JPH0424898B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates generally to transmitters for AM stereo radio broadcast systems, and more particularly to techniques for reducing distortion by modifying the stereo signal encoding multiplex modulation section of such transmitters. It depends.

In the AM stereo radio broadcast system,
The carrier wave is amplitude modulated with a signal component representing the sum (L _T +R _T ) of the left (L) and right (R) input stereo audio signals. Using phase or frequency modulation techniques,
The second signal representing the difference between the L signal and the R signal (L _T −R _T )
The same carrier wave is multiplex modulated with the signal components of . The (L _T +R _T ) component is equivalent to monaural sound information, while the (L _T -R _T ) component carries stereo sound information. Both of these components, when received at an AM stereo receiver, are combined to produce two output audio signals L _R and R _R representing the original L and R stereo input signals provided to the transmitter.

To ensure accurate stereo reproduction in an AM stereo receiver, it is obvious that L _T = L _R and
It is desirable that R _T =R _R. However, distortion occurs due to various causes related to the transmitter (including the antenna system), during propagation from the transmitter to the receiver, and the receiver itself. It is therefore desirable to reduce such distortion to improve stereo reproduction accuracy at the receiver.

Although known techniques can reduce the distortion introduced into AM stereo transmitters to levels known to be acceptable in practical listening tests,
It is desired to further reduce distortion in stereo transmission and reception.

For example, according to my U.S. Pat. No. 3,908,090, which is an improvement over the basic independent sideband (ISB) AM stereo system disclosed in my U.S. Pat. No. 3,218,393, the basic system Some distortions that existed in the .

The present invention relates to one or both of two techniques applied to the multiplex modulation (ie, L-R) channels of an AM stereo transmitter to reduce distortion to very low levels. According to one feature of the invention, distortion is reduced by anticipating the distortion and creating a distortion cancellation component that is subtractively combined with the fundamental stereo difference signal of the multiplex modulation channel. According to a second feature of the invention, distortion in all multiplex modulation channels is reduced using a novel feedback structure that provides distortion prediction. Although each technique alone is effective in reducing distortion, they are especially effective when used together.

It is therefore an object of the present invention to provide an improved AM stereo transmitter that provides less distortion in the multiplex modulation channel than known transmitters.

Another object of the invention is to provide an apparatus for reducing distortion in the multiplex modulation channels of an AM stereo transmitter to very small levels.

It is a further object of the present invention to provide two separate predictive techniques for reducing distortion in multiplexed modulation channels of AM stereo transmitters which, when used together, provide substantial distortion reduction and practical benefits. The purpose is to provide two techniques that bring the device into play.

For a better understanding of the invention as well as further objects thereof, the invention will be described in detail below with reference to the accompanying drawings. The scope of the invention is defined in the claims.

1 and 2 are block diagrams showing a transmitter 10 and a receiver 30, respectively, used in an AM stereo radio broadcast system according to the inventor's known US patent.

In the transmitter 10 of FIG. 1, separate left (L _T ) and right (R _T ) stereo audio signals are sent to a sum circuit 12 and a difference circuit 14, which circuits
Signals are generated representing the sum (L _T +R _{T )} and difference (L _T -R _T ) of the L _T and RT stereo audio signals, respectively. These difference and sum signals are provided to respective phase shift circuits 16 and 24 where both signals are +45° and -45°
are phase-shifted separately and relatively to each other. Thereby, the first and second modulated signals applied to connection lines 17 and 25, respectively, are in quadrature with respect to each other. The first modulated signal (L _T −R _T ) is connected to connection line 1
7 to a phase modulator 20 which modulates the carrier signal output from oscillator 18.
Typically, phase modulation of the carrier signal is performed at some selected relatively low first carrier frequency, and the phase modulated carrier is frequency translated and amplified in circuitry 22, which is well known to those skilled in the art.

The second modulating signal (L _T +R _T ) is on line 2
5 to an amplitude modulator 26 which amplitude modulates the phase modulated carrier signal and sends its output composite signal to a transmit antenna 28. This output composite signal has a phase modulation component due to the first modulating signal (representing the stereo difference signal) and an amplitude modulating component due to the second modulating signal (representing the stereo sum signal). Additional amplifiers may be provided between amplitude modulator 26 and antenna 28, as will be apparent to those skilled in the art. The signal transmitted from antenna 28 is a composite independent sideband (ISB) AM stereo signal of the type disclosed in my US Pat. No. 3,218,393.

The composite signal broadcast from the transmitter 10 in Fig. 1 can be received by a general AM monaural radio receiver.
The receiver detects the signal envelope including the upper and lower sidebands and produces an output audio signal representing the stereo sum signal (L+R). 1 pair of general AM
By tuning the receiver to frequencies slightly higher and lower than the carrier signal, primarily left and right stereo signals are received, respectively, thereby providing a simple form of stereo reception. However, the preferred form of AM stereo receiver 30 shown in FIG.
separately demodulates the amplitude and phase modulation components of the transmitted composite signal, and derives left and right stereo signals using these demodulated signals.

Receiver 30 includes an antenna 32 for receiving the transmitted composite signal and an RF/IF circuit 34 of conventional design which converts the received composite signal to a suitable intermediate frequency. The intermediate frequency composite signal is given to the envelope detector 36, whose output is the signal (L _R
+R _R ), which is the second waveform applied to the amplitude modulator 26 via the transmitter connection line 25 in FIG.
represents the modulated signal (L _T +R _T ).
The output signal from detector 36 is sent to phase shift network 38, which effectively compensates for the original phase shift provided by network 24 of FIG. The resulting phase shifted stereo sum signal is then sent to sum and difference circuits 54 and 56.

In the known AM stereo receiver of FIG. 2, the received intermediate frequency composite signal is
A carrier tracking circuit 44 of the type disclosed in US Pat. The intermediate frequency composite signal is also provided to an inverse modulator 42 for distortion reduction according to the inventor's US Pat. No. 4,018,994. The intermediate frequency composite signal is inversely amplitude modulated with the output signal from envelope detector 36 to form an intermediate signal, which is in turn amplitude modulated by the recovered carrier signal from carrier tracking circuit 44 (90° in phase shift circuit 48). (transferred) to the product demodulator 46. product demodulator 46
is responsive to said intermediate signal and said regenerated phase-shifted carrier signal to demodulate the quadrature component of the intermediate signal and to phase modulator 20 via transmitter interconnection line 17 of FIG. An output signal ₍ L _{R −}
R _R ) is applied to the connection line 47. This stereo difference signal is phase shifted in phase network 50 and sent to the other input of sum and difference circuits 54 and 56;
Thus, L _R and R _R output signals are generated which represent the original L _T and R _T input stereo audio signals applied to the transmitter of FIG.

The simplified transmitter of FIG. 1 does not include a mechanism to compensate for undesired second-order components caused by amplitude modulating the phase modulated signal with amplitude modulator 26. This results in systematic error components in the (L-R) channels of a stereo receiver as shown in FIG. According to the inventor's US Pat. No. 3,908,090, a circuit is provided in the transmitter to reduce this undesired secondary component. Furthermore, an inverse modulation circuit 42 is provided in the receiver of FIG. 2 primarily to compensate for certain distortion components caused by the signal weight nature of the transmitter (phase modulation followed by amplitude modulation). .

Even with such known correction circuitry in the transmitter and receiver, there is still a systematic error component in the demodulated output signal of the stereo difference signal channel of the receiver 30 of FIG. In addition to the error component caused by amplitude modulating the phase modulated signal at the transmitter, an additional system is generated by detecting the quadrature phase of the phase modulated component of the received signal in the L-R channel of the receiver. An error component occurs. Product demodulator 46 responds to the quadrature reference carrier recovered by carrier tracking circuit 44 and phase shifted by circuit 48 to generate a portion of the intermediate signal that is in phase with the recovered quadrature carrier (inverse modulator 42 ) is detected. Product demodulator 46 therefore acts as a quadrature synchronous detector to detect the quadrature components of the intermediate signal. It is known that the quadrature component of a phase modulated signal represents the sine value of the phase modulation angle rather than the phase modulation angle itself. Therefore, systematic errors occur when a quadrature detector is used to detect the phase modulation component of a composite signal received by an AM stereo receiver of the type shown in FIG.

However, according to the present invention, systematic errors inherent in detecting the quadrature of the phase modulated component in an AM stereo receiver as well as amplitude modulation of the phase modulated signal in the transmitter are eliminated. Modifications can be made to the AM stereo transmitter to compensate for the phase modulated signal to correct for the systematic errors that occur. In accordance with one feature of the present invention, such compensation is provided by a novel distortion predictive feedback technique as demonstrated by the AM stereo transmitter of FIG.

The transmitter 61 shown in FIG. 3 includes sum and difference circuits 12 and 14 and phase shift networks 16 and 24, which circuits are similar to those provided in the known transmitter 10 shown in FIG. It is the same thing. In the transmitter 61 of FIG. 3, the phase-shifted stereo difference signal (L _T -R _T ) from the phase shift network 16;
A modulating signal in combination with the negative feedback signal is provided to a phase modulator 20. The negative feedback signal is combined with the phase-shifted stereo difference signal in summation circuit 72.
The output of phase modulator 20, similar to the output of phase modulator 20 of transmitter 10 of FIG. 1, is sent via frequency converter and amplifier 22 to one input of amplitude modulator 26. Therefore, the main difference in the transmitter 61 of FIG. 3 is that it is connected via a connection line 70 to a combining circuit 72 for combining with the phase-shifted stereo difference signal before phase modulating the carrier wave with the phase-shifted stereo difference signal. The goal is to provide a feedback signal.

In the transmitter 61, carrier phase modulation is performed at a first selected low carrier frequency, which is the frequency of the oscillator 18. This phase modulated signal is converted to a broadcast carrier frequency in frequency converter circuit 22. The phase modulated signal from modulator 20 is also sent to amplitude modulator 58 which receives the phase shifted stereo sum signal (L _T +R _T ) from phase shift network 24. will also be sent. The phase modulated signal from phase modulator 20 is therefore amplitude modulated in modulator 58 to generate a low carrier frequency signal on connection line 60, which is phase and amplitude modulated. , which simulates the high carrier frequency complex signal transmitted by the antenna. line 60
is inverse amplitude modulated with the phase-shifted stereo sum signal on line 62 in an inverse modulator 64, which simulates the operation of the inverse modulator 42 of the known receiver 30 of FIG. . The output of inverse modulator 64 is provided to product demodulator 66, where oscillator 18
A feedback signal is generated on line 70 by demodulating the quadrature phase using a 90° phase shifted version of the carrier signal from the carrier signal as a reference signal. This feedback signal represents the stereo difference signal produced in the product demodulator of a receiver of the type shown in FIG. 2 in response to the composite signal transmitted at antenna 28. Elements 64, 66, and 68 thus constitute a "mock receiver" 62 that anticipates the results that would occur when an actual receiver, such as that of FIG. 2, would receive and demodulate a broadcast ISB AM stereo signal. It is something to do. Similarly, amplitude modulator 58 simulates the result produced by the last amplitude modulator 26 of FIG.

The output signal of connection line 70 in FIG. 3 is applied as a negative feedback signal to synthesis circuit 72,
It is combined with the stereo difference signal sent from phase shift network 16. This negative feedback signal represents the combination of the systematic error components caused by the operation of the modulation and demodulation elements of the entire system and the phase modulated stereo difference signal. By using this signal as a negative feedback, the systematic error component of the final composite signal transmitted by antenna 28 of FIG. 3 is reduced.

The improvement provided by incorporating a negative feedback structure for distortion estimation into the transmitter of FIG. 3 reduces the systematic error component present at the output of an ISB AM stereo receiver of the type shown in FIG. be able to. L-R of the transmitter in Figure 3
By using a simulated transmitter and receiver in the channel, the feedback circuit can generate a signal that accurately predicts the error components that can occur in the L-R channels of a typical ISB stereo receiver. , the phase modulated signal can be compensated to reduce such systematic errors.

With the strain prediction feedback technology shown in Figure 3,
Although it is possible to reduce distortion in AM stereo systems, it is particularly effective when other distortion reduction techniques are used in conjunction with this feedback technique. In particular, distortion reduction can also be achieved by using subtractive distortion reduction techniques before the phase modulator 20 of FIG. A suitable subtractive distortion reduction circuit based on the distortion prediction technique is shown in FIG. 5, which may be used in conjunction with the feedback technique as shown in FIG.

The subtractive distortion reduction technique shown in Figures 4 and 5 is similar to the type used with the transmitter of Figure 1.
It is based on generating a signal that predicts the distortion components occurring in the L-R output of an AM stereo receiver.
A typical known receiver configuration is shown in FIG.

As shown in FIG. 5, the distortion reduction circuit 100 includes a simulated transmitter 103 and a simulated receiver 107, as well as a delay network 116 and a synthesis circuit 112. As shown, the input signal to the distortion reduction circuit 100 comes from either the phase shifting network 16 of FIG. 1 or the combining circuit 72 of FIG. 4. A signal applied to the input of circuit 100 phase modulates a carrier wave from reference oscillator 114. The resulting signal is amplitude modulated with the (L+R) signal and then demodulated by the simulated receiver 107. At the same time, the above input signal is bypassed through the simulated transmitter 103 and the simulated receiver 107, and the delay network 11
6 is subjected to delay compensation and then provided to the combining circuit 112. In an ideal case, the simulated receiver 10
The output signal from 7 is the same as the input signal provided to circuit 100 and therefore the same signal provided to synthesis circuit 112. For example, if the signal sent from delay network 116 to combining circuit 112 is
(LR) and if the signal from the simulated receiver 107 is equal to (LR), then the combining circuit 11
When the latter signal is subtracted from the former signal in 2, the output signal is simply (LR).
However, from the simulated receiver 107 to the synthesis circuit 112
The signal given to the receiver will contain distortion components to the extent that distortion is introduced by the transmission and reception of the signal (simulated by the simulated transmitter 103 and receiver 107). By subtractively combining the two signals in the combining circuit 112, these distortion components can be introduced into the composite signal, and thus these distortion components will be removed later when this composite signal is used at the actual transmitter. It is also possible to cancel the distortion components that occur when the signal is processed, transmitted, and received and processed by the actual receiver.
Therefore, the actual receiver output will have reduced distortion components that would have occurred if the transmitter signal had not been processed by the predictive distortion reduction circuit 100.

Referring to FIG. 5, the simulated transmitter 103 includes a phase modulator 102 followed by an amplitude modulator 10.
4 is connected, and the phase modulator 102 is connected to the reference oscillator 1.
14. It will be apparent that this combination operates similarly to the known transmitter units 18, 20 and 26 shown in FIG. Similarly,
The simulated receiver 107 of FIG. 5 comprises an inverse modulator 106 and a product demodulator 108, the inverse modulator 106 being obtained, for example, from the output of the phase shifting network 24 of the transmitter of FIG. 1 or 3. L _T +R _T ) signal. The product demodulator 108 is connected to the reference oscillator 114
90° phase shifted by phase shift network 110. It will be apparent that these units function similarly to units 42 and 46 of the known receiver shown in FIG.

From the above description of the structure shown in FIG.
Using the illustrated subtractive distortion reduction circuit without using feedback, the input stereo difference signal (L _T −R _T ) is processed by a simulated transmitter and receiver,
It is clear that distortion reduction can be achieved at the transmitter by generating a signal that simulates the stereo difference signal that would be generated at the output of the LR channels of an actual receiver. If the signal from the simulated receiver 107 contains a distortion component, the signal is subtractively synthesized with the delay-compensated original input signal (L _T −R _T ) in the synthesis circuit 112 to eliminate the negative signal. A composite (L _T -R _T ) signal can be generated that includes a distortion component of , and this negative distortion component can cancel the distortion component introduced by the actual transmission and reception. Thus, the (L _R −R _R ) signal produced at the stereo difference signal output of an actual AM stereo receiver, such as the type shown in FIG. This also results in small distortion.

As mentioned above, the subtractive distortion reduction technique shown in FIG. 5 and the feedback distortion reduction technique shown in FIG. 3 are particularly effective when used in combination as shown in FIG. Become something. Third
As shown, when using only feedback techniques, the amount of feedback required to obtain the desired degree of distortion reduction is such that under certain conditions the feedback loop becomes unstable. As shown in FIG. 4, by introducing a subtractive distortion reduction circuit 100 before the position where the signal is sent by the feedback loop, the distortion reduction function can be achieved by the subtractive technique and the feedback technique. will be shared. As a result, the amount of feedback required to obtain the desired degree of distortion reduction is less than when using feedback techniques alone, and therefore it is possible to use a moderate feedback level in the feedback loop. It is possible to achieve considerable distortion reduction while ensuring loop stability.

Although the present invention has been described with respect to an independent sideband format AM stereo system, those skilled in the art will appreciate that the distortion predictive feedback and subtractive distortion reduction techniques disclosed herein are applicable to transmitters for other formats of AM stereo radio broadcasting. It can also be applied to

Having described what is presently believed to be a preferred embodiment of this invention, it will be apparent to those skilled in the art that various changes and modifications may be made thereto without departing from the invention. and all modifications are intended to be included within the true spirit and scope of the invention.

[Brief explanation of drawings]

1 is a block diagram of a known transmitter for an AM stereo broadcast system of the type disclosed in my U.S. Pat. No. 3,218,393, and FIG. 2 is a block diagram of a known transmitter for an AM stereo broadcast system of the type disclosed in my U.S. Pat. No. 4,018,994. known AM of
FIG. 3 is a block diagram of a stereo receiver, FIG. 3 is a block diagram of a transmitter for an AM stereo system according to the invention, and FIGS. 4 and 5 are block diagrams of another transmitter according to the invention. 10... Transmitter, 12... Sum circuit, 14...
Difference circuit, 16, 24... Phase shifter, 18... Oscillator, 20... Phase modulator, 22... Frequency converter/amplifier, 26... Amplitude modulator, 28... Transmitting antenna, 30 ... Receiver, 32 ... Antenna, 34 ... RF/IF circuit, 36 ... Detection circuit,
38, 48, 50... Phase shifter, 42... Inverse modulator, 44... Carrier tracking circuit, 46...
Product demodulator, 54... Sum circuit, 56... Difference circuit, 58... Amplitude modulator, 61... Transmitter, 62
... Simulation receiver, 64 ... Inverse modulator, 68 ... Product demodulator, 72 ... Synthesis circuit, 100 ... Distortion reduction circuit, 103 ... Simulation transmitter, 107 ... Simulation receiver, 116 ... ...Delay circuit network, 112...Synthesis circuit.

Claims (1)

[Scope of Claims] 1. A device for generating a composite AM stereo radio broadcast signal consisting of a carrier wave amplitude-modulated by a stereo sum signal and angle-modulated by a stereo difference signal, comprising: transmitting and receiving the composite signal and demodulating its angle-modulated component. means for generating a signal for predicting distortion that may occur due to means for combining with a supplied stereo difference signal to generate a modified stereo difference signal and using the modified stereo difference signal to angularly modulate said carrier wave, thereby reducing said distortion. A device characterized by: 2. The apparatus according to claim 1, wherein the combining means combines the expected signal with the stereo difference signal based on a predetermined mathematical function. 3. A stereo difference signal is provided, the expected signal generating means is responsive to the stereo difference signal, and the modified stereo difference signal is used directly to angularly modulate the carrier wave.
Equipment described in Section. 4. A stereo sum signal is supplied, the expected signal generating means also responds to the stereo sum signal, and the transmission simulating means includes means for angularly modulating the supplied carrier wave with the stereo difference signal, and a means for angularly modulating the supplied carrier wave with the stereo difference signal; 4. Apparatus according to claim 3, comprising means for modulating with said stereo sum signal, thereby generating a simulated composite AM stereo radio broadcast signal. 5. Apparatus according to claim 4, further comprising means for simulating reception and demodulation of angle modulated components thereof in response to said simulated composite signal, thereby generating said expected signal. 6. The apparatus of claim 1, wherein said anticipatory signal generating means is responsive to said angle modulated carrier wave. 7. The expected signal generating means also responds to the supplied stereo sum signal, and the transmission simulating means includes means for amplitude modulating the angle-modulated carrier wave with the supplied stereo sum signal, thereby generating the simulated signal. 7. Apparatus as claimed in claim 6 for generating a composite AM stereo radio broadcast signal. 8. According to claim 7, the expected signal generating means further includes means for simulating reception and demodulation of the angle modulated component in response to the simulated composite signal, thereby generating the expected signal. The device described. 9. A second expected signal generating means and a second signal combining means are provided, the second expected signal generating means being the first expected signal generating means.
in response to the first modified stereo difference signal generated by the signal combining means of the actual composite AM
The second predicted signal generates a second predicted signal that predicts distortion that may occur due to the transmission and reception of a stereo radio broadcast signal and the demodulation of its angle modulation component, and the second signal synthesis means combining the expected signal from the first modified stereo difference signal with the first modified stereo difference signal from the first signal combining means to generate a second modified stereo difference signal;
2. The apparatus of claim 1, wherein this is used to angularly modulate the carrier wave to further reduce the distortion. 10. The device according to any one of claims 1 to 9, wherein the angle modulation is phase modulation. 11. An apparatus for generating a composite AM stereo radio broadcast signal, comprising: means for supplying a stereo sum signal and a stereo difference signal; means for supplying a first carrier; responsive to said stereo sum and difference signals and said first carrier; Simulation transmitting means for angle-modulating the first carrier wave with the stereo difference signal and amplitude-modulating the angle-modulated carrier wave with the stereo sum signal, thereby generating a simulated composite AM stereo radio broadcast signal; Means for generating a signal that simulates reception and demodulation of its angle modulation component and predicts distortion that may occur due to actual transmission and reception of the composite signal and demodulation of its angle modulation component, the stereo difference signal and the predicted signal means for angularly modulating the supplied second carrier wave with the modified signal; and means for angularly modulating the supplied second carrier wave with the corrected signal; An apparatus characterized in that it comprises means for amplitude modulating a stereo sum signal, thereby generating said actual composite AM stereo radio broadcast signal. 12. The apparatus of claim 11, wherein the angular modulation is a phase modulation. 13 In an apparatus for generating a composite AM stereo radio broadcast signal, means for supplying a stereo sum signal and a stereo difference signal, means for supplying a first carrier wave, combining the stereo difference signal with the supplied expected signal, and modifying the signal. means for angularly modulating the supplied second carrier wave with the modified signal; and means for amplitude modulating the angularly modulated signal with the stereo sum signal to produce an actual composite signal. means for generating an AM stereo radio broadcast signal; simulated transmitting means responsive to the angle modulated signal and the stereo sum signal for amplitude modulating the former with the latter, thereby generating a simulated composite AM stereo radio broadcast signal; and generating a signal that simulates the reception of the simulated signal and the demodulation of its angle modulation component, and predicts distortion that may occur due to the actual transmission and reception of the composite signal and the demodulation of its angle modulation component; and Apparatus characterized in that it comprises means for feeding back to the input of said synthesis means, thereby reducing said distortion. 14. The apparatus of claim 13, wherein the angular modulation is a phase modulation.