JPH0416970B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention aims to reduce unnecessary spectrum.
This invention relates to a multiplex modulation circuit (hereinafter referred to as an MPX modulation circuit) for FM stereo broadcasting.

[Conventional technology]

Conventionally, matrix type and switching type modulation circuits have been used in MPX modulation circuits for FM broadcasting.

The FM stereo broadcast signal will be explained with reference to FIG. Sum signal (L+R) of left and right audio signals (L, R) 31
occupies 50Hz to 15kHz as the main signal, and the difference signal (L-R) occupies the band of 38kHz±15kHz, that is, 23kHz to 53kHz, as the sub signal 32 which is carrier suppression modulated with a 38kHz carrier wave. . The pilot signal 33 required to reproduce the carrier wave during demodulation has a frequency of 19kHz, which is half the frequency of the 38kHz carrier wave.
The phases are also synchronized.

The matrix-type MPX modulation circuit replaces the two stereo audio signals, the left audio signal and the right audio signal, with two signals, a sum signal and a difference signal, and performs carrier suppression amplitude modulation on the difference signal using a 38kHz carrier wave. The signal is then combined with the sum signal to obtain a composite signal. There is also a method of obtaining a composite signal by time-division synthesis, which is called a switching type MPX modulation circuit.

[Problem that the invention seeks to solve]

Conventional MPX modulation circuits use a square wave with a duty ratio of 50% as a carrier wave when performing carrier suppression amplitude modulation, so even the high frequency of the square wave is also subjected to carrier suppression amplitude modulation, resulting in a large amount of unnecessary spectrum. Therefore, it is necessary to remove it using a low-pass filter (hereinafter referred to as LPF).

For example, the effect of the third harmonic of the carrier wave is
In terms of level, 1/3 of the fundamental modulated wave is approximately -10 dB. The frequency is three times that of the carrier wave 38kHz.
114kHz, and audio modulation is applied to this, so 114k
Affects Hz±15kHz, that is, the range from 99kHz to 129kHz. In this way, the lower limit of the band occupied by the third harmonic of the carrier wave is only about 0.9 octave apart from the upper limit of 53 kHz of the band occupied by the fundamental wave, which has already been shown. For this reason, the unnecessary spectrum is −
To achieve a reduction of 440 dB (1/100) or less, it is necessary to use an LPF with a steep cut-off characteristic of -30 dB/oct or more.

Normally, as an LPF with such steep characteristics,
A 7th to 11th order Tievichiev type LPF is used, but the phase characteristics of the LPF are very poor, and a phase difference between the composite signal and the pilot signal is likely to occur. Furthermore, ripples are likely to appear within the pass band, and the side band plates around 50 kHz are also slightly removed, resulting in a level difference between the modulated waves of the sum signal and the difference signal. For this reason, the entire audio input band from 50Hz to 15kHz,
It is extremely difficult to ensure high separation, especially in high frequencies.

Additionally, the LPF itself is susceptible to temperature changes and moisture, and tends to change over time, resulting in poor reliability.

In addition, in the case of a matrix type MPX modulation circuit,
If carrier suppression amplitude modulation is performed with an appropriate amount of 38 kHz sine wave using an analog multiplier, harmonics will not be generated (this is called a non-saturated matrix type), but the signal path will be divided into a sum signal and a difference signal. Because of this difference, delay time differences and level differences are likely to occur. Further, level fluctuations in the carrier wave directly lead to level fluctuations in the modulated wave of the difference signal, and changes in the transfer function of the analog multiplier due to changes in temperature or power supply voltage also directly lead to level fluctuations in the modulated wave of the difference signal. Such a level fluctuation of the modulated wave of the difference signal results in a gain difference between the sum signal and the difference signal, which also deteriorates the degree of separation. For example, if there is a gain difference of only 0.1 dB between the sum signal and the difference signal, the degree of separation can only be ensured at about 40 dB.

The purpose of the present invention is to provide an LPF with steep characteristics,
The object of the present invention is to provide an MPX modulation circuit that achieves both high reliability and high performance without using an analog multiplier.

[Means for solving the problem] According to the present invention, a first input terminal into which a left audio signal is input, a first attenuator that divides the voltage of this left audio signal in multiple stages, and this first attenuator. a first multi-channel input analog switch that inputs the output of the switch, a second input terminal that inputs the variable attenuator and the right audio signal, and a second input terminal that inputs the right audio signal and divides the right audio signal into multiple stages. an attenuator, and a second multi-channel analog switch inputting the output of the second attenuator;
a synthesis circuit for synthesizing each output of the first and second multi-channel input analog switches;
MPX that supplies a timing signal whose transfer characteristics change in a quasi-sinusoidal manner with a period of 38kHz to the first and second multi-channel input analog switches.
Obtain a modulation circuit.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

First, the principle of the present invention will be explained using FIG. A left audio signal is received at an input terminal 1, applied to a multichannel analog switch 5 via a resistor attenuator 3, and further applied to one end of a resistor 9 via a buffer amplifier 7. Similarly, the right audio signal is applied to the input terminal 2 and supplied to the multi-channel analog switch 6 via the resistor attenuator 4,
Furthermore, it is applied to one end of a resistor 10 via a buffer amplifier 8. The other ends of the resistors 9 and 10 are connected in common, and the connection point is applied to the phase shifter 12 via the LPF 11, and the output thereof is applied to the output terminal 13. Furthermore, it has a timing decoder 14 that provides timing signals to the multi-channel analog switches 5 and 7.

Next, the operation will be explained.

Input the left audio signal to input terminal 1. The input audio signal enters a resistor attenuator 3, and the resistor attenuator 3 is combined with a multi-channel analog switch 5 to form an attenuator with variable attenuation. By setting the resistance value to an appropriate value and applying a signal from the timing decoder 14 so that the channels of the analog switch can be sequentially switched, a pseudo sine wave whose transfer characteristic can be approximated to the transfer function f _L (t) shown in equation (1) is generated. The modulated audio signal constitutes an analog multiplier with a transfer characteristic of
It is applied to a resistor 9 via a buffer 7 to obtain a left audio modulation signal.

f _L (t) = 1/2 (1 + coswt) (w = 2πf, f = 38kHz) ... (1) Similarly, the right audio signal applied to input terminal 2 is connected to resistor attenuator 4 and multi-channel analog switch 6. The timing decoder 14 constitutes an analog multiplier having a transfer function approximated by the transfer function f _L (t) shown in equation (2), and supplies it to the buffer 8 and resistor 10 to obtain a modulated signal for the right audio.

f _R (t)=1/2(1-coswt) (w=2πf, f=38kHz) ...(2) The left and right audio modulation signals synthesized in this way are connected in series with resistors 9 and 10. Added by the circuit. Thereafter, the LPF 11 removes slight harmonics of the pseudo sine wave, the phase shifter 12 corrects the phase rotation of the LPF, and outputs a composite signal to the output terminal 13.

By doing this, the harmonic components generated during modulation are equivalently multiplied by a pseudo sine wave, so they are greatly reduced, and the remaining harmonics are also of high order, so they can be easily removed.

With this method, the LPF can be simplified, the ON resistance of the analog switch can be ignored, and the accuracy is determined mostly by the relative value of the resistor attenuator, and the temperature coefficient of the resistance is also canceled out, resulting in excellent stability and high reproducibility.

As a result, the problems caused by the conventional switching type and non-saturated matrix type can be solved.

First, compared to the conventional switching type,
Since a simple LPF is sufficient, there is almost no phase rotation or attenuation of the modulation band of the difference signal due to the LPF.
Since it is easy to correct, the degree of separation is very good over the entire audio band from 50Hz to 15kHz, and it is almost unaffected by things such as temperature changes, humidity, vibrations, and changes over time, and adjustments after assembly are easy. It's easy.

Also, compared to the non-saturated matrix type, the modulated waves of the sum signal and the difference signal take the same path, so the gain difference and delay time difference between the modulated wave of the sum signal and the difference signal due to going through independent paths is Since no analog multiplier is used, the modulation level of the difference signal will not change due to changes in the carrier wave level, temperature, or power supply voltage, and the degree of separation will never deteriorate.

As described above, the present invention has both the advantages of a switching type that does not require a matrix circuit or an analog multiplier, and the advantages of a non-saturated matrix type that is not affected by the adverse effects of an LPF.

Next, an embodiment in which the present invention is applied to a stereo MPX modulation circuit for FM broadcasting will be described using FIG.
Note that parts corresponding to those in FIG. 1 are given the same reference numerals. The left channel input terminal 1 receives the left audio signal, and the right channel input terminal 2 receives the right audio signal. Resistance attenuators 3 and 4 are each 16
It has an output of Therefore, the multi-channel input analog multiplexers 5 and 6 each have 16 inputs. In timing decoder 14
608kHz (Q1), which is obtained by dividing the original oscillation signal of 1216kHz,
The polarity of each timing signal of 304kHz (Q2), 152kHz (Q3), and 76kHz (Q4) is inverted by the polarity of the timing signal of 38kHz (Q5).
6 as a control signal. This results in
Analog switches 5 and 6 are I ₁ , I ₂ , I ₃ , ..., I ₁₅ ,
The input contacts are switched in the order of I ₁₆ , I ₁₆ , I ₁₅ , . . . , I ₂ , I ₁ , I ₁ , I ₂ , . . . at a cycle of 38 kHz to receive audio signals. Each resistance Rn (n=1, 2,...,
N-1, N; N is the number of analog switch channels) is Rn=ΣR _N /2 [1-cos {180/N (n-1/2)}]-ΣR _o-1 (R ₀ = 0) )...(3) If the gains of the buffer amplifiers 7 and 8 are set to "1", the transfer characteristic from the input terminal 1 to the buffer amplifier 7 will be as shown in FIG. FIG. 4a shows the transfer characteristic, and FIG. 4b shows the 38kHz clock signal, and shows their mutual phase relationship. The transfer characteristics from the input terminal 2 to the output of the buffer amplifier 8 are of opposite phase to those shown in FIG.

In the case of the transfer characteristic shown in Figure 4, there are no harmonics below the 31st order, and the band is 1163kHz or higher, which is more than 4 octaves away from the upper limit of the fundamental modulation wave, 53kHz, and the harmonic level is also approximately -30dB, and even with a -6dB/oct CR 1-stage LPF, the harmonic level can be easily suppressed to -40dB or less.

In this way, the outputs of buffer amplifiers 7 and 8 are combined and output through resistors 9 and 10, respectively, and the LPF 11 removes harmonics of the 31st order and above.
Phase shifter 12.

Next, an embodiment in which the present invention is applied to a stereo MPX modulation circuit for FM broadcasting will be described using FIG.
Note that parts corresponding to those in FIG. 1 are given the same reference numerals. The left channel input terminal 1 receives the left audio signal, and the right channel input terminal 2 receives the right audio signal. Resistance attenuators 3 and 4 are each 16
It has an output of Therefore, the multi-channel input analog multiplexers 5 and 6 each have 16 inputs. In timing decoder 14
608kHz (Q1), which is obtained by dividing the original oscillation signal of 1216kHz,
The polarity of each timing signal of 304kHz (Q2), 152kHz (Q3), and 76kHz (Q4) is inverted by the polarity of the timing signal of 38kHz (Q5).
6 as a control signal. This results in
Analog switches 5 and 6 are I ₁ , I ₂ , I ₃ , ..., I ₁₅ ,
The input contacts are switched in the order of I ₁₆ , I ₁₆ , I ₁₅ , . . . , I ₂ , I ₁ , I ₁ , I ₂ , . . . at a cycle of 38 kHz to receive audio signals. Each resistance Rn (n=1, 2,...,
N-1, N; N is the number of analog switch channels) is Rn = ΣR _N /2 [1-cos {180/N (n-1/2)}]-ΣR _o-1 (R ₀ = 0) )……(3
) and the gains of the buffer amplifiers 7 and 8 are set to "1", the transfer characteristic from the input terminal 1 to the buffer amplifier 7 is as shown in FIG. FIG. 4a shows the transfer characteristic, and FIG. 4b shows the 38kHz clock signal, and shows their mutual phase relationship. The transfer characteristics from the input terminal 2 to the output of the buffer amplifier 8 are of opposite phase to those shown in FIG.

In the case of the transfer characteristic shown in Figure 4, there are no harmonics below the 31st order, and the band is 1163kHz or higher, which is more than 4 octaves away from the upper limit of the fundamental modulation wave, 53kHz, and the harmonic level is also approximately -30dB, and even with a -6dB/oct CR 1-stage LPF, the harmonic level can be easily suppressed to -40dB or less.

In this way, the outputs of buffer amplifiers 7 and 8 are combined and output through resistors 9 and 10, respectively, and the LPF 11 removes harmonics of the 31st order and above.
A phase shifter 12 performs phase correction, and the signal is output to an output terminal 13.

In the case of this embodiment, since the LPF 11 is constituted by one stage of filter consisting of a resistor and a capacitor, phase rotation can be completely corrected by the phase shifter 12. The pilot signal is input from the input terminal 15.

Although the method of generating the pilot signal is not mentioned in the above embodiment, if a method of generating the pilot signal based on a pseudo sine wave is used, it is possible to avoid the adverse effects of the LPF.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to provide an MPX stereo broadcast modulator for FM broadcast that has both high performance and high reliability.

[Brief explanation of drawings]

FIG. 1 is a circuit block diagram showing the principle of the present invention, FIG. 2 is a diagram showing the frequency spectrum of a stereo composite signal, FIG. 3 is a circuit diagram according to an embodiment of the present invention, and FIG. Figure 4 shows the pseudo-sinusoidal transfer characteristic of the left audio signal modulation section, in which Figure a shows the output waveform of the buffer amplifier, and Figure b shows the 38kHz clock signal. 1...Input terminal for left audio signal, 2...Input terminal for right audio signal, 3...Resistance attenuator for left audio signal, 4...Resistance attenuator for right audio signal, 5
...Multi-channel input analog switch for left audio signal, 6...Multi-channel input analog switch for right audio signal, 7...Buffer amplifier for left audio signal, 8...Buffer amplifier for right audio signal, 9
...Resistance, 10...Resistance, 11...Low pass filter, 12...Phase shifter, 13...
Output terminal, 14... Timing decoder, 15
... Pilot signal input terminal, 31 ... Spectrum of sum signal (L+R), 32 ... Difference signal (L
-R) carrier suppression amplitude modulated spectrum,
33...Pilot signal.

Claims (1)

[Claims] 1 a first input terminal into which a left audio signal is input; a first attenuator that divides the left audio signal from the first input terminal into multiple stages; and a first attenuator that selectively outputs the left audio signal from the first attenuator. a first multi-channel input analog switch that receives the output of the first multi-channel input analog switch; a first synthesis resistor that receives the output of the first multi-channel input analog switch; a second input terminal that receives the right audio signal; a second attenuator that divides the right audio signal from the input terminal into multiple stages; a second multi-channel analog switch that selectively receives the output from the second attenuator; and from the second multi-channel analog switch. The first and second multi-channel input analog switches are supplied with a timing signal whose transfer characteristic changes in a quasi-sinusoidal manner at a period of 38 kHz from a timing decoder. A stereo modulation circuit characterized by supplying.