JPS6086923A - Audio muting circuit - Google Patents

Abstract

PURPOSE:To prevent transmission of noises in a reception start mode and to set freely the muting characteristics by muting a sound circuit for a period of time needed for the rise of a sound demodulating circuit and transmitting a sound signal after releasing said muting state. CONSTITUTION:When a carry input (a) is supplied for a prescribed period of time through a squelch circuit 11 of an audio muting circuit, a squelch output (b) is changed to level L from level H. While a muting signal (d) given from a muting signal generating circuit 15 is set at level H after it is kept at level L for a prescribed period of time and then changed to level L again when the input (a) does not exist any more. At the same time, a rise/fall detecting circuit 13 detects the rise and fall of the output (b). A timer 14 counts a prescribed period of time, and the circuit 15 is controlled by an output (c) of the CPU12. Then an analog switch part 16 is controlled by the signal (d) of the circuit 15. Thus a sound signal is transmitted just for a prescribed period of time. This prevents transmission of noises in a reception start mode.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a microprocessor program controlled speech and noise audio mute circuit for use in communication systems such as digital radio systems.

Conventional Structure and Problems Therein FIG. 1 is a block diagram showing an example of the structure of a mute circuit used in a digital wireless communication system or the like. In the figure, 1 is a squelch circuit that generates a system squelch signal by carrier input, etc., 2 is an audio demodulation circuit controlled by the output (mute signal) from squelch circuit 1, and 3 is a squelch circuit between squelch circuit 1 and audio demodulation circuit 2. inserted capacitor C
It is a saw delay circuit consisting of 7 resistors R, etc.

Next, the operation of the above configuration will be explained based on the time chart of FIG. ;ifi The usual audio demodulation circuit 2 has a carrier input (
From when the squelch signal (b in Fig. 2) starts to be transmitted due to a) in Fig. 2 until the audio demodulation signal (d in Fig. 2) is output, it takes 1'J of several tens of m5OC to 100 ms.
′, it takes time to go up. Therefore, it is necessary to apply a mute function to prevent noise output from the audio circuit during the time (T1) until the audio demodulation circuit 2 performs normal operation. As a muting method for this purpose, in the conventional example, a mute signal (FIG. 2C) is obtained from the output of the squelch circuit 1 of the receiving section via a delay circuit 3, and this is used as a control signal for muting.

However, in the conventional example shown in FIG. 1, the mute signal C is controlled by the squelch circuit 1 in terms of quantity, so there are problems as shown below.

(1) The delay effect of the mute signal C occurs not only at the rising edge of reception but also at the falling edge, generating noise during this period.

(2) The characteristics of the delay circuit components change due to temperature and other factors, resulting in a change in delay time.

In particular, in the case of digital radio, the rise time of audio demodulation is 100 ms due to the addition of decoding time of the encoded code.
In addition, the noise level when there is no signal is often on the same level as the audio signal level (weak electric field), and for this reason, the above-mentioned problem is often emphasized and becomes a problem.

Purpose of the Invention The present invention eliminates the above-mentioned conventional drawbacks, and provides a stable audio mute that can quickly respond to changes in radio equipment operation, does not generate noise even at the end of received signals, and can freely set mute characteristics. The purpose is to provide circuits.

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention performs the mute operation by program control of a microprocessor, maintains the mute operation for a predetermined period of time after the start of signal reception, and outputs audio as mute release after the timer expires. The squelch output rise/fall detection means and timer means are controlled so that the squelch output rise/fall detection means and timer means are returned to the mute state at the same time as the reception of the signal ends.

Description of examples An embodiment of the present invention will be described below with reference to the drawings.

Figure 3 is a diagram showing the basic configuration of the present invention, and Figure 4 is a diagram showing the basic configuration of the present invention.
FIG. 5 is a waveform diagram of each part of the circuit of FIG. 3. In FIG. 3, 11 is a squelch circuit, 12 is a microprocessor (CPU), and 13 is a rise/fall detector stage.

14 is a timer means; 16 is a mute signal generation circuit; 1
6 is an analog switch section. Also, in Figures 3 and 5, a is the carrier input, b is the squelch output, and C is the C
The PU output signal, d is a mute signal, and e is an audio signal.

Next, the operation will be explained. While the receiver is in operation, a signal is received and the carrier signal a is detected at time T as shown in FIG. From T
When input for 2 seconds, the squelch output changes from H (high) to L (o-). On the other hand, the mute signal d from the mute signal generating circuit 15 becomes H after maintaining the predetermined time (1), and changes to L again when the carrier input a ends.

On the other hand, since the analog switch section 16 is configured to be non-conductive when the mute signal d is L and conductive when it is H, it passes the audio signal from the output of the 11 output from time T1 to T2. .

Next, the operation of the CPU 12 will be explained using the flowchart shown in FIG. In step 31, the receiver is powered on and becomes operational. When carrier a is input to the squelch circuit 11 in this state, its output changes from H to L in step 32, and the rise/fall detection means 1
At step 3, the fall of the output is detected, and at step 33, the mute signal d is maintained at L as described later, and at the same time, at step 34, the timer means 13 is started. During this time, the output of the timer means 13 is kept at L. When the timer means 13 counts a predetermined time (1), the timer exceeds in step 35, and in step 36 the output (= c, p U output C) of the timer means 13 becomes H, and as a result, the mute signal also becomes H ( mute off). As described above, while the mute signal d is H, the analog switch section 16 is conductive, so as shown in step 37, the audio signal e is passed. Further, in step 38, when the squelch output changes from L to H again at time T2, the rising/falling detection means 13 detects the rising edge of the input and directly turns on the CPU output C without going through the timer means 14 (
c). In this step 39, Mu 1
- The signal generation circuit 15 outputs L again, and the audio signal e returns to the cutoff state.

Next, the operations of the mute signal generation circuit 16 and the analog switch section 16 will be explained in more detail using FIG. 6. In the same figure, 17 and 18 are mute signal generation circuits 16, respectively.
19 is an amplifier including an operational amplifier 19A, 20 is an input terminal for an audio signal (demodulated output), and 21 is an output terminal. In the configuration shown in the figure, when the squelch is output to the NOR circuit 17, the CPU output is high, and between times T0 and T1 when the squelch output is low and the CPU output is high, the mu I signal d is low.
become. When the timer exceeds at time T2, the timer output, and therefore the CPU output, changes to a large output and the NOR circuit 17
The output (=mute output d) becomes H, and an audio signal is output.

On the other hand, when the CPU output becomes high at time T2, the mute signal d returns to low again and cuts off the audio. Here, N
The output of the OR circuit 17 is directly input to the control terminal of the analog switch 16A, and is also input to the control terminal of the analog switch 16B in reverse phase by the inverter 18. Further, the analog switch 16A is configured to be non-conductive when the control terminal is at L, and conductive when the control terminal is at H. Therefore, when the output of the NOR circuit 17 is H, the analog switch 1eAi,j is on and the analog switch 16B is off, presenting a low impedance to the straddle signal.

On the other hand, when the output of the NOR circuit 17 is L, the analog switch 16A is turned off and the analog switch 16B is turned off.
By turning on and short-circuiting the audio signal with low impedance, the mute effect is further ensured. In addition, as analog switches 16A and 16B,
Even if the control characteristic is turned off at H and then turned on, the inverter 17 (terminal) is
The same characteristics can be easily obtained by changing to 6A.

Effects of the Invention The present invention has the above-described configuration, and the following effects can be obtained.

(1) By configuring the system so that the audio circuit is muted for the time required for the rise of one audio demodulation path, and then unmuted to allow the audio signal to pass, it is possible to prevent noise from being transmitted at the start of reception.

(2) Mute is turned on immediately upon completion of reception, so no noise is generated.

(3) Since the mute signal generation mechanism is digitalized, it is stable without changes such as variations in temperature characteristics.

(4) Since a microprocessor-based timer is used, there is a large degree of freedom in setting mute characteristics, making it particularly suitable for use in digital radio.

[Brief explanation of the drawing]

Figures 1 and 2 are block diagrams and time charts of a conventional audio mute circuit, Figure 3 is a basic configuration diagram of an audio mute circuit according to an embodiment of the present invention, and Figure 4 is a flowchart of the circuit. The time chart of the circuit, FIG. 6, is a block diagram showing the configuration of the main parts of the circuit. 11...Squelch circuit, 12...Microprocessor, 13...Rise/fall detection means, 14...Timer means, 16...・
Mute signal generation (circuit, 16...analog switch section. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1)
Figure 2 Figure

Claims (1)

[Claims] Literature 9 of the output of the squelch circuit is provided with fall detection means and a timer means, detects a change in the output of the squelch circuit at the start of reception, and transmits the change in output of the squelch circuit to an analog switch section via a mute signal generation circuit. A mute operation is performed to cut off the audio signal, and the timer means is started, and when a predetermined timer of one hour elapses, the mute operation is canceled and the audio signal is passed.
an audio mute circuit which detects a change in the output of the squelch circuit at the end of reception and controls the analog switch section to return to the LI and mute operation without using the timer means. .