JP3160031B2 - Audio signal modulation method - Google Patents

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 is to obtain a modulated wave which is not easily disturbed by external interference by combining a digital modulation method and an analog modulation method (amplitude modulation method) in a narrow band audio transmission line. is there.

[0003]

2. Description of the Related Art Conventionally, amplitude modulation is used in medium-wave broadcasting and short-wave broadcasting.

[0004] However, this amplitude modulation method cannot prevent interference caused by foreign waves.

[0005]

As described above, according to the conventional amplitude modulation method, when a plurality of broadcast waves interfere, it is impossible to separate the desired wave from the interference wave.

On the other hand, it is conceivable to transmit a medium-wave broadcast or a short-wave broadcast by a digital signal. However, in general, a voice of a telephone level requires a band of about 32 to 64 kHz. Could not apply digital technology.

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

[0008]

In order to achieve the above object, a voice signal modulation method according to the present invention comprises: PCM-encoding a low-frequency side signal in a program audio signal; , And the modulated carrier is amplitude-modulated by the high-band signal or the full-band signal in the program audio signal.

[0009]

In general, in medium-wave broadcasting and short-wave broadcasting, the clarity of conversational speech often becomes a problem. The present invention focuses on the fact that the energy component of the speech sound is concentrated on the low frequency side, and the low frequency component is MSK-modulated and transmitted, thereby making it possible to reduce the deterioration of the desired signal due to the interference wave.

[0010]

FIG. 1 is a block diagram showing an embodiment of a narrow band audio digital transmission system to which the present invention is applied. In this embodiment, the present invention is applied to a medium frequency radio broadcasting system. Things.

In the figure, the configuration on the transmitting side is composed of an audio distributor 1 for distributing a program audio signal into two parts, a low-pass filter 2 for dividing the audio into two sub-bands of a low-frequency signal and a high-frequency signal, and a high-pass filter. A filter 3 and an ADPCM (Adaptive) for converting a low-frequency signal into a digital signal.
Differential PCM (coder) encoder 4, MSK modulator 5 that modulates a carrier by MSK (Minimum Shift Keying) with a digital signal, carrier oscillator 6 that supplies a carrier to MSK modulator 5, and amplifies the level of a high-frequency signal by limiting. Amplifier 7 that modulates an MSK-modulated carrier with the high-frequency side signal output from the limiting amplifier 7, and supplies the entire band signal to the limiting amplifier 7 by bypassing the high-pass filter 3. And a through-sizing device 9.

On the other hand, on the receiving side, a high-frequency distributor 10 for splitting the received signal into two parts, a limiter 11 for removing an amplitude modulation component from the split received signal, and an MSK demodulator for MSK demodulating the output signal of the limiter 11 An ADPCM decoder 13 for obtaining a low-frequency side signal from the demodulated signal;
An AM demodulator 14 for demodulating the received signal distributed from the high-frequency distributor 10 to extract an audio signal by AM demodulation; a high-pass filter 15 for removing a low-frequency component from the demodulated audio signal to obtain a high-frequency signal; And a voice synthesizer 16 for synthesizing the high-frequency side signal and the low-frequency side signal to obtain an 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 this embodiment, the signal is divided into a low-frequency signal of 1.2 kHz or less and a high-frequency signal of higher frequency.

Next, the low-frequency side signal of 1.2 kHz or less extracted by the low-pass filter 2 is converted by the ADPCM encoder 4 into a 4-bit digital signal.
The sampling frequency used is 2.5 kHz. In the present embodiment, ADP is used as the PCM encoding method.
The reason why the CM encoder 4 is used is that the speech can be encoded at a smaller bit rate while the speech quality is maintained as much as possible, and a dedicated LSI can be used.

Using the 4-bit digital signal obtained by the ADPCM encoder 4, the harmonic signal generated by the carrier oscillator 6 is directly digitally modulated in the MSK modulator 5. With this MSK modulator 6, a frequency-modulated wave having a bandwidth of 15 kHz, a constant amplitude, and including 99.5% of the total energy in the main lobe is 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 adder 7 and input to the AM modulator 8. The AM modulator 8 amplitude-modulates the MSK-modulated carrier with the high-frequency side signal to obtain a carrier. At this time, the level of the high-frequency signal is limited by the limiting amplifier 7 so that the modulation factor does not become too high.

When a signal to be subjected to amplitude modulation is used as a full-band signal in order to achieve compatibility with the current amplitude modulation method, the high pass filter 3 is bypassed by the through converter 9.

On the receiving side, the received signal is
Is divided into two. One of the distribution signals is a limiter 1
The amplitude modulation component is removed by 1 and input to the MSK demodulator 12. The MSK demodulated digital signal is
The signal is returned to the audio signal by the M decoder 13 to obtain a low-frequency side signal of the program audio signal.

The other signal of the distribution signal is an AM demodulator 14
The high-pass signal from which the low-pass component has been removed by the high-pass filter 15 is converted to a speech signal by the speech synthesizer 1.
6 is input.

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

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

The low-frequency component having a large amount of information in the desired wave signal is digitally transmitted, so that the deterioration due to the transmission path is extremely small, and it is hard to be affected by the amplitude modulated foreign interference wave.

Further, by making the amplitude modulation signal a full-band signal, compatibility with the current amplitude modulation method can be achieved. In this case, the receiver of the conventional method can receive the signal as it is, but in the present method, the service area is slightly narrowed because the amplitude modulation degree is shallower than the conventional method.

Further, according to the present embodiment, the MSK modulator (demodulator) and the ADPCM modulator (decoder) are connected to a dedicated IC.
Since a circuit can be produced with a simple configuration, there is an effect that the economy is excellent.

[0026]

As described above, according to the present invention, a low-frequency component having a large amount of information in a program audio signal is M
Since the transmission is performed after the SK modulation, the deterioration of the program audio signal due to the interference wave can be reduced.

[Brief description of the drawings]

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

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 audio 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 through converter 10 high-frequency distributor 11 limiter 12 MSK demodulator 13 ADPCM decoder 14 AM demodulator 15 High-pass filter 16 Voice synthesizer

Continuation of the front page (58) Fields investigated (Int. Cl. ⁷ , DB name) H04B 14/00-14/06 H04L 27/10 H03C 1/00

Claims (1)

(57) [Claims] 1. A low-frequency side signal in a program audio signal is represented by P
After CM encoding, MSK is generated by this PCM encoded signal.
An audio signal modulation method, wherein modulation is performed, and the modulated carrier is amplitude-modulated by a high-frequency signal or a full-band signal in a program audio signal.