JPH02288424A - Digital signal input switching device - Google Patents

Abstract

PURPOSE:To prevent a noise inputted to an amplifier circuit from being supplied to a D/A converter by applying the control signal of a selection circuit to select a digital signal transmitted in an optical signal on an inverter at the initial stage of the amplifier circuit. CONSTITUTION:The control signals of (H) and (L) levels are used in a control circuit 15 which switches the selection circuit 12 to select an electrical signal 1 and the digital signal transmitted in the optical signal 10. At a state selecting the digital signal transmitted in the optical signal by the selection circuit 12, output from the amplifier circuit 3 is fixed by applying the control signal to control the selection circuit 12 on the inverter at the initial stage of the amplifier circuit 3. Also, at the state where no audio information is inputted with the digital signal, the selection circuit 12 is switched to the state to select the digital signal of the optical signal. Thereby, it is possible to prevent the noise inputted to the amplifier circuit from being vocalized from a speaker.

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Industrial Application Field The present invention is applicable to a digital signal transmitted as an electric signal using a coaxial cable, and an optical signal transmitted as an optical signal using an optical fiber. The present invention relates to a digital signal input switching device that switches and inputs a digital signal that is converted into an electrical signal.

(b) Conventional technology Recently, CD players, DAT (digital audio
Many digital audio source devices that handle audio information as digital signals, such as tape recorders) or BS tuners (satellite broadcast reception tuners),
A digital output function is provided to output the audio information as a digital signal. The digital signal output from the digital audio source device is EIAJ (
The digital audio interface standard (EIAJ standard CP) standardized by the Electronics Industries Association of Japan)
With the advent of digital audio source devices equipped with the above-mentioned digital output function, some audio equipment, including audio amplifiers, have a digital input function compliant with the above-mentioned standard. A device that can directly input a digital signal from the digital audio source device has been commercialized.

Incidentally, an audio device having a digital input function generally has a plurality of digital input terminals so that a plurality of digital signals can be inputted, and is also provided with a selection circuit so that the digital signal to be inputted can be selected. Here, as methods for transmitting digital signals, there are two methods: a method of transmitting an electric signal using a coaxial cable, and a method of transmitting an optical signal using an optical fiber. After converting a digital signal into an electrical signal, the desired digital signal is generated regardless of whether it is an electrical signal or an optical signal by supplying it to a selection circuit that selects a digital signal transmitted by an electrical signal or a digital signal transmitted by an optical signal. It is designed so that you can choose.

(c) Problems to be Solved by the Invention By the way, since the signal level of the input digital signal complies with the above-mentioned standards, the digital signal is converted to a signal level corresponding to the subsequent circuit, for example, when the subsequent circuit TT
When configured with L-IC, it is necessary to set the threshold level of TTL-IC, so-called TTL level. Therefore, the digital signal transmitted as an electrical signal is amplified by the amplifier circuit and the signal level is adjusted according to the subsequent circuit, so the selection circuit selects the digital signal transmitted as an optical signal. However, if no digital signal is input, internal noise and external noise mixed in the stage before the amplifier circuit will be amplified by the amplifier circuit,
It has reached a level that cannot be ignored. In other words, when audio information is input as a digital signal, the noise is masked by the input digital signal, and is not included in the analog signal generated from the DA converter provided after the amplifier circuit. Although this is rare, when audio information is not input as a digital signal, only noise is input to the DA converter, so the noise is converted into an analog signal by the DA converter and emitted from the speaker. In particular, if the input terminal and the amplifier circuit are placed apart from each other due to the location of the input terminal to which the coaxial cable is connected or the circuit design, noise may be generated between the input terminal and the amplifier circuit. becomes easily mixed in, which becomes a big problem.

(d) Means for Solving the Problems The present invention has been made in view of the above-mentioned points, and provides a first digital signal and an optical signal output from an amplifier circuit that amplifies a digital signal transmitted by an electrical signal. a selection circuit that selectively outputs a second digital signal output from an optical receiving circuit that receives the digital signal transmitted by the optical receiver and converts it into an electrical signal; a control circuit that switches and controls the selection state of the selection circuit by generating a second digital signal, and when a control signal for selecting the second digital signal by the selection circuit is generated from the control circuit, the control signal is It is applied to the input terminal of an inverter constituting the first stage of the amplifier circuit to fix the output from the inverter.

(*) Effect The present invention normally uses "H" and rL level signals as control signals for switching a selection circuit that selects a digital signal transmitted by an electrical signal and a digital signal transmitted by an optical signal. Focusing on the fact that a digital signal transmitted as an electrical signal is amplified by an amplifier circuit configured by connecting a plurality of inverters in cascade, the digital signal transmitted as an optical signal is selected by the selection circuit. In a state in which the output from the amplifier circuit is fixed by applying a control signal for controlling the selection circuit to an inverter at the first stage of the amplifier circuit, and in a state in which audio information is not input as a digital signal, By switching the selection circuit to a state in which the digital signal of the optical signal is selected, the noise input to the amplifier circuit is prevented from being emitted from the speaker.

(f) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention, in which (1) shows the electric power output from the first digital audio source device and transmitted via a coaxial cable (not shown). a first digital input terminal into which a digital signal of the signal is input; (2)
is a first resistor for matching the input impedance at the first input terminal (1) to the coaxial cable, and (3) is a first amplifier circuit for amplifying the digital signal input to the first digital input terminal (1). , (4) is a second digital input terminal into which a digital signal of an electric signal outputted from a second digital audio device and transmitted via a coaxial cable (not shown) is input, and (5) is a second digital input terminal. A second resistor (6) is used to match the input impedance at the digital input terminal (4) to the coaxial cable, and (6) is a second resistor for amplifying the digital signal input to the second digital input terminal (4).
The amplifier circuits (7) and (8) are DC blocking capacitors that block DC component signals from being input to the first and second amplifier circuits (3) and (6), respectively, and (9) is a DC blocking capacitor. A first selection circuit (10) is for selecting an input digital signal from among the digital signals output from the first and second amplification circuits (3) and (6); A third digital input terminal into which a digital signal of an optical signal output from the second or other digital audio device and transmitted via an optical fiber (not shown) is input; (11) is the third digital input terminal; An optical receiving circuit (12) receives a digital optical signal input to the terminal (10) and converts it into a digital electrical signal, from the first selection circuit (9) and the optical receiving circuit (11). A second selection circuit (13) is used to select the input digital signal from among the output digital signals, and the second selection circuit (13) is configured to perform various functions necessary for demodulating and reproducing the digital signal output from the second selection circuit (12). an interface circuit that performs signal processing; (14) a DA converter that converts the digital signal of audio information obtained from the interface circuit (13) into an analog signal; (15)
controls the selection state of the first selection circuit (9) according to the control signal output from the first terminal (15a), and controls the selection state of the first selection circuit (9) according to the control signal output from the second terminal (15b).
a control circuit (1) that controls the selection state of the selection circuit (12);
6) and (17) are the second terminals (
These are first and second diodes for applying the control signals output from the first and second amplifier circuits (3) and (6) to the input terminals of the first and second amplifier circuits (3) and (6), respectively.

The first and second terminals (15a) and (
15b), control signals of "H4" and "L" levels are generated, respectively, and the first selection circuit (9) is r
When H and LJ level control signals are applied, digital signals from the first and second amplifier circuits (3> and (6), respectively) are selected.

In addition, the second selection circuit (12) is 1H" and "1H, respectively.
When the L and level control signals are applied, the digital signal from the optical receiving circuit (11) and the digital signal from the first selection circuit (9) are respectively selected.

Therefore, from the second terminal (15b) of the control circuit (15)
H.

When the level control signal is generated, a digital signal from the optical receiving circuit (11) is input to the interface circuit (13). On the other hand, the second terminal (
When a “L” level control signal is generated from isb),
Since the digital signal selected by the first selection circuit (9) is input to the interface circuit (13), the level of the control signal generated from the first terminal (15a) of the control circuit (15) A digital signal from the first or second amplifier circuit (3) or (6) is input to the interface circuit (13) according to the input signal.

By the way, the first amplifier circuit (3) receives the digital signal of the electrical signal input to the first digital input terminal (1).
Since the signal is supplied via the capacitor (7), the first amplifier circuit (3) outputs a digital signal obtained by amplifying the digital signal input to the first digital input terminal (1). Further, the second amplifier circuit (6) is supplied with the digital signal of the electric signal input to the second digital input terminal (4) via the second capacitor (8).
From the amplifier circuit (6), the second digital input terminal (
A digital signal obtained by amplifying the digital signal input to step 4) is output. Further, since the optical receiving circuit (11) is supplied with a digital signal of the optical signal input to the third digital input terminal (10), from the optical receiving circuit (11),
A digital signal obtained by converting the digital signal input to the third digital input terminal (10) into an electrical signal is output.

On the other hand, the digital signal input to the interface circuit (13) is subjected to various signal processing necessary for demodulation and reproduction by the interface circuit (13).Then, the audio information extracted by the interface circuit (13) is processed by the interface circuit (13). Only the digital signal becomes DA converter (1
4), the DA converter (14) outputs an analog signal corresponding to the audio information.

Therefore, when a control signal of "1L" level is generated from the second terminal (15b) of the control circuit (15), a control signal of "H" level is generated from the first terminal (15a) of the control circuit (15). and the first one from the DA converter (14).
When an analog signal of audio information corresponding to the digital signal input to the digital input terminal (1) is output and a control signal of level r L is generated from the first terminal (15a), the DA humbater (14) outputs an analog signal of audio information corresponding to the digital signal inputted to the second digital input terminal (4), and on the other hand, when a control signal of r H level is generated from the second terminal (15b), Regardless of the level of the control signal generated from the first terminal (15a), the DA converter (15a)
4) outputs an analog signal of audio information corresponding to the digital signal input to the third digital input terminal (10).

That is, the first and second terminals (15) of the control circuit (15)
a) and (15b) from the DA converter (14) in response to the control signals from the first. It is possible to selectively output an analog signal of audio information corresponding to a digital signal input to the second or third digital input terminal (1), (4) or (10), respectively.

By the way, the first and second amplifier circuits (3) and (6) are
It is constructed by connecting a plurality of inverters operating at high speed in cascade, and is constructed as shown in FIG. 2, for example. In FIG. 2, the first to third inverters (
Three inverters (18) to (20) are connected in series, and the first inverter (18) forming the first stage of the first and second amplifier circuits (3) and (6) is connected to the feedback resistor (2).
By connecting the output end to the input end via 1), it is configured to operate as a linear amplifier. For that reason,
The digital signal input to the input terminal (22) serving as the input terminal of the first and second amplifier circuits (3) and (6) is linearly amplified by the first inverter (18), and then the digital signal is linearly amplified by the first inverter (18). Output terminal (23) which is amplified and waveform-shaped by the third inverters (19) and (20) and becomes the output terminal of the first and second amplifier circuits (3) and (6).
is output from. Here, the digital signals input to the first and second digital input terminals (1) and (4) are EI
Compliant with AJ standard CP-340, input terminal (22
A digital signal at a level (o, 5vp-p) corresponding to the standard is input to the output terminal (23), and a digital signal at a level corresponding to the subsequent interface circuit (13) is output from the output terminal (23). . In this case, if the interface circuit (13) is composed of TTL-IC,
The digital signal output from the output terminal (23) is T
The first to third inverters (18) to reach the TL level.
The digital signal input to the input terminal (22>) is amplified by (20>).

By the way, the second terminal (15b) of the control circuit (15) and the respective input terminals of the first and second amplifier circuits (3) and (6) are connected to the first and second diodes (16) and (17), respectively. ) are connected through. Therefore, when the control signal of r H level is output from the second terminal (15b), that is, when the digital signal from the optical receiving circuit (11) is selected by the second selection circuit (12). , the input terminals of the first and second amplifier circuits (3) and (6) are supplied with a voltage lower than the rH level of the control signal by the conduction voltage of the first and second diodes (16) and (17), respectively. is applied. Here, unless the "H" level of the control signal is set to a low voltage that is close to the threshold level of the "H" level, the first
and r H at the input terminals of the second amplifier circuits (3) and (6).
, the first level signal will be applied.
The output levels of the second amplifier circuits (3) and (6) are fixed. In other words, the input terminal of the first inverter (18) is at the "H" level, which is approximately half the power supply voltage, due to the feedback path.
Although the voltage is not defined as the "L" level, it is fixed at the "H" level of the control signal at the rH level, and the output of the first inverter (18) is rL.
The output of the second inverter (19) is "H4" and the output of the third inverter (20) is "LJ, so the output terminals of the first and second amplifier circuits (3) and (6) are fixed at rL" level. be done. Therefore, when the second selection circuit (12) selects the digital signal from the optical receiver circuit (11), even if noise is input to the first and second amplifier circuits (3) and (6), , the first and second amplifier circuits (3) and (6
) generates an output at a constant level, so the noise component is not supplied to the DA converter (14).

Therefore, the state in which audio information is not input as a digital signal, that is, the first state. Rather than reproducing audio information from any of the digital signals input to the second or third digital input terminals (1), (4), or (10), a circuit not shown in FIG. In a state where audio information input as an analog signal is reproduced, the second selection circuit (12) selects the optical receiving circuit (
By selecting the digital signal from 11), the noise input to the first and second amplifier circuits (3) and (6) can be prevented from being emitted from the speaker (not shown). .

Furthermore, it is possible to detect that the audio information to be reproduced is input as an analog signal, and in response to this detection, automatically set the second selection circuit (12) to a state in which the digital signal from the optical receiver circuit (11) is selected. For example, the first and second amplifier circuits (3) and (6) can be connected from the speaker to the first and second amplifier circuits (3) and (6) without any extra operations.
It is possible to prevent noise that is input into the system from being emitted.

Also, from the second terminal (15b) of the control circuit (15), “L”
” level control signal is generated and the second selection circuit (12)
When the digital signal of the electrical signal input to the first or second digital input terminal (1) or (4) is selected by the first and second diodes (16) and (
17) is reverse biased, so that the digital signal input to the first digital input terminal (1) is reverse biased.
There is no problem in the digital signal input to the second digital input terminal (4) being supplied to the amplifier circuit (3) and the second amplifier circuit (6>).

Further, in the above embodiment, the second
When the control signal from the terminal (15b) is “H” level,
The second selection circuit (12) selects the digital signal from the optical receiving circuit (11), but conversely, when the control signal is at "L" level, the optical receiving circuit (11)
If it is configured to select a digital signal from the first and second diodes (16) and (1
7) should be connected in opposite directions. By doing this, the second selection circuit (12) is connected to the optical receiver circuit (11).
In the state where the digital signal from is selected, the input terminals of the first and second amplifier circuits (3) and (6) are
Since the output level of the first and second amplifier circuits (3) and (6) can be fixed to "H", the output level of the first and second amplifier circuits (3) can be fixed to "H". )
The noise input to (6) can be prevented from being emitted from the speaker.

(G) Effects of the Invention As described above, according to the present invention, when a digital signal transmitted as an optical signal is selected by a selection circuit, a control signal for controlling the selection circuit is transmitted to the first stage of the amplifier circuit. By applying the voltage to the inverter, the output from the amplifier circuit is fixed, and the noise input to the amplifier circuit is prevented from being supplied to the DA humbverter. Therefore, when audio information is not input as a digital signal, the output from the amplifier circuit is It is possible to prevent noise input to the circuit from being emitted from the speaker.

[Brief explanation of the drawing]

FIG. 1 is a circuit block diagram showing one embodiment of the present invention, and FIG.
The figure is a circuit diagram showing the configuration of an amplifier circuit. Explanation of main drawing numbers (1) (4) (to)...Digital input terminal, (3
)(6)...Amplification circuit, (9)(12)...Selection circuit, (11)...Optical receiving circuit, (13)...Interface circuit, (14)...DA converter , (15)...Control circuit, (16)(17)...Diode.

Claims (1)

[Claims] (1) The first stage is composed of an inverter with a feedback path formed, and an amplifier circuit that amplifies the digital signal transmitted as an electrical signal via a coaxial cable, and the digital signal transmitted as an optical signal via an optical fiber. an optical receiver circuit that receives a signal and converts it into an electrical signal; and a selection circuit that selectively outputs a first digital signal output from the amplifier circuit and a second digital signal output from the optical receiver circuit. and set the selection state of the selection circuit to "H" and "L".
a control circuit that performs switching and control based on a level control signal, and when a control signal for selecting a second digital signal by the selection circuit is generated from the control circuit, the control signal is transmitted to the input terminal of the inverter. A digital signal input switching device characterized in that the output from the inverter is fixed by applying a voltage.