JP2003143017A - Code quantity changing method and apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a code amount changing method and an apparatus therefor, which reduces required process for cutting down the code amount of element- coded data contained in multiplexed coded data and which has no need of special controls, even when editing of the multiplexed coded data. SOLUTION: For cutting down the code amount of specified element-coded data contained in multiplexed coded data, e.g. video data, the code amount of video data (indicated by hatching is reduced) and a corresponding code amount of invalid data packets which are indicated by cross hatching to the cut code amount is inserted. Time-related information, such as position, PTS or PCR of audio data or system data are not changed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a code amount conversion method and device for converting the code amount of multiplexed coded data created by packet-multiplexing a plurality of element coded data.

[0002]

2. Description of the Related Art As a conventional example of a code amount conversion device, for example, one disclosed in Japanese Patent Laid-Open No. 2000-244872 is known. This device can arbitrarily change the bit rate of the encoded signal, and minimizes the time lag between input and re-encoding caused by the change of the bit rate to prevent the deterioration of image quality. In order to achieve this, the process of deleting part of the variable length code is performed on the video element data signal, and the deleted signal is restored to the coded signal again at an arbitrary bit rate, and the transport stream signal A PCR signal which is a time reference signal included in is extracted, and the content of this PCR signal is rewritten to the content according to the state in which the bit sequence increases or decreases due to the code deletion processing and the restoration processing of the video element data signal. .

[0003]

However, in the above-mentioned conventional apparatus, when the element data code amount is changed by some method, the PCR changes due to the change in the bit sequence.
Signal rewriting and re-multiplexing are required, and when implemented by hardware, the circuit scale becomes complicated, and when implemented by software, the algorithm becomes complicated and the number of steps increases. Therefore, a large increase in cost cannot be avoided.

In addition, when the transport stream is edited and connected, there are few problems because MPEG2 audio and AC-3 have a fixed rate, but MPEG3 audio and AAC (Advanced Audio C
Audio such as oding) and MPEG video are variable-length coded, and the buffer of the element decoder (VBV (Video Buffer Verifie for video) is used when editing and connecting.
r), AAC requires fine adjustment of the code amount of element data so that the connection of adts_buffer fullness) is consistent. At that time, the code amount of the element coded data is set to a value larger than the start occupied value of the decoder buffer of the element coded data in the latter half of the connection position so as not to cause loss in the buffer of the element decoder (code amount It is important to control so that
There has never been a simple method for realizing such a function.

The present invention has been made by paying attention to the above-mentioned points, and it is possible to reduce the processing required when reducing the code amount of element coded data included in multiplexed coded data. Moreover, it is an object of the present invention to provide a code amount conversion method and apparatus that do not require special control even when editing multiplexed coded data.

[0006]

In order to achieve the above object, the invention according to claim 1 is a code amount for converting the code amount of multiplexed coded data created by packet multiplexing a plurality of element coded data. In the conversion method, the code amount of the specific element encoded data is reduced without changing the positions of packets other than the specific element encoded data, and the number of packets corresponding to the specific element encoded data is reduced. A feature is that an invalid data packet that does not belong to any element encoded data is inserted according to the amount.

According to a second aspect of the present invention, in the code amount conversion method according to the first aspect, the start position of the access unit of the specific element coded data and time-related information are not changed.

According to a third aspect of the invention, in a code amount conversion device for converting the code amount of multiplexed coded data created by packet-multiplexing a plurality of element coded data, other than specific element coded data. A code amount reducing means for reducing the code amount of the specific element encoded data without changing the position of the packet, and which element according to the amount by which the number of packets corresponding to the specific element encoded data is reduced. And invalid data insertion means for inserting an invalid data packet that does not belong to the encoded data.

According to a fourth aspect of the present invention, in the code amount converting apparatus according to the third aspect, the code amount reducing means changes the start position and time-related information of the access unit of the specific element encoded data. Characterized by not doing.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. First, M used in this embodiment
The PEG video (video encoding method) and the MPEG system (audio video multiplexing method) will be described with reference to FIGS.

MPEG was 1988 ISO / IEC
Name of the organization (Movi) established in JTC1 / SC2 (International Organization for Standardization / International Electrotechnical Commission, Technical Committee 1 / Special Committee 2, current SC29) to study video coding standards.
ng Pictures Expert Group). MPEG1
Video (MPEG Phase 1) is a standard for storage media of about 1.5 Mbps, and is intended for JPEG for still image encoding and moving image compression for low transfer rates of ISDN video conferences and video phones. H. 261
It inherits the basic technology of (CCITT SGXV, standardized by the current ITU-T SG15) and introduced a new technology for storage media. These were established as ISO / IEC 11172 in August 1993.

[0012] MPEG2 (MPEG Phase 2) video was established in November 1994 with the aim of being a general standard so that it can be applied to various applications such as communication and broadcasting.
O / IEC 13818-2, H.O. It is established as 262. MPEG2 system will be 13818-at the same time
1, H.I. It is established as 222.0.

MPEG2 video is created by combining several technologies. A general encoding device that performs encoding conforming to the MPEG2 video standard is configured as shown in FIG. 4, for example. This device includes an adder 21, a DCT
(Discrete Cosine Transform) Device 22, Quantizer 23, VLC
(Variable length coding) device 24, buffer 25, code amount controller 26, inverse quantizer 27, inverse DCT device 28, adder 29,
An image memory 30 and a motion compensator 31 are provided.

The time redundancy portion of the input image is reduced by taking the difference from the image decoded by the motion compensation predictor 31. There are three prediction directions, past and future. Also, these can be switched and used for each macro block of 16 pixels × 16 pixels. The prediction direction is determined by the picture type given to the input image. There are two modes, a prediction mode from the past and a mode in which the macroblock is independently coded without prediction.
It is a picture. A B picture has four modes: a prediction mode from the future, a prediction mode from the past, a prediction mode from both, and a mode of independently encoding. It is an I picture that independently encodes all macroblocks.

In motion compensation, pattern matching is performed for each macroblock in a motion area, a motion vector is detected with half-pel accuracy, and a motion amount is shifted before prediction. A motion vector has a horizontal direction and a vertical direction, and is transmitted as additional information of a macroblock together with an MC (Motion Compensation) mode indicating where the prediction is from. A picture from an I picture to a picture before the next I picture is called a GOP (Group Of Picture), and when used in a storage medium, a GOP is generally composed of about 15 pictures.

The difference image output from the adder 21 is DC
The orthogonal transformation is performed in the T unit 22. DCT (Discre
te Cosine Transform) is an orthogonal transform that transforms the integral transform with the cosine function as the integral kernel into a finite space. M
In PEG, a macro block is divided into four 8 × 8 DCT
Two-dimensional DCT is performed on the block. In general, a video signal has many low-frequency components and few high-frequency components, so that when DCT is performed, the coefficients are concentrated in the low frequency band.

The DCT image data (DCT coefficient) is quantized by the quantizer 23. The quantization is a quantization matrix, which is a value obtained by weighting an 8 × 8 two-dimensional frequency with visual characteristics and a value obtained by multiplying a whole by a value called a quantization scale, which is a scalar multiplication, and the DCT coefficient is quantized Divide by the value. When performing inverse quantization in the decoder, a value that approximates the original DCT coefficient is obtained by multiplying the quantization value.

The quantized data is variable length coded by the VLC unit 24. Regarding the direct current (DC) component of the quantized value, DPCM (Di
fferential Pulse Code Modulation) is used. For alternating current (AC) components, zigzag scanning is performed from the low frequency region to the high frequency region, and the run length of zero and the effective coefficient value are set to 1
Huffman coding is performed by assigning a code having a shorter code length to a phenomenon having a higher appearance probability.

The variable-length coded data is stored in the temporary buffer 25 and output as coded data at a predetermined transfer rate. The generated code amount of each macroblock of the output data is transmitted to the code amount controller 26, and the error code amount of the generated code amount with respect to the target code amount is fed back to the quantizer to adjust the quantization scale. By doing so, the code amount is controlled. The quantized image data is inversely quantized by the inverse quantizer 27, and inverse DCT is performed by the inverse DCT unit 28.
(Inverse discrete cosine transform). The image data output from the inverse DCT unit 28 is input to the adder 29 and added to the difference image data output from the motion compensation predictor 31. After being temporarily stored in the image memory 30,
The motion compensation predictor 31 is input and used as a reference decoded image for calculating a difference image.

FIG. 5 is a block diagram showing the configuration of a general decoding device for decoding encoded data. This apparatus includes a buffer 41, a VLD (variable length decoding) device 42,
It includes an inverse quantizer 43, an inverse DCT unit 44, an adder 45, an image memory 46, and a motion compensation predictor 47.

The encoded stream is buffered by the buffer 41, and the data from the buffer 41 is input to the VLD unit 42. The VLD unit 42 performs variable length decoding, and outputs a direct current (DC) component and an alternating current (AC) component. The alternating current (AC) component data is arranged in an 8 × 8 matrix in the order of zigzag scanning from low frequency to high frequency. This data is input to the dequantizer 43 and dequantized by the quantization matrix. The inversely quantized data is input to the inverse DCT unit 44, inversely DCTed, and output as image data (decoded data). In addition, after the decoded data is temporarily stored in the image memory 46,
In the motion compensation predictor 47, it is used as a reference decoded image for calculating a difference image.

The MPEG system defines a method of multiplexing a bit stream encoded by MPEG video and audio into one bit stream and reproducing the same while ensuring synchronization. The contents specified by the system are roughly divided into the following 5 points. 1) Synchronous reproduction of a plurality of encoded bitstreams 2) Multiplexing of a plurality of encoded bitstreams into a single bitstream 3) Initialization of a buffer at the start of reproduction 4) Continuous buffer management 5) Determining time for decryption and playback

In order to perform multiplexing in the MPEG system, it is necessary to packetize information. Packet multiplexing means, for example, when video and audio are multiplexed,
This is a method of dividing each into a stream called a packet, having an appropriate length, adding additional information such as a header, and appropriately switching between video and audio packets for time division transmission. The header stores information for identifying video and audio, and time information for synchronization. The packet length depends on the transmission medium and application, and is 5 like ATM (Asynchronous Transfer Mode).
There are from 3 bytes to as long as 4 Kbytes like an optical disc. In MPEG, the packet length is variable and can be arbitrarily specified.

The data is packetized and then packed. One pack is composed of several packets. The pack_start_code and SCR (System
Clock Referance), stream_id at the beginning of the packet
And the time stamp are described. The time stamp describes time information for synchronizing audio, video, etc., and includes DTS (Decoding Time Stamp) and PTS.
There are two types, (Presentation Time Stamp). P
CR (Program Clock Reference) is described with a time accuracy of 27 MHz, and the reference clock of the decoder is P
Locked to CR.

DTS indicates the decoding start time of the first access unit (1 picture for video, 1152 samples for audio, for example) in the packet data, and PTS indicates the display (reproduction) start time.

FIG. 6 is a block diagram showing a general structure of an apparatus for decoding video data and audio data from a packet stream. This device includes a system decoder 51 for decoding system data, an audio decoder 52 for decoding audio data, a video decoder 53 for decoding video data, and a PCR.
A clock control unit 54 for generating a 27 MHz reference clock synchronized with the reference clock, a comparison unit 55 for comparing the reference clock with DTS and PTS, an audio data memory 56, and a video data memory 57. .

The comparison section 55 constantly monitors the common reference clock locked by the PCR, and when it coincides with the time of the DTS or PTS, decodes and displays the data.
It is a mechanism to control 6,57. The multiplexed data is buffered by each decoder, and a virtual decoder for performing synchronized display is used as an STD (System Tar).
This STD must be multiplexed so as not to cause overflow or underflow.

The MPEG system is roughly divided into TS (Transport Stream) and PS (Program Stream).
Exists. These are PES (Packetized Elementary)
Stream) and other necessary information. PES is defined as an intermediate stream to enable conversion between both streams,
In addition to MPEG encoded video data and audio data, a private stream is packetized.

The PS is capable of video and audio multiplexing of programs having a common reference time. The packet layer is called PES, and this structure is shown in FIG.
As shown in FIG. 3, it is used in common with the TS described later, and enables mutual compatibility of these. In the STD model of PS, streams are switched by stream_id in PES packets.

Like the PS, the TS can also multiplex the video and audio of the program having the common reference time, but the TS can further multiplex the multiple programs such as communication and broadcasting having the different reference times. Is possible. The TS is composed of a fixed length packet of 188 bytes in consideration of the ATM cell length and error correction coding, and is considered so that it can be used even in a system in which an error exists. Although the structure of the TS packet itself is not so complicated, its operation is complicated because it is a multi-program stream.

What is characteristic of PS is that it is (usually) shorter than a PES packet, even though the TS packet has a higher-level structure, and the PES packet is divided and placed on the TS packet for transmission. In the STD model of TS, streams are switched by PID (packet ID) in TS packets.

The TS of the MPEG system has a mechanism for instructing which PID is the packet relating to the information of the multiplexed program. It will be described with reference to FIG. First, the TS packet group is searched for a PID = 0. It is PAT (Program Association Tabl)
In the information packet called e), the information PID corresponding to the program number PR is described in the packet in a linked form. Next, when reading the packet of the PID corresponding to the target program number PR, the PMT
There is an information packet called (Program Map Table), and the PID of the video packet of the program corresponding to the program number PR and the PID information of the audio packet are described in the packet. PAT and P
MT refers to PSI (Program Specific Information)
, And has an information system that enables access (entry) to the channel of the target program.

Next, the concept of the present invention will be described with reference to FIG. The state of FIG. 1 (1) shows the state of packetized data of the MPEG transport stream.
The packet described as V is a video packet, the packet described as A is an audio packet, and the packet described as S is an information packet such as PAT or PMT used in the system. Each MPEG2
It is recorded according to the system rules.

Of these, in the present embodiment, a video packet is selected as a specific element data packet. In general, audio is often fixed-rate encoded such as MPEG audio or AC3, and the target of adjusting the code amount is often MPEG video that is variable-length encoded. For the identified video packet,
It is hatched. The state showing all of them is shown in FIG. A state in which only these video packets are collected is shown in FIG. The beginning of the contents of this video packet is I, as shown in FIG.
It is a picture, then two B pictures, and then P
A typical example is one picture, and so on. The code amount is reduced in one or more of these pictures. The reduced state is shown in FIG. The code amount of each picture is small, but the feature is that the start position is not changed.

Packetized data in that state is shown in FIG. MP in the part where the code amount has decreased
NULL packet specified by EG (invalid data)
Insert. This can be described as data that does not belong to any element data by using a predetermined PID of PID = 0x1FFF. This N
The UL packet is cross-hatched. This state is represented in its entirety in FIG. 7 (7). FIG. 8 is an enlarged view of FIG. 7 (7). In FIG. 8 (8), "N" is displayed in the NULL packet. As a result, a part of the V packet changes to a NULL packet, and the other element data packets do not change in position or code amount. Therefore, it is not necessary to change the PCR clock information and the time stamp information described in each element packet.

According to the MPEG system regulations, PCR clock information is recorded once every 100 msec. Further, in the video packet, PTS and DTS are described in the packet in which the leading PES header of the access unit (picture unit of frame or field) exists. In the audio packet, PTS is described in a packet in which one or a plurality of audio frames are packed by PES and the leading PES header is present. The clock-related information and the time-related information such as the time stamp information do not need to be replaced unless the position of the packet is changed. Therefore, it is not necessary to perform the extremely complicated PCR and the process of rewriting the time stamp information, which is necessary when re-multiplexing.

FIG. 2 is a block diagram showing an example of the configuration of a code amount conversion device that executes the processing described with reference to FIG. This device includes a CPU 11 that performs overall control,
Specific element data indicator 12 for instructing specific element data for reducing the code amount, and T for temporarily storing a TS packet
An S packet memory 14 and an input / output memory manager 13 that manages input / output of data to / from the TS packet memory 14,
A specific element data separator 15 that separates the specified specific element data, a code amount reducer 16 that reduces the code amount of the separated specific element data, and the reduced code amount is calculated by converting it into the number of packets. A reduction amount counter 17 and an invalid packet inserter 18 for inserting an invalid packet corresponding to the reduced code amount are provided.

The input TS data is input / output memory manager 1
3 to be stored in the TS packet memory 14. Here, the TS packet memory 14 is for 1 GOP of video data.
It is desirable to be able to store the TS data including the minutes. The video data depends on the video data reduction method described later, but in the case of simply re-encoding, GOP from the I picture, which is the reset timing of the predictive encoding, is used.
This is because the unit is easy to handle. When the GOPs are not independent, that is, when the B picture at the boundary is predicted across both GOPs (when GOP closed_gop = 0), the last reference picture of the previous GOP is decoded. In some cases, it may be necessary to store the data in a memory (not shown) for image re-encoding.

On the other hand, a CPU 11 for controlling the entire apparatus
Outputs a command for instructing specific element data for which the code amount should be reduced, to the specific element data indicator 12. When receiving the command, the specific element data indicator 12 receives the TS packet memory 1
Among the elements of the TS data stored in No. 4, a signal for designating the data whose code amount should be changed is transmitted to the specific element data separator 15. Specific element data separator 15
Then, the data to be specified is extracted from the TS packet memory 14 and transmitted to the code amount reducer 16. The code amount reducer 16 receives from the CPU 11 a signal instructing how much the code amount should be reduced. The reduction amount is, for example, “1GO
It is also possible to transmit information having a ratio of "reduce 10% by P" as 8-bit information from -128% to + 127%. Alternatively, a direct code amount of reducing n bytes (bits) may be transmitted with a predetermined number of bits.

The code amount reducer 16 is equipped with re-encoding devices for the types of specific elements. MPEG here
Since video is selected, the code amount of MPEG video is reduced. As a specific reduction method, for example, as described in detail in Japanese Patent Laid-Open No. 11-234677 by the present inventor, the VLC code may be deleted, or the target encoded data may be decoded and The decoded data may be input and re-encoded. The code amount reducer 16 transmits the element data with the reduced code amount to the invalid packet inserter 18, and at the same time, transmits the actually reduced code amount to the reduction amount counter 17. In the reduction amount counter 17, for video packets, the reduction amount in increments of access units (picture units of frames and fields) is added, and for audio packets, one or more audio frames are packed in PES.
The total reduction amount is calculated by adding the reduction amounts in ES units, and the total reduction amount (bytes) is divided by 184 bytes to calculate the necessary number of invalid packets. Then, the number of invalid packets is transmitted to the invalid packet inserter 18. The TS packet is configured as shown in FIG. 3, and 184 bytes of the payload is an effective NULL data area. Since it is calculated that the amount of element coded data transmitted in that area has been reduced, the number of packets is obtained by dividing by 184 bytes. Invalid packet inserter 18
The TS stream in which the invalid packet is inserted by
It is temporarily stored in the S packet memory 14 and output as output TS data via the input / output memory manager 13.

As described above, in the present embodiment, the code amount of specific element coded data (for example, video coded data) is reduced, and the number of NULL packets corresponding to the reduced data amount is inserted. , Like conventional technology, PC
Since it is not necessary to perform R rewriting processing or re-multiplexing processing, cost increase can be suppressed. Further, it is not necessary to finely adjust the code amount when editing the TS.

In the above-mentioned embodiment, the code amount of the TS data is reduced for each 1 GOP. However, the TS data may be reduced in units of one picture or several GOP. Is also good. The TS data may be recorded or may be transmitted and received via various transmission media such as communication and broadcasting.

[0043]

As described above in detail, according to the present invention, the code amount of the specific element encoded data is reduced without changing the position of the packet other than the specific element encoded data, and the specific element encoded data is reduced. Invalid data packets that do not belong to any element coded data are inserted according to the amount of decrease in the number of packets corresponding to coded data, so it is necessary when reducing the code amount of specific element coded data. It is possible to reduce the amount of processing required, and it is possible to eliminate the need for special control even when editing multiplexed coded data.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the outline of the present invention.

FIG. 2 is a block diagram showing a configuration of a code amount conversion device according to an embodiment of the present invention.

FIG. 3 is a diagram showing a structure of a transport stream packet.

FIG. 4 is a diagram showing a configuration of a general image data encoding device.

FIG. 5 is a diagram showing a configuration of a general image data decoding device.

FIG. 6 is a block diagram showing a configuration of a decoding device for a multiplexed packet stream.

FIG. 7 is a diagram showing the structure of a transport stream.

FIG. 8 is a diagram for explaining a mechanism for extracting a necessary information packet from multiplexed information.

[Explanation of symbols]

11 CPU 12 Specific element data indicator 13 I / O memory manager 14 TS packet memory 15 Specific element data separator 16 Code amount reducer (code amount reducing means) 17 Reduction amount counter 18 Invalid Packet Inserter (Invalid Data Insertion Means)

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Seiji Higurashi             3-12 Moriya-cho, Kanagawa-ku, Yokohama-shi, Kanagawa             Local Victor Company of Japan, Ltd. (72) Inventor Toshio Kuroiwa             3-12 Moriya-cho, Kanagawa-ku, Yokohama-shi, Kanagawa             Local Victor Company of Japan, Ltd. (72) Inventor Kenjiro Ueda             3-12 Moriya-cho, Kanagawa-ku, Yokohama-shi, Kanagawa             Local Victor Company of Japan, Ltd. F-term (reference) 5C059 KK39 KK41 MA00 MA05 MA14                       MA23 MC11 MC33 MC34 MC38                       ME02 PP05 PP06 PP07 RB02                       RB10 RC02 RC03 RC04 RC09                       RC32 SS01 SS06 UA02 UA32                 5C063 AB03 AB07 AC01 AC10 CA11                       CA14 CA23 CA34 CA40                 5J064 AA02 AA05 BC01 BC26 BD02                 5K028 AA06 EE03 KK01 KK32 MM16

Claims (4)

[Claims] 1. A code amount conversion method for converting the code amount of multiplexed encoded data created by packet-multiplexing a plurality of element encoded data, wherein the position of a packet other than specific element encoded data is changed. Without, the code amount of the specific element coded data is reduced, and an invalid data packet that does not belong to any element coded data according to the amount by which the number of packets corresponding to the specific element coded data is reduced. A code amount conversion method characterized by inserting a code. 2. The code amount conversion method according to claim 1, wherein the start position and time-related information of the access unit of the specific element coded data are not changed. 3. A code amount conversion device for converting the code amount of multiplexed encoded data created by packet-multiplexing a plurality of element encoded data, wherein the position of a packet other than specific element encoded data is changed. Without any code amount reduction means for reducing the code amount of the specific element coded data, and in accordance with the reduced amount of packets corresponding to the specific element coded data A code amount conversion device, comprising: invalid data insertion means for inserting an invalid data packet that does not belong. 4. The code amount conversion device according to claim 3, wherein the code amount reduction means does not change the start position and time-related information of the access unit of the specific element coded data.