JPH0685671A - Converter - Google Patents

Abstract

PURPOSE:To efficiently perform conversion with small errors even when the frequency of an inputted analog signal is flucturated largely. CONSTITUTION:The levels of analog signals passing filters 1, 2 different in frequency characteristics are compared with each other by a comparator 3, and a control circuit 8 switches selectively either output of the filters 1, 2 at a switching circuit 4 based on a comparison result. The signal is A/D-converted with respect a corresponding sampling frequency in an A/D conversion circuit 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a converter for converting an analog signal into a digital signal suitable for transmission or recording.

[0002]

2. Description of the Related Art As a conversion device of this type, there is a voice synthesis device which converts a voice signal into a digital signal, stores it in a memory, and extracts voice when necessary.

FIG. 4 is a block diagram showing the structure of a recording system in a conventional speech synthesizer.

[0004] In the figure, 1 is a low-pass filter for removing high frequency components from the analog signal, 5 ₀ A / D converter circuit for converting an analog signal into a digital signal, the encoding circuit for compressing the digital signal data is 6, 7 Is a data memory in which digital signal data is stored, 8 ₀ is each circuit 5 ₀ ,
It is a control circuit for controlling 6, 7.

The analog voice signal from the input terminal 9 has its high-frequency component removed by the low-pass filter 1, converted into a digital signal by the A / D conversion circuit ₅₀ , and encoded by the encoding circuit 6.
For example, it is stored in the data memory 7 after being compressed by the ADPCM method.

In the case of reproduction, the operation reverse to the above,
That is, the data is read from the data memory 7, decoded, further D / A converted, and then the unnecessary frequency component is removed and output.

[0007]

However, in such a conventional example, since data is collected and stored at a predetermined sampling frequency, the efficiency of storage or transmission is high for an analog signal with a large frequency fluctuation. It has the drawback of getting worse and having more errors, and
An analog signal whose frequency characteristic is not clear has a drawback that the sampling frequency must be determined by trial and error.

The present invention has been made in view of the above points, and an object of the present invention is to enable efficient conversion with less error even when the frequency fluctuation of an analog signal is large. To do.

[0009]

In order to achieve the above-mentioned object, the present invention is constructed as follows.

That is, the present invention is a conversion device for converting an input analog signal into a digital signal, wherein a plurality of filters having different frequency characteristics to which the analog signal is input and the filters output from the respective filters are output. Circuit for comparing the levels of analog signals output from the filters, a switching circuit for switching and outputting any of the analog signals output from the filters, and an A / D for A / D converting the analog signals output from the switching circuits. A conversion circuit and a control circuit for controlling the switching of the switching circuit based on the output of the comparison circuit and controlling the sampling frequency of the A / D conversion circuit are provided.

[0011]

According to the above construction, the levels of the analog signals that have passed through the filters having different frequency characteristics are compared, and based on this, the output of the filter having the optimum frequency characteristic is switched and selected, and at the corresponding sampling frequency A Since the D / D conversion is performed, even when the frequency fluctuation of the input analog signal is large, it is possible to perform the conversion efficiently and with a small error.

[0012]

Embodiments of the present invention will now be described in detail with reference to the drawings.

Embodiment 1. FIG. 1 is a block diagram of an embodiment of the present invention. In this embodiment, description will be made by applying it to a recording system of a speech synthesizer similar to the conventional example of FIG.

The voice synthesizer according to this embodiment has first and second low-pass filters 1 and 2 having different frequency characteristics to which an analog voice signal is input, and analog voice signals output from the low-pass filters 1 and 2. Of the analog audio signals output from the low-pass filters 1 and 2 and a switching circuit 4 that switches and outputs one of the analog audio signals output from the low-pass filters 1 and 2, and the analog audio signal output from the switching circuit 4 is converted into an A / D signal. A / D conversion circuit 5 for conversion and encoding circuit 6 for compressing A / D converted digital signal data
Based on the output of the comparison circuit 3 and the data memory 7 that stores the digital signal data from the encoding circuit 6,
The control circuit 8 controls the switching of the switching circuit 4, controls the sampling frequency of the A / D conversion circuit 5, and further controls the encoding circuit 6 and the data memory 7.

The first low-pass filter 1 to which the analog audio signal from the input terminal 9 is input is a filter that passes a frequency component of, for example, 8 kHz or less, and the analog audio signal from the first low-pass filter 1 is input. The second low-pass filter 2 is a filter that passes a frequency component of 4 kHz or less, for example.

The comparison circuit 3 includes the low-pass filters 1 and 2.
Of the output levels are compared and the ratio is given to the control circuit 8.

In the control circuit 8, the ratio given from the comparison circuit 3 is about 1, that is, each low-pass filter 1,
When the output levels of 2 are almost equal,
The output of the low-pass filter 2 is selected by the switching circuit 4, and the sampling frequency of the A / D conversion circuit 5 is set to the second
The output of the first low-pass filter 1 having a wide band is selected by the switching circuit 4 and the sampling of the A / D conversion circuit 5 is performed at 8 kHz corresponding to the low-pass filter 2, and when the ratio from the comparison circuit 3 is not about 1. The frequency is set to 16 kHz corresponding to the first low pass filter 1.

Further, which of the low pass filters 1 and 2 is used?
And the sampling frequency has been selected, the control circuit 8 controls the second low-pass filter 2
When the output of is selected, the second low-pass filter 2
And the code indicating that the corresponding sampling frequency has been selected are stored in the data memory 7 together with the encoded data.

FIG. 2 is a flowchart of such control.

First, at first, the switching circuit 4 selects the analog audio signal from the wideband first low-pass filter 1 and A / D-converts it at the corresponding sampling frequency (step n1). That is, it is determined whether or not the ratio of the output levels of the low pass filters 1 and 2 (= the output level of the second low pass filter 2 / the output level of the first low pass filter 1) is within a certain range substantially equal to 1. (Step n2) If it is within a certain range, the switching circuit 4 selects the analog audio signal from the narrow band second low-pass filter 2 and A / D-converts it at the corresponding sampling frequency (Step n3). If it is not within the fixed range, the process returns to step n1.

In this way, the output levels of the low-pass filters 1 and 2 are compared, the optimum outputs of the low-pass filters 1 and 2 are switched and selected according to the comparison result, and the low-pass filters 1 and 2 are selected. A / D conversion is performed at the sampling frequency, and the data is further encoded and stored in the data memory 7 together with its history, that is, a code indicating which low-pass filter and sampling frequency is selected.

For example, as an input analog voice signal, 2
Considering the case where a sine wave of kHz and a cosine wave of 6 kHz are mixed and input, the first and second low-pass filters 1, 2
Output characteristics are different as shown in FIGS. 3 (A) and 3 (B), respectively. Therefore, when comparing the outputs of the low-pass filters 1 and 2, the ratio is as shown in FIG.
It will change as shown in. This Figure 3 (C)
, There is no period in which the ratio is about 1. Therefore, in this case, the output of the wideband first low-pass filter 1 is selected and the sampling frequency is set to 16
It will be A / D converted as kHz.

In this way, the output of the filter having the optimum frequency characteristic is switched and selected, and A / D conversion is performed at the corresponding sampling frequency, so that the efficiency is improved even when the frequency fluctuation of the input analog audio signal is large. It is possible to perform conversion that is good and has few errors.

In the case of reproduction, the code indicating which low-pass filter 1 or 2 and the sampling frequency is selected is stored in the data memory 7 as described above, and the corresponding decoding is performed based on the code. Conversion, D
A / A conversion is performed and a filter is selected.

In the above embodiment, the low pass filter 1,
Although 2 is cascade-connected, the low-pass filters 1 and 2 may be connected in parallel with the analog audio signal.

In the above-mentioned embodiment, the case where two low-pass filters 1 and 2 having different frequency characteristics are provided has been described. However, as another embodiment of the present invention, three or more may be provided. is there. For example, when a 2 kHz third low-pass filter is provided in addition to the first and second low-pass filters 1 and 2 of the above-described embodiment, the output ratio of the second and third low-pass filters may be about 1. For example, when the third low-pass filter is selected and the output ratio of the second and third low-pass filters is not about 1, the output levels of the first and second low-pass filters are compared. If the ratio is about 1, then The second low-pass filter is selected, and when it is not about 1, the first low-pass filter is selected.

Although the above embodiment has been described by applying it to the speech synthesizer, the present invention is not limited to the speech synthesizer, and the converted digital signal is transmitted instead of being stored in the memory. You may

[0028]

As described above, according to the present invention, by comparing the levels of analog signals that have passed through a plurality of filters having different frequency characteristics, the output of the filter having the optimum frequency characteristic is switched and selected. Since the A / D conversion is performed at the sampling frequency that is set, even if the frequency fluctuation of the input analog signal is large, it is efficient and
The conversion with less error can be automatically performed.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a flowchart for explaining the operation of the embodiment of FIG.

FIG. 3 is a waveform diagram for explaining the operation of the embodiment of FIG.

FIG. 4 is a block diagram of a conventional example.

[Explanation of symbols]

 1, 2 1st, 2nd low-pass filter 3 Comparison circuit 4 Switching circuit 5 A / D conversion circuit 8 Control circuit

Claims (1)

[Claims] 1. A converter for converting an input analog signal into a digital signal, wherein a plurality of filters having different frequency characteristics to which the analog signal is input, and an analog signal output from each of the filters are provided. A comparison circuit for comparing levels; a switching circuit for switching and outputting any of the analog signals output from the filters; an A / D conversion circuit for A / D converting the analog signals output from the switching circuit; And a control circuit that controls switching of the switching circuit based on an output of the comparison circuit and controls a sampling frequency of the A / D conversion circuit.