JPH05122162A - Sound signal modulating system - Google Patents

Abstract

PURPOSE:To reduce the deterioration of a program sound signal due to interruption waves by digitally modulating a low band component having much information volume out of the program sound signal and transmitting the digitally modulated component. the direct digital modulation of CONSTITUTION:An MSK modulator 5 executes the direct digital modulation of a higher harmonic signal formed by a carrier oscillator 6 by using a 4-bit digital signal obtained by an ADPCM coder 4. A frequency-modulated wave having 15kHz band width and fixed amplitude and including 99.5% of the whole energy in a main lobe is obtained from the modulator 5. A high band side signal >=1.2kHz extracted by an HPF 3 is level-limited by a limiting amplifier 7 and inputted to an AM modulator 8. The modulator 8 executes the amplitude modulation of the MSK-modulated carrier by a higher frequency side signal to obtain a carrier. On the receiving side, a receiving signal is distributed into two signals by a high frequency distributer 10. An amplitude-modulated component is removed from one signal by a limiter 11 and the component-removed signal is inputted to an MSK demodulator 12. A digital signal is decoded to a sound signal by a PCM decoder 13 to obtain the low band side signal in the program sound signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an audio signal modulation system capable of effectively preventing external interference.

SUMMARY OF THE INVENTION The present invention provides a modulated wave that is less likely to be disturbed by external interference by combining a digital modulation system and an analog modulation system (amplitude modulation system) in a narrow band voice transmission line. is there.

[0003]

2. Description of the Related Art Conventionally, an amplitude modulation method has been used in medium-wave broadcasting and short-wave broadcasting.

However, this amplitude modulation method cannot prevent interference due to foreign waves.

[0005]

As described above, the conventional amplitude modulation system cannot separate the desired wave and the interference wave when a plurality of broadcast waves interfere with each other.

On the other hand, it is possible to transmit medium-wave broadcasting and short-wave broadcasting by digital signals. However, since generally telephone-level voice also requires a band of about 32 to 64 kHz, a narrow-band transmission line for medium-wave broadcasting or the like is required. The digital technology could not be applied to.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an audio signal modulation system capable of reducing deterioration of a desired wave signal (program audio signal) due to an interfering wave. ..

[0008]

In order to achieve the above object, a voice signal modulation system according to the present invention is such that a low frequency side signal in a program voice signal is PCM coded and then digitally modulated by this PCM coded signal. Is performed, and the modulated carrier wave is amplitude-modulated by the high-frequency side signal or the full-band signal in the program voice signal.

[0009]

In general, in medium-wave broadcasting and short-wave broadcasting, intelligibility of conversational voice is often a problem. The present invention focuses on the fact that energy components of conversational voice are concentrated on the low frequency side, and digitally modulates this low frequency component and transmits it, thereby making it possible to reduce deterioration of a desired wave signal due to an interfering wave.

[0010]

FIG. 1 is a block diagram showing the configuration of an embodiment of a narrow band audio digital transmission system to which the system of the present invention is applied. In the present embodiment, the system of the present invention is applied to a medium-wave radio broadcasting system. It is a thing.

In the figure, the configuration of the transmitting side is as follows: a voice distributor 1 for dividing a program voice signal into two, a low pass filter 2 for dividing a voice into two sub-bands of a low band side signal and a high band side signal, and a high pass band. The filter 3 and ADPCM (Adaptive for converting the low-frequency side signal to a digital signal)
Differential PCM (Encoder) encoder 4, MSK modulator 5 that modulates a carrier wave by a digital signal to MSK (Minimum shift keying), carrier oscillator 6 that supplies the carrier wave to MSK modulator 5, and limit amplification of the level of the high frequency side signal. Limiting amplifier 7, an AM modulator 8 that amplitude-modulates the MSK-modulated carrier wave by the high-frequency side signal that is the output of limiting amplifier 7, and high-pass filter 3 is bypassed to supply a full-band signal to limiting amplifier 7. It consists of a slewing device 9.

On the other hand, the configuration of the receiving side is a high frequency distributor 10 for dividing the received signal into two, a limiter 11 for removing an amplitude modulation component from the distributed received signal, and an MSK demodulation for MSK demodulating an output signal of the limiter 11. And an ADPCM decoder 13 for obtaining a low frequency side signal from this demodulated signal,
An AM demodulator 14 for AM demodulating the received signal distributed from the high frequency distributor 10 to extract a voice signal, a high pass filter 15 for removing a low frequency component from the demodulated voice signal to obtain a high frequency side signal, It comprises a voice synthesizer 16 for synthesizing the high frequency side signal and the low frequency side signal to obtain the original program voice signal.

Next, the operation of this embodiment will be described.

On the transmitting side, the program audio signal is divided into two by the audio distributor 1 and then divided into two sub-bands by the low pass filter 2 and the high pass filter 3. In the case of the present embodiment, it is divided into a low frequency side signal of 1.2 kHz or less and a high frequency side signal of more than that.

Next, the low frequency side signal of 1.2 kHz or less extracted by the low pass filter 2 is converted into a 4-bit digital signal by the ADPCM encoder 4.
The sampling frequency used is 2.5 kHz. Further, in the present embodiment, ADP is used as the PCM encoding method.
The CM encoder 4 is used because it is possible to encode voice at a smaller bit rate while maintaining the voice quality as much as possible, and to use a dedicated LSI.

The MSK modulator 5 directly digitally modulates the harmonic signal generated by the carrier oscillator 6 using the 4-bit digital signal obtained by the ADPCM encoder 4. With this MSK modulator 6, a frequency modulated wave having a bandwidth of 15 kHz, a constant amplitude, and a main lobe containing 99.5% of the total energy can be obtained.

On the other hand, the high-frequency side signal of 1.2 kHz or more extracted by the high-pass filter 3 is level-limited by the limiting / amplifying device 7 and input to the AM modulator 8. The AM modulator 8 amplitude-modulates the MSK-modulated carrier wave with the high frequency side signal to obtain a carrier wave. At this time, the level of the high frequency side signal is limited by the limiting amplifier 7 so that the modulation degree does not become too high.

In order to achieve compatibility with the current amplitude modulation method, when the signal to be amplitude-modulated is a full band signal, the high pass filter 3 is bypassed by the slewing device 9.

On the receiving side, the received signal is the high frequency distributor 10
Will be distributed in two. One of the distribution signals is the limiter 1
The amplitude modulation component is removed by 1 and is input to the MSK demodulator 12. The digital signal that is MSK demodulated is the PC
The M decoder 13 restores the audio signal to obtain the low frequency side signal of the program audio signal.

The other signal of the distribution signal is the AM demodulator 14
Is converted into a voice signal by the high pass filter 15 and the high frequency side signal after the low frequency side component is removed by the high pass filter 15
6 is input.

The speech synthesizer 16 adjusts the levels of the low-frequency side signal and the high-frequency side signal and then synthesizes them to obtain the original program speech signal.

As described above, according to this embodiment, it is possible to reduce the deterioration of the desired wave signal due to the interfering wave. In particular, when the program of the disturbing wave is conversational speech, the clarity of the desired wave signal is often significantly deteriorated. However, according to the present embodiment, the low-frequency component of the demodulated disturbing wave is high-pass component. It is cut by the filter 15. Since the energy of this interfering wave is also concentrated in the low range, the effect of reducing the interfering wave by this filter 15 is great.

Since the low-frequency component having a large amount of information in the desired wave signal is digitally transmitted, the deterioration due to the transmission line is extremely small, and the disturbance due to the amplitude-modulated foreign interference wave is not easily received.

Furthermore, by making the amplitude modulation signal a full band signal, it is possible to achieve compatibility with the current amplitude modulation method. In this case, although the receiver of the conventional system can receive the signal as it is, in this system, the amplitude modulation degree becomes shallower than that of the conventional system, and thus the service area becomes slightly narrow.

Further, according to this embodiment, an MSK modulator (demodulator) or an ADPCM modulator (decoder) is used as a dedicated IC.
Since it is possible to produce a circuit with a simple configuration, it is possible to achieve an effect of being excellent in economical efficiency.

[0026]

As described above, according to the present invention, the low-frequency component having a large amount of information in the program voice signal is configured to be transmitted after being digitally modulated. It is possible to reduce signal deterioration.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment to which an audio signal modulation method according to the present invention is applied.

[Explanation of symbols]

 1 Voice Distributor 2 Low Pass Filter 3 High Pass Filter 4 ADPCM Encoder 5 MSK Modulator 6 Carrier Oscillator 7 Limiting Amplifier 8 AM Modulator 9 Slewing Device 10 High Frequency Distributor 11 Limiter 12 MSK Demodulator 13 ADPCM Decoder 14 AM Demodulator 15 High-pass filter 16 Speech synthesizer

Claims (1)

[Claims] 1. A low-frequency side signal in a program voice signal is set to P
A voice signal modulation characterized in that after CM coding, digital modulation is carried out by this PCM coded signal, and the modulated carrier wave is amplitude-modulated by a high band side signal or a full band signal in a program voice signal. method.