JP3416403B2 - MPEG audio decoder - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is, MPEG (Moving
Picture Expert Group) Audio decoder.

[0002]

2. Description of the Related Art Information handled by multimedia is enormous and diverse, and it is necessary to process such information at high speed in order to put multimedia into practical use. In order to process information at high speed, data compression / decompression technology is essential. As such a data compression / decompression technique, there is an “MPEG” method. This MPEG system is based on ISO (International Organization)
for Standardization) / IEC (Intarnational Elec)
trotechnical Commission (MPEG Committee) (IS
O / IEC JTC1 / SC29 / WG11) is being standardized.

MPEG is composed of three parts. Part 1, "MPEG System Part" (ISO / IEC
IS 11172 Part1: Systems) specifies the multiplexing structure and synchronization method for video data and audio data. Part 2 “MPE
In the "G video part" (ISO / IEC IS 11172 Part2: Video), a high-efficiency coding method of video data and a format of video data are specified. Part 3 “MPEG
Audio part "(ISO / IEC IS 11172 Part3: Audio)
Defines a high-efficiency encoding method for audio data and an audio data format.

The video data handled in the MPEG video part relates to a moving image, and the moving image is composed of several tens (for example, 30) frames (still images, frames) per second. The video data includes a sequence (Sequence), a GOP (Group Of Pictures), a picture, a slice (Slice), and a macroblock (Macrob).
lock), and a block in order of 6 layers.

In addition, MPEG is currently different from MPEG-1 and MPEG mainly due to the difference in encoding rate.
There are two methods of -2. In MPEG-1, a frame corresponds to a picture. In MPEG-2, a frame or field can be associated with a picture. Two fields make up one frame. A structure in which a frame corresponds to a picture is called a frame structure, and a structure in which a field corresponds to a picture is called a field structure.

In MPEG, a compression technique called interframe prediction is used. Inter-frame prediction compresses data between frames based on temporal correlation. Bi-directional prediction is performed in inter-frame prediction. Bidirectional prediction is a combination of forward prediction that predicts a currently reproduced image from a past reproduced image (or picture) and backward prediction that predicts a currently reproduced image from a future reproduced image. .

This bidirectional prediction is based on the I picture (Intra-Pi
cture), P picture (Predictive-Picture), and B picture (Bidirectionally predictive-Picture). The I picture is independently generated regardless of past and future reproduced images. P pictures are generated by forward prediction (prediction from past I pictures or P pictures). B pictures are generated by bidirectional prediction. In bidirectional prediction, a B picture is generated by any one of the following three predictions. Forward prediction; prediction from past I picture or P picture, backward prediction; prediction from future I picture or P picture, bidirectional prediction; prediction from past and future I picture or P picture. Then, these I, P and B pictures are encoded respectively. That is, the I picture is generated even if there is no past or future picture. On the other hand, a P picture is not generated without a past picture, and a B picture is not generated without a past or future picture.

In inter-frame prediction, first, I pictures are periodically generated. Next, a frame several frames ahead of the I picture is generated as a P picture. This P
A picture is generated by unidirectional (forward) prediction from the past to the present. Subsequently, frames located before the I picture and after the P picture are generated as B pictures. When this B picture is generated, the optimum prediction method is selected from the three methods of forward prediction, backward prediction, and bidirectional prediction. Generally, in a continuous moving image, the current image and the images before and after it are very similar to each other, and only a part thereof is different. So, the previous frame (eg I
It is assumed that the (picture) and the next frame (for example, P picture) are the same, and if there is a change between the two frames, only the difference (B picture data) is extracted and compressed.
Thereby, data between frames can be compressed based on temporal correlation.

A data string (bit stream) of video data encoded in accordance with the MPEG video part in this way is called an MPEG video stream.
A data string of audio data encoded according to the MPEG audio part is called an MPEG audio stream. Then, the video stream and the audio stream are time-division multiplexed in accordance with the MPEG system part, and M as one data string is generated.
It becomes a PEG system stream. The system stream is also called a multiplex stream.

By the way, MPEG-1 is mainly a video CD.
(Compact Disc), CD-ROM (CD-Read Only Memor
y), DVD (Digital Video Disk), and other storage media are used, and MPEG-2 is M
It supports a wide range of applications including PEG-1.

For MPEG audio, a layer (Layer)
There are 3 modes, I, Layer II and Layer III.
Higher layers can achieve higher sound quality and higher compression rate. One frame of the audio stream is AAU (Audio Ac
cess Unit). Each AAU is a minimum unit that can be independently decoded, and includes a fixed number of samples (384 samples for layer I and 1152 samples for layer II and layer III) set for each layer.

The structure of the AAU is such that an error check (CRC; Cyclic Redundancy Co) is performed from the header to the header.
de 16 bits), followed by audio data. The data from the header to the audio data is used to reproduce the audio signal.

A sampling frequency is specified in the header. Sampling frequency is a field that specifies the sampling rate, and has three types of frequencies (32kHz
, 44.1kHz, 48kHz).

The audio data is variable length data, and when the end of the audio data does not reach the end of AAU, the remaining part (gap portion from the end of audio data to the end of AAU) is ancillary data (Ancillary data). Data) is called. Arbitrary data other than MPEG audio can be inserted into this ancillary data. In MPEG-2, multi-channel and multi-lingual data is inserted in the ancillary data.

The layer I audio data includes allocation and scale factor (Scale Fact).
or) and a sample (Sample). layer
II and Layer III audio data is allocated in the scale factor scale information.
Information), scale factor, and sample.

The scale factor is a magnification factor at the time of reproducing the waveform of each subband and each channel. It is represented by 6 bits for each subband and channel, and +6 to −
Up to 118 dB can be specified in units of approximately 2 dB. Since the value of the scale factor corresponds to the sound pressure level of the reproduced voice, when the value of the scale factor falls below a certain level, the reproduced sound becomes a sound pressure level that is inaudible to humans (that is, silence).

Human auditory characteristics (auditory psychological model) used in MPEG audio include a masking effect and a minimum audible limit characteristic. The masking effect is that when a loud sound is emitted at a certain frequency, the sound below the certain level of the frequencies in the vicinity of the sound becomes inaudible or hard to hear. In addition, the minimum audible limit characteristic is a constant frequency characteristic in which the human ear is most sensitive to the human voice band of several hundreds Hz, and sounds below a certain sound pressure level cannot be heard in the ultra-low and ultra-high frequencies. Is to have. Therefore,
Set the mask level that dynamically changes with the audio signal by synthesizing the masking effect and the minimum audible limit characteristic,
Data compression is applied to signals below that level. As a result, in layer I, encoding rate; 192k, 128kbps, compression rate;
1/4, sound quality is CD-DA (CD Digital Audio) and P
Same as CM (Pulse Code Modulation), with layer II encoding rate: 128k, 96kbps, compression rate: 1/6 to 1/8
, Sound quality is equal to MD and DCC, Layer III encoding rate: 128k, 96k, 64kbps, compression rate: 1/6 ~ 1 /
A compression effect of 12 and sound quality can be obtained.

In the MPEG audio encoder, first, the input audio signal is divided into 32 subbands by using a band division filter. Next, in the quantization, the masking effect and the minimum audibility characteristic are used as described above, and the bit amount is not assigned to the masked inaudible voice, so that the information amount is reduced and the data compression is performed.

FIG. 12 shows a block circuit of a main part of a conventional MPEG audio decoder 301. The MPEG audio decoder 301 is composed of a bit buffer 302 and a decode core circuit 303. The decoding core circuit 303 includes an inverse quantizer 304 and a band synthesizer 30.
5, the PCM output unit 306 and the control circuit 307, each AAU (frame) forming an audio stream is decoded according to the MPEG audio part.

The bit buffer 302 is a FIFO (First-
In-First-Out) RAM (Random Access Memory)
The audio stream transferred from an external device (recording medium such as video CD or DVD , information device such as personal computer, etc.) is sequentially stored.

The control circuit 307 includes a bit buffer 302.
AAUs that make up the audio stream stored in
The header attached to the beginning of the A is detected, and an audio stream for each AAU is read from the bit buffer 302 based on the detection result. In addition, the control circuit 307
Detects the sampling frequency specified in the header and generates a pipeline signal which is a pulse corresponding to the sampling frequency.

The operations of the respective units 304 to 306 are controlled according to pipeline signals. That is, each unit 304 to 30
The operation speed of 6 corresponds to the pipeline signal. The inverse quantization unit 304 performs inverse quantization of the above-described quantization in the encoder on each AAU read from the bit buffer 302.

The band synthesizing unit 305 performs a product-sum operation by a butterfly operation on the output of the dequantizing unit 304, and synthesizes the data divided into 32 subbands by the above encoder into one.

The PCM output unit 306 is composed of an output interface and a cross attenuator, and generates an audio signal (PCM output signal) from the output of the band synthesis unit 305.

The audio signal is D / A converted by a D / A converter (not shown), amplified by an audio amplifier (not shown), and sent to a speaker (not shown). Then, the sound is reproduced from the speaker.

[0026]

When the bit rate of the audio stream transferred from the external device is higher than the specified value, the bit buffer 302 may overflow.

Since the bit buffer 302 is composed of a ring buffer, if it overflows, the audio stream already stored in the bit buffer 302 is overwritten with the newly input audio stream. Then, the audio stream already stored in the bit buffer 302 is destroyed and lost. As a result, it becomes impossible to reproduce the audio of the lost audio stream, and the reproduced sound is cut off and the user feels uncomfortable.

The case where the bit rate of the audio stream is higher than the specified value is the following case. (1) When playing audio at a higher speed than normal (standard) playback speed. Such high-speed playback is, for example, when a recording medium is used as an external device, when the user performs fast-forward playback to listen to a voice in a short time, or fast-forward playback or fast-forward reverse to search for a desired voice. Used when playing.

(2) When an information device is used as the external device. For recording media such as video CDs and DVDs, MPEG is used.
The bit rate of the audio stream is set according to the audio part. However, in an information device such as a microcomputer, the audio stream is not always encoded according to the standard, and the bit rate of the audio stream may be out of the standard.

The present invention has been made to solve the above problems, and an object of the present invention is to prevent MPEG audio from overflowing a bit buffer.
It is to provide a decoder .

[0031]

[Means for Solving the Problems]

[0032]

[0033]

The invention described in claim 1 is an audio stream.
Bit buffer that stores reams and bit buffer
From each of the frames that make up the audio stream read from
To decode the video according to the MPEG audio part.
Decoding core circuit and bit buffer occupancy detected
Occupancy comparing the occupancy with a predetermined threshold
Based on the determination result of the amount determination circuit and the occupancy determination circuit,
Predetermine frames from the audio stream
Skip to place and transfer the remaining frames to the bit buffer.
And a frame skip means for transmitting the
If it is less than the threshold, the audio stream
Transfer to the bit buffer without skipping
Next, if the occupancy becomes larger than the first threshold,
Skip audio stream frame by frame
And the occupancy is less than the second threshold, which is less than the first threshold.
Frames from audio stream if smaller
Transfer to bit buffer without skipping
Is the gist.

The invention described in claim 2 is an audio stream.
Bit buffer that stores reams and bit buffer
From each of the frames that make up the audio stream read from
To decode the video according to the MPEG audio part.
Decoding core circuit and bit buffer occupancy detected
Occupancy judgment circuit and the judgment result of the occupancy judgment circuit.
As the bit buffer occupancy increases,
Equipped with speed control means to increase the operating speed of the code core circuit
What I got is the summary.

The invention according to claim 3 is the audio stream.
Bit buffer that stores reams and bit buffer
From each of the frames that make up the audio stream read from
To decode the video according to the MPEG audio part.
Decoding core circuit and bit buffer occupancy
Pipeline signal generation
And a decode core circuit comprising pipeline signal
Operating speed according to the pipeline signal generated by the generating means
Is controlled.

The invention according to claim 4 is the audio stream.
Bit buffer that stores reams and bit buffer
From each of the frames that make up the audio stream read from
To decode the video according to the MPEG audio part.
And a decode core circuit that
Scale included in audio data included in the frame
If the factor value falls below a specified value, the frame
The point is to skip without decoding.

The invention as defined in claim 5 is an audio stream.
Bit buffer that stores reams and bit buffer
From each of the frames that make up the audio stream read from
To decode the video according to the MPEG audio part.
And a decode core circuit that
The occupancy of the read buffer exceeds a specified value, and
Scale included in the audio data included in the
When the value of the actor becomes less than the specified value, the frame is
The point is to skip without decoding.

The invention according to claim 6 is the same as that of claims 1 to 5.
The MPEG audio decoder described in any one of
The decode core circuit reverses the audio stream.
The product of the inverse quantizer to quantize and the output of the inverse quantizer
Audio divided into subbands by performing a sum operation
Band combiner that combines streams and output of band combiner
And an output unit for generating an audio signal from
The summary will be given.

(Operation) According to the invention described in claim 1, the bit buffer is occupied.
If the volume exceeds the threshold, the audio stream
You can skip predetermined frames from the
And reduce the number of frames input to the bit buffer.
And become possible. Furthermore, the first and second thresholds are appropriately set.
By setting, the audio frame will be
The transfer time can be adjusted arbitrarily.

[0041]

[0042]

[0043]

[0044]

Therefore, the bit buffer overflow
Can be avoided.

[0046]

According to the second or third aspect of the invention, by increasing the operating speed of the decode core circuit,
The speed at which the audio stream is read from the bit buffer can be increased. Therefore, even if the bit rate of the audio stream is higher than the specified value, overflow can be avoided by reading the audio stream from the bit buffer at a speed higher than the bit rate.

According to the third aspect of the invention, the operation speed of the decode core circuit can be set arbitrarily by controlling the pipeline signal. Claim 4
According to the invention described in claim 5, when the value of the scale factor included in the audio data included in the frame becomes equal to or less than a predetermined value, the frame is skipped without decoding. Then, the operation speed of the decode core circuit is increased by not decoding the frame. Therefore, even if the bit rate of the audio stream is higher than the specified value, if you read the audio stream from the bit buffer at a speed higher than that bit rate,
Overflow can be avoided.

Further, according to the invention described in claim 5 , since the frame is skipped based on the occupied amount of the bit buffer, the skip control of the frame can be more accurately performed.

According to the invention described in claim 6 , the decode core circuit can be embodied with a simple structure.

[0051]

DETAILED DESCRIPTION OF THE INVENTION

(First Embodiment) A first embodiment of the present invention will be described below with reference to FIG.

FIG. 1 shows a block circuit of a main part of the MPEG audio decoder 1 of this embodiment. The MPEG audio decoder 1 is composed of a bit buffer 2, a decode core circuit 3, an analysis circuit 9, and a skip circuit 8. The circuits 2, 3, 8 and 9 are mounted on a 1-chip LSI.

The decode core circuit 3 is composed of an inverse quantization unit 4, a band synthesis unit 5, a PCM output unit 6 and a control circuit 7, and each A constituting an MPEG audio stream.
The AU (frame) is decoded according to the MPEG audio part.

As will be described later, an audio stream transferred from an external device (recording medium such as a video CD or DVD, information device such as a personal computer) is sent to the bit buffer 2 via the skip circuit 8. .

The skip circuit 8 has two nodes 8a and 8b.
And each node 8a, 8b according to the control of the analysis circuit 9.
The connection to the side is switched. When connected to the node 8a side, the audio stream transferred from the external device is transferred to the bit buffer 2 as it is.
When connected to the node 8b side, the audio stream transferred from the external device is skipped. As a result, the audio stream transferred to the bit buffer 2 is thinned by the amount skipped by the skip circuit 8.

The analysis circuit 9 analyzes each AAU forming the audio stream transferred from the external device,
The skip circuit 8 is controlled based on the analysis result. That is, the analysis circuit 9 transfers each node 8a, 8b of the skip circuit 8 so that only the data (header, error check, audio data) necessary for the decode core circuit 3 of each AAU is transferred to the bit buffer 2. Control the switching of. That is, the analysis circuit 9 connects the skip circuit 8 to the node 8a side when the data of the audio stream transferred from the external device is required, and transfers the data as it is to the bit buffer 2. Also, data that is not necessary in the decode core circuit 3 (for example, ancillary data, error data, etc.)
When, the skip circuit 8 is connected to the node 8b side,
Skip the data.

The bit buffer 2 is a RAM having a FIFO structure.
It is composed of a ring buffer consisting of the following and sequentially accumulates audio streams. The control circuit 7 detects the header attached to the head of each AAU forming the audio stream accumulated in the bit buffer 2, and reads the audio stream for each AAU from the bit buffer 2 based on the detection result. In addition, the control circuit 7
Detect the sampling frequency specified in the header,
A pipeline signal that is a pulse corresponding to the sampling frequency is generated.

The operations of the respective units 4 to 6 are controlled according to pipeline signals. That is, the operating speed of each unit 4 to 6 corresponds to the pipeline signal. Inverse quantizer 4
Performs inverse quantization of the above-described quantization in the encoder on each AAU read from the bit buffer 2.

The band synthesizing unit 5 performs a product-sum operation by a butterfly operation on the output of the inverse quantizing unit 4, and synthesizes the data divided into 32 sub-bands in the above encoder into one.

The PCM output section 6 is composed of an output interface and a cross attenuator, and generates an audio signal (PCM output signal) from the output of the band synthesis section 5. As will be described later, the audio signal is D / A converted by the D / A converter, then amplified by the audio amplifier and sent to the speaker. Then, the sound is reproduced from the speaker.

As described above, according to the present embodiment, the following actions and effects can be obtained with the above-mentioned configuration. By providing the analysis circuit 9 and the skip circuit 8, only the data (header, error check, audio data) necessary for the decode core circuit 3 is stored in the bit buffer 2.
Transfer to. As a result, the bit buffer 2 can store only necessary data.

Conventional MPEG Audio Decoder 301
However, since all the audio streams transferred from the external device are transferred to the bit buffer 302, the bit buffer 302 also stores unnecessary data. Therefore, in particular, when the bit rate of the audio stream transferred from the external device is higher than the specified value, the bit buffer 302 may overflow.

However, in this embodiment, since the bit buffer 2 stores only the necessary data, the amount of data stored in the bit buffer 2 (the amount occupied by the bit buffer 2)
Is reduced by eliminating unnecessary data. Therefore, even if the bit rate of the audio stream transferred from the external device is higher than the specified value, overflow of the bit buffer 2 can be avoided.

To put it the other way round, the bit buffer 2
If there is no risk of overflow of the bit buffer 2,
The capacity of can be reduced.

Since only the necessary data is stored in the bit buffer 2, the control circuit 7 is
It is possible to reduce the number of accesses from the control circuit 7 to the bit buffer 2 when reading AAU from.

(Second Embodiment) A second embodiment of the present invention will be described below with reference to FIGS. In this embodiment, the same components as those in the first embodiment are designated by the same reference numerals and detailed description thereof will be omitted.

FIG. 2 shows a block circuit of a main part of the MPEG audio decoder 11 of this embodiment. The MPEG audio decoder 11 includes a bit buffer 2, a decode core circuit 3, a skip circuit 8, an occupancy determination circuit 12, and an analysis circuit 13. In addition, each circuit 2, 3, 8,
12 and 13 are mounted on a one-chip LSI.

The skip circuit 8 has two nodes 8a and 8b.
, And the connection to each of the nodes 8a and 8b is switched under the control of the occupancy determination circuit 12 and the analysis circuit 13. The occupancy determination circuit 12 detects the occupancy B of the bit buffer 2 and determines whether there is a risk of overflow.

The analysis circuit 13 analyzes each AAU forming the audio stream transferred from the external device, and switches each node 8a, 8b of the skip circuit 8 so that the AAU transferred to the bit buffer 2 is not interrupted. . As a result, the audio stream transferred to the bit buffer 2 is thinned in AAU units by the amount skipped by the skip circuit 8.

Next, the operation of this embodiment will be described with reference to FIG. The occupancy determination circuit 12 determines that there is no risk of overflow of the bit buffer 2 when the occupancy B is smaller than a preset threshold TH1. In this case, the analysis circuit 13 connects the skip circuit 8 to the node 8a side so that the AAU transferred to the bit buffer 2 is not interrupted, and transfers the audio stream transferred from the external device to the bit buffer 2 as it is. Then,
The occupancy B of the bit buffer 2 rises in accordance with the bit rate of the audio stream transferred from the external device (period α in the figure).

Next, the occupancy determination circuit 12 determines that the bit buffer 2 may overflow when the occupancy B becomes larger than the preset threshold TH1. In this case, the analysis circuit 13 connects the skip circuit 8 to the node 8b side so that the AAU transferred to the bit buffer 2 is not interrupted, and skips the audio stream transferred from the external device in AAU units. Then, the occupied amount B decreases as AAU is read from the bit buffer 2 (period β in the figure).

Then, the occupation amount determination circuit 12 determines that the occupation amount B
Is smaller than a preset threshold value TH2, it is determined that there is no risk of the bit buffer 2 overflowing. In this case, the analysis circuit 13 connects the skip circuit 8 to the node 8a side so that the AAU transferred to the bit buffer 2 is not interrupted, and transfers the audio stream transferred from the external device to the bit buffer 2 as it is ( Illustrated period α).

As described above, AAU is not input to the bit buffer 2 during the period β. Therefore, the audio signal cannot be generated for the AAU corresponding to the period β, and the time when the audio signal is continuously generated (that is, the time when the voice is continuously reproduced).
Corresponds to the above period α. If the time during which the sound is continuously reproduced becomes too short, the reproduced sound is cut off and the user feels uncomfortable. Therefore, each threshold TH
It is necessary to set 1 and TH2 to optimum values by actual listening.

As described above, according to this embodiment, the following actions and effects can be obtained by the above-mentioned configuration. By providing the occupation amount determination circuit 12 and the skip circuit 8, the occupation amount B of the bit buffer 2 can be optimally controlled. Therefore, even if the bit rate of the audio stream transferred from the external device is higher than the specified value, overflow of the bit buffer 2 can be reliably avoided.

Since the audio stream skipped by the skip circuit 8 is in AAU units, the audio stream stored in the bit buffer 2 is also in AAU units, and the decoding core circuit 3 can generate an audio signal for each AAU.

A simulation of the effect at the time of high speed reproduction in this embodiment has confirmed that the overflow of the bit buffer 2 can be avoided even at the high speed reproduction of 8 × speed reproduction or higher, and that the reproduced sound is not interrupted. It was

As shown in FIG. 4, the thresholds TH1 and TH2 may be set to the same value. In this case, since the period α is shortened, the time during which the sound is continuously reproduced is shortened, and the sound breakage easily occurs. On the other hand, the occupancy determination circuit 12
Since the process of 1 is simplified, the circuit scale of the occupation amount determination circuit 12 can be reduced.

(Third Embodiment) The third embodiment of the present invention will be described below with reference to FIG. In the present embodiment, the same components as those in the first and second embodiments are designated by the same reference numerals, and detailed description thereof will be omitted.

FIG. 5 shows a block circuit of a main part of the MPEG audio decoder 21 of this embodiment. The MPEG audio decoder 21 includes a bit buffer 2, a decode core circuit 3, and an occupation amount determination circuit 12.
The circuits 2, 3 and 12 are mounted on a 1-chip LSI.

The decode core circuit 3 is composed of an inverse quantization unit 4, a band synthesis unit 5, a PCM output unit 6, and a control circuit 22. The audio stream transferred from the external device is directly transferred to the bit buffer 2. The occupancy determination circuit 12 detects the occupancy B of the bit buffer 2 and determines whether there is a risk of overflow.

The control circuit 22 detects the header at the beginning of each AAU forming the audio stream accumulated in the bit buffer 2, and based on the detection result, the audio stream for each AAU from the bit buffer 2 is detected. Read out. The control circuit 22 also generates a pipeline signal that is a pulse based on the determination result of the occupation amount determination circuit 12. That is, the control circuit 22 shortens the generation period of the pipeline signal as the occupied amount B of the bit buffer 2 increases.

The operation of each unit 4 to 6 is controlled according to the pipeline signal. That is, the operating speed of each unit 4 to 6 corresponds to the generation period of the pipeline signal. Therefore, as the occupied amount B of the bit buffer 2 increases, the generation period of the pipeline signal becomes shorter and the operating speed of each unit 4 to 6 becomes faster.

The speed at which the audio stream is read from the bit buffer 2 depends on the processing speed of the decode core circuit 3 (that is, the operating speed of each section 4 to 6). Therefore, if the operating speed of each unit 4 to 6 is increased, the speed at which the audio stream is read from the bit buffer 2 is also increased. That is, even if the bit rate of the audio stream transferred from the external device is higher than the specified value, overflow can be avoided by reading the audio stream from the bit buffer 2 at a speed equal to or higher than the bit rate.

As described above, according to this embodiment, even when the bit rate of the audio stream transferred from the external device is higher than the specified value, the overflow of the bit buffer 2 can be avoided.

In the present embodiment, the decode core circuit 3
The bit rate of the audio signal is increased as the processing speed of is increased. As a result, not only the pitch (pitch) of the reproduced voice rises but also the utterance speed (speech speed) increases. Therefore, if the processing speed of the decode core circuit 3 is set too high, the reproduced sound will not be cut off, but the user still feels uncomfortable. Therefore, it is important that the generation period of the pipeline signal is not too short, and it is necessary to set the generation period to an optimum value by actual listening.

(Fourth Embodiment) A fourth embodiment of the present invention will be described below with reference to FIG. In addition, in this embodiment, the same components as those in the first to third embodiments are designated by the same reference numerals, and detailed description thereof will be omitted.

FIG. 6 shows a block circuit of a main part of the MPEG audio decoder 31 of this embodiment. The MPEG audio decoder 31 includes a bit buffer 2, a decode core circuit 3, and an occupation amount determination circuit 12.
The circuits 2, 3 and 12 are mounted on a 1-chip LSI.

The decoding core circuit 3 includes an inverse quantizer 32,
It is composed of a band synthesis unit 5, a PCM output unit 6, and a control circuit 7. The audio stream transferred from the external device is directly transferred to the bit buffer 2.

The occupancy determination circuit 12 uses the bit buffer 2
The occupancy B is detected and it is determined whether there is a risk of overflow. The band synthesizing unit 5 performs a product-sum operation by a butterfly operation on the output of the dequantization unit 32, and synthesizes the data divided into 32 subbands in the encoder described above into one.

The dequantization unit 32 dequantizes the AAUs read from the bit buffer 2 by the quantization in the encoder. Also, the inverse quantizer 32
Determines, based on the determination result of the occupancy determination circuit 12 and the scale factor included in the audio data included in the AAU, whether to perform the inverse quantization process or skip the AUU.

That is, since the value of the scale factor corresponds to the sound pressure level of the reproduced voice, when the value of the scale factor becomes equal to or less than the predetermined value, the reproduced sound of the AUU cannot be heard by humans. (Ie silence)
become. That is, even if the silent AUU is skipped, the reproduced voice section (the section in which the voice exists) does not change. Therefore, by skipping the silent AUU from the dequantization unit 32, the band synthesis unit 5 and the PCM output unit 6 do not need to process the AUU, and the speed at which the audio stream is read from the bit buffer 2 is correspondingly increased. Can be faster. That is, even if the bit rate of the audio stream transferred from the external device is higher than the specified value, overflow can be avoided by reading the audio stream from the bit buffer 2 at a speed equal to or higher than the bit rate.

However, if the silent AUU is skipped, the user feels unnatural because there is no silent section (the time when there is no voice) in the reproduced voice. Therefore, only when the bit buffer 2 may overflow based on the determination result of the occupancy determination circuit 12,
Skip silent AUUs. By doing so, it is possible to avoid the overflow of the bit buffer 2 while keeping the reproduced sound as natural as possible.

FIG. 8 shows a block circuit of the MPEG system decoder. The MPEG system decoder 101 is
Audio Video Parser (AV Parser) 102, MPE
The G video decoder 201 and any one MPEG audio decoder among the above-mentioned MPEG audio decoders 1, 11, 21, 31 and 41 are provided.

The AV parser 102 has a demultiplexer (DMUX; DeMUltipleXer) 103, and receives the MPEG system stream transferred from the external device 104. The DMUX 103 separates the system stream into an MPEG video stream and an MPEG audio stream. The video stream is output to the video decoder 201, and the audio stream is output to the audio decoders 1, 11, 21, 31, 41.

The video decoder 201 decodes the video stream in accordance with the MPEG video part and generates a video signal. The video signal is the display 10
5, and the moving image is reproduced on the display 105.

Audio decoders 1, 11, 21, 3
1, 41 generate the audio signal as described above,
The audio signal is D / A converted by the D / A converter 106, amplified by the audio amplifier 107, and sent to the speaker 108. Then, the speaker 10
Audio is reproduced from 8.

The bit rate of the system stream transferred from the external device 104 corresponds to the reading speed. Then, the bit rates of the video stream and the audio stream are the same as those of the system stream.

Therefore, the video decoder 12 produces a video output corresponding to the bit rate of the system stream. That is, if the bit rate of the system stream is higher than that during normal reproduction (during standard reproduction), the moving image is reproduced at high speed on the display 105, and if smaller than during normal reproduction, the moving image is reproduced at low speed on the display 105.

FIG. 9 shows a block circuit of a main part of the MPEG video decoder 201 having a high speed reproduction function. MPE
The G video decoder 201 includes a bit buffer 202, a picture header detection circuit 203, an MPEG video decode core circuit (hereinafter abbreviated as a decode core circuit) 204, a variable threshold overflow judgment circuit (hereinafter abbreviated as a judgment circuit) 205, and a picture skip circuit. 206 and a control core circuit 207. Each circuit 203 to 20
7 is mounted on a 1-chip LSI.

The control core circuit 207 includes the circuits 202 to 20.
Control 6 The MPEG video stream transferred from the external device is transferred to the bit buffer 202.

The bit buffer 202 is an R having a FIFO structure.
It is composed of an AM ring buffer, and sequentially stores the transferred video stream as it is. The picture header detection circuit 203 includes a bit buffer 202.
The picture header attached to the beginning of each picture of the video stream stored in 1 is detected, and the picture type (I, P, B) specified in each picture header is detected.

The control core circuit 207 reads an appropriate video stream for each frame period from the bit buffer 202 based on the detection result of the picture header detection circuit 203 and the determination result of the determination circuit 205 described later. Note that the video stream read from the bit buffer 202 remains in the bit buffer 2 even after being read.
It is left as it is in 02.

Each picture read from the bit buffer 202 is transferred to the decode core circuit 204 via the picture skip circuit 206. Decode core circuit 2
04 decodes each picture in accordance with the MPEG video part, and generates a video signal for each picture. The video signal is output to the display 105 provided outside the video decoder 201.

The picture skip circuit 206, under the control of the control core circuit 207, each node 206a, 206b.
The connection to the side is switched. When the picture skip circuit 206 is connected to the node 206a side, the picture read from the bit buffer 202 is transferred to the decode core circuit 204 as it is. Also, node 2
When connected to the 06b side, the picture read from the bit buffer 202 is skipped without being transferred to the decode core circuit 204 . As a result, the decode core circuit 2
The picture transferred to 04 is the picture skip circuit 2
The amount skipped by 06 is thinned out in picture units.

The judgment circuit 205 sets the threshold value Bthn of the occupied amount Bm of the bit buffer 202 based on the reproduction speed designated from the outside, and the occupied amount Bm of the bit buffer 202 is set.
Compare m with the threshold value Bthn. Incidentally, the designation of the reproduction speed from the outside is performed by the magnification n with respect to the normal reproduction speed. For example, during 2 × speed reproduction, the magnification n = 2 and the threshold Bthn = Bth2. Further, during normal reproduction, the magnification n = 1 and the threshold value Bthn = Bth1.

If the occupied amount Bm of the bit buffer 202 does not exceed the threshold value Bthn, the decision circuit 205 decides that the bit buffer 202 is normal without fear of overflow. In this case, the control core circuit 2
07 reads the video stream for one picture from the bit buffer 202. Then, the control core circuit 207
Connects the picture skip circuit 206 to the node 206a side and transfers the picture read from the bit buffer 202 to the decode core circuit 204.

Further, the judgment circuit 205 judges that the bit buffer 202 may overflow when the occupied amount Bm of the bit buffer 202 exceeds the threshold value Bthn. In this case, the control core circuit 207 continues until the occupied amount Bm of the bit buffer 202 falls below the threshold value Bthn.
A video stream for an appropriate picture is read from the bit buffer 202. Then, the control core circuit 207
The picture skip circuit 206 is connected to the node 206b side to skip all video streams of appropriate pictures read from the bit buffer 202.

FIG. 10 shows changes in the occupied amount Bm of the bit buffer 202. Occupancy Bm of bit buffer 202
Increases the bit rate RB as the slope of the graph. The bit rate RB is defined as shown in Expression (1) according to BR (Bit Rate) of the sequence header attached to the beginning of the sequence. Also, the picture rate RP of the video stream transferred from the external device is defined by the PR (Picture Rate) of the sequence header. And
The capacity B of the bit buffer 202 is the VBV (Vbv [Video Buffering Verifier] Buffer Siz) of the sequence header.
According to e), it is defined as shown in equation (2). Then, for each frame period, the video stream for one picture to be decoded by the decode core circuit 204 at that time is read from the bit buffer 202 at once. Here, the data amount X of the video stream input to the bit buffer 202 in one frame period is defined as shown in Expression (3) according to the bit rate RB and the picture rate RP. Therefore, the bit buffer 202
The occupancy Bm (= B0 of the bit buffer 202 immediately after the video stream for one picture is read out at once from
To B6) are defined so as to satisfy the condition shown in Expression (4) based on the data amount X and the capacity B of the bit buffer 202.

RB = 400 × BR (1) B = 16 × 1024 × VBV (2) X = RB / RP (3) 0 <Bm <BX−B− (RB / RP) ... (4) The bit buffer 20 is set so as to satisfy the condition shown in Expression (4).
If the occupancy Bm of 2 is specified, the bit buffer 2
02 does not overflow or underflow. Conversely, when the occupied amount Bm of the bit buffer 202 exceeds the threshold value (BX), there is a high possibility that the bit buffer 202 will overflow due to the video stream input to the bit buffer 202 in the next one frame period. Become.

In the video decoder 201, the values of the bit rate RB, the picture rate RP, and the capacity B are defined so that the expression (4) is satisfied during normal reproduction. That is, as shown in equation (2), the bit buffer 2
If the capacity B of 02 is set, the bit buffer 202 will not overflow or underflow in an ideal state even if the connection of the picture skip circuit 206 is fixed to the node 206a side. .

Therefore, during normal reproduction, the occupied amount Bm (= B0 to B4) immediately after the data for one picture is read from the bit buffer 202 at once is the threshold value Bth.
Based on 1, it is defined to satisfy the condition shown in equation (5). The threshold value Bth1 is set as shown in Expression (6) based on Expression (4).

0 <Bm <Bth1 <B (5) Bth1 = B−X = B− (RB / RP) (6) By the way, in the actual state, as shown in the formula (2), Even if the capacity B of the bit buffer 202 is set, if the connection of the picture skip circuit 206 is fixed to the node 206a side, the bit buffer 202 may overflow.

However, in the video decoder 201, the occupied amount Bm of the bit buffer 202 during normal reproduction
Is greater than the threshold value Bth1, it is determined that the bit buffer 202 may overflow. Then, a video stream of an appropriate picture is read from the bit buffer 202 until the occupied amount Bm of the bit buffer 202 falls below the threshold value Bth1. Then, the picture skip circuit 206 is connected to the node 206b side, and all the video streams of appropriate pictures read from the bit buffer 202 are skipped. Therefore, according to the video decoder 201, during normal reproduction,
The bit buffer 202 never overflows.

Bit buffer 202 during high speed reproduction
The occupancy Bm rises with the bit rate n × RB as the slope of the graph. For example, the occupied amount Bm of the bit buffer 202 at the time of double speed reproduction increases with the bit rate 2 × RB as the slope of the graph.

Therefore, at the time of high speed reproduction, the occupied amount Bm (= B0 to B4) immediately after the data of one picture is read from the bit buffer 202 at once is the threshold value Bthn.
Is defined so as to satisfy the condition shown in Expression (7). The threshold value Bthn is set as shown in equation (8).

0 <Bm <Bthn (7) Bthn = B−n × X = B− (n × RB / RP) (8) The occupied amount Bm of the bit buffer 202 during high speed reproduction Exceeds the threshold value Bthn, the bit buffer 202
Is determined to be likely to overflow. For example, the occupancy amount Bm is equal to the threshold value Bth2 (= B-
(2 × RB / RP)), the occupied amount Bm is the threshold value Bth3 (= B− (3 × RB / RP)) at the time of 3 × speed reproduction.
It is determined that the bit buffer 202 may overflow when the value exceeds. Then, a video stream of an appropriate picture is read from the bit buffer 202 until the occupied amount Bm of the bit buffer 202 falls below a threshold value Bthn, and the video stream is skipped. Therefore, according to the video decoder 201, the bit buffer 202 does not overflow during high speed reproduction.

During decoding of an arbitrary picture in the decode core circuit 204, the bit buffer 202
Overflows, the newly input video stream is overwritten on the portion of the picture being decoded that remains in the bit buffer 202. As a result, the bit buffer 202 of the picture being decoded
The part remaining in is destroyed and lost. Then, the decoding core circuit 204 cannot complete the decoding of the picture, and cannot generate the video signal of the picture. Therefore, it is absolutely necessary to prevent the bit buffer 202 from overflowing while the decode core circuit 204 is decoding an arbitrary picture.

Therefore, it is necessary to determine whether or not the bit buffer 202 may overflow before the decoding core circuit 204 starts decoding any picture. More precisely, when the picture header detection circuit 203 detects the picture header,
It is necessary to determine if the bit buffer 202 may overflow and to decide whether to skip the picture via the picture skip circuit 206.

By the way, the data amount of one picture is 0.
Although it is up to 40 bytes, the amount of data cannot be known until the decoding is completed in the decoding core circuit 204. Also, the decoding processing time for one picture is
Although it depends on the data amount of the picture and the operation speed of the decode core circuit 204, it is usually 1 / one frame period.
It is about 3 to 3/4.

When the data amount of the picture read from the bit buffer 202 is 0 bytes, the occupied amount Bm of the bit buffer 202 does not change before and after the reading of the picture, and therefore overflow is avoided even if the picture is skipped. It is not possible. Conversely, even if the data amount of the picture read from the bit buffer 202 is 0 bytes, overflow does not occur if the bit buffer 202 has sufficient free space.

Therefore, a free space corresponding to the data amount of the video stream input to the bit buffer 202 in one frame period is secured in the bit buffer 202. Then, even if the data amount of the picture read from the bit buffer 202 is 0 byte, overflow does not occur.

The data amount of the video stream input to the bit buffer 202 in one frame period is (n × X =
n * RB / RP). If the free capacity of the bit buffer 202 is equal to or larger than this data amount, it will not overflow. Therefore, if the threshold value Bthn is set as shown in Expression (8), overflow of the bit buffer 202 can be reliably avoided.

That is, the decision circuit 205 checks the free space of the bit buffer 202 at the time when the picture header detection circuit 203 detects the picture header, and secures a sufficient free space (n × X = n × RB / RP). It is determined whether it has been done. If the sufficient free space is not secured, the control core circuit 2 based on the picture header
The picture read by the 07 from the bit buffer 202 is skipped via the picture skip circuit 206. Subsequently, the determination circuit 205 checks the free space of the bit buffer 202 again when the picture header detection circuit 203 detects the next picture header. Since the time required for these processes is much shorter than the decode processing time of the decode core circuit 204, it is sufficient to start the decode processing of the decode core circuit 204 after securing sufficient free space in the bit buffer 202. Be in time.

By the way, the picture header detection circuit 203
Bit buffer 20 when a picture header is detected, or after the decoding core circuit 204 starts decoding.
2 may underflow. In this case, as soon as the video stream is input to the bit buffer 202,
Since it suffices to sequentially read the video stream for one picture from the bit buffer 202, there is no particular problem.

As described in detail above, the video decoder 20
According to 1, it is possible to obtain the following effects. It is possible to avoid overflow of the bit buffer 202 during normal reproduction.

Bit buffer 2 during high-speed reproduction
02 overflow can be avoided. By providing the determination circuit 205 and the picture skip circuit 206, overflow of the bit buffer 202 can be avoided. Since the control of the determination circuit 205 and the picture skip circuit 206 is simple as described above, the control core circuit 207 does not need to be configured using a microcomputer. Then, each circuit 203
˜207 are mounted on a one-chip LSI. Therefore, the video decoder 201 can be downsized.

Node 2 of the picture skip circuit 206
The video stream skipped from the 06b side is in picture units. Therefore, the data is not interrupted in the middle of the picture transferred to the decode core circuit 204. Therefore, the decode core circuit 204 can decode not only I pictures but also P pictures and B pictures. As a result, frame drop occurring in the moving image reproduced on the display 105 is reduced. Therefore, 2-
It is possible to display several frames / second during high-speed playback, which is relatively slow at 4 times. Therefore, it is possible to smooth the motion of the moving image during high-speed reproduction and significantly improve the image quality.

By the way, the above video decoder 201
In, the two thresholds B2thn and B3thn may be set so as to satisfy the formula (9). Each threshold B2t
The values of hn and B3thn are set according to the reproduction speed as described above, and may be set appropriately by actually examining the image quality of the moving image reproduced on the display 105.

0 <B3thn <B2thn <B (9) The determination circuit 205 determines the occupied amount Bm of the bit buffer 202.
And the threshold values B3thn and B2thn are compared with each other to determine in which region the occupancy Bm is included in the expressions (10) to (12).

Bm <B3thn (10) B3thn <Bm <B2thn (11) B2thn <Bm (12) The decision circuit 205 determines the bit buffer 202 of the bit buffer 202 as shown in equation (10). If the occupied amount Bm does not exceed the threshold value B3thn, it is determined that the bit buffer 202 is normal without fear of overflow. In this case, the control core circuit 207 reads the video stream for one picture from the bit buffer 202. Then, the control core circuit 20
7 connects the picture skip circuit 206 to the node 206a side and transfers the picture read from the bit buffer 202 to the decoding core circuit 204.

The decision circuit 205 determines that the picture read from the bit buffer 202 is an I picture when the occupied amount Bm of the bit buffer 202 exceeds the threshold value B2thn and does not exceed the threshold value Bthn, as shown in equation (12). Or P
If it is a picture, the first flag is set. Also, the formula (1
As shown in 1), the occupied amount Bm of the bit buffer 202
Is above the threshold B3thn and not above the threshold B2thn,
If the picture read from the bit buffer 202 is a P picture, the second flag is set. First or second
If the flag is set, the control core circuit 207 connects the picture skip circuit 206 to the node 206b side if the picture read from the bit buffer 202 is a B picture, even in the case of the expression (10). Skip the picture.

In FIG. 11, the occupied amount Bm of the bit buffer 202 when two threshold values B2thn and B3thn are set.
Shows the change of. When the occupied amount Bm exceeds the threshold value B3thn,
If the picture read from the bit buffer 202 is a B picture, the picture is skipped without decoding (illustration *
1). Even if the occupied amount Bm still exceeds the threshold value B3thn after the B picture is skipped, the bit buffer 202
If the next picture read from is an I or P picture, it is decoded (* 2 in the figure).

Even if the occupied amount Bm exceeds the threshold value B3thn, if the picture read from the bit buffer 202 is an I picture or a P picture, it is decoded (* 3 in the figure). Here, if the occupied amount Bm still exceeds the threshold value B3thn after the decoding of the I picture or P picture, if the next picture read from the bit buffer 202 is a B picture, it is skipped without decoding (illustration *). 4). The skip of this B picture is the occupancy B
Repeat until m falls below the threshold B3thn.
5).

If the occupied amount Bm exceeds the threshold value B2thn, and if the picture read from the bit buffer 202 is an I picture or a P picture, the decision circuit 205 makes the first decision.
Set the flag (see * 6). When the first flag is set and the picture read next from the bit buffer 202 is a B picture, the occupied amount Bm is the threshold value B3t.
Even if it falls below hn, the B picture is skipped (Fig. * 7).

The occupied amount Bm exceeds the threshold value B3thn and the threshold value B3
If it does not exceed 2thn, if the picture read from the bit buffer 202 is a P picture, the determination circuit 205
Sets a second flag (figure * 8). When the second flag is set and the picture read next from the bit buffer 202 is a B picture, the B picture is skipped even if the occupancy Bm is below the threshold value B3thn (Fig. 9). ).

The occupied amount Bm exceeds the threshold value B3thn and the threshold value B3
If it does not exceed 2thn and the picture read from the bit buffer 202 is an I picture, the determination circuit 20
5 does not set the second flag (illustration * 10). When the second flag is not set and the occupation amount Bm is below the threshold value B3thn, the picture read next from the bit buffer 202 is decoded even if it is a B picture.

As described above, the two threshold values B2thn and B3thn
In case of setting, the following effects can be obtained in addition to the effects of the video decoder 201 described above.
When the occupied amount Bm of the bit buffer 202 exceeds the threshold value B3thn and does not exceed the threshold value B2thn , the I picture and the P picture are decoded as much as possible, and the B picture is skipped preferentially.

Since B pictures are generated by bidirectional prediction, their importance is lower than that of I pictures and P pictures. Therefore, by skipping B-pictures of low importance with priority, it is possible to further reduce dropped frames that occur in a moving image played on the display 105. As a result, it is possible to further smooth the motion of the moving image during high-speed reproduction and further improve the image quality.

By setting the first flag, the bit buffer 2 is decoded after the I picture or P picture is decoded.
Even if the occupancy Bm of 02 falls below the threshold value B3thn, the B picture read next from the bit buffer 202 can be skipped in advance with a margin. Further, by setting the second flag, even if the occupied amount Bm of the bit buffer 202 is below the threshold value B3thn after the P picture is decoded, the B picture read next from the bit buffer 202 is skipped in advance with a margin. You can

As described above, skipping a B picture in advance is nothing but taking precautions against the next overflow of the bit buffer 202. Therefore, the overflow of the bit buffer 202 can be avoided more reliably.

The data amount of the I picture is as large as 2-3 times that of the P picture. Therefore, the degree of decrease in the occupied amount Bm of the bit buffer 202 is larger when the I picture is read than when the P picture is read. Therefore, rather than after the P picture is read
The bit buffer 20 after the I picture is read
2 is less likely to overflow. Therefore,
By setting the first and second flags, the precautionary measures are differentiated between I pictures and P pictures. That is, the threshold B2thn of the preventive measures for the I picture is set to P
By setting a value higher than the threshold B3thn of the preventive measure for the picture, the preventive measure for the I picture can be set looser than that for the P picture. As a result, it is possible to reduce unnecessary skipping of B pictures.

When the video stream having the GOP configuration (arrangement of picture types) shown in a) and b) below was transferred from an external device, simulation was performed, and the following results were obtained.

A) IBPBPBPBP ... b) IBBPBBPBBPBBPBBIBP ... [1] During double speed reproduction; In the case of a), all I pictures and P pictures can be decoded, and as a result, 30 frames / second full rate Can be displayed with. In the case of b), all the I and P pictures and part of the B pictures can be decoded, and as a result, they can be displayed at 25 frames / second or more.

[2] At 4 × speed reproduction; a) and b), I picture and 3 to 4 P pictures following it can be decoded, and as a result, it can be displayed at 15 frames / second or more. The above-described embodiments may be modified as follows, and in that case, the same operation and effect can be obtained.

(1) Any two or more of the first to fourth embodiments are combined as appropriate. In this way, a more excellent effect can be obtained by the synergistic action of the combined embodiments.

FIG. 7 shows a block circuit of a main part of the MPEG audio decoder 41 in the case where all the first to fourth embodiments are combined (fifth embodiment). The MPEG audio decoder 41 includes a bit buffer 2, a decode core circuit 3, an occupancy determination circuit 12, an analysis circuit 42, and a skip circuit 8. The decoding core circuit 3 includes an inverse quantization unit 32, a band synthesis unit 5, a PCM output unit 6, and a control circuit 2.
It consists of two. The analysis circuit 42 switches the operations of the analysis circuits 9 and 13 to perform the operations. By doing so, an extremely excellent effect can be obtained by the synergistic action of the first to fourth embodiments.

(2) The first to fourth embodiments are replaced by software processing using a CPU. That is, the signal processing in each circuit (2 to 32) is replaced with software-like signal processing using a CPU. (3) In the embodiment shown in FIGS. 1, 2 and 7,
In order to make the explanation easy to understand, the skip circuit 8 has the nodes 8a and 8b, and the connection is switched according to the data. However, instead of this configuration, the skip circuit 8 is replaced with the analysis circuit. It may be configured by a logic circuit that passes only necessary data in the audio stream in accordance with the signals from 9, 13, and 42.

Similarly, the picture skip circuit 206 shown in FIG. 9 may be constructed by a logic circuit for passing only the picture data to be decoded. (4) As shown in FIG. 13, in the embodiment shown in FIGS. 1, 2 and 7, the skip circuit 8 may be omitted and the control circuits 7 and 22 may have the same function as the skip circuit 8. .

In this case, the control circuits 7 and 22 are connected to the related analysis circuits 9, 13 and 42, and the analysis circuits 9 and 1 are connected.
When the necessary data is supplied to the bit buffer 2 based on the analysis results from 3, 42, the necessary data is stored in the regular address An in the bit buffer 2. When unnecessary data is supplied based on the analysis result, the address An + 1 next to the regular address An is added.
Temporarily stored in.

Following this, when another data is newly supplied, the control circuits 7 and 22 determine the next address An.
Memory control of the bit buffer 2 is performed so that unnecessary data stored in +1 is replaced with new necessary data. When unnecessary data is newly supplied while unnecessary data is stored at the next address An + 1,
The control circuits 7 and 22 perform memory control so that the unnecessary data stored at the next address An + 1 is replaced with new unnecessary data.

Rewriting of unnecessary data by this memory control prevents overflow of the bit buffer 2.
The unnecessary data referred to here means the first embodiment or the second data.
In the embodiment, it refers to data to be skipped by the skip circuit 8.

By the way, the second one of Japanese Patent Application Laid-Open No. 7-307674 (H03M 7/30, G10L 7/04, G10L 7/06, G10L 9/18)
In rows 40 to 46, the transfer rate (corresponding to the bit rate of the audio stream in each of the above embodiments) of the data input to the decoder (corresponding to the decode core circuit 3 in each of the above embodiments) is increased. It is described that the processing speed of the decoder is improved and the data is instantly decoded.

However, there is no suggestion in the publication that the occupancy of the bit buffer 2 is determined and the processing speed of the decode core circuit 3 is increased accordingly, as in the third embodiment. Therefore, the operation and effect of the third embodiment cannot be achieved by the technique described in the publication.

[0154] Also, the eighth column, line 29 to the ninth column 11 of the publication.
In the row, it is described that the data thinning process to be input to the decoder is executed by controlling the writing of the data to the FIFO memory (corresponding to the bit buffer 2 in each of the above embodiments).

However, the publication does not even suggest skipping unnecessary data in the decode core circuit 3 as in the first embodiment. Further, there is no disclosure at all in the same publication of determining the occupancy of the bit buffer 2 and skipping the data accordingly, as in the second embodiment. Therefore, with the technique described in the publication, the operation and effect of the first embodiment or the second embodiment cannot be achieved at all.

As described above, the above-described embodiments differ from the invention described in the publication in the structure, operation, and effect. Further, it is difficult for a person skilled in the art to conceive the operation and effect of each of the above-mentioned embodiments based on the publication.

Although the respective embodiments have been described above, technical ideas other than the claims which can be understood from the respective embodiments will be described below together with their effects. (A) In the MPEG audio decoder according to any one of claims 1 to 5 , a D / A converter (10) for D / A converting the output of the decode core circuit (3).
6) and an MPEG audio decoder comprising an audio amplifier (107) for amplifying the output of the D / A converter.

By doing so, an analog signal for driving the speaker can be generated from the digital audio signal .

[0159]

[0160]

As described above in detail, according to the present invention, it is possible to avoid the overflow of the bit buffer.
An MPEG audio decoder can be provided.

[Brief description of drawings]

FIG. 1 is a block circuit diagram of a main part of a first embodiment.

FIG. 2 is a block circuit diagram of a main part of a second embodiment.

FIG. 3 is a graph for explaining the operation of the second embodiment.

FIG. 4 is a graph for explaining the operation of the second embodiment.

FIG. 5 is a block circuit diagram of an essential part of a third embodiment.

FIG. 6 is a block circuit diagram of an essential part of a fourth embodiment.

FIG. 7 is a block circuit diagram of an essential part of a fifth embodiment.

FIG. 8 is a block circuit diagram of a main part of an MPEG system decoder.

FIG. 9 is a block circuit diagram of a main part of an MPEG video decoder.

FIG. 10 is a graph for explaining the operation of the MPEG video decoder.

FIG. 11 is a graph for explaining the operation of the MPEG video decoder.

FIG. 12 is a block circuit diagram of a main part of a conventional form.

FIG. 13 is a block circuit diagram of a main part of another embodiment.

[Explanation of symbols]

1, 11, 21, 31, 41 ... MPEG audio decoder 2 ... Bit buffer 3 ... Decoding core circuit 4, 32 ... Dequantization unit 5 ... Band synthesis unit 6 ... PCM output unit 7, 22 ... Control circuit 8 ... Skip means , Frame skip means, frame control means, skip circuit 9 constituting data extraction means ... analysis circuit 12 constituting data extraction means ... speed control means, occupation determination circuit 13 constituting frame control means ... frame skip means, frame Analysis circuit 22 constituting the control means ... Control circuit constituting the speed control means

─────────────────────────────────────────────────── --- Continuation of the front page (56) Reference JP-A-7-143010 (JP, A) JP-A-6-61950 (JP, A) JP-A-7-79411 (JP, A) JP-A-7- 111043 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H03M 7/00 G11B 20/10

Claims (6)

(57) [Claims] 1. A bit buffer for accumulating an audio stream, a decoding core circuit for decoding each frame constituting an audio stream read from the bit buffer in accordance with an MPEG audio part, and detecting an occupied amount of the bit buffer. Then, based on the result of the occupancy determination circuit that compares the occupancy amount with a predetermined threshold value and the determination result of the occupancy determination circuit, a predetermined frame is skipped frame by frame and the remaining frames are A frame skip means for transferring to the buffer, and when the occupied amount is smaller than the first threshold value,
Bitback without skipping frames from the ream
Then the occupancy is greater than the first threshold
The audio stream in frames,
KIP, and then the occupancy is less than the first threshold
Audio stream if less than 2 threshold
To a bit buffer without skipping frames
MPEG audio decoder to transfer. 2. A bit buffer for accumulating an audio stream, a decoding core circuit for decoding each frame constituting the audio stream read from the bit buffer according to the MPEG audio part, and detecting an occupied amount of the bit buffer. An MPEG audio decoder having an occupancy determining circuit and a speed control means for increasing the operating speed of the decoding core circuit as the occupying amount of the bit buffer increases based on the determination result of the occupying amount determining circuit. 3. A bit buffer for accumulating an audio stream, a decoding core circuit for decoding each frame constituting the audio stream read from the bit buffer in accordance with the MPEG audio part, and corresponding to an occupied amount of the bit buffer. And a pipeline signal generating means for generating a pipeline signal generated by the decoding core circuit. The decode core circuit has an operation speed controlled according to the pipeline signal generated by the pipeline signal generating means.
PEG audio decoder. 4. A bit buffer for accumulating an audio stream, and a decode core circuit for decoding each frame constituting the audio stream read from the bit buffer in accordance with an MPEG audio part, the decode core circuit comprising: An MPEG audio decoder that skips without decoding the frame when the value of the scale factor included in the audio data included in the frame becomes equal to or less than a predetermined value. 5. A bit buffer for accumulating an audio stream, and a decode core circuit for decoding each frame constituting the audio stream read from the bit buffer according to the MPEG audio part, the decode core circuit comprising: An MPEG audio decoder that skips without decoding the frame when the occupancy of the bit buffer exceeds a predetermined value and the value of the scale factor included in the audio data included in the frame falls below the predetermined value. 6. M according to any one of claims 1 to 5.
In the PEG audio decoder, the decoding core circuit synthesizes an audio stream divided into subbands by performing a product-sum operation on an output of the inverse quantization section and an inverse quantization section which inversely quantizes the audio stream. An MPEG audio decoder having a band synthesizing unit for generating and an output unit for generating an audio signal from the output of the band synthesizing unit.