JP3200887B2 - Audio waveform decoding device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an audio waveform decoding device, and more particularly to an audio waveform decoding device for decoding an audio waveform in a digital transmission cordless telephone.

[0002]

2. Description of the Related Art In recent years, cordless telephones based on the analog transmission system have become widespread, but there have been problems of limitations on channel frequency allocation and voice leakage. Therefore, as a method of solving these problems, against the background of the development of digital signal processing theory and the advancement of high integration technology of IC,
Cordless telephones based on digital transmission systems have been developed. Among them, a speech codec that performs speech encoding and decoding in order to reduce the amount of data in a transmission path has been developed. An adaptive differential PCM (ADPCM) scheme, which is a waveform encoding scheme that is easy to process, is being adopted as a speech encoding scheme for cordless telephones.

The transmission path of a cordless telephone is a wireless transmission path. Therefore, there are many transmission errors as compared with the wired transmission path. If the audio signal encoded on the transmitting side is erroneous on the transmission path, the voice decoded on the receiving side becomes noise and the contents cannot be heard correctly. Also, it makes the listener more uncomfortable. Therefore, when a transmission error occurs, a decoded signal is not output and a silent signal is output instead. This prevents the listener from feeling uncomfortable. This is the mute function.

FIG. 4 is a block diagram showing an example of a conventional audio waveform decoding apparatus which is a backward ADPCM decoding apparatus having a mute function. As shown in FIG. 4, a conventional audio waveform decoding apparatus includes an inverse quantization circuit 1 and
The configuration includes an adder circuit 2, a prediction circuit 3, a switch 4, and an output buffer 5.

[0005] As shown in FIG.
An inverse quantization table 11 in which a DPCM code is associated with an inverse quantization value, a coefficient table 12 in which an ADPCM code is associated with a quantization width coefficient, multiplication circuits 13 and 14, and a quantization width buffer 15 are provided. Was composed.

Table 1 shows an example of the inverse quantization value and the quantization width coefficient corresponding to the ADPCM code, which are the contents of the inverse quantization table 11 and the coefficient table 12, respectively.

[0007]

[Table 1]

Next, the operation of the conventional speech waveform decoding device will be described.

It is assumed that the ADPC input at time t
The M code is a binary number '101', and the quantization width buffer 15
Contains the quantization width calculated from the past ADPCM code '
It is assumed that 1 ′ is stored.

First, A when there is no transmission error at time t
The decoding of the DPCM code will be described.

First, the input ADPCM code '10
1 ′ is transferred to the inverse quantization table 11 and coefficient table 12 in the inverse quantization circuit 1. ADPC transferred
The M code '101' is converted into an inverse quantization value '-2' and a quantization width coefficient '1' in the inverse quantization table 11 and coefficient table 12, respectively. The multiplication circuit 14 multiplies the output “−2” of the inverse quantization table 11 by the quantization width “1” stored in the quantization width buffer 15. On the other hand, the multiplication circuit 13 multiplies the quantization width coefficient “1” by the quantization width “1” stored in the quantization width buffer 15, and the output “1” is returned to the quantization width buffer 15.

Next, the output “−2” of the multiplication circuit 14 is added to the output of the prediction circuit 3 by the addition circuit 2 as a difference value. The result of the addition becomes a PCM code, which is output via the switch 4 and the output buffer 5. The addition result is transferred to the prediction circuit 3 for the next sample.

Next, the output of a silent signal when a transmission error occurs at time t + 1 and is detected will be described.

In this case, a silence signal output instruction signal M for instructing to output a silence signal is sent to the switch 4, and the switch 4 is turned off so that the output of the adder circuit 2 is not transferred to the output buffer 5. That is, the PCM code immediately before disconnection, that is, the DC waveform outside the audible range is continuously output until the switch 4 is connected next time. A curve C shown in FIG. 2 is an example of an output waveform of the conventional audio waveform decoding device. The waveform of the silent signal when the sample after time t + 1 is lost due to a transmission error is shown. The portion indicated by the dotted line of the curve C is a waveform assuming that the sample has not disappeared. It is shown that the output waveform changes abruptly at time t + 1, which is the boundary point of the silence signal output.

[0015]

The above-mentioned conventional speech waveform decoding apparatus has a drawback that the output waveform changes abruptly at the boundary point of the output of the silent signal, thereby generating high-frequency noise.

[0016]

A speech waveform decoding apparatus according to the present invention comprises an inverse quantization circuit for converting an adaptive difference code into an inversely quantized value ;
Corresponding to a quantization width coefficient smaller than 1 at the time of erasure due to a transmission error , the sign of the sign coincides with the sign of the inverse quantization value before erasure, which is the inverse quantization value immediately before the erasure, and the numerical value is inverted A minimum code output circuit that outputs a minimum code that is an adaptive difference code having a value smaller than the value of the quantization value; and a selection that selects one of the adaptive difference code and the minimum code and inputs the selected code to the inverse quantization circuit. A transmission error detection circuit.
Silence output instruction to output a silence signal when going out
The minimum code is input to the inverse quantization circuit by signal control.
It is characterized by force .

[0017]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing a first embodiment of the speech waveform decoding apparatus according to the present invention. This embodiment is a backward type AD having a mute function similar to the conventional example.
It is an example of a PCM decoding device.

As shown in FIG. 1, the speech waveform decoding apparatus according to the present embodiment includes, in addition to an inverse quantization circuit 1, an addition circuit 2, and a prediction circuit 3 similar to those of the conventional example, a preceding one. The positive minimum code '0 corresponding to the positive or negative of the inverse quantization value of the ADPCM code
A minimum code output circuit 6 for outputting 00 'or a negative minimum code' 100 '; and a selection circuit 7 for switching between the input ADPCM code and the output of the minimum code output circuit 6 and inputting them to the inverse quantization circuit 1. It is configured.

The inverse quantization circuit 1 includes an inverse quantization table 11, a coefficient table 12, multiplication circuits 13 and 14, and a quantization width buffer 15, as described in the conventional example. .

Next, the operation of this embodiment will be described.

As in the above-described conventional example, it is assumed that the ADPCM code input at time t is a binary number '101', and the quantization width buffer 15 stores the quantization width calculated from the past ADPCM code. It is assumed that 1 ′ is stored.

First, A when there is no transmission error at time t
The decoding of the DPCM code will be described.

First, the input ADPCM code '10
1 ′ is transferred to the inverse quantization table 11 and coefficient table 12 in the inverse quantization circuit 1 via the selection circuit 7. The transferred ADPCM code '101' is converted into an inverse quantization value '-2' and a quantization width coefficient '1' by the inverse quantization table 11 and the coefficient table 12, respectively.
The output “−2” of the inverse quantization table 11 and the quantization width “1” stored in the quantization width buffer 15 are multiplied by the multiplication circuit 1
Multiplied by four. On the other hand, the multiplication circuit 13 multiplies the quantization width coefficient “1” by the quantization width “1” stored in the quantization width buffer 15, and the output “1” is returned to the quantization width buffer 15.

Next, the output “−2” of the multiplication circuit 14 is added as a difference value by the addition circuit 2 to the output of the prediction circuit 3. The result of the addition becomes a PCM code and is output. The addition result is transferred to the prediction circuit 3 for the next sample.

Next, the output of a silent signal when a transmission error occurs at time t + 1 and is detected will be described.

In this case, a silence output instruction signal M for instructing to output a silence signal is sent to the selection circuit 7. When the silence output instruction signal M is input, the selection circuit 7 switches the input of the ADPCM code to the output of the minimum code output circuit 6 and inputs the same to the inverse quantization circuit 1. As described above, the minimum code output circuit 6 outputs the minimum positive code '000' or the minimum negative code '100' corresponding to the positive or negative of the inverse quantization value of the immediately preceding ADPCM code. . Time t
The inverse quantization value at is “−2” and is negative. The minimum code '100' is transferred to the inverse quantization table 11 and coefficient table 12 of the inverse quantization circuit 1 via the selection circuit 7. Inverse quantization table 11 and coefficient table 1
In 2, the input minimum code '100' is converted into an inverse quantization value '-1' and a quantization width coefficient '0.8', respectively. The multiplication circuit 14 multiplies the output “−1” of the inverse quantization table 11 by the quantization width “1” stored in the quantization width buffer 15. On the other hand, the multiplication circuit 13 multiplies the quantization width coefficient '0.8' by the quantization width '1' stored in the quantization width buffer 15, and outputs the output '0.
8 'is returned to the quantization width buffer 15.

Next, the output "-1" of the multiplication circuit 14 is added as a difference value by the addition circuit 2 to the output of the prediction circuit 3. The result of the addition becomes a PCM code and is output. The addition result is transferred to the prediction circuit 3 for the next sample.

The above processing is repeated until the silence output instruction signal M has expired. A curve A shown in FIG. 2 is an example of an output waveform of the audio waveform decoding device of the present embodiment. Time t + 1
The waveform of a silent signal when the subsequent sample is lost due to a transmission error is shown. The figures after the decimal point are rounded off. The portion indicated by the dotted line of the curve A is a waveform assuming that the sample has not disappeared. It is shown that the output waveform gradually changes from time t + 1 which is a boundary point of the silent signal output.

Next, a second embodiment of the present invention will be described.

FIG. 3 is a block diagram showing a second embodiment of the speech waveform decoding apparatus according to the present invention. This embodiment is different from the first embodiment in that the present embodiment is an example of a forward adaptive DM (ADM) decoding device having a mute function. It is changing.

As shown in FIG. 3, the speech waveform decoding apparatus according to the present embodiment includes an adding circuit 2 and a predicting circuit 3 according to the first embodiment.
In addition to the multiplication circuits 13 and 14 and the quantization width buffer 15, when the input DM code is binary "1", "1"
In addition, an inverse quantization table 16 for converting to '-1' when it is '0', a silent signal buffer 17 for storing the previous inversely quantized value, and a value smaller than '1.'
It comprises a silence signal coefficient output circuit 18 for outputting 6 ′, and selection circuits 19 and 20.

Next, the operation of this embodiment will be described.

As a premise, it is assumed that the DM code input at time t is a binary number “0” and a quantization width coefficient “1.2” is input. It is also assumed that the quantization width buffer 15 stores the quantization width '1.67' calculated from the past DM code.

First, decoding of the ADM code of the forward system when there is no transmission error at time t will be described.

The input quantization width coefficient '1.2' is transferred to the multiplication circuit 13 via the selection circuit 20 and multiplied by the quantization width '1.67' which is the content of the quantization width buffer 15. Is done. The output '2' is returned to the quantization width buffer 15. On the other hand, the input DM code “0” is converted into an inverse quantization value “−1” by the inverse quantization table 16. The output “−1” is stored in the silence signal buffer 17 and, at the same time, transferred to the multiplication circuit 14 via the selection circuit 19. The inverse quantization value “−1” and the output “2” of the multiplication circuit 13 are multiplied by the multiplication circuit 14. The output “−2” of the multiplication circuit 14 is added to the output of the prediction circuit 3 by the addition circuit 2 as a difference signal. The result of the addition becomes a PCM code and is output. The addition result is transferred to the prediction circuit 3 for the next sample.

Next, a description will be given of a silent signal output when a transmission error occurs at time t + 1 and is detected.

In this case, a silence output instruction signal M for instructing to output a silence signal is sent to the selection circuits 19 and 20. When the silence output instruction signal M is input, the selection circuit 20 switches the input of the quantization width coefficient to the output '0.6' of the silence signal coefficient output circuit 18 and inputs it to the multiplication circuit 13. Silence signal coefficient output circuit 18 by multiplication circuit 13
Output of '0.6' and the contents of the quantization width buffer 15 '
Multiplied by 2 '. The output '1.2' is returned to the quantization width buffer 15. At the same time, it is transferred to the multiplication circuit 14. On the other hand, the selection circuit 19 outputs the output of the silence signal buffer 17 from the output of the inverse quantization table 16 so far.
-1 'is input to the multiplication circuit 14. Multiplication circuit 14
Thus, the output “−1” of the silence signal buffer 17 and the output “1.2” of the multiplication circuit 13 are multiplied. The output '1.2' of the multiplication circuit 14 is added to the output of the prediction circuit 3 by the addition circuit 2 as a difference signal. The result of the addition becomes a PCM code and is output. The addition result is transferred to the prediction circuit 3 for the next sample.

The above processing is repeated until the silence output instruction signal M is turned off. A curve B shown in FIG. 2 is an example of an output waveform of the audio waveform decoding device of the present embodiment. Time t + 1
The waveform of a silent signal when the subsequent sample is lost due to a transmission error is shown. The figures after the decimal point are rounded off. The portion indicated by the dotted line of the curve B is a waveform assuming that the sample has not disappeared. It is shown that the output waveform gradually changes from time t + 1 which is a boundary point of the silent signal output.

[0040]

As described above, the speech waveform decoding apparatus according to the present invention provides a minimum code output circuit for outputting a minimum code and an inverse quantization circuit for selecting one of an adaptive difference code and a minimum code. Since the output waveform does not gradually change from the boundary point of the silent signal output and does not suddenly change, no unsightly high-frequency noise is generated, and the receiver This has the effect of not giving any discomfort to the user.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of a speech waveform decoding device according to the present invention.

FIG. 2 is an output waveform diagram showing an example of an operation in the first and second embodiments and a conventional speech waveform decoding device.

FIG. 3 is a block diagram showing a second embodiment of the speech waveform decoding device of the present invention.

FIG. 4 is a block diagram illustrating an example of a conventional audio waveform decoding device.

FIG. 5 is a block diagram illustrating an example of an inverse quantization circuit.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 inverse quantization circuit 2 addition circuit 3 prediction circuit 4 switch circuit 5 output buffer 6 minimum code output circuit 7, 19, 20 selection circuit 11, 16 inverse quantization table 12 coefficient table 13, 14 multiplication circuit 15 quantization width buffer 17 Silence signal buffer 18 Silence signal coefficient output circuit

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G10L 19/00

Claims (2)

(57) [Claims] An inverse quantization circuit for converting an adaptive differential code into an inverse quantized value, and a transmission error of the adaptive differential code which receives the inverse quantized value.
Corresponds to a quantization width coefficient smaller than 1 at the time of erasure, the sign of which corresponds to the sign of the inverse quantization value before erasure, which is the inverse quantization value immediately before the erasure, and the numerical value is the inverse quantization before erasure. A minimum code output circuit that outputs a minimum code that is an adaptive difference code of a value smaller than the value of the value, and a selection circuit that selects one of the adaptive difference code and the minimum code and inputs the selected code to the inverse quantization circuit. equipped with, before
Instructs to output silence signal upon detection of transmission error
By controlling the silence output instruction signal, the minimum code
An audio waveform decoding device for inputting to a decoding circuit . 2. A quantization width storage circuit for storing a first quantization width which is a quantization width calculated immediately before in time, and a quantization value smaller than 1 when the first quantization width coefficient or the difference code disappears. A coefficient output circuit that outputs a second quantization width coefficient that is a quantization width coefficient, and the sign thereof matches the sign of the pre-erasure inverse quantization value that is the inverse quantization value immediately before the erasure, and the numerical value is A dequantized value output circuit that outputs a second dequantized value that is a value smaller than the numerical value of the pre-erasure dequantized value, and the first quantized width coefficient and the second quantized width coefficient A first selection circuit that selects any one of the following: a second selection circuit that selects any one of a first dequantized value and the second dequantized value; and the first selection. A second quantization width is calculated by multiplying the output of the circuit by the first quantization width stored in the quantization width storage circuit. First multiplying circuit and said second voice waveform decoding unit, characterized in that it comprises a second multiplying circuit for multiplying the output of the selection circuit and the second quantization width.