JPS6316928B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a muting circuit that removes glitches by changing the aperture ratio of an aperture signal according to the level of a muting control signal and changing the output energy of an output PAM signal. , no distortion and no deterioration of the SN ratio,
An object of the present invention is to provide a muting circuit that requires a simple circuit configuration.

Generally, in a digital signal transmission system such as a PCM signal, error detection and error correction are performed on the transmitted digital signal, but if an error occurs in this digital signal that exceeds the error correction ability, the transmitted digital signal The output of the reproduced signal obtained from the above is muted. This is to prevent noise such as abnormal sounds from occurring in the reproduced sound when the reproduced signal is an audio signal. If this muting of _the reproduced audio signal is performed suddenly as shown in FIG _.
As shown in FIG. 1B, the generation of click noise is prevented by a muting method, so-called soft muting, in which the level of the reproduced audio signal is gradually attenuated when disconnected, and the audio signal level is gradually raised when conductive. As a method of gradually attenuating and raising the reproduced audio signal, there is a conventional method of digitally calculating the transmitted digital signal, but this method has the disadvantage that it requires a digital calculation circuit, making the circuit complicated and expensive. . Also,
Conventionally, there is a method of gradually changing the gain of the reproduced audio signal obtained by D/A converting a digital signal, but this method has disadvantages such as increasing distortion and deteriorating the S/N ratio because it operates on an analog signal system. It was hot.

The present invention eliminates the above-mentioned drawbacks, and an embodiment thereof will be described with reference to FIG. 2 and the following figures.

First, before explaining the circuit of the present invention, a 1-bit transmitted digital signal as shown in FIG. 2A is converted into an analog signal as shown in FIG. 2B by a D/A converter. At this time, a glitch g occurs at each step of the analog signal, and a settling time is required, resulting in a conversion error.
In order to eliminate this conversion error, there are two methods: sample and hold the analog signal in FIG. 2B using a sampling pulse synchronized with the digital signal shown in FIG. 2C to obtain the output signal shown in FIG. 2D; Diagram E
There is a method of obtaining the output signal shown in FIG. 2F by extracting the analog signal shown in FIG. 2B only during the high level period of the aperture pulse synchronized with the digital signal shown in FIG.

Here, the high frequency components of the output signals of FIGS. 2D and F are attenuated due to the aperture effect, but the amount of attenuation is different for the output signals of FIGS. 2D and F. The smaller the aperture ratio t ₀ /T expressed by the period T of the aperture pulse and the aperture time t ₀ for gating shown in FIG. 2E, the smaller the above-mentioned attenuation amount becomes. For example, the aperture ratio t ₀ /T is 1/
4, the attenuation at the transmission upper limit frequency, that is, 1/2 the sampling frequency, is 0.22 dB, but the output signal in Figure 2 D has the maximum aperture ratio.
Equivalent to that gated by the aperture pulse in (1), the amount of attenuation at the transmission upper limit frequency, that is, 1/2 the sampling frequency, is quite large at 3.9 dB, which becomes a problem when the output signal is an audio signal. Therefore, a method using an aperture circuit that extracts an analog signal only during the high level period of the aperture pulse is advantageous in reducing the aperture effect, and the present invention is applied to a method using this aperture circuit.

FIG. 3 shows a block diagram of an embodiment of the muting circuit according to the present invention.

In the figure, reference numeral 1 denotes an input terminal to which a transmitted digital signal is input after error detection and error correction, and this digital signal is supplied to a D/A converter 2. The D/A converter 2 converts the digital signal into
A conversion is performed to obtain an analog signal a as shown in FIG. 4A, which is supplied to the aperture circuit 3. next,
As shown in FIG. 4B, an aperture pulse b whose high level period corresponds to the stable period of each step of the analog signal a is input from the input terminal 4 in synchronization with the digital signal, and is input to one input of the AND circuit 5. Supplied to the terminal. Further, a mute control signal c comes in from the input terminal 6 and is supplied to the pulse generation circuit 7. When correcting an error in the transmitted digital signal, the mute control signal is at a low level when error correction is possible, and is at a high level when error correction is not possible. ), as shown in FIG. 4C, which requires a predetermined time to transition to. However, in FIG. 4C, the transition time is shortened for convenience of explanation, and the rising and falling points of this mute control signal c are synchronized with the digital signal.

The pulse generating circuit 7 synchronizes its cycle with the aperture pulse b (rises a certain period of time before the rise of the aperture pulse b) by, for example, a built-in oscillator or a separately supplied high-frequency clock signal.
and generates a pulse d as shown in FIG. 4D whose pulse width corresponds to the level of the mute control signal c,
This is supplied to the other input terminal of the AND circuit 5. The AND circuit 5 calculates the logical product of the aperture pulse b and the pulse d to obtain a control pulse e whose aperture ratio corresponds to the level of the mute control signal c as shown in FIG. 4E, and supplies it to the control input terminal of the aperture circuit 3. . The aperture circuit 3 extracts the analog signal a only during the high level period of the control pulse e and performs PAM (pulse amplitude modulation) as shown in FIG. 4F.
A signal is obtained and output from the output terminal 8.

In this way, the aperture ratio is mutated by the control signal c
By controlling the level of the mute control signal c, the PAM signal is output without being muted during the high level period of the mute control signal c, and is completely muted and not output during the low level period of the mute control signal, and the transition of the mute control signal During the period, the duty ratio of the PAM signal f changes, and the resulting output energy of the PAM signal f gradually attenuates or rises, resulting in so-called soft mutating. Here, since the aperture ratio during muting is smaller than normal, the frequency characteristics of the output signal during muting will not deteriorate more than normal. Therefore, in order to perform muting, it is only necessary to add the pulse generation circuit 7 and the AND circuit 5, and the circuit configuration is very simple.Also, since the analog signal system is not manipulated, distortion increases and the SN ratio deteriorates. There's nothing to do.

Next, a circuit diagram of an embodiment of the pulse generating circuit 7 is shown in FIG. In the figure, 10 is a general-purpose timer IC
(semiconductor integrated circuit) and NE555 is used.
This timer IC 10 is supplied with power from a power supply terminal 11, is supplied with a timing pulse shown in FIG. 6A from an input terminal 12, and is further supplied with a mute control signal c shown in FIG. 6B from an input terminal 6. It constitutes a monostable multivibrator triggered by a timing pulse. At the falling edge of the timing pulse in Figure 6A, capacitor C ₁
starts charging with the time constant of capacitor C ₁ and resistor R ₁ , and is instantly discharged when the voltage across this capacitor C ₁ matches the level of the miute control signal c, and the voltage of capacitor C ₁ is as shown in Fig. 6. It changes as shown in C. The timer IC ₁₀ generates a pulse d as shown in FIG.
is generated and supplied from the output terminal 13 to the input terminal of the AND circuit 5 shown in the third figure. This figure 6 B, D
The mute control signals and pulses shown respectively are the same as those shown in FIGS. 4C and 4D. In this way, the pulse generator 7 can be realized with a simple circuit. Further, the aperture circuit 3 shown in FIG. 4 is composed of, for example, an analog switch.

In the above embodiment, the miute control signal c is changed linearly from a high level (low level) to a low level (high level), but this change characteristic may be exponential or trigonometric. This transition time may be arbitrarily set and is not limited to the above embodiment.

As described above, the muting circuit according to the present invention includes a D/A converter that converts an error-detected and error-corrected digital signal into an analog signal, and a D/A converter whose predetermined level corresponds to a stable period of each step of the analog signal. and an AND circuit in which an aperture pulse is supplied to one input terminal, and when correcting an error in the digital signal, the first level is used when error correction is possible, and the second level is used when error correction is not possible. and a mute control signal that requires a predetermined time to transition from the first level to the second level is supplied, and the period thereof is synchronized with the aperture pulse, and the pulse width is equal to that of the mute control signal. a pulse generating circuit that supplies a pulse corresponding to the amplitude level of to the other input terminal of the AND circuit; and a period during which the control pulse from the AND circuit is supplied to the control input terminal;
an aperture circuit that outputs the analog signal from the D/A converter, and mutating by changing the output energy of the PAM signal by changing the aperture ratio of the aperture pulse according to the level of the muting control signal. Because of this, distortion does not occur during muting and the SN
It has the advantage that the ratio does not deteriorate, and a simple circuit configuration is sufficient without requiring a complicated and expensive circuit.

[Brief explanation of the drawing]

1A and 1B are waveform diagrams for explaining the muting method, FIGS. 2A to 2F are waveform diagrams for explaining the method for removing glitches, and FIG. 3 is a block diagram of an embodiment of the circuit of the present invention. Systematic diagram, Figure 4 A-F
5 is a circuit diagram of a part of the third illustrated circuit, and FIGS. 6A to 6D are waveform diagrams of various parts of the fifth illustrated circuit. 1, 4, 6, 12...Input terminal, 2...D/A
Converter, 3...Aperture circuit, 5...AND circuit, 7...Pulse generation circuit, 8, 13...Output terminal.

Claims (1)

[Claims] 1. A D/A converter that converts an error-detected and error-corrected digital signal into an analog signal, and an aperture pulse whose predetermined level corresponds to a stable period of each step of the analog signal is supplied to one input terminal. an AND circuit; when correcting an error in the digital signal, the first level is used when error correction is possible; the second level is used when error correction is not possible; A mute control signal that requires a predetermined time to transition to a second level is supplied, and a pulse whose period is synchronized with the aperture pulse and whose pulse width corresponds to the amplitude level of the mute control signal is supplied to the AND circuit. a pulse generation circuit that supplies the other input terminal of the AND circuit; and a period during which the control pulse from the AND circuit is supplied to the control input terminal;
an aperture circuit that outputs the analog signal from the D/A converter, and mutating by changing the output energy of the PAM signal by changing the aperture ratio of the aperture pulse according to the level of the muting control signal. A mutating circuit characterized by performing the following.