JPH0441539B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Industrial Application Field The present invention is a method for receiving AM stereo broadcasting using ISB (Independent Side Band) system.
The present invention relates to an AM stereo receiver, and particularly to a difference signal demodulation circuit for an AM stereo receiver that can obtain a distortion-free stereo difference signal.

(b) Prior art A number of AM stereo broadcasting systems have been proposed in the past, one of which is the left and right stereo signal L.
and R are placed on the upper and lower bands of the carrier wave, respectively.
ISB format AM stereo broadcasting exists. Said
ISB system AM stereo broadcasting was published in Japanese Patent Publication No. 55-695.
As shown in the figure, the carrier wave is amplitude-modulated by a sum signal (L+R) whose phase is shifted by +45 degrees, and phase-modulated by a difference signal (L-R) whose phase is shifted by -45 degrees. The left stereo signal L is modulated into the lower band, and the right stereo signal R is modulated into the upper band. However, if the carrier wave is simply modulated, the phase modulation vector will not necessarily be at 90 degrees with respect to the carrier wave, and an error will occur.
In No.-695, the distortion is corrected by an operation called inverse modulation.

FIG. 1 shows a transmitter that transmits such AM stereo broadcast waves, and 1 is a first input terminal to which a left stereo signal L is applied, and 2 is a second input terminal to which a right stereo signal R is applied. Input terminal 3 is a matrix circuit that matrixes the left and right stereo signals L and R to generate a sum signal (L+R) and a difference signal (L-R); 4 is a difference obtained at the first output terminal of the matrix circuit 3; A first phase shift circuit that shifts the phase of the signal (L-R) by +45 degrees, 5 a second phase shift circuit that shifts the phase of the sum signal (L+R) obtained at the second output terminal of the matrix circuit 3 by -45 degrees, 6 is an inverse modulation circuit that performs inverse modulation using the output signal of the first phase shift circuit 4 and the output signal of the second phase shift circuit 5; 7 is the output signal of the inverse modulation circuit 6, which is the output signal of the carrier wave generation circuit 8; 9 is a limiter; 10 is the second phase modulation circuit;
An amplitude modulation circuit amplitude modulates the output signal of the limiter 9 with the output signal of the phase shift circuit 5, and an antenna 11 radiates the output signal of the amplitude modulation circuit 10 into the air as a radio wave.

Generally, when a carrier wave is phase-modulated and then amplitude-modulated, the modulated signal S is S=(1-m _p ² /16)+m _a /2cosω _a t -jm _p /2sinω _a t+m _p ² /16cos2ω _a t -jm _a m _p /8sin2ω _a t ...(1) (where m _a is the amplitude modulation index, m _p is the phase modulation index, and ω _a is the angular frequency). As is clear from equation (1), independent sidebands of the left and right stereo signals L and R can be obtained by simple modulation, but the amplitude modulation component and phase modulation component of the second harmonic remain, resulting in distortion. cause the occurrence.

On the other hand, as shown in FIG. 1, if the inverse modulation circuit 6 is used to inversely modulate the difference signal (L-R) with the sum signal (L+R) using a predetermined modulation function, the second harmonic component is also unified. It can be a sideband wave. That is,
If the modulation function is 1/1+mtm _a /2cosω _a t (where m _t is the inverse modulation coefficient) and the phase component of equation (1) is inversely modulated, then Q=-jm _p /2sinω _a t-jm _p / 2sinω _a t (m _a /2−m _t m _a /2) cosω _a t ...(2), and in equation (2), m _a = m _p = m, m _t
= 1/2, then Q=-jm/2sinω _at -jm ² /16sin2ω _at ...(3). As can be seen by replacing the imaginary part of equation (1) with equation (3), by performing inverse modulation, the second harmonic component also becomes a single sideband, resulting in a signal waveform suitable for transmission.

As mentioned above, the AM stereo radio waves radiated from the antenna 11 have a single sideband configuration up to the second harmonic, but since the second harmonic component is distortion, the receiver The second harmonic component is removed and a distortion-free stereo difference signal (L-
R) must be generated. For this purpose, in JP-A-55-695, as shown in Figure 2, an inverse modulation circuit 12 is provided in the stereo difference signal path of the receiver, and inverse modulation is performed again to remove distortion components. There is. In FIG. 2, the AM stereo signal received and converted into an intermediate frequency signal is applied to the envelope detection circuit 13, and the AM component, that is, the stereo sum signal (L+R) is output to the output terminal of the envelope detection circuit 13. Occur. This stereo sum signal (L
+R) is phase-shifted by +45 degrees in the first phase shift circuit 14,
The signal is applied to the matrix circuit 15, and is also applied to the inverse modulation circuit 12 for performing inverse modulation. On the other hand, the phase component in the AM stereo signal is inversely modulated by an inverse modulation circuit 12, and then is modulated by a synchronous detection circuit 16.
is applied to The synchronous detection circuit 16 generates a stereo difference signal (LR) using a synchronous signal created using a PLL (phase locked loop) circuit 17 that responds to an intermediate frequency signal and a carrier wave 90 degree phase shift circuit 18. However, the synchronous detection circuit 1
Stereo difference signal (L-R) obtained at the output end of 6
is further phase-shifted by -45 degrees in the second phase shift circuit 19 and then applied to the matrix circuit 15. The matrix circuit 15 matrixes the stereo sum signal (L+R) applied from the first phase shift circuit 14 and the stereo difference signal (L-R) applied from the second phase shift circuit 19, and Left stereo signal L
A right stereo signal R is generated at the second output terminal 21.

As mentioned above, by introducing the concept of inverse modulation to the transmission and reception of AM stereo broadcasting using the ISB system, it is possible to demodulate the stereo difference signal without distortion. However, using the technology of JP-A No. 55-695,
The disadvantage is that the circuit configuration is complicated, and if distortion correction is to be performed strictly, the inverse modulation function must be set more precisely.

(c) Purpose of the Invention The present invention has been made in view of the above points, and provides a difference signal demodulation circuit for an AM stereo receiver that can generate a distortion-free stereo difference signal simply by performing multiplication and addition. We aim to provide the following.

(d) Configuration of the Invention The difference signal demodulation circuit of the AM stereo receiver according to the present invention includes a first detection circuit that detects the level of an intermediate frequency signal of a received signal, and a second detection circuit that detects the phase of the intermediate frequency signal. a multiplication circuit that multiplies the output signals of the first and second detection circuits, and an addition circuit that adds the output signals of the multiplication circuit and the output signal of the second detection circuit.

(e) Embodiment FIG. 3 is a circuit block diagram showing an embodiment of the difference signal demodulation circuit of the AM stereo receiver according to the present invention, and 22 is an input terminal to which an intermediate frequency signal corresponding to the received signal is applied. , 23 is a first detection circuit that detects the envelope of the intermediate frequency signal applied to the input terminal 22, 24 is a second detection circuit that detects the phase of the intermediate frequency signal, and 25 is the output of the first detection circuit 23. 26 is a multiplication circuit that multiplies the output signal of the first amplification circuit 25 and the output signal of the second detection circuit 24; 27 is a multiplication circuit that amplifies the output signal of the multiplication circuit 26; 2 amplifier circuits, and 2
Reference numeral 8 denotes an adder circuit that adds the output signal of the second amplifier circuit 27 and the output signal of the second detection circuit 24.

The transmission signal generated from the transmitter in Figure 1 is F = (1 + L + R) cos (ωt + φ) ... (4) (However, φ = L - R / 1 + m _t (L + R), m _t = 1/2 ). In equation (4), (1+L+R) is an amplitude modulation component, and φ is a phase modulation component subjected to inverse modulation. Note that equations (1) to (3) and equation (4) are different in form, but for convenience of explanation, equations (1) to (3) are expressed as vectors, and equation (4) is expressed as a vector. This is because they are expressed in scalar expression, and the two are substantially the same. When the signal represented by the above equation (4) is received, converted into an intermediate frequency signal, and applied to the input terminal 22, the stereo sum signal (L+R) is output to the output terminal of the first detection circuit 23. Occur. Then, the stereo sum signal (L+R) is amplified by a times in the first amplifier circuit 25 to become a(L+R). On the other hand, the intermediate frequency signal applied to the input terminal 22 is phase-detected by the second detection circuit 24, and a signal b(L-R)/1+1/2(L+R) is generated at the output terminal. The multiplier circuit 26 multiplies the output signal of the first amplifier circuit 25 and the output signal of the second detection circuit 24, and outputs the signal at the output terminal.
Generate ab(L+R)(L-R)/1+1/2(L+R). The output signal of the multiplier circuit 26 is amplified by c times in a second amplifier circuit 27, and then amplified by a factor of c in an adder circuit 28.
is added to the output signal of As a result, adder circuit 2
The output terminal of 8 receives the signal b(L-R) {1+ac(L+R)}/1+1/2(L+
R) occurs. Then, if the amplification factors of the first and second amplifier circuits 25 and 27 are set so that ac=1/2, the output signal of the adder circuit 28 becomes b(LR),
A distortion-free stereo difference signal can be obtained at the output terminal 29.

By using the circuit shown in Figure 3, it is possible to obtain a stereo difference signal (LR) without distortion as described above, and it is possible to obtain a stereo difference signal (L-R) without distortion. The stereo difference signal can be demodulated using only a circuit that can be easily converted into a circuit.

Note that the stereo sum signal a (L+R) obtained at the output end of the first detection circuit 23 and the stereo difference signal b (L-R) obtained at the output end of the adder circuit 28 are as follows.
After level matching, the signals are applied to the matrix circuit 15 through first and second phase shift circuits 14 and 19 shown in FIG. 2, respectively, and are separated into left and right stereo signals L and R. Further, the first and second amplifier circuits 25 and 27 are arranged for level adjustment, and one of them can be omitted.

(f) Effects of the invention As stated above, according to the present invention, the ISB method can be improved.
It has the advantage of being able to demodulate the stereo difference signal of AM stereo broadcasting without distortion. Further, according to the present invention, a stereo difference signal can be demodulated with a simple circuit configuration, so it is particularly suitable for IC implementation.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a transmitter for AM stereo broadcasting using the ISB system, FIG. 2 is a circuit diagram showing a conventional example of the receiver, and FIG. 3 is a circuit diagram showing an embodiment of the present invention. be. Explanation of main drawing numbers, 23, 24...Detection circuit, 2
6...multiplication circuit, 28...addition circuit.

Claims (1)

[Claims] 1 AM receiving ISB format AM stereo broadcasting
A difference signal demodulation circuit of a stereo receiver includes a first detection circuit that detects the level of an intermediate frequency signal of a received signal, and a second detection circuit that detects the phase of the intermediate frequency signal.
The addition circuit comprises a detection circuit, a multiplication circuit that multiplies the output signals of the first and second detection circuits, and an addition circuit that adds the output signals of the multiplication circuit and the output signal of the second detection circuit. A difference signal demodulation circuit for an AM stereo receiver that obtains a distortion-free stereo difference signal at the output end of the AM stereo receiver.