DE2619440A1 - COMPATIBLE AM STEREO RECEIVER - Google Patents

Description

Compatible AM stereo receiver

The invention relates to AM stereo receiver for receiving a carrier wave with stereo message content, which is in the corresponding upper and lower sidebands of the transmitted signal appear. Such a receiver is out the German patent application P 14 16 141.2 known.

AM stereo receivers according to the invention can increase the carrier to reduce the signal distortion at low level of the received signal strength and / or amplitude counter modulation of the carrier and / or quadrature demodulation of the received Carriers for deriving the message content of a stereo difference (L - R) signal have and in-phase demodulation (e.g. hill curve demodulation or product demodulation) to obtain the message content of the stereo sums Have (L + R) signals with subsequent phase shift and unification of the stereo sum and difference signals in a known way for generating stereo output signals (L and R). Modulation of the carrier wave with an infrasound frequency (e.g. 15 Hz) is preferably used to detect the presence of a in the receiver stereo-modulated signal. The invention relates to

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also a special receiver circuit with the possibility of stereophonic and monophonic operation and automatic Switching between the two.

Compatible stereophonic AM broadcast and reception with stereophonically connected upper and lower sidebands, the message content of the difference stereo signal (L - R) phase modulates the carrier wave and the message content of the summed stereo signal (L + R) the carrier wave envelope modulated, together with certain Types of receivers for stereophonic reception of a carrier wave modulated in this way in German Patent application P 14 16 141.2 described. Another explanation of this compatible AM stereophonic modulation technique can be found in the article "A Stereophony System For Amplitude Modulated Broadcast Stations", IEEE Transactions on Broadcasting, Volume BX-17, No. 2, June 1971, Pages 50 - 55. As far as this publication is relevant here, reference is made to it.

It is also the so-called "compatible" stereophonic AM system from ÜS-PS 3 102 167 known, in which a approximate quadrature modulation technique is used. This system is used to reduce monophonic distortion a relative phase angle between the support and the sidebands of ί 25 to 30 degrees is used, the two being Channel signals using a phase shifted raised carrier and product demodulator, but without a combination of the demodulated signals, to be developed =

From US-PS 3 009 151 are also stereophonic radiation and receiving systems including a two-channel FM-AM stereo system known, in which stereophonically related

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low frequency signals on FM band and AM band carrier waves or amplitude-modulated. From U.S. Patent 3,068,475 a stereo broadcasting and receiving system is known in which the one stereo signal is amplitude-modulated onto a carrier wave and the other stereo signal is frequency-modulated onto the same carrier wave. From US-PS 3,167,614 it is known to use an infrasonic sound to indicate the presence of a stereo signal in an AM / PM broadcast system use. Finally, US Pat. No. 3,231,672 discloses an AM stereo system with linearly added carrier waves of the same frequency, but known in different phase, in which the Carrier waves are amplitude modulated with the stereo signals.

The features and advantages of the present invention are demonstrated by various embodiments of AM stereo receivers for receiving radiant energy carrier waves modulated with two stereo signals (L and R) from each of which appears as an independent first order single sideband, the carrier with the stereo sum message content (L + R) essentially amplitude modulated and phase modulated with the stereo difference message content (L - R) isti With such receivers, quadrature demodulation is used phase modulation, which represents the stereo difference message content (LR) of the received signal, and one Use one or more of the following methods to optimize the quality of the stereo output signal:

1. Use of amplitude counter modulation of the carrier, i. H. Use of the envelope fundamental (and as preferred embodiments one or more harmonics) for additional and inverse modulation of the receiver carrier in such a way that the distortion of the stereo difference signal, that from the additionally modulated

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Carrier is derived, is reduced;

2. Using the beam lift, d. H. a raised beam, preferably with a phase-locked loop or similar control of the carrier gain input such that the carrier gain is substantially no phase modulation distortion occurs and that the stereo difference signal emanating from the raised carrier is derived, even if the received bearer is complete, i. H. 100%, is modulated, no Contains noise pulses;

3. In conjunction with either of the above two procedures for demodulating and processing an infrasound tone (e.g. 15 Hz), which is used as an indication of the presence a stereo signal is modulated on the received carrier, such a tone preferably for automatic control of the receiver output mode is used.

Further features and advantages result from the invention Receivers in that circuit components such as sideband filters are omitted, whereby the circuits readily suitable for the use of integrated circuits.

A feature and advantage of the AM stereo transmission and reception system and method of the present invention is optional modulation of the carrier with an infrasound frequency signal for displaying an existing stereo signal in the receiver. This signal can be used for automatic switching the reception mode from stereophonic to monophonic and vice versa and / or as a carrier voting indicator.

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Further features and advantages of the invention will become apparent from the following description and explanation of certain typical embodiments of the invention.

Fig. 1 shows in a block diagram a compatible AM stereo receiver for receiving an AM stereo signal _f as z. B. is developed in the illustrated control transmitter of the transmitter. The receiver has a phase-modulated carrier wave boost and a carrier tracking circuit with carrier selection through a phase-locked loop. The receiver also has in-phase demodulation in the form of an envelope detector for deriving the stereo sum signal (L + R) and counter-modulation of the phase-modulated raised carrier by either the fundamental of the envelope or the fundamental and harmonics derived from the received carrier envelope, in connection with quadrature demodulation of the phase-modulated and inverse amplitude-modulated raised carrier wave for deriving a stereo difference signal (L - R) of low distortion. The receiver shown in Fig. 1 also has a demodulation device for an infrasound sound and electronic switching devices which respond to this and automatically change the output mode of the receiver.

2 shows a modified procedure in a block diagram the amplitude counter-modulation of the phase-modulated raised carrier with that derived from the envelope curve Fundamental oscillation and components of the second harmonic to reduce signal distortion. The circuit possesses in this example separate amplitude modulators for the envelope curve fundamental and the second harmonic component the envelope curve,

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3 shows, in a block diagram, a further modified method for amplitude counter-modulation of the phase-modulated increased carrier wave to further reduce the distortion of the stereo differential output signal (LR). That In this example, the method includes an additional amplitude modulation of the phase-modulated raised carrier with components of the second and third harmonics derived from the envelope curve and also the fundamental envelope curve component.

Fig. 4 is a representation of the spectrum of the received signal with the carrier raised, as it is in Fig. 1 at the input 26 of the amplitude modulator 28 occurs in the case where the received signal is a carrier wave which is in the one stereo channel (L) is fully modulated (phase modulation of 0.5 radians and amplitude modulation of 50%) and has no stereo modulation in the other stereo channel (R).

Fig. 5 is a diagram of the output signal spectrum of the amplitude modulator, as it occurs in Fig. 1 at 48, and corresponds to the spectrum of the received signal shown in Fig. 4. Both for the fundamental as well as for the second harmonic of the envelope curve of the received carrier counter modulation is used ο

Fig. 6 shows in detail a block diagram of a typical carrier tracking circuit as used in Fig. 1;

Fig. 7 shows in detail a modified part of the in. shown circuit »

Fig. 8 shows in a block diagram a part of a modification of the receiver shown in Fig. 1 , with a product demodulator as in-phase detector instead of the envelope curve detector.

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gate, which derives the L + R signal, is used.

Fig. 1 shows in a block diagram a typical invention compatible AM stereo receiver. The antenna 10 is connected via a line 12 to a conventional HF-IF superhet circuit 14, which has an intermediate frequency output signal 16 generated, part of which to an in-phase demodulation device is performed, such as B. an envelope curve detector 18, as what a conventional Diode detector circuit is suitable. The IF output signal 16 becomes the carrier selection without introducing phase distortion also passed to a carrier tracking filter circuit 20, e.g. B. a phase-locked loop. This The circuit is of a construction known per se (e.g. Signetics IC No. 562).

The output signal 22 of the carrier tracking circuit 20 represents the carrier component of the received carrier wave and is fed to the summing circuit 24 by this output signal 22 is added to the received carrier wave (at ZF). Part of the output signal 16 of the HF / IF stages 14 is also provided as an input to the sum circuit 24. The combined output 26 of the Summing circuit 24 suitably, as exemplified in FIG. 4, to such a degree Carrier raising that the raised carrier is about half higher is as the received carrier, i.e. H. in the particular example shown, a relative voltage of 1.5 volts compared to a relative carrier level in the received signal of 0.9415 volts. This phase modulated, raised carrier wave output signal is more suitable for demodulation to gain phase modulation (after it in the amplitude modulator 28, as detailed below

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is explained, was amplitude-counter-modulated), since the raised Carrier wave no instantaneous zero amplitude, i.e. H. negative 100 percent modulation.

The output signal 22 of the carrier tracking circuit is also fed to a phase shifter network 30, which the carrier phase offset by 90 degrees. Network 30 then feeds the phase shifted carrier output signal 32 to the product demodulator 34 to. The network 30 and the demodulator 34 together represent a quadrature demodulator known per se, which is designated as a whole by 36. The phase shifter network 30 can be of a conventional type known per se, as described in "Radio Engineer Handbook," Keith Henny, 5th Edition, McGraw-Hill Company, New York, New York, 1959, chapter 12, and Pages 16 - 52. A suitable, per se known product demodulator 34 can also, for. B. the module Motorola IC No. MC 1596 L be.

The output signal 38 of the in-phase detector 18 is the amplitude modulation envelope curve of the received carrier wave and is in particular the fundamental oscillation, since the amplitude modulation or envelope curve in this type of AM stereo transmission, a substantially distortion-free reproduction of the stereo summary message content (L + R) is in the sidebands of the received carrier wave, as is known per se. This Envelope fundamental signal 38 is fed to amplifier 40 and the amplified output signal x 42 is applied the summing circuit 44 which then receives the low frequency modulation input signal 46 for the amplitude modulation of the phase-modulated, raised carrier wave (output signal 26) in the amplitude modulator 28 supplies. The output signal 48 of the Amplitude modulator 28 can be used as an inversely amplitude-modulated (amplitude-counter-modulated), phase-modulated, raised carrier wave and is called the second input

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signal for the product demodulator 34 in the quadrature demodulator circuit 36 used.

As an essential feature of the present invention, the additionally reduced on the carrier wave in the amplitude modulator 28 applied amplitude modulation significantly reduces the distortion that is otherwise in the output signal of the quadrature demodulator 36 would occur. Uses the envelope fundamental (output signal 38) as the only input signal of the amplitude modulator 28 is used (this operating state is achieved by the manual switch 52 is left open), the distortion in the demodulated output signal is reduced to about 4% (comparison basis is the tension). If a further reduction in the distortion factor in the stereo difference signal (L - R) is desired or required, the frequency doubler 54 can be switched on by closing the switch 52 will. In this mode of operation, the second harmonic output signal is 56 of the frequency doubler 54 in the summing circuit 44 with the envelope curve fundamental (Output signal 42) combined, whereby both fundamental as also harmonic envelope components at the low frequency input 46 of the amplitude modulator 28 are present and there is a further reduction in distortion. The approximate optimum of the relative heights of the Amplitude modulation in the amplitude modulator 28 is approximately 26% for the fundamental wave modulation and the second harmonic wave modulation about 8% of the total modulation voltage (the basis of comparison is the voltage). With these values for amplitude counter modulation, the total harmonic distortion of the second or higher order is that in the output signal 50 of the quadrature deitiodulator 36 occurs to about 0.8% (Voltage comparison) reduced.

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A phase shift network known per se is used to combine the stereo difference output signal (LR) 50 with the envelope curve fundamental wave output signal (L + R) 38 is used in a known per se and described in more detail below Way to generate relatively distortion-free stereo signals (L and R).

The receiver system shown in Fig. 1 responds to ultrasonic modulation of the wearer indicates the presence of a Stereo signal and preferably automatically sets the receiver output to stereo mode and maintains this at when a stereo presence signal is present. The automatic one Switching the receiver output mode is accomplished by controlling an electronic switch 58, as is is also explained in detail below. When closed, the electronic switch 58 connects the demodulator output 50 with a 0-45 '' phase shifter network 60, its output signal 62 to the sum circuit 64 and the Difference circuit 66 is performed. The stereo composite signal (L + R), which occurs as detector output signal 38, is assigned to an associated 0 + 45 ° phase shifter 68 in the same way is fed, the output signal 70 of which is also fed to the sum and difference circuits 64, 66. As stated, the phase shifters 60 and 68 are a phase shifter pair (0-45 ° and 0 + 45 °), which are of a known type and a relatively constant relative phase difference of substantially 90 degrees within an effective audio frequency range delivers and maintains relatively constant signal amplitudes. With this type of output circuit In general, the sum circuit 64 prefers the stereo message content of the left or L channel and will consequently the L speaker 68 is driven by the L stereo signal. Similarly, differential circuit 66 prefers the right or R channel and message content

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drives the R speaker 70. As it is known of course stereophonically indistinguishable message content occurs in the received signal sidebands (i.e. monophonic news content) simply as double first-order sidebands in the received carrier wave, d. H. occurs as conventional double sideband amplitude modulation and appears as part of the demodulated envelope and drives both the L speaker 68 and the R speaker 70 monophonic.

The electronic switch 58 is controlled by the infrasound tone (e.g. 15 Hz), which acts as a modulation on the carrier of the received signal occurs. It is assumed that the infrasound sound is due to amplitude modulation of the carrier is broadcast (this is currently believed to be the preferred type of modulation), switch 72 is in Fig. 1 is shown in the correct position for responding to the infrasound tone display for stereo, since the infrasound envelope curve component, occurring in the detector output signal 38, through the switch 72 to the band pass filter 74 is performed, the isolated infrasound-from output signal 76 leads to the amplifier 78, the output signal 80 energizes the stereo presence indicator lamp 82. The infrasonic sound output signal 80 is also sent to the detector placed, which generates a DC voltage component at the output 86, which closes the electronic switch 58 when the Stereo infrasound sound is present. Another possible way of working is that the infrasound sound that the Indicates the presence of a stereo signal onto which the carrier wave is phase modulated.

In this working mode, the quadrature demodulator generates of the receiver shown in Fig. 1 as part of its output signal 50 the infrasound tone, which indicates the presence of a

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Stereo signal, and the switch 72 is switched to its second position 72 'to the demodulator output signal 50 to the band pass filter 74, the filter output signal 76 the stereo indicator 82 and the electronic Switch 58 controls in the same manner as indicated above. If to modulate the infrasound sound on the carrier phase modulation or the like (e.g. quadrature modulation) is used, it should be noted that the carrier tracking circuit 20 must be sufficiently narrow in its output signal · 22 so that it does not follow the infrasound modulation of the carrier or runs after. If it had to follow, the infrasound sound would be strongly attenuated and the stereo response circuit (e.g. the electronic switch 58) is put out of operation. In Fig. 1, the hand switch 88 is also shown, which is closed, if the receiver can only be used for stereo reception target. In this mode of operation with switch 88 closed, switch 72, bandpass filter 74, amplifier 78, the stereo lamp 82, the detector 84 and the electronic switch 58 are unnecessary since the switch 88 is the output signal 50 of the product demodulator and the associated phase shifter 60 directly connects.

FIG. 2 shows part of a modified form of an AM stereo receiver according to the invention, the receiver circuit corresponding to FIG. 1, apart from the part shown in FIG. 2 and explained below. According to FIG. 2, instead of the one modulator 28 of FIG. 1, the modified circuit has two amplitude modulators 28 'and 28 *'. The amplifier 40, whose input signal is the output signal 38 of the detector, provides the fundamental component input signal 42 (as in FIG. 1) for the first amplitude modulator 28 '. In this modified circuit, the output signal of the first amplitude modulator 28 'is the input signal 90 for the second Airiplitudenniodulafcor 28 ⁸ ' and becomes an additional one

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Low frequency input signal 92 for amplitude modulator 28 ″ derived from frequency doubler 54. The output signal 9.4 of the second amplitude modulator 28 ″ is then used as the input signal of the product demodulator 34. Although this arrangement requires a second amplitude modulator 28 ″, it causes somewhat less distortion than the circuit arrangement shown in FIG.

FIG. 3 shows a further modified amplitude counter-modulation circuit _# which reduces the distortion in the demodulated stereo difference output signal even below that of the one shown in FIG.

2 is reduced. In the circuit of Fig.

3, the output 42 of the amplifier 40 is fed to the frequency doubler 54 (as in FIG. 1) which in turn provides a second harmonic output 56 (again as in FIG. 1) to the summing circuit 44 '. The operation of the amplifier 4O ₇ is of the frequency doubler 54 and the frequency doubler output signal 56 which is combined in the summing circuit 44 'with the fundamental wave output signal 42, the same as in the circuit in Fig. 1, when the switch 52 is closed. In Fig. 3, however, use is made of a further envelope curve fundamental component 42 of the amplifier 40, which is connected to a frequency tripler 96, the third harmonic output signal 98 of which is also fed to the summing circuit 44 'and to the fundamental component 42 and the second Harmonic component 56 is combined in such a way that the summing circuit 44 'applies an output signal 100 to the amplitude modulator 28 which is free to an even greater extent from the fourth-order sideband components, which would otherwise cause distortion in the stereo Difference output signal (e.g. with the signal 50 of the quadrature demodulator 36 in Fig. 1), Another selection of harmonic component input

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Signals for the sum circuit (44 or 44 '} to further reduce the distortion of the demodulated output signal of the stereo difference signal results for the person skilled in the art.

From the preceding explanation of various embodiments of receiver circuits according to the invention, there is The basic concept of the invention in this type of AM stereo receiver is applied, generally in reducing stereo signal distortion by using an or several components that are derived from the envelope curves of the received carrier wave for counter-modulation of the entire wave (with Carrier boost or otherwise), eliminating unwanted spectral components that would otherwise cause distortion would if the stereo difference signal through the quadrature demodulator or similar devices demodulated will be completely or almost completely wiped out.

Fig. 4 shows schematically the spectrum of the modulated carrier received by the receiver shown in Fig. 1, wherein the carrier is lifted by the carrier tracking filter circuit is. In the typical example of the received signal, the carrier wave in the one stereo channel (L) is complete modulated and not modulated in the other stereo channel (R).

Theoretical analysis shows that this signal can be demodulated by a quadrature demodulator and without any additional Amplitude modulation, d. H. if the received signal with the raised carrier output signal 26 in FIG would be fed directly to the product demodulator 34 without any additional modulation in the amplitude modulator 28, provides a stereo differential output signal at output 50 which by a second order distortion factor of

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about 13%, exactly 13.05%, and a third harmonic distortion Order of 2 1/2%, exactly 2.33%, when compared on the basis of the voltage. Although these Harmonic distortion or total harmonic distortion values can be considered tolerable in some system applications however, such distortion values are obvious from the point of view of commercial broadcasting too large. However, it is an essential feature and advantage of the present invention that such Distortion factor values through selective counter-modulation of the phase-modulated carrier wave with the demodulated envelope curve wave can be reduced to commercially acceptable levels. If such an amplitude counter modulation to almost complete Elimination of the second order distortion factor is to be used, theoretical considerations show that the counter modulation input signal increases the modulation percentage of the phase modulated carrier wave by a factor of should decrease about 26%. This is the process of counter-modulating the phase-modulated carrier wave with the fundamental oscillation of the envelope is applied (i.e. switch 52 in Fig. 1 is open and the output 42 of the amplifier 40 applied via the summation circuit 44 to the input 46 of the amplitude modulator 28), then the distortion factor becomes second Order is reduced to around 0.30% and the cubic distortion factor is around 4.1%. (In a specific example, the value of the counter modulation resulted in a reduction in the carrier modulation of 26.09%, the distortion factor being of the second order 0.30% and the cubic total harmonic distortion was 4.13% at the voltage as a basis of comparison.) Although such a value of the cubic The distortion factor is tolerable, however, its substantial value makes it desirable, the cubic distortion factor as well to reduce. This can be done in the receiver shown in Fig. 1 by closing the switch 52, whereby

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a frequency-doubled envelope curve component (that is a component of the second harmonic of the envelope curve) dem Counter modulation input signal 46 of the amplitude modulator 28 is added. In an example with an envelope fundamental input signal (at 42) of about 26% and an envelope input of the second harmonic (at 56 coming from the frequency doubler 54) of about 8% when comparing the voltage, the amplitude-counter-modulated, phase-modulated raised carrier wave, as occurs at the output signal 48 of the amplitude modulator 28, essentially the spectral distribution shown in FIG. (In a special case, the value of the fundamental component was the out-of-phase amplitude modulation 26.09% and the amplitude counter modulation of the second harmonic 8.115%, where the spectral distribution shown in FIG. 5 resulted.) With such an application of the amplitude counter modulation of the In the specific example cited, the fundamental and second harmonic was the second order distortion factor 0.295%, the third-order distortion factor 0.145% and the fourth-order distortion factor 0.725%.

As already mentioned, an alternative method for amplitude counter-modulating the phase-modulated, raised carrier wave with both the fundamental envelope curve and the envelope curve components of the second harmonic is shown in FIG. In this case, the raised carrier wave is modulated with the fundamental in a first amplitude modulator 28 ″ and with the second harmonic of the envelope curve in a second amplitude modulator 28 ¹¹ . Harmonic counter-modulation components result in slightly less distortion compared to using a single amplitude modulator stage as in FIG.

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As a further improvement in the direction of optimizing the distortion reduction in the stereo differential output signal In Fig. 1, Fig. 3 shows a further modified circuit with a frequency tripler 96 and a frequency doubler 54, the corresponding envelope harmonics output signals 98 and 56 provide the sum circuit 44 'to further reduce out-of-band distortion can be combined with the fundamental envelope output signal 42.

The lowering of the higher order sideband distortion can be achieved by envelope component input signals accordingly higher order can be further improved. For the person skilled in the art it follows that the technique of reducing the Distortion due to counter-modulation of the phase-modulated carrier wave with fundamental and harmonic components the envelope curve of the received wave and various relative values to achieve the desired goal Allows selection of components and component values.

Figure 6 shows in detail a typical carrier tracking filter circuit (carrier track filter circuit) of the type designated as a whole by 20 in FIG. 1. If one assumes that a typical receiver will follow carrier frequency errors and a carrier frequency drift in the range of about 1,800 Hz or must lag, so requires a good carrier lag to realize a raised carrier signal without significant Phase distortion that the carrier tracking circuit is much narrower than * 800 Hz because of such a passband in addition to the desired carrier, would pass many sideband signal components, especially since these Sideband components are not necessarily symmetrical in stereo operation and the carrier tracking circuit of the resulting phase modulation components of the stereo wave

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would follow instead of just letting the carrier pass if the passband is too wide. The carrier tracking circuit shown in FIG For this reason, the input signal 16 of the received carrier first performs a first phase-locked Phase locked loop circuit (PLLA) 100 to, which may be of suitable design, such as. B. Signetics IC No. 562 B, and a pass band of ί 800 Hz owns. The output signal 102 of the phase locked loop 100 is then fed to a frequency divider circuit 104, by dividing the frequency of the carrier by an appropriate integer, e.g. B. 16, is shared. The frequency division serves to also divide the frequency error by an equal amount (The sidebands, however, are not pushed closer together because the sideband spacing is achieved by frequency division or frequency multiplication is not changed). With the carrier and any frequency error divided by the chosen integer, the output signal 106 of reduced frequency is sent to a second phase-locked loop circuit (PLLB) 108 (e.g. also a Signetics IC No. 562 B) is supplied, which in the selected example has a pass band of 50 Hz. This circuit 108 effectively acts as a carrier tracking filter (carrier tracking filter, that is a bandpass filter, the center frequency of which corresponds to the average value of the frequency of the input signal), however, is sufficiently narrow to avoid any to allow a substantial amount of sixth band modulation to pass through, so that the filtered output signal 110 is essentially only the available carrier at a reduced frequency. The filtered one is used to isolate the original carrier frequency Output 110 then to a frequency multiplier circuit 112 by multiplying it by a suitable whole number (in the chosen example by 16) and which provides an output x 22 that is the tracked carrier is at the received carrier frequency and that is the sum circuit 24 and the phase shifter 30 (Fig. 1)

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The carrier tracking circuit 20 is generally intended to have a bandwidth that can follow the expected frequency drifts in the transmitter and receiver. This consideration leaves in some cases do not agree with the use of phase modulation for the infrasound sound. For this Reason such a tone is preferably amplitude-modulated on the carrier, thereby avoiding any difficulty in the circuit 20 in tracking the infrasonic sound.

Since the frequency division in stage 104, as explained above, takes place, it is necessary to match the phase of the output signal 22 of the filter system with the phase of the received carrier gers to compare. In Fig. 6 this is done in that a portion of the output signal 22 passes to the phase detector 114 (z. B. Motorola IC No. MC 4004 P) and through a Low pass filter (LPF) 116 with a typical time constant of 15 milliseconds, creating a control voltage input signal 118 for the phase locked loop circuit 100 is provided will. The carrier tracking output signal 22 becomes with respect to the phase in the phase detector 114 with the phase of Input wave 16 is compared and the control voltage output 118 of the phase detector corrects any larger Phase errors between the input signal and the output signal (with a frequency division of 16 there are 16 different phase stable points to which the phase locked loop circuit 108 can lock, if not the phase control -would be present by the phase detector exerted on the phase-locked loop 100). The one exerted on the phase-locked loop 100 by the phase detector 114 Control is effected relatively slowly by means of a low pass filter circuit 116 and works in principle in such a way that it corrects for major phase errors that can occur when the device is switched on or when

there is significant carrier fading affecting the frequency divider circuit 104 "lets slip one tooth further".

Fig. 7 shows in block form a further modified circuit detail, which brings a simplification of the carrier tracking circuit of FIG. Basically it can be phase-locked Loop 108 and the frequency multiplier stage 112 of the in Fig. 6 can be replaced by the circuit shown in Fig. 7, which is known per se. At this Type of circuit, the frequency-divided carrier output signal 106 is applied to a phase detector 102, its output signal 122 is a voltage controlled oscillator (VCO) 124 which drives the tracked carrier output 22 generated at the desired frequency. The voltage controlled oscillator 124 operates at 16 times the frequency the input frequency occurring at the input signal 106. This output signal 22 is also fed to the frequency divider 126, which in turn divides the frequency precisely by 16. The output signal 128 of the frequency divider 126 arrives at the phase detector 120 by dividing the phase of the frequency divided output signal 128 with the. Phase of the input signal 106 is compared, the phase detector 120 the Output signal 122 generated, which is used in the voltage-controlled oscillator 124, the phase of the output signal in phase with the phase of the input signal. In other words, the circuit shown in Fig. 7 works as a normal phase-locked loop, but with a frequency division of 16 on the feedback path and with operating the voltage controlled oscillator at 16 times the input frequency, thereby providing exact in-phase frequency multiplication is achieved.

Fig. 8 shows part of a further modified form of a AM stereo receiver according to the invention, apart from that

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8 and explained below has the circuit shown in FIG. 1 (or FIG. 2 or 3). Instead of the envelope curve detector 18 of FIG. 1 where an input signal 22 of the carrier being tracked is also supplied to the demodulator 18 'is a product demodulator 18 ¹ as the in-phase detector for deriving the L + R output signal 38 is used, in this modified embodiment. Although the product demodulator 18 'is somewhat more complicated than the envelope detector 18, it is advantageous in terms of signal to noise ratio and is currently preferred, especially in noisy conditions.

For a person skilled in the art, the exemplary embodiments explained above result Within the scope of the following claims, further possibilities of application and adaptation of the invention Stereo receiver.

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Claims (3)

573 905 May 3 -f9η £ Claims 1. Compatible AM stereo receiver for receiving a carrier wave, which is amplitude-modulated with the signal of the stereo sum message content (L + R) and with the signal of the stereo difference message content (L - R) is phase modulated by ι a) In-phase demodulation devices (18) for deriving a L + R low frequency signal; b) a quadrature demodulator (36) for deriving an L-R low frequency signal and c) means (68,64; 60,66) for combining the L + R and L-R signals in phase to generate L and R stereo output signals. 2. Receiver according to claim 1, characterized by means (28) for amplitude counter-modulating the phase-modulated carrier wave with part of the output signal of the in-phase detector (18) before the quadrature demodulation of the carrier wave, thereby eliminating the distortion introduced by quadrature demodulation is decreased in the demodulated L - R output signal will. 3. Receiver according to claim 2, characterized in that the phase-modulated Carrier wave amplitude-counter-modulated at least with the fundamental oscillation of the output signal of the in-phase detector (18) will. 4. Receiver according to claim 3, characterized in that the • 609846/0796 - 23 - Amplitude counter modulation of the carrier wave the modulation percentage of the carrier wave, thereby reducing the demodulated output signal derived from the resulting wave is essentially free of second order harmonic distortion. 5. Receiver according to claim 4, characterized in that the modulation percentage of the carrier wave by the input signal, derived from the envelope fundamental by about 26% when comparing the voltage with the fully modulated received carrier wave is reduced in size. 6. Receiver according to claim 2, characterized in that the phase-modulated carrier wave with at least the fundamental oscillation and the second harmonic of the output signal of the in-phase detector (18) is counter-modulated in amplitude. 7. Receiver according to claim 6, characterized in that the amplitude counter modulation of the carrier wave reduces the modulation percentage of the carrier wave by an amount that the demodulated output signal derived from the resulting wave is substantially free of one Second order distortion factor makes. 8. Receiver according to claim 7, characterized in that the modulation percentage of the carrier wave by the input signal, which is derived from the fundamental oscillation of the output signal of the in-phase detector (18) by about 26% and by the input signal that results from the second harmonic of the output signal of the in-phase detector (18) is derived by about 8% when comparing the voltage with the fully modulated, received carrier wave is reduced. 609846/0796
- 24 - 573 905 26194 9. Receiver according to claim 2, characterized in that the phase-modulated carrier wave with the fundamental and the second and third harmonic of the output signal of the in-phase detector (18) is amplitude-counter-modulated. 10. Receiver according to claim 9, characterized in that the amplitude counter modulation of the carrier wave reduces the modulation percentage of the carrier wave to a value that the demodulated output signal obtained from the resulting wave is substantially free of distortion factor second order makes. 11. Receiver according to claim 2 to 10, characterized by a summation circuit (24) which the carrier of the received carrier wave increases before the amplitude counter modulation by the signal derived from the envelope curve. 12. Receiver according to claim 11, characterized in that the device for raising the carrier has a carrier tracking circuit (20) for isolating the carrier frequency from the received carrier and means (24) for combining the Output of the carrier tracking circuit having a portion of the received carrier. 13. Receiver according to claim 12, characterized in that the Carrier tracking circuit (20) a phase-locked loop circuit contains. 14. Receiver according to claim 13, characterized in that the in-phase detector contains a product demodulator (18 ¹ ) which receives an input signal from the carrier tracking circuit (20). 15. Receiver according to claim 12, characterized in that the 609846/0796 - 25 - 573 905 281944Q Carrier tracking circuit (20) a first phase-locked Loop circuit (100), a frequency divider (104), a second phase locked loop circuit (108) having a passband which is narrower than the passband the first phase-locked circuit (100), and a frequency multiplier (112) which the frequency the output signal of the second phase-locked loop circuit (108) back to the frequency of the carrier input signal of the first phase-locked circuit (100) and has a phase detector (114), the phase of the carrier input signal of the first phase-locked circuit with the carrier output signal of the second phase-locked circuit compares and phase-locks the phase of the output of the first Schlexfenschaltkreises controls in such a way that the input signal and the output signal of the carrier tracking circuitry be kept in phase. 16. Receiver according to claim 15, characterized in that the second phase-locked loop circuit and the frequency multiplier a voltage controlled oscillator (124). 17. Receiver according to claim 2 or 12, wherein the received Carrier is additionally modulated with an infrasound tone, which indicates the presence of a stereo signal, characterized by a) devices (74 , 78, 84) for filtering out and demodulating the infrasound sound and b) means (58) for controlling the audio frequency output channels of the receiver thereby producing L and R stereo output signals in response to the presence of such a tone and monophonic output in the absence of one 609846/0796 - 26 - such tones are generated, and / or c) a visual indicator (82) responsive to the isolated infrasound sound responds and indicates the presence of a stereo signal. 18. Receiver according to claim 17, characterized in that the In fr aschall tone has a frequency of about 15 Hz. 19. Receiver according to claim 17, characterized in that the Carrier wave is phase or frequency modulated with the infrasound sound. 20. Receiver according to claim 17, characterized in that the carrier wave is amplitude-modulated with the infrasound sound. 21. Compatible AM stereo receiver for receiving a carrier wave, which are amplitude-modulated with the message content of the stereo composite signal (L + R) and with the message content of the Stereo difference signal (L r R) is phase modulated, marked by a) an in-phase detector (18) for obtaining an L + R signal ? b) means (28) for amplitude counter-modulating the modulated Carrier wave having a portion of the output of the in-phase detector (18); c) a quadrature demodulator (36) for obtaining an L-R signal from the amplitude-counter-modulated, phase-modulated carrier wave; and d) Means (64, 66) to unite the L + R- and L-R- 609846/0796 - 27 - Signals in phase to produce L and R stereo output signals. 22. Receiver according to claim 21, characterized in that the in-phase detector is a hull curve detector (18). 23. Receiver according to claim 21, characterized by a device (24) to raise the carrier wave before its amplitude counter-modulation. 24. Receiver according to claim 23, characterized in that the in-phase detector contains a product demodulator (18 ¹ ). 25. Compatible AM stereo receiver for receiving a carrier wave, which are amplitude-modulated with the message content of the stereo composite signal (L + R) and with the message content of the stereo difference signal (L - R) is phase modulated, characterized by a) an in-phase detector (18) for obtaining an L-R signal; b) a carrier tracking circuit (20) for isolating the Carrier frequency from the received carrier wave; c) means (24) for combining the output signal of the carrier tracking circuit with part of the caught carrier wave for generating a received carrier wave with the carrier raised; d) a quadrature demodulator (36) for obtaining an L-R signal from the received carrier wave with the carrier raised; and 609846/0796
- 28 - e) means (64, 66) for combining the L + R and L-R signals in phase to generate L and R stereo output signals . 26. Receiver according to claim 25, characterized in that at a comparison of the voltages the level of the raised carrier is about half higher than the level of the received one Carrier, and that the effective pass band of the carrier tracking circuit (20) is approximately ± 50 Hz. 27. Receiver according to claim 26, characterized in that the in-phase detector is an envelope curve detector (18). 28. Receiver according to claim 26, characterized in that the in-phase detector is a product demodulator (18 ¹ ). 29. Receiver for receiving a compatible single sideband radio frequency carrier wave with stereophonically related message content (L + R), which is a modulation of the upper and lower sideband of the carrier wave occurs, the carrier wave having the message content of the stereo composite signal (L + R) amplitude modulated and phase modulated with the message content of the stereo difference signal (L - R) is indicated by a) a superhet (14) for generating an IF signal the received RF signal; b) an in-phase detector (18, 18 ') for obtaining the fundamental oscillation the L + R message content from a portion of the IF signal; c) means (24) for raising the carrier of a portion of the IF signal; 609846/0796
- 29 - 573 905 d) means (28) for amplitude counter-modulating a Part of the raised carrier IF signal with at least the fundamental of the L + R message content; e) a demodulator (36) for obtaining a low-frequency output signal, which has essentially the fundamental of the L - R message content, from the raised Carrier IF signal and the additionally amplitude-modulated IF signal; f) phase shifter (60, 68), which the fundamental L + R- and -L - put R signals in phase; g) means (64, 66) for combining the L + R and L-R signals and for deriving L and R stereo signals therefrom; and h) corresponding acoustic output devices (68, 70) for the L and R signals. 609846/0 7 96 - 30 - Blank page