JPH046293B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to an audio signal muting circuit in an audio digital communication device using a carrier wave.

(b) Prior art As an audio digital communication system, for example, there is a
There is a PCM subcarrier system, which is shown in Figure 1a,
As shown in b, audio input children 1, 2, 3, 4
The input audio signal is converted into a digital signal by an A/D converter 5, and then encoded by an encoder 6 comprising an error correction circuit, a scrambling circuit, etc. The coded signal is 4-phase DPSK
(4-phase Differential Phase Shift Keying) circuit 7
After being converted into a subcarrier signal by , it is added to the video signal input from the video signal input terminal 8 and converted to an FM signal by the frequency modulator 9 . This FM signal is transmitted as a radio wave by a 12 GHz band transmitter 10 from a parabolic antenna 11. Radio waves transmitted from the parabolic antenna 11 are received by a receiver via a broadcasting satellite 12.

On the receiving side, the signal is received by the receiving parabolic antenna 13, then supplied to the 12 GHz band receiver 14, and applied to the FM demodulator 15 as an intermediate frequency signal. The signal demodulated by the demodulator is separated into a video signal and a 4-phase DPSK subcarrier signal, which are output from output terminals 17 and 16, respectively. The subcarrier signal is further demodulated by a 4-phase DPSK demodulation circuit 18 and returned to a baseband digital signal, and then sent to a decoder 19 consisting of a descrambler circuit, an error correction circuit, etc.
The signal is returned to the original audio signal through the A conversion circuit 20,
The audio is output from audio output terminals 21, 22, 23, and 24.

Now, in such a digital voice communication system,
Due to the decrease in S/N (signal-to-noise ratio) in the subcarrier after FM demodulation due to the decrease in the received carrier signal level,
Digital data errors after 4-phase DPSK demodulation become a problem. Although the data errors can be corrected to some extent by the error correction circuit of the decoder 19 in FIG. Noise occurs. Since this noise reaches the maximum output level of the audio signal, it is extremely harmful to the sense of hearing, and as a countermeasure against this, the output audio signal is usually suppressed by a muting circuit. This will be briefly explained with reference to FIG. Digital data demodulated by the four-phase DPSK demodulation circuit 18 is input to a decoder indicated by a broken line 19. In the decoder 19,
First, a synchronization signal for each frame in the data is detected by the synchronization detection circuit 25, and after the data is descrambled by the descramble circuit 26, it is input to the error correction circuit 27 and the error detection circuit 28. A data error is detected by the error detection circuit 28, and a correction operation is performed by the error correction circuit 27 based on the detection signal.If the frequency of data errors is high, the error correction circuit 27
The audio signal output from the D/A converter 20 via the data extraction circuit 29 is sent to the switches 30, 31, 3 controlled by the error detection circuit 28.
2, 33, audio output terminals 21, 2
The output states at 2, 23, and 24 are set to a no-signal state. It is assumed that the error detection circuit 28 has both an instantaneous error detection function and an error frequency detection function within a certain period of time.

With this configuration, when the frequency of data errors increases, the output audio signal is blocked at the output terminal, making it possible to avoid loud noises that are harmful to the auditory senses. However, in a reception state where the error frequency further increases, for example, when the synchronization detection circuit 25 has difficulty even detecting the synchronization signal, the error detection circuit 25
A detection error occurs at 8, and therefore, an error also occurs in the operation of switches 30, 31, 32, and 33 that operate based on the error detection signal, so that a strong noise is output to the audio output terminals 21, 22, 23, and 24. There is a possibility that

(c) Purpose In order to solve this problem, the present invention proposes so-called audio output signal muting, which makes it possible to cut off the audio signal to the output terminal without malfunctioning even in a reception state where errors are extremely frequent. It provides a circuit.

(d) Configuration The present invention does not operate the muting circuit using the error detection signal of the error detection circuit 28 of the decoder 19 shown in FIG. It is configured to obtain an accurate muting operation signal by detecting the reception state.

(e) Embodiment An embodiment of the present invention will be described with reference to FIG.
A 4-phase DPSK signal input from the input terminal 16 is input to a 4-phase DPSK demodulation circuit indicated by a broken line 18. First, a 4-phase DPSK signal is input to a carrier regenerator circuit (Carrier Regenerator Circuit) 41, where the phase is set to a constant value +π/2 or -
Two types of carriers, π/2, are regenerated. The reproduced carrier is multiplied by the input 4-phase DPSK signal by multipliers 34 and 37, respectively. After a carrier component and a component twice the carrier frequency are removed from the multiplicable signals by low-pass filters 35 and 38, respectively, they are passed to a zero-cross discriminator 3.
6 and 39, it is converted into a binary signal. The original digital data of these two series of binary signals is reproduced by a data regenerator circuit (Code Regenerator Circuit) 40. At this time, the bit clock receives the output signal of the zero-cross discriminator 39 and uses a timing regeneration circuit (Retiming circuit).
(Cireuit) 42. The digital data is input to the decoder 19 in the same manner as described above, and then restored to an audio signal by the D/A conversion circuit 20, and output from the output terminals 21, 22, 23, 24 via the switches 30, 31, 32, 33. be done.
The switches 30, 31, 32, and 33 are audio signal muting switches, and output signals obtained by smoothing the output signal of an amplitude discriminator 43, which identifies the amplitude of the signal output from the low-pass filter 38, by an integrator 44. It is configured to be opened and closed by.

The mutating operation according to the present invention is further explained in a fourth manner.
To explain with a diagram, the multiplier 3 in FIG.
When the carrier of 7 and the 4-phase DPSK signal have a normal phase relationship, that is, π/4, the signal after passing through the low-pass filter 39 of the multiplied signal has the waveform shown in Figure 4a, and its amplitude is approximately constant (Vh ). That is, the 4-phase DPSK signal is S(t), and the signal and π/4
Let C(t) be a carrier signal with a phase relationship of
Assuming that these signals are expressed by equations (1) and (2), respectively, the result of their multiplication is expressed by equation (3). In addition, wc is the carrier frequency, l is 0,
It represents four phase states of 1, 2, and 3.

S(t)=Acos(ωct+lπ/2)...(1) C(t)=Bcos(ωct+π/4)...(2) S(t)・C(t)={Acos(ωet+lπ/2)
}{Bcos(ωct+π/4)} =1/2AB{cos(2ωct+2l−1/4π)+co
s2l+1/4π}...(3) When the double component of the carrier frequency in equation (3) is removed, the result is expressed by equation (4). That is, S(t)・C(t)=1/2ABcos2l+1/4π...(
4) In equation (4), A=B=1, l=0, 1, 2,
By substituting 3, it can be seen that the amplitude of the signal expressed by equation (4) becomes 1/√2. That is, the above-mentioned (Vh) becomes 1/√2.

On the other hand, if the phase of the carrier signal is set to π/2 with respect to the 4-phase DPSK signal, using the same calculation, (5)
The result is expressed by Eq.

S(t)・C(t)={Acos(ωct+lπ/2)
}{Bocs(ωct+x/2)} =1/2AB{cos(2ωct+l+1/2x)+co
sl-1/2x}...(5) In equation (5), after removing the double carrier component, set A=B=1 and substitute l=0, 1, 2, 3, then ( The amplitude of the signal expressed by equation 5) is 1. Low pass filter 38 in this phase state
The output signal of is shown in FIG. 4b. That is, the amplitude Vh' becomes 1.

Now, the state shown in FIG. 4b is caused by a phase shift of the regenerated carrier in the carrier regenerator 41 in FIG.
This is because the S/N of the phase DPSK signal decreases and the phase lock in the carrier regeneration circuit 41 is lost. 4
In a receiver using phase DPSK, errors caused by this condition cause strong noise.
Therefore, by detecting the change in the amplitude of the demodulated signal due to the phase shift, the switches 30, 31, 3
By opening 2 and 33, the above noise can be avoided. In actual operation, the signals shown in Figure 4b are not always obtained;
Since there are signals shown in FIG. 3 and signals having intermediate amplitudes, it is desirable that the threshold Vt of the amplitude discriminator 43 shown in FIG.

Vh<Vt<Vh' (6) In FIG. 3, the input signal to the amplitude discriminator 43 is the output of the low-pass filter 38, but it goes without saying that the output of the low-pass filter 35 may also be used. Further, although the present invention has been described using a four-phase phase modulation method, it is also applicable to a two-phase modulation method.

(f) Effects As described above, according to the present invention, when a phase lock is lost in the carrier regeneration circuit, the state of the phase lock is detected, and the audio signal is blocked by the muting circuit. In normal digital communication systems, the carrier regeneration circuit does not operate normally due to the deterioration of the S/N due to the decrease in the received signal level, and the abnormal operating state generates strong noise in the demodulated audio signal. According to the audio signal muting circuit of the present invention, it is possible to completely avoid loud noises that are harmful to the auditory senses even if the reception condition becomes extremely poor.

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram for explaining an audio digital communication system, FIG. 2 is a conventional example of an audio signal muting method, and FIG. 3 is a block circuit diagram showing an audio signal muting circuit according to the present invention. FIG. 4 is an explanatory diagram of the operation of the present invention. 16...Input terminal, 34, 37...Multiplication circuit,
35, 38...LPF, 36, 39...Zero cross identification circuit, 40...Data reproducing circuit, 41...
Carrier regeneration circuit, 42...timing recovery circuit, 43...amplitude discrimination circuit, 44...integration circuit.

Claims (1)

[Claims] 1. A carrier reproducing circuit for reproducing a carrier from a modulated wave signal modulated by a digital signal, a multiplier circuit for multiplying the carrier regenerated by the circuit and the modulated wave signal, and an output of the multiplier circuit. A connected low-pass filter, a zero-cross identification circuit for converting the output signal of the low-pass filter into a binary signal, a decoding circuit for receiving the output of the identification circuit and decoding digital data, and the decoding circuit. a digital-to-analog conversion circuit connected to the output; a switch circuit connected between the output terminal and the output terminal of the digital-to-analog conversion circuit; and an amplitude identification circuit for identifying the amplitude of the output signal of the low-pass filter. , and an integrating circuit connected to the identification circuit, and controlling opening/closing of the switch circuit based on the output of the integrating circuit.