time the transistor 11 is switched through. Signal distortions are considerably reduced thereby.
The gate of the field effect transistor 11 is connected to the output of a threshold value switch 12 which renders the field effect transistor 11 conductive when the voltage at its input decreases to below a pre-detsrmined threshold value. The input of the threshold value switch 12 is connected to the output of a rectifier 13 which has a time constant chosen between the period of the intermediate frequency carrier and the period of the amplitude-modulating signal. l~hen using a full-wave rectifier for the rectifier 13, the time constant should be chosen between half the period of the inte-rmediate fre-quency carrier and half the period of the amplitude-mo-dulating signal. The input of the rectifier 13 is connect_ ed to a terminal of the limiter 5 at which the voltage has not yet been limited. Said input may however also be con-nected directly to the output of the intermediate fre-quency amplifier 3. So the output voltage of the rectifier 13 follows the envelope of the intermediate frequency sig-nal. Owing to the disappearance of the carrier, which in practice occurs in the event of overmodulation, the output -~ 1 17~4~9 PHD 80180 7 21.5.81 signal of the rec~ifier 13 has zero value during the o~er-modulation or assumes at least a very low value.
The limiter stage 5, the FM-demodulator 6 with exception of its resonant circuits and the rectifier 13 may in practice be realized by means of an integrated circuit of the Valvo/Philips types TCA 420A or TDA 1576.i Each of these integra*ed circuits has two output terminals for field strength indication, at which a voltage is pre-sent which corresponds to the logarithm of the amplitude of the input signal of the limiter stage 5. For this pur-pose signals, which corresponds to the logarithm of themagnitude of the input signal are formed in the said cir-cuits, integration elements ensuxing that the output voltage does not follow the input signal itself (and sig-nals having double the frequency of the input signal, res-pectively) but fluctua*ions in the amplitude of the inputsignal. This output voltage may then be applied to the threshold value switch 12.
The resonant circuits, not shown, of the FM de-modulator 6 ensure that the signal in the signal channel is subjected to such a delay that in the event of overmo-- dulation the switch 11 is already energized before the ef-fects produced by the overmodulation occur at the output of the FM-modulator.
But also if the switch were activated some micro-seconds too late this would not be disturbing, as the vol-tage shift of the output voltage of the discriminator then occurring would, at the occurrence of overmodulation with respect to the average outpu-t voltage outside overmodula-tion7 be integrated only during this comparatively short period of time by the integrator 8 to 10, inclusive. It is therefore in principle also possible to derive the cri-terion for the operation of the switch 11 from the output voltage of the FM demodulator, it being possible to uti-lize the fact that an overmodulation is accompanied by a sudden change of the output signal of the FM demodulator, which change can be used for a switching control. To this 1 1~548g PHD 80180 8 Z1.5.81 end, a threshold value switch which is energized when it~ output signal exceeds a predetermined threshold value must be connected to the output o~ the FM demodulator, preferably via a differentiating element and a high-pass filter, respectively, which amplifies the sudden change of the output voltage.
Just as it is not really annoying that switch 11 does not become active until shortly after the oc-currence of the distortions produced by the overmodula-tion at the output of the FM demodulator, it is also not annoying that the switch 11 is already adjusted to its normal state, which in this case corresponds to blocking, before the effect occurring during the overmodulation at the output of the ~M-demodulator ends. Optionally, how-ever, the return of the switch to the normal state may be effected with some delay. To that end it is, for example, possible to connect to the gate electrode of the field effect transistor 11 a capacitor the other end of which is connected to ground and which at the occurrence of a disturbance is rapidly charged by the threshold value switched 12 via a suitably poled diode and, after-change-over of the threshold value switch 12, is slowly discharg-ed via a parallel-arranged resistor. It is alternatively possible to arrange behind the threshold value switch 12 a monostable multivibrator which maintains the switch 11 in the conducting state during a time constant which would have to be larger than the duration of an average over-modulation. As a result thereof the output signal of the integrator is indeed kept longer than necessary at a constant value, in certain circumstances during several periods of the sum signal, which howe~er is not annoying in a stereo receiver, as then the change from mono to stereo reception is only delayed for a short period of time.
If the carrier is frequency-modulated instead of phase-modulated and has a pre-emphasis, it is suffi-cient to add the resistor 14 (shown by means of a dashed 1 ~754~
PI-~ 80180 9 21.5.81 line) arranged in parallel with the capacitor 10 of the integrator 8,..10 to the circuit shown in Figure 1 with the requirement that the resistor 14 and the capaci-tor 10 together h~ve a time constant which corresponds to the re-quired de-emphasis.
Figure 2 shows a portion of the block schematic circuit diagram of an embodiment which employs a PLL loop as a phase demodulator. The output signal of the inter-mediate frequency amplifier 3 is then applied to a first input of a phase comparator stage 15, for example a mul-tiplier. A second inFut is connected to the output of an oscillator 16, which produces a signal the frequency of which depends on a d.c. voltage which is applied to a control input of the oscillator 16. Via a switch 11', lS which is normally closed but open during an overmodula-tion the output of the phase comparator circuit 15 is con-nected to the further portion, not shown, of the receiver (for example the matrixing circuit) and via a low-pass filter 17, which removes the audio signal components from the output signal of the phase Gomparator stage 15, to the control input of the oscillator 16, so that the frequency thereof is adjusted in accordance with the average value of the frequency of the input signal. The switch 11 ' which in all other respects can be controlled in a simi-lar manner as the switch 11 in the circuit shown in Figure1 forms in conjunction with a capacitor 18 which connects the signal path behind the capacitor 11' -to ground, a sample-and-hold circuit which in the event of overmodula-tion maintains the output signal at the value present 3~ prior to the ove~nodulation.
Although the invention is described in the fore-going as relating to the reception of stereo signals, the invention may alternatively be used if there is no rela-tionship as to contents between the signals modulating the amplitude and phase or the frequency respectively.
ing ~M-signals the carrier frequency or carrier phase being modulated which receiver comprises a signal channel having a frequency or phase demodulator. A receiver of this type is particularly suitable for the reception of medium-wave stereo signals, the carrier being amplitude-modulated by the sum signal and phase-modulated by the difference signal. Such a receiver is described in the U.S. Patent 4,408,097 which issued on October 4, 1983 and is assigned to U.S. Philips Corporation.
With a receiver of the above-described type amplitude overmodulation may result in significant dis-tortions on reception. In the event of overmodulation the amplitude of the signal which is amplitude-modulated on the carrier or the so-called envelope, is larger than or equal to the amplitude of the carrier or, put differ-ently: the amplitude modulation factor is larger than or equal to 1 (or 100%). Such distortions are particularly noticeable in the (difference) signal channel in which the phase demodulator is comprised, while they are not very disturbing in the other (sum) signal channel in which the amplitude demodulator is comprised, particularly when the overmodulation is moderate.
The invention has for its object to reduce in a receiver of the type defined in the preamble the dis-tortions in the output signal of the signal channel in which the frequency or phase demodulator, respectivelyr is included at the occurrence of overmodulation.
~ccording to the invention, a receiver is there-fore characterized by an electronic switch for blockingand releasing the last-mentioned signal channel, said electronic switch being controlled by an overmodulation detector which is energized by an overmodulation of the 3~-~ 175489 carrier by the AM signal.
An overmodulation results in sudden frequency and phase transients, which manifest themselves at the output of the frequency or phase demodulator, respec-tively, as interference pulses.
An overmodulation may alternatively occur for example if the carrier drops out. The envelope of a sig-nal with overmodulation passes through zero, the carrier phase then changing over 180. In practice the transmit-ter signal is, however, equal to zero for the duration ofan overmodulation. On the one hand, these effects are caused by the distortions mentioned in the preamble; on the other hand they also represent the criteria by which the overmodulation detector may be energized.
The switch must then be arranged such that the signal path to the overmodulation detector is not inter-rupted by the blocking of the signal channel. If possi-ble, the delay of the signal in the signal channel must be such that in the event of an overmodulation the switch blocks the signal channel when the distortion resulting from the overmodulation reaches the switch or just prior thereto.
It should be noted that the previously mentioned U.S. Patent 4,408,097 also shows a switch for blocking and releasing the signal channel. However, said switch serves only as a mono-stereo switch, the control of which does not follow a rapid change in the receiving conditions.
A further embodiment of a receiver in accordance with the invention is characterized in that the input sig-nal of the overmodulation detector is derived from thesignal path before the demodulator and that the overmodu-lation detector is of such a form that it is activated at the disappearance of the carrier.
In this further embodiment use is made of the above-mentioned fact that in practice the carrier drops out during an overmodulation. If then the switch is pro-vided in that part of the signal channel behind the demo-~ 17~89 PHD 80180 3 21.5.81 dulator, the delay of the signal in the demodulator it-self is generally of a sufficient duration -to ensure that the signal distorted by the overmodulation does not reach the switch until after it has already been opened.
It is, however, also possible to connect the ove~nodulation detector to the output of the frequency and phase demodulator, respectively.
A still further embodiment of the receiver in accordance with the invention is characterized in that the overmodulation detector comprises a threshold value switch to which the input signal is applied via a recti-fier circuit, the time constant of which is small compared to the period of the signal which is amplitude-modulated on the carrier, but large compared to the period of the carrier.
The rectifier circuit produces a signal which has only one polarity, the time constant ensuring that the output signal thereof and the input signal of the threshold value switch, respectively can indeed follow the envelope, but not the carrier signal. The threshold value switch must then be adjusted such that it generates a control signal for blocking the signal channel.
A further embodiment of such a receiver in ac-cordance with the invention is characterized in that the demodulator comprises a FM-demodulator and a subsequent integrator and that the switch is connected between the FM-demodulator and the integrator.
This further embodiment is based on the recog-nition of the fact that at a frequency-demodulator which is not accurately adjusted to the intermediate frequency a voltage shift is produced which is integrated by the integrator, so that the output voltage thereof may attain a maximum value already in the event of a single overmo-dulation carrier drop out. The switch provided between the FM-demodulator and the integrator prevents such a voltage shift, so that the voltage at the output of the integrator remains constant for the duration of the over-PHD 80180 4 21.5.81 modulation.
Preferably, the switch is then arranged in a signal short-circuiting path and controlled such by the overmodulation detector that it 8hort-circuits the inte-grator input in the event of overmodulation. When arrang-ing the switch serially in the signal path the integrator output voltage might charge during the overmodulation as a result of the leakage currents which inevitably flow through the electronic switch.
The invention will now be further explained by way of non-limitative example with reference to the ac-companying drawing.
Figure 1 shows a block-schematic circuit diagram of a receiver in accordance with the invention and Figure 2 shows an embodiment in which a P~L loop isiprovided as a phase demodulator.
Figure 1 shows a portion of the block schematic circuit diagram of a medium wave receiver which is suit-able for receiving a stereo signal, the sum signal being 2D amplitude-modulated on the carrier and the difference sig nal being phase-modulated on the carrier. The input signal is applied by an aerial 1 to the input of the radio-fre-quency stage 2, which is provided in known rnanner with an input stage (filter stage), a tunable oscillator and a mixer stage and which produces an output signal in the intermediate frequency range, for example at 455 K~Iz.
The output of the radio-frequency s-tage is connected to an intermediate frequency arnplifier 3 in which the signal is selected and amplified.
The output signal of the intermediate-frequency amplifier 3 is applied to an amplitude-demodulator 4, whose output ~ignal corresponds to the sum signal L + R.
In addition, the output signal of the intermediate-fre-quency amplifier 3 is applied to a limiter stage 5~ which produces an output signal the amplitude of which is con-stant in a wide range of the input voltage and therefore independent of the amplitude of the input voltage. In ~ 17548g this limiter stage 5 the superimposed (L + R) amplitude modulation is removed from the input signal of the limiter stage. The output signal of the limiter stage 5 is applied to a FM-demodulator 6 and via a decoupling capac-itor 1 to an integrator comprising an operational ampli-fier 9 the inverting input of which is connected to the capacitor 7 vla a resistor 8 and to the amplifier output v the capacitor lO. The non-inverting input of the operational amplifier 9 is connected to a reference volt-age UR.
The frequency demodulator 6 forms a phase demod-ulator in conjunction with the integrator 8...10. The output signal of said phase demodulator corresponds nor-mally to the difference signal L-R. This difference sig-nal, eventually after phase reversal, is combined with the output signal of the amplitude demodulator 4 in a demat-rixing circuit, not shown, at the output of which the sig-nals L and R are separately available. Up to this point the circuit is already described in the previously men-tioned U.S. Patent 4,408,097.
In the event of overmodulation by the (sum) sig-nal (L + R), which modulates the amplitude of the carrier, the output voltage of the intermediate frequency amplifier 3 is zero or almost zero. Consequently, the input voltage of the limiter stage 5 has a constant value, of zero or almost zero as well as the output voltage thereof. If the FM demodulator 6 is not accurately tuned to the inter-mediate frequency of 455 KHz, its output voltage deviates in that case from the temporary average value of the out-put signal of the limiter 5 prior to the appearance of theovermodulation. As a result thereof there is produced at the output of the FM-demodulator 6 a step-wise voltage change which reaches the input of the integrator 8...10 via the capacitor 7 and is integrated by said integrator.
The output signal of the integrator 8...10 increases lin-early and may assume values which exceed the amplitude of the normal modulation, particularly if the frequency of the sum signal caused by the overmodulation is rela-`'` ~ 17548g PHD 80180 6 21.5.S1 tively low and the overmodulation consequently continues for a comparatively long period of time, or if the over-modulation occurs during the several consecutive signal periods.
The distortions resulting therefrom are sup-pressed by means of an electronic switch in the form of a field effect transistor 11. During the overmodulation the source-drain path of said field effect transistor con-nects the non-inverting input of the operational amplifier 9 to the junction of the elements 7 and 8. During normal reception the field effect transistor 11 is cutoff.
During the overmodulation the integrator input is short-circuited thereby, so that the output voltage of the integrator remains constant for the duration of the overmodulation, that is to say for the period of time the transistor 11 is switched through. Signal distortions are considerably reduced thereby.
The gate of the field effect transistor 11 is connected to the output of a threshold value switch 12 which renders the field effect transistor 11 conductive when the voltage at its input decreases to below a pre-detsrmined threshold value. The input of the threshold value switch 12 is connected to the output of a rectifier 13 which has a time constant chosen between the period of the intermediate frequency carrier and the period of the amplitude-modulating signal. l~hen using a full-wave rectifier for the rectifier 13, the time constant should be chosen between half the period of the inte-rmediate fre-quency carrier and half the period of the amplitude-mo-dulating signal. The input of the rectifier 13 is connect_ ed to a terminal of the limiter 5 at which the voltage has not yet been limited. Said input may however also be con-nected directly to the output of the intermediate fre-quency amplifier 3. So the output voltage of the rectifier 13 follows the envelope of the intermediate frequency sig-nal. Owing to the disappearance of the carrier, which in practice occurs in the event of overmodulation, the output -~ 1 17~4~9 PHD 80180 7 21.5.81 signal of the rec~ifier 13 has zero value during the o~er-modulation or assumes at least a very low value.
The limiter stage 5, the FM-demodulator 6 with exception of its resonant circuits and the rectifier 13 may in practice be realized by means of an integrated circuit of the Valvo/Philips types TCA 420A or TDA 1576.i Each of these integra*ed circuits has two output terminals for field strength indication, at which a voltage is pre-sent which corresponds to the logarithm of the amplitude of the input signal of the limiter stage 5. For this pur-pose signals, which corresponds to the logarithm of themagnitude of the input signal are formed in the said cir-cuits, integration elements ensuxing that the output voltage does not follow the input signal itself (and sig-nals having double the frequency of the input signal, res-pectively) but fluctua*ions in the amplitude of the inputsignal. This output voltage may then be applied to the threshold value switch 12.
The resonant circuits, not shown, of the FM de-modulator 6 ensure that the signal in the signal channel is subjected to such a delay that in the event of overmo-- dulation the switch 11 is already energized before the ef-fects produced by the overmodulation occur at the output of the FM-modulator.
But also if the switch were activated some micro-seconds too late this would not be disturbing, as the vol-tage shift of the output voltage of the discriminator then occurring would, at the occurrence of overmodulation with respect to the average outpu-t voltage outside overmodula-tion7 be integrated only during this comparatively short period of time by the integrator 8 to 10, inclusive. It is therefore in principle also possible to derive the cri-terion for the operation of the switch 11 from the output voltage of the FM demodulator, it being possible to uti-lize the fact that an overmodulation is accompanied by a sudden change of the output signal of the FM demodulator, which change can be used for a switching control. To this 1 1~548g PHD 80180 8 Z1.5.81 end, a threshold value switch which is energized when it~ output signal exceeds a predetermined threshold value must be connected to the output o~ the FM demodulator, preferably via a differentiating element and a high-pass filter, respectively, which amplifies the sudden change of the output voltage.
Just as it is not really annoying that switch 11 does not become active until shortly after the oc-currence of the distortions produced by the overmodula-tion at the output of the FM demodulator, it is also not annoying that the switch 11 is already adjusted to its normal state, which in this case corresponds to blocking, before the effect occurring during the overmodulation at the output of the ~M-demodulator ends. Optionally, how-ever, the return of the switch to the normal state may be effected with some delay. To that end it is, for example, possible to connect to the gate electrode of the field effect transistor 11 a capacitor the other end of which is connected to ground and which at the occurrence of a disturbance is rapidly charged by the threshold value switched 12 via a suitably poled diode and, after-change-over of the threshold value switch 12, is slowly discharg-ed via a parallel-arranged resistor. It is alternatively possible to arrange behind the threshold value switch 12 a monostable multivibrator which maintains the switch 11 in the conducting state during a time constant which would have to be larger than the duration of an average over-modulation. As a result thereof the output signal of the integrator is indeed kept longer than necessary at a constant value, in certain circumstances during several periods of the sum signal, which howe~er is not annoying in a stereo receiver, as then the change from mono to stereo reception is only delayed for a short period of time.
If the carrier is frequency-modulated instead of phase-modulated and has a pre-emphasis, it is suffi-cient to add the resistor 14 (shown by means of a dashed 1 ~754~
PI-~ 80180 9 21.5.81 line) arranged in parallel with the capacitor 10 of the integrator 8,..10 to the circuit shown in Figure 1 with the requirement that the resistor 14 and the capaci-tor 10 together h~ve a time constant which corresponds to the re-quired de-emphasis.
Figure 2 shows a portion of the block schematic circuit diagram of an embodiment which employs a PLL loop as a phase demodulator. The output signal of the inter-mediate frequency amplifier 3 is then applied to a first input of a phase comparator stage 15, for example a mul-tiplier. A second inFut is connected to the output of an oscillator 16, which produces a signal the frequency of which depends on a d.c. voltage which is applied to a control input of the oscillator 16. Via a switch 11', lS which is normally closed but open during an overmodula-tion the output of the phase comparator circuit 15 is con-nected to the further portion, not shown, of the receiver (for example the matrixing circuit) and via a low-pass filter 17, which removes the audio signal components from the output signal of the phase Gomparator stage 15, to the control input of the oscillator 16, so that the frequency thereof is adjusted in accordance with the average value of the frequency of the input signal. The switch 11 ' which in all other respects can be controlled in a simi-lar manner as the switch 11 in the circuit shown in Figure1 forms in conjunction with a capacitor 18 which connects the signal path behind the capacitor 11' -to ground, a sample-and-hold circuit which in the event of overmodula-tion maintains the output signal at the value present 3~ prior to the ove~nodulation.
Although the invention is described in the fore-going as relating to the reception of stereo signals, the invention may alternatively be used if there is no rela-tionship as to contents between the signals modulating the amplitude and phase or the frequency respectively.
Claims (3)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A receiver for receiving AM-signals the carrier being amplitude modulated with a first signal (L+R) and phase modulated with a second signal (L-R), said receiver comprising a signal channel having a phase demodulator as well as an electronic switch for blocking and releasing the signal channel, said electronic switch being con-trolled by an overmodulation detector, which is energized by an amplitude overmodulation of the carrier, being char-acterized in that said phase demodulator comprises an FM-demodulator and a subsequent integrator and that the switch is connected between the FM-demodulator and the integrator.
2. A receiver as claimed in Claim 1 being charac-terized in that said switch is mass-connected for short-circuiting the integrator input at the occurrence of an amplitude-overmodulation of the carrier.
3. A receiver for receiving AM-signals the carrier being amplitude modulated with a first signal (L+R) and phase modulated with a second signal (L-R), said receiver comprising a signal channel having a phase demodulator as well as an electronic switch for blocking and releasing the signal channel, said electronic switch being con-trolled by an overmodulation detector, which is energized by an amplitude overmodulation of the carrier, being char-acterized in that said phase-demodulator comprises a phase locked loop, the switch being included within the phase locked loop between a phase comparator of the phase locked loop and an integrator, the integrator being connected to an output of the phase locked loop and through a low pass loop filter to a control input of a voltage controlled oscillator of the phase locked loop.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19803047386 DE3047386A1 (en) | 1980-12-16 | 1980-12-16 | RECEIVER FOR RECEIVING AM SIGNALS WHOSE CARRIER IS FREQUENCY OR PHASE MODULATED |
| DEP3047386.0 | 1980-12-16 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1175489A true CA1175489A (en) | 1984-10-02 |
Family
ID=6119310
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000391918A Expired CA1175489A (en) | 1980-12-16 | 1981-12-10 | Receiver for am stereo signals having a circuit for reducing distortion due to overmodulation |
Country Status (7)
| Country | Link |
|---|---|
| US (1) | US4405835A (en) |
| JP (1) | JPS57124943A (en) |
| KR (1) | KR830008566A (en) |
| AU (1) | AU543982B2 (en) |
| CA (1) | CA1175489A (en) |
| DE (1) | DE3047386A1 (en) |
| GB (1) | GB2089610B (en) |
Families Citing this family (23)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4479233A (en) * | 1982-03-03 | 1984-10-23 | Hazeltine Corporation | Distortion correcting AM stereo receiver with non-flat AGC |
| EP0089853B1 (en) * | 1982-03-23 | 1987-04-01 | Nec Corporation | Interference wave detection circuit for use in radio receiver |
| US4747141A (en) * | 1983-10-24 | 1988-05-24 | Kahn Leonard R | AM stereo signal decoder |
| JP3674166B2 (en) * | 1996-02-14 | 2005-07-20 | ソニー株式会社 | Broadcast wave signal receiver |
| US6047170A (en) * | 1997-07-29 | 2000-04-04 | Honeywell Inc. | Automatic squelch control for narrow band AM radio |
| US7515896B1 (en) | 1998-10-21 | 2009-04-07 | Parkervision, Inc. | Method and system for down-converting an electromagnetic signal, and transforms for same, and aperture relationships |
| US6061551A (en) * | 1998-10-21 | 2000-05-09 | Parkervision, Inc. | Method and system for down-converting electromagnetic signals |
| US7236754B2 (en) | 1999-08-23 | 2007-06-26 | Parkervision, Inc. | Method and system for frequency up-conversion |
| US7039372B1 (en) | 1998-10-21 | 2006-05-02 | Parkervision, Inc. | Method and system for frequency up-conversion with modulation embodiments |
| US6542722B1 (en) * | 1998-10-21 | 2003-04-01 | Parkervision, Inc. | Method and system for frequency up-conversion with variety of transmitter configurations |
| US6370371B1 (en) * | 1998-10-21 | 2002-04-09 | Parkervision, Inc. | Applications of universal frequency translation |
| US6879817B1 (en) | 1999-04-16 | 2005-04-12 | Parkervision, Inc. | DC offset, re-radiation, and I/Q solutions using universal frequency translation technology |
| US6853690B1 (en) | 1999-04-16 | 2005-02-08 | Parkervision, Inc. | Method, system and apparatus for balanced frequency up-conversion of a baseband signal and 4-phase receiver and transceiver embodiments |
| US7110444B1 (en) | 1999-08-04 | 2006-09-19 | Parkervision, Inc. | Wireless local area network (WLAN) using universal frequency translation technology including multi-phase embodiments and circuit implementations |
| US7693230B2 (en) | 1999-04-16 | 2010-04-06 | Parkervision, Inc. | Apparatus and method of differential IQ frequency up-conversion |
| US7065162B1 (en) | 1999-04-16 | 2006-06-20 | Parkervision, Inc. | Method and system for down-converting an electromagnetic signal, and transforms for same |
| US8295406B1 (en) | 1999-08-04 | 2012-10-23 | Parkervision, Inc. | Universal platform module for a plurality of communication protocols |
| US7010286B2 (en) | 2000-04-14 | 2006-03-07 | Parkervision, Inc. | Apparatus, system, and method for down-converting and up-converting electromagnetic signals |
| US6697576B1 (en) * | 2000-07-20 | 2004-02-24 | Northrop Grumman Corporation | Wideband, high-fidelity analog optical link design |
| US7454453B2 (en) | 2000-11-14 | 2008-11-18 | Parkervision, Inc. | Methods, systems, and computer program products for parallel correlation and applications thereof |
| US7072427B2 (en) | 2001-11-09 | 2006-07-04 | Parkervision, Inc. | Method and apparatus for reducing DC offsets in a communication system |
| US7379883B2 (en) | 2002-07-18 | 2008-05-27 | Parkervision, Inc. | Networking methods and systems |
| GB2481237A (en) * | 2010-06-17 | 2011-12-21 | Nokia Corp | Protecting a speaker arrangement against over-modulated broadcasts |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3076057A (en) * | 1959-04-22 | 1963-01-29 | Westinghouse Electric Corp | Broadcast stereo receiver |
| US3714583A (en) * | 1970-08-26 | 1973-01-30 | Rca Corp | Muting circuit |
| US4107613A (en) * | 1977-02-11 | 1978-08-15 | Perma-Power Inc. | Wireless microphone with FM receiver muting system responsive to excessive undesired AM level or low AGC control level |
| US4172966A (en) * | 1978-02-23 | 1979-10-30 | Motorola, Inc. | AM stereophonic receiver |
| JPS5624838A (en) * | 1979-08-08 | 1981-03-10 | Pioneer Electronic Corp | Demodulating circuit for am stereo signal |
| US4340782A (en) * | 1980-06-13 | 1982-07-20 | Magnavox Consumer Electronics Co. | Circuit for demodulating amplitude and angle modulated broadcast signals |
| DE3024085A1 (en) * | 1980-06-27 | 1982-01-21 | Philips Patentverwaltung Gmbh, 2000 Hamburg | AM-FM BROADCAST RECEIVER FOR RECEIVING AM STEREO SIGNALS |
-
1980
- 1980-12-16 DE DE19803047386 patent/DE3047386A1/en active Granted
-
1981
- 1981-09-14 US US06/301,567 patent/US4405835A/en not_active Expired - Fee Related
- 1981-12-10 CA CA000391918A patent/CA1175489A/en not_active Expired
- 1981-12-11 GB GB8137481A patent/GB2089610B/en not_active Expired
- 1981-12-14 AU AU78487/81A patent/AU543982B2/en not_active Ceased
- 1981-12-14 JP JP56200257A patent/JPS57124943A/en active Pending
- 1981-12-16 KR KR1019810004955A patent/KR830008566A/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| DE3047386C2 (en) | 1989-05-11 |
| AU543982B2 (en) | 1985-05-09 |
| GB2089610B (en) | 1985-05-15 |
| GB2089610A (en) | 1982-06-23 |
| JPS57124943A (en) | 1982-08-04 |
| KR830008566A (en) | 1983-12-10 |
| AU7848781A (en) | 1982-06-24 |
| US4405835A (en) | 1983-09-20 |
| DE3047386A1 (en) | 1982-07-15 |
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Effective date: 20011210 |