JPH10190745A - Encoding signal transmitting method/device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably reproduce a signal without the failure of a buffer on a decoder system-side even if a transmission bit rate changes. SOLUTION: The size of the encoder buffer 53 which temporarily accumulates the encoded signal on an encoder system-side in encoding the digital signal is controlled in accordance with an encoding bit rate RT. The encoder buffer 53 has a code buffer used for rate control by an encoder system. The size of the code buffer is decided based on the reception buffer size of a decoder system from a terminal 60, the maximum value Rmax of the encoding bit rate from a terminal 59 and the present encoding bit rate RT.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coded signal transmission method and apparatus suitable for use in a case where a digital signal is encoded on a transmission side at a variable bit rate and transmitted to the reception side at a variable bit rate. .

[0002]

2. Description of the Related Art As a next-generation television broadcast, a plan for digitizing a moving image signal has been developed in order to realize high-quality transmission of the moving image signal. Here, if the moving image signal is digitized as it is, the amount of data becomes enormous, so that data must be encoded (information compression) in order to efficiently transmit it over a limited transmission path.

[0003] Generally, moving images are not stationary.
The picture and movement of the screen change with time. Also, even in the screen, the picture and the movement may be greatly different between the central part and the peripheral part of the image. Therefore, the amount of information generated at the time of encoding by the encoder changes depending on the nature of such an image. To transmit this at a fixed transmission bit rate, a transmission buffer is prepared at the last stage of the encoder system. That is, the coded output whose generation amount changes is temporarily stored in a transmission buffer, and is read out at a determined transmission bit rate and output to a transmission path.

FIG. 4 shows a conventional encoder having a constant output bit rate (hereinafter referred to as an encoder system).
FIG. In the encoder system shown in FIG. 4, a transmission buffer (hereinafter, referred to as an encoder buffer) 13 is provided between a video encoder 12 to which a video input is supplied via a terminal 11 and a transmission path, The variation in the amount of generated bits over time is smoothed, and control is performed so that a bit stream can be output from the encoder buffer 13 at a constant bit rate.

[0005] The rate controller 15 generates a coded picture generation bit amount S 21 from the video encoder 12.
And the bit rate R from the terminal 16 and the decoder buffer size B from the terminal 10 as inputs, for example, based on a VBV (Video Buffering Verifier) model described later, the size B of the size provided on the decoder system side. To prevent the decoder buffer from overflowing or underflowing, the allocated bit amount S22 of the picture to be encoded next is calculated, and the information on the allocated bit amount S22 is sent to the video encoder 12 for designation.

The encoder buffer 13 to which the video bit stream from the video encoder 12 is supplied has a code buffer at least equal to the decoder buffer size B. Generally, the code buffer is included in the transmission buffer.

[0007] The bit stream output from the encoder buffer 13 is input to a multiplexer 14.
Although not shown, an encoded bit stream of an audio signal and the like are also input to the multiplexer 14. The multiplexer 14 performs system encoding and multiplexing of a plurality of input bit streams, and outputs a multiplexed stream from a terminal 17.

The start controller 19 issues an instruction to start outputting a bit stream from the encoder buffer. This is illustrated in FIG.
3 shows a configuration in which a switch 20 provided on the output side of No. 3 is controlled by a start controller 19. The start time is determined by the bit rate R and the terminal 18 as described later.
Is calculated from the information on the bit occupation amount b0 at the start of decoding of the decoder buffer from.

FIG. 5 shows a block diagram of a conventional decoder (hereinafter, referred to as a decoder system). The multiplexed stream from the terminal 25 is input to the demultiplexer 26, and the video bit stream separated by the demultiplexer 26 is stored in a reception buffer (hereinafter, referred to as a decoder buffer) 27. The decoder buffer 27 is provided to absorb a change in the amount of bits read by the video decoder 28 in a short time. Since the decoder system is passive with respect to the transmitted bit stream, in order for the video decoder 28 to stably perform video reproduction, the encoder system should not cause the decoder buffer 27 to overflow and underflow. Be careful, you have to encode.

Here, a typical moving picture coding method is MPEG (Moving Picture Coding Experts Groove).
up) Standards are known. This MPEG is an ISO
/ IEC JTC1 / SC29 (International Organi
zation for Standardization / International Electrot
echnical Commission, Joint Technical Committee 1 /
Sub Committee 29: International Organization for Standardization / International Electrotechnical Commission Joint technical committee 1 / special subcommittee 29) is an abbreviation for the study organization of video coding for storage, which is ISO11172 as MPEG1 standard and ISO13 as MPEG2 standard.
818. In these international standards, the items of multimedia multiplexing include ISO117172-1 and I.
SO13818-1 is ISO11 as a video item.
172-2 and ISO13818-2, and voice items are ISO11172-3 and ISO13818.
-3 are standardized.

In the MPEG, an ideal input / output model of the decoder buffer 27 of the decoder system is defined. The encoder system assumes the decoder buffer model (ideal model of the decoder buffer) while assuming the decoder buffer model (ideal model of the decoder buffer). It is specified that the buffer 27 should be encoded with care so as not to cause overflow and underflow. The input / output model of the decoder buffer 27 of this decoder system is VBV (Video Buffer).
ing Verifier) As a model, ISO / IEC1117
2-2 Annex C or ISO / IEC 138
18-2 Annex C. VB
The buffer of the V model is called a VBV buffer.

[0012] The VBV buffer size of the decoder system is determined by the identifier "vbv_buff" in the MPEG bit stream.
er_size ". The standard size is, for example, 1.7 in MP @ ML (Main Profile at Main Level).
5 Mbit.

The VBV of the decoder system is assumed to operate under the following ideal conditions:

(1) The bit stream for each picture is output instantaneously from the decoder buffer, and the decoding of each picture is performed instantaneously. When transmitting a bit stream from the encoder system to the decoder system in real time under these conditions, the transmission buffer (encoder buffer) on the encoder system side needs to operate under the following ideal conditions.

(2) Encoding of each picture is performed instantaneously, and the bit stream for each picture is instantaneously input to the encoder buffer.

A VBV model in a case where an encoder system and a decoder system operate in real time with a transmission line interposed therebetween, such as broadcasting and communication, will be described. Here, the encoder system outputs a bit stream from the encoder buffer 13 at a constant bit rate, as described with reference to FIG. Therefore, the decoder buffer 2 shown in FIG.
7, a bit stream is input at a constant bit rate.

FIG. 6 shows an example of a change in the bit occupancy of each buffer of the encoder system and the decoder system according to the VBV model. In FIG.
The right side from the line cd represents a change in the bit occupancy of the decoder buffer, and the left side from the line cd represents a change in the bit occupancy of the encoder buffer.

The horizontal axis t represents the passage of time. Here, two time axes are drawn, the upper time axis represents the passage of time on the encoder system side, and the lower time axis represents the passage of time on the decoder system side. For simplicity, the straight line cd is shared between the encoder system side and the decoder system side, and there is no time difference between them.
Between the encoder system side and the decoder system side,
There is a certain transmission path delay time D0. Accordingly, the time at the point c is the origin t = 0 on the time axis of the encoder system, but is t = 0 on the time axis of the decoder system.
= D0. What is included in D0 is the processing time and transmission time of the multiplexer 14 on the encoder system side, the processing time of the demultiplexer 26 on the decoder system side, and the like.

The vertical axis represents the cumulative value of the bit amount of the bit stream output from the encoder buffer up to a certain time on the encoder system side, and the total bit amount of the bit stream input to the decoder buffer up to a certain time on the decoder system side. Indicates the cumulative value of the bit amount.

The inclination (Δd / Δt) of the straight line cd is viewed from the encoder system side by the encoder buffer 13.
And a constant input bit rate R to the decoder buffer 27 when viewed from the decoder system side.

The width in the vertical axis direction between the straight line cd and the straight line ef represents the size B of the decoder buffer, and B is constant. The width in the vertical axis direction between the straight line cd and the straight line ab represents the size B of the encoder buffer, where B is constant. The respective buffer sizes of the encoder system and the decoder system are always equal.

A (n) represents the n-th coded picture, and its size represents the bit amount of the coded picture. Each picture is coded by one of three types of pictures: an I picture, a P picture, and a B picture, as shown in FIG. An I picture is coded using intra coding, that is, using only its own image signal. The P picture is motion-compensated from the immediately preceding I picture or P picture, and the prediction residual is encoded. The B picture is motion-compensated from the I and P pictures before and after the B picture, and the prediction residual is encoded. The bit amount of each coded picture A (n) varies depending on the picture type or picture of I, P or B.

ETS (n) represents the time at which the n-th encoded picture A (n) is encoded. The interval between pictures to be encoded (ie, ETS (n + 1) −ETS (n)) is
For example, the time is 1 / 29.97 seconds for an NTSC video signal and 1/25 seconds for a PAL video signal. DTS (n) represents the time at which the n-th encoded picture A (n) is decoded. The interval between pictures to be decoded (that is, DTS (n + 1) -DTS (n)) is equal to the interval between pictures to be encoded.

On the encoder system side, the area below the zigzag step-like trajectory in the figure indicates the change in the bit occupancy of the encoder buffer. That is, the distance in the vertical axis direction from the point at the time t on the straight line cd to the step-like locus indicates the bit occupancy at the time t. The vertical movement of the step-like trajectory indicates that the bit stream is instantaneously input from the video encoder 12 to the encoder buffer 13, and the horizontal movement of the step-like trajectory indicates that the video encoder 12 transmits the bit stream to the encoder buffer 13. 13 indicates that the input of the bit stream to the bit buffer 13 is stopped (the encoding is stopped), and the bit stream is output from the encoder buffer 13 at the bit rate R.

The manner in which the bit occupancy of the encoder buffer changes on the encoder system side will be described below. Before time t = ETS (0), the bit occupancy of the encoder buffer is zero. The data of the 0th picture A (0) encoded at time t = ETS (0) is instantaneously input to the encoder buffer, whereby the bit occupancy of the encoder buffer is instantaneously changed to the 0th encoded picture. It increases by the bit amount of A (0). The output of the bit stream from the encoder buffer starts at t = 0. This start instruction is issued by the start controller 19 of the encoder system shown in FIG.
The start time do is calculated from ETS (0) + do = 0 do = (B-b0) / R based on the bit rate R and the bit occupation amount b0 of the decoder buffer at the start of decoding.

The first picture A (1) following t = 0
Until the encoding time ETS (1), since the bit stream is output from the encoder buffer at the bit rate R, the bit occupancy of the encoder buffer decreases with time. At the encoding time ETS (1), since the first picture A (1) is encoded and supplied to the encoder buffer, the bit occupancy of the encoder buffer is the bit occupancy of the first picture A (1). It increases instantaneously by the amount. Then, from t = ETS (1) to ETS (2), a bit stream is output from the encoder buffer at a bit rate R, so that the bit occupancy of the encoder buffer decreases with time. Hereinafter, similarly, encoding of each picture is continued at a predetermined time interval.

The change in the bit occupancy of the decoder buffer changes in conjunction with the change in the bit occupancy of the encoder buffer. On the decoder system side, the area above the staircase locus indicates a change in the bit occupancy of the decoder buffer. That is, the distance in the vertical axis direction from the point at the time t on the straight line cd to the step-like locus indicates the bit occupancy of the decoder buffer at the time t. The vertical movement of the step-like trajectory indicates that the video decoder 28 is instantaneously reading the bit stream from the decoder buffer 27, and the horizontal movement of the step-like trajectory indicates that the video decoder 28 This indicates that the reading of the bit stream from the buffer 27 has been stopped (the decoding has been stopped), and that the bit stream has been input to the decoder buffer 27 at the bit rate R.

A description will be given of how the bit occupancy of the decoder buffer changes on the decoder system side.

At t = D0, the input of the bit stream to the decoder buffer at the bit rate R starts. Then, at time DTS (0) after a lapse of time di, that is, di = b0 / R, the 0th coded picture A (0) is decoded.

This time di or time DTS (0) is indicated in the received bit stream. The bit occupancy of the decoder buffer is instantaneously reduced by the bit amount of the 0th coded picture A (0) by decoding the 0th coded picture A (0) at the time DTS (0). I do. Subsequently, the bit stream is input to the decoder buffer at the bit rate R until the next time DTS (1), so that the bit occupancy of the decoder buffer increases with time. At time DTS (1), the first coded picture A (0) is decoded, so that the bit occupancy of the decoder buffer is instantaneously equal to the bit amount of the first coded picture A (1). To decrease. Similarly, decoding of each picture is continued at a predetermined decoding time interval.

Here, T (i) is from the time ETS (i) at which the i-th encoded picture A (i) was encoded to the time DTS (i) at which the encoded picture A (i) is decoded. A time interval (hereinafter, T is called a delay time), that is, T (i) = DTS (i) -ETS (i).

In order to perform stable image reproduction on the decoder system (receiving) side, the delay time T (i) is constant for encoding / decoding of all encoded pictures.
That is, it is necessary that T = T (0) = T (1) =... = T (n).

Therefore, as shown in FIG. 6, the trajectory of the bit occupancy of the decoder buffer is obtained by advancing the trajectory of the bit occupancy of the encoder buffer to the future by the delay time T (moving horizontally horizontally to the right). Become.

Here, when the buffer size is B and the nth
The bit occupancy of the encoder buffer immediately before encoding the coded picture A (n) is Oe (n), and
The amount of free space in the encoder buffer immediately before encoding the coded picture A (n) is Ve (n), and the bit occupancy of the decoder buffer immediately before decoding the coded picture A (n) is If Od (n) and the amount of free space in the decoder buffer immediately before decoding the n-th coded picture A (n) is Vd (n), Ve (n) = B−Oe (n) Vd (n) ) = B-Od (n) Oe (n) = Vd (n) Ve (n) = Od (n) B = Oe (n) + Ve (n) = Od (n) + Vd (n) = Oe (n) +
Od (n).

That is, as shown in FIG. 8, through the delay time T, the sum of the bit occupation amounts of the encoding buffer on the encoder system side and the VBV buffer (decoder buffer) on the decoder system side is always a constant value (buffer). (A value corresponding to the size B).

The delay time T is calculated by the following equation: T = τe (n) + τd (n) + D0 = Oe (n) / R + Od (n) / R + D0 = B / R + D0 D0: Transmission path delay amount (constant) You.

The time required to change the bit occupancy of the encoder buffer from the buffer size B to 0 at the output bit rate R, or the time required to change the bit occupancy of the decoder buffer from 0 to B at the input bit rate R Assuming that this time is τ, there is a relationship of τ = B / R = τe (n) + τd (n) (constant) T = τ + D0 (constant)

Assuming this buffer model, the encoder system must perform encoding and transmission with care so as not to cause the decoder buffer of the decoder system to overflow and underflow. If the step-like trajectory on the decoder system side falls between the straight line cd and the straight line ef so as not to exceed the buffer size B,
The decoder system can decode pictures stably. If this is not observed, i.e., the step-like trajectory is above the line cd, it means that the decoder buffer underflows, and the step-like trajectory is a straight line e.
Being below -f indicates that the decoder buffer overflows.

When the encoder system encodes the k-th picture A (k), the encoder A assumes the state of the bit occupancy of the decoder buffer when the picture A (k) is decoded. Encode k). At this time, the generated bit amount of the k-th picture A (k) needs to satisfy the following condition.

[0040]

(Equation 1)

In the encoder system shown in FIG. 4, the bit amount of the picture A (i) for i <k corresponds to the bit amount S21 generated from the video encoder 12. The rate controller 15 indicates a value of the bit amount (the size of the picture A (k)) that satisfies the expression (3) as the allocated bit amount S22 of the k-th picture A (k). By performing such control, the encoder system performs encoding so that the decoder buffer on the decoder system side does not overflow or underflow.

[0042]

In the conventional technique described above, there is no problem when the data transfer rate between the encoder system and the decoder system is constant,
If this is transferred at a variable bit rate, a problem may occur that the picture cannot be reproduced stably on the decoder system side. An example of the occurrence of the problem will be described with reference to FIG.

Here, as in the above-described conventional technique, the size of the encoder buffer on the encoder system side is the same as the size B of the VBV buffer (decoder buffer) on the decoder system side and is constant.

On the encoder system side when transferring at a variable bit rate, for example, the n-th picture A
When encoding (n), set the encoding bit rate to R1.
To R2, and in synchronization therewith, the output bit rate R from the encoder buffer is changed from R1 to R2. This is shown in FIG. 9 by changing the slope of the output bit rate R from the encoder buffer from t = ETS (n), which is a joint between the straight line ef and the straight line fg. That is, the relationship between the output bit rate R and the encoding bit rates R1 and R2 is as follows: R = R1: 0 ≦ t <ETS (n) R = R2: ETS (n) ≧ t, where R1> R2 I have.

At this time, the encoder system performs the above (1)
According to the relations of the equations (2) and (3), the area where the trajectory of the bit occupancy of the VBV buffer on the decoder system side can pass is defined by the broken line efpq and the broken line hir. Assume that Then, it is assumed that the encoder system encodes the trajectory of the bit occupancy of the encoder buffer so as to fall between the broken lines efg and abd as shown in the figure.

In this case, as apparent from the figure, a problem of underflow of the decoder buffer occurs when decoding the n-th picture A (n).

In the example of FIG. 9, this is the case when the output bit rate R is the same as the encoding bit rate R1.
This is because the time interval between the time at which a certain picture is encoded and the time at which it is decoded is different between when the rate is the same as that at 2. That is, assuming that the time interval when R = R1 is T1 and the time interval when R = R2 is T2, B = Oe (1) + Od (1) = Oe (n) + Od (n) (Oe in FIG. 9) (n) = 0) When R = R1, T1 = Oe (1) / R1 + Od (1) / R1 +
D0 = B / R1 + D0 When R = R2, T2 = Oe (n) / R2 + Od (n) / R2 +
D0 = B / R2 + D0. D0 is the transmission path delay time (constant), R1>
Since R2, T1 <T2.

From FIG. 9, on the decoder system side,
From the 0th picture A (0) to the (n-1) th picture A (n-1), the time interval from when the picture is encoded to when it is decoded is constant T1, and the picture is stably decoded. it can.

However, a problem occurs when decoding the picture A (n). That is, when decoding the picture A (n), at time t = (ETS (n) + T1), the picture A (n) has not completely arrived at the decoder buffer, so that an underflow problem occurs. From the locus of X in the figure when the picture A (n) is to be decoded at the time t = (ETS (n) + T1) in the figure, it can be seen that an underflow of the decoder buffer has occurred. The picture A (n) can be correctly decoded at time t = D
TS (n) = ETS (n) + T2. Therefore, on the decoder system side, t = (ETS (n) + T1)
= DTS (n) during the time indicated by F in the figure, decoding cannot be performed normally, and therefore, a problem occurs in image display. That is, for example, during this time, the display of the picture A (n-1) immediately before the last decoding is continued, so that there is a problem that the display is frozen (still).

The present invention has been made in view of the above-described problems. A digital signal is encoded at a variable bit rate from an encoder system (transmitting) side, and the encoded digital signal is transmitted to a decoder system (receiving) side at a variable bit rate. It is an object of the present invention to provide a coded signal transmission method and apparatus for controlling a decoder buffer on the decoder system side so as not to overflow or underflow when transmitting data.

[0051]

In order to solve the above problem, when a video sequence is encoded at a variable bit rate and transmitted to a receiving (decoder system) side in real time, a certain picture is included in all pictures. The time interval from encoding to decoding must be constant.

Therefore, in the coded signal transmission method according to the present invention, when the digital signal is coded at a variable bit rate and transmitted, the size of the transmission buffer for temporarily storing the coded signal on the encoder system side is coded. Is controlled according to the bit rate.

As a first means for this, for the transmission buffer of the encoder system, the code buffer size that can be used by the encoder system for rate control is changed according to the encoding bit rate. Here, it is assumed that the code buffer is included in the transmission buffer.

In a concrete means for doing this, the encoder system determines the size of the code buffer which can be currently used by the receiving buffer size of the decoder system, the maximum value of the coding bit rate and the current coding bit rate. .

Specifically, the size B of the code buffer of the transmission buffer is Bmax for the reception buffer size of the decoder system, Rmax for the maximum value of the encoding bit rate,
Assuming that the current encoding bit rate is RT, it is determined by B = Bmax × RT / Rmax. Here, the switching time of the size of the code buffer of the transmission buffer is a time (β−τ) preceding the time β at which the encoding bit rate changes by a time τ obtained by τ = Bmax / Rmax.

Note that the decoder system on the receiving side is
O / IEC 11172-2 or ISO / IEC 1
In the case of conformity with 3818-2 (MPEG1 or MPEG2), the encoder system side sets the receiving buffer of the decoder system to ISO / IEC 1117.
2-2 Annex C or ISO / IEC 138
18-2 VBV (Vi) defined in Annex C
Deo Buffering Verifier). In this case, when encoding a picture,
The code buffer size that can be used for rate control depends on the VB required when decoding the picture.
The output bit rate from the encoder buffer for a coded picture is equal to the input bit rate to the VBV buffer for that picture.

Using the VBV buffer size of the decoder system calculated in this way and the input bit rate to the VBV buffer, calculation of the allocated bit amount when encoding the input picture based on the VBV and generation bits By controlling the amount, stable image reproduction can be performed without overflow or underflow on the decoder system (reception) side.

[0058]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a coded signal transmission apparatus to which a coded signal transmission method according to the present invention is applied will be described with reference to the drawings. In the following description of the embodiments, encoding and transmission of a video signal will be described. However, the scope of the present invention is not limited to a video signal, but can be applied to an audio signal and the like.

An embodiment of the present invention will be described with reference to a configuration example of an encoder (hereinafter, referred to as an encoder system) shown in FIG.

In the encoder system shown in FIG. 1, an input video sequence is encoded at a variable bit rate, and is output from an encoder buffer (transmission buffer) 53 at a variable bit rate. Here, a major difference from the encoder system of FIG. 4 described as the related art is a function of an output bit rate controller 56 and a buffer size controller 57 provided for controlling the encoder buffer 53.

That is, in the encoder system shown in FIG.
Is the output bit rate R from the encoder buffer 13 as it is. In FIG. 4, the code buffer size that can be used by the encoder system is always equal to the VBV buffer size of the decoder system.

On the other hand, in the encoder system of the present embodiment shown in FIG. 1, the output bit rate controller (designator) 56 controls the encoder buffer 53 at time t.
, And a buffer size controller (designator) 57 controls a code buffer size B that can be used on the encoder system side at time t.

In the encoder system shown in FIG.
A video signal is input from the terminal 51, and the user can freely specify the encoding bit rate RT for the current input video from the terminal 50. The encoding bit rate RT can be changed.

The video encoder 52 encodes the current input picture so as to approach the assigned bit amount S61 specified by the rate controller 55, and supplies a bit stream of the coded picture to the encoder buffer 53.

The rate controller 55 calculates the bit occupation amount b0 of the decoder buffer at the start of decoding, the generated bit amount S62 of the coded picture, and the output bit rate R from the encoder buffer 53 specified by the output bit rate controller 56. And the buffer size B instructed from the buffer size controller 57, and the VBV on the decoder system side
In order to prevent the buffer (decoder buffer) from overflowing or underflowing, the allocated bit amount S61 of the picture to be encoded next is calculated and specified to the video encoder 52.

The start controller 66 instructs the start of the output of the bit stream from the encoder buffer 53, as in the case of the prior art described with reference to FIG.

The bit stream output from the encoder buffer 53 at the bit rate R is
Is input to Although not shown, the multiplexer 54
Is also input with an encoded bit stream of an audio signal. The multiplexer 54 performs system encoding and multiplexing of a plurality of input bit streams, and outputs a multiplexed transmission stream from a terminal 58.

The rate controller 55 controls the encoding bit rate in the video encoder 52, the output bit rate controller 56 controls the reading rate from the encoder buffer 53, and the buffer size controller 57 limits the size of the encoder buffer 53. The controller 55 controls the rate controller 55 as described above. The controllers 55, 56, and 57 will be described in detail below.

FIGS. 2 and 3 show first and second specific examples of changes in the bit occupancy of each buffer on the encoder system side and the decoder system side in this embodiment. At this time, the same decoder system as that shown in FIG. 5 described as a conventional technique can be used.

In FIGS. 2 and 3, the broken line efg-
The left side from h indicates a change in the bit occupancy of the encoder buffer 53 on the encoder system side, and the right side from the broken line indicates a change in the bit occupancy of the decoder buffer 27 on the decoder system side.

The slope of the polygonal line efgh at any time t represents the bit rate R output from the encoder buffer 53 at time t when viewed from the encoder system side, and when viewed from the decoder system side. , Represents a change in the bit rate R input to the decoder buffer 27 at time t.

The broken lines efgh and the broken lines abc
The width in the vertical axis direction at −t at −d represents a code buffer size B that the encoder system can use for rate control at t. Also, the broken line efg-
h and the width of the broken line ijkl in the vertical axis direction at time t are VBV required on the decoder system side at time t.
Represents the buffer size B. The buffer size B that the encoder system can use at time t = ETS (i) is equal to the buffer size B that the decoder system requires at t = DTS (i).

D0, A (n), ETS (n), DTS (n), T
Symbols such as (n) shown in FIG. 6 have the same meaning. That is, D0 is a fixed transmission path delay time existing between the encoder system side and the decoder system side. A (n) represents the n-th coded picture,
The size indicates the bit amount of the n-th encoded picture A (n). ETS (n) indicates the time at which the n-th encoded picture A (n) is encoded, and DTS (n) indicates the time at which the n-th encoded picture A (n) is decoded. T (n) is the n-th coded picture A (n)
Is encoded from time ETS (n) to time DTS (n) at which the n-th encoded picture A (n) is decoded.

Here, for the input video sequence,
Consider a case where the encoding bit rate RT is indicated as follows. RT = RX: 0th encoded picture A (0) to nth picture
-The encoding bit rate between the first encoded pictures A (n-1). (ETS (0) ≦ t <ETS (n)) RT = RY: From the n-th coded picture A (n) to the m-th
-The encoding bit rate between the first encoded pictures A (m-1). (ETS (n) ≦ t <ETS (m)) RT = RZ: coding bit rate after the m-th coded picture A (m). (ETS (m) ≦ t) The above encoding bit rates RX, RY, and RZ are
Instructed from the terminal 50 corresponding to the input video input from the terminal 1. In this example, the encoding bit rate RX is the maximum bit rate, and RX>RZ> RY.

Here, T (i) is the time DTS (i) at which the i-th encoded picture A (i) is decoded from the time ETS (i) at which the i-th encoded picture A (i) is encoded. )
, Ie, T (i) = DTS (i) −ETS
(i). In order to enable stable image reproduction on the decoder system side, the time interval T (i) is constant for the i-th coded picture, that is, for all coded pictures as in the following equation (1-1). Must be constant.

T = T (0) = TT (1) =... = T (n) (1-1) Therefore, as shown in FIG. 2 or FIG. 3, the locus of the bit occupancy of the decoder buffer is It is necessary that the trajectory of the bit occupancy of the encoder buffer be advanced to the future by the time T (translated horizontally horizontally to the right in the figure). How to determine the buffer size B and the output bit rate R to satisfy the condition of the expression (1-1) will be described.

In this case, the output bit rate R from the encoder buffer 53 is changed as follows: R = RX: 0 ≦ t <α1 R = RY: α1 ≦ t <α2 R = RZ: α2 ≦ t How to determine the times α1 and α2 will be described later.

The buffer size controller 57 determines the code buffer size B that can be used by the encoder system at time t, based on the VBV buffer size on the decoder system side, the maximum value of the encoding bit rate, and the encoding bit rate RT at time t. Decide.

The buffer size B at time t = 0, α1, α2 will be described. Here, B = BX: code buffer size on the encoder system side at t = 0 B = BY: code buffer size on the encoder system side at t = α1 B = BZ: code buffer size on the encoder system side at t = α2 Size R = RX: Output bit rate from encoder buffer at t = 0 R = RY: Output bit rate from encoder buffer at t = α1 R = RZ: Output bit rate from encoder buffer at t = α2 I do. BY and BZ can be calculated based on the encoder buffer size BX at the maximum bit rate RX.
For example, BX is made equal to the VBV buffer size on the decoder system side, and BX = 1.75 Mbit.

Then, each output bit rate (R
X, RY, RZ), the time τ required for changing the bit occupancy of the encoder buffer from BX, BY, BZ to 0 is made constant. That is, the bit occupancy amounts BY and BZ are determined so that τ = BX / RX = BY / RY = BZ / RZ Equation (1-2) That is, BY = BX × RY / RX BZ = BX × RZ / RX Here, since RX>RZ> RY, BX>BZ> BY.

If the relationship of equation (1-2) is satisfied for the buffer size B and the output bit rate R, T = T (i) = τ + D0 (constant) τ = Rmax / Bmax where Bmax: VBV on the decoder system side Buffer size.
For example, 1.75 Mbit Rmax: the maximum value of the encoding bit rate. In this example, RX D0: transmission path delay amount (constant).

Next, the values of α1 and α2, and (α1−τ) ≦
The buffer size B on the encoder system side in the section of t <α1 and (α2−τ) ≦ t <α2 will be described.

Here, α1 and α2 are times when the output bit rate from the encoder buffer changes, and these times α1 and α2 are the minimum values, respectively, and therefore, larger values α1 + Δτ, α2 + Δτ (Δτ
≧ 0).

The output bit rate R from the encoder buffer changes from RX to RY at t = α1, and changes from RY to RZ at t = α2.

The buffer size B is (α1−τ) ≦ t
In the section of <α1, it changes continuously from BX to BY so as to gradually decrease, and in the section of (α2−τ) ≦ t <α2, it continuously changes from BY to BZ so as to gradually increase. There are two ways to determine the buffer size B in such a section as described below.

The first method is to decode the bit occupancy Oe (n) of the encoder buffer immediately before encoding the picture A (n) at the time ETS (n) or the picture A (n) at the time DTS (n). This is a method that can be used when the bit occupancy Od (n) of the encoder buffer immediately before the execution is determined in advance.

FIG. 2 is a diagram for explaining the first method. In this case, in order to set the bit occupancy of the encoder buffer at time ETS (n) in FIG. 2 to Oe (n), the rate controller 55 in FIG. There is.

In this case, the time t = α1 at which the output bit rate R is changed from RX to RY on the encoder system side
Can be predetermined as follows. τe (n) = Oe / RX or τe (n) = τ−Od (n) / RY α1 = ETS (n) + τe (n) Formula (1-3) Also, (α1−τ) ≦ t <α1 Can be determined as follows.

X = t− (α1−τ) B = BX × (τ−x) / τ + BY × x / τ Equation (1-4) Thus, in the first method, the time t = α1
Can be determined in advance, so that the output bit rate R at time t and the usable buffer size B can be determined based on this. (α2−τ) ≦ t <α2
The same can be done for the section of.

The second method is to decode the bit occupancy Oe (n) of the encoder buffer immediately before encoding the picture A (n) at the time ETS (n) (or decode the picture A (n) at the time DTS (n)). This method is used when the bit occupation amount Od (n) of the decoder buffer immediately before the execution is not determined in advance.

FIG. 3 is a diagram for explaining the second method. Generally, this is considered to be the case in many cases.

In this second method, as shown in FIG. 3, α1 = ETS (n) Equation (1-5). Then, the encoder buffer size B at (α1−τ) ≦ t <α1 is determined from Expression (1-4). The output bit rate R from the encoder buffer is calculated at time t = α1 (=
The output bit rate is changed from RX to RY to ETS (n)).

Thus, in this second method,
The output bit rate R and the buffer size B from the encoder buffer can be determined. (α2−τ) ≦ t <
The same applies to the section of α2.

When the method shown in FIG. 3 is used, it is not necessary for the rate controller 55 of FIG. 1 to obtain the bit occupancy Oe (n) of the encoder buffer, and the bit occupancy Oe (n) is output. This eliminates the need to send to the bit rate controller 56 and the buffer size controller 57, respectively.

2 and 3, the size B and the output bit rate R that can be used as a code buffer on the encoder system side at the time t are summarized as follows.

Do = (BX-b0) / RX: ETS
Delay time from (0) to the start of bit stream output from the encoder buffer. R = RX: 0 ≦ t <α1 R = RY: α1 ≦ t <α2 R = RZ: α2 ≦ t B = BX: −τ ≦ t <(α1-τ) B = BY: α1 ≦ t <(α2- τ) B = BY: α2 ≦ t The above-mentioned bit occupancy amounts BX, BY, and BZ are calculated from Expression (1-2). α1, α2 and (α1−τ) ≦ t <α
The code buffer size B at the time t of 1, (α2−τ) ≦ t <α2 can be calculated by using the above two methods, Equation (1-3) and Equation (1-4) or Equation (1). -5) and Equation (1-4).

On the encoder system side, the buffer model as described above must be assumed and the decoder buffer must be encoded with care not to cause overflow and underflow. That is, the broken line efgh and the broken line i are set such that the step-like trajectory on the decoder system side does not exceed the decoder buffer size B at the time t.
−j−k−l, the rate controller 55 must be controlled.

When the k-th picture A (k) is encoded, the encoder system assumes that the picture is occupied by the decoder buffer when the picture A (k) is decoded. Encode. At this time, the amount of generated bits (the magnitude of A (k)) of the k-th picture A (k) needs to satisfy the following condition.

[0099]

(Equation 2)

Here, R is the input bit rate to the decoder buffer 27 at time t, and B is the size required for the decoder buffer 27 at time t.

When the underflow of the encoder buffer does not matter, the equation (1-9) may be used instead of the equation (1-8).

A (k) ≦ Od (k) (1-9) where A (k) ≧ 0, 0 ≦ Od (k) ≦ B When this (1-9) is used, the encoder buffer shown in FIG. 5
If the output bit rate from 3 is less than the specified R, the output bit rate is increased to R by performing bit stuffing in the multiplexer 54.

The input bit rate R to the decoder buffer at time t is equal to the output bit rate from the encoder buffer at time D0 in the past, so that: R = RX: D0 ≦ t <α1 + D0 R = RY: α1 + D0 ≦ t <α2 + D0 R = RZ: α2 + D0 ≦ t Also, the size B required for the decoder buffer at time t
Is equal to the size used in the code buffer on the encoder system side at the time (D0 + τ) in the past, and B = BX: D0 ≦ t <(α1 + D0) B = BY: (α1 + D0 + τ) ≦ t <(α2 + D0) ) B = BZ: (α2 + D0 + τ) ≦ t

Α1 and α2 are determined from equations (1-3) and (1-5) for the above two methods.

Section (α1 + D0) ≦ t <(α1 + D0 + τ)
The size B required for the decoder buffer for can be calculated as x = t− (α1 + D0) B = BX × (τ + x) / τ + BY × x / τ, corresponding to the above two methods.

Section (α2 + D0) ≦ t <(α2 + D0 + τ)
The size B can be calculated in the same manner.

For simplicity, it may be considered that D0 = 0. Then, since the bit stream input to the decoder buffer can be considered from t = 0, the handling of time becomes easy.

On the encoder system side in FIG. 1, i <k
Corresponds to the signal S62. The rate controller 55 indicates a value (the size of the k-th encoded picture A (k)) that satisfies the condition of Expression (1-8) as the allocated bit amount S61 of the k-th picture A (k). I do. By controlling in this manner, encoding can be performed so that the decoder buffer does not overflow or underflow.

The operation of such an encoder system will be described with reference to FIG. The terminal 50 shown in FIG. 1 receives the above-mentioned encoding bit rate RT, and the encoding bit rate RT is given earlier than the input video sequence at the terminal 51.

The start controller 66 has a terminal 65
Occupancy b0 at the start of decoding of the decoder buffer from the beginning, and the first encoding bit rate RT (= RX)
And the first decoder buffer size from the rate controller 55, the output start delay time do is obtained by do = (BX-b0) / RX, and the encoding is performed after delaying this time do from the start of encoding. Control to start outputting the signal.

In the output bit rate controller 56,
For example, when the encoding bit rate RT = RX up to the (n−1) th picture A (n−1) is changed from the next picture A (n) to the bit rate RT = RY, the encoding bit rate Is changed, and the encoder buffer 53 is controlled so that the code buffer size B of the encoder buffer 53 is switched from BX to BY at a time (β−τ) preceding the time β by a time τ.
Here, in the first method shown in FIG. 2, based on the bit occupancy Oe (n) of the buffer immediately before encoding the picture A (n) at the time ETS (n) from the rate controller 55, The time β is obtained by the equation β = ETS (n) + Oe (n) / RX. In the second method shown in FIG. 3, the time ETS (n) for encoding the picture A (n) is It is calculated as β.

The rate controller 55 receives as input the information of the code amount S62 from the video encoder 52, the output bit rate R from the output bit rate controller 56, and the decoder buffer size B from the buffer size controller 57. Based on the VBV model thus calculated, the allocated bit amount S61 of the picture to be encoded next is calculated so that the decoder buffer of size B does not overflow or underflow, and the video encoder 5
2 to control the code amount. Regarding the buffer size B at the time of this rate control, the buffer size B is initially set to BX, but the time τ (= Rmax / Bmax) becomes longer than the coding rate switching time β obtained by the output bit rate controller 56. ) Preceding time (β-
From τ) until the time β, the buffer size B is gradually reduced from BX to BY. After the time β, the buffer size B is changed to BY so that the decoder buffer does not overflow or underflow. Assigned bit amount S61 at the time of encoding by encoder 52
Is controlling.

In summary of the technology of the embodiment of the present invention described above, when a digital signal is encoded at a variable bit rate and transmitted, a transmission buffer (encoder) for temporarily storing the encoded signal on the encoder system side is used. The size of the buffer 53) is controlled according to the encoding bit rate. The transmission buffer has a code buffer used for rate control by the encoder system, and the size of the code buffer is determined by the decoder system. Is determined based on the receiving buffer size, the maximum value of the encoding bit rate, and the current encoding bit rate.

Here, when the size B of the code buffer of the transmission buffer is Bmax for the reception buffer size of the decoder system, Rmax for the maximum encoding bit rate, and RT for the current encoding bit rate, B = It is determined by Bmax × RT / Rmax.

The switching time of the code buffer size of the transmission buffer is the time β at which the encoding bit rate changes.
, The time (β−τ) preceding by the time τ obtained by τ = Bmax / Rmax.

The switching time β of the code buffer size B of the transmission buffer can be determined in two ways.
In the first method, it is determined based on the time when encoding or decoding a picture immediately after the change of the encoding bit rate, the buffer occupancy at that time, and the old encoding bit rate before the change. And 2
In the second method, the encoding time of a picture immediately after the change of the encoding bit rate is set to the above-mentioned time β.

The encoder system of the above embodiment has an advantage that the transmission delay time between the encoder system and the decoder system is small. Therefore, the encoder system of the present embodiment is particularly effective when low delay is required for a news program or the like.

The present invention is not limited to the above-described embodiment. For example, digital signals to be handled are not limited to video signals, but can be applied to audio signals and the like. In addition, it goes without saying that various changes can be made without departing from the spirit of the present invention.

[0119]

According to the method and apparatus for transmitting an encoded signal according to the present invention, the size of a transmission buffer for temporarily storing an encoded signal on the encoder system side is controlled according to the encoding bit rate. When a digital signal is encoded at a variable bit rate from the encoder system (transmission) side and transmitted in real time to the decoder system (reception) side at a variable bit rate, the reception buffer on the decoder system side may overflow or underflow. Since the signal disappears, stable signal reproduction becomes possible.

A specific example of the control of the size of the transmission buffer is to change the code buffer size that can be used by the encoder system for rate control in accordance with the encoding bit rate. Here, it is assumed that the code buffer is included in the transmission buffer. In this case, the size of the code buffer that can be currently used by the encoder system is determined by the reception buffer size of the decoder system, the maximum value of the encoding bit rate, and the current encoding bit rate.

According to this specific example, stable signal reproduction corresponding to a variable bit rate can be specifically realized. Also,
This specific example has an advantage that the delay time between the encoder system and the decoder system is small, and is particularly effective when a low delay is required for a news program or the like.

[Brief description of the drawings]

FIG. 1 is a block circuit diagram illustrating a schematic configuration of an encoder system according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a change in bit occupancy of a buffer in the encoder system and the decoder system according to the embodiment;

FIG. 3 is a diagram illustrating a change in bit occupancy of a buffer of the encoder system and the decoder system according to the embodiment;

FIG. 4 is a block circuit diagram of a conventional encoder system when an output bit rate is constant.

FIG. 5 is a diagram showing a conventional decoder system.

FIG. 6 is a diagram illustrating a change in bit occupancy of each buffer of the encoder system and the decoder system when the output bit rate is constant.

FIG. 7 is a diagram illustrating types of pictures.

FIG. 8 is a diagram illustrating the relationship between the bit occupancy of each buffer of the encoder system and the decoder system.

9 is a diagram illustrating a problem that occurs when the output bit rate is variable in the encoder system of FIG. 7;

[Explanation of symbols]

52 video encoder, 53 encoder buffer, 55 rate controller, 56 output bit rate controller, 57 buffer size controller, 66 start controller

Claims (6)

[Claims] 1. An encoded signal transmission method for encoding a digital signal at a variable bit rate and transmitting the encoded signal, wherein a size of a transmission buffer for temporarily storing an encoded signal on an encoder side is controlled in accordance with the encoding bit rate. A method for transmitting an encoded signal, comprising: 2. The transmission buffer has a code buffer used by an encoder for rate control. The size of the code buffer is determined by a reception buffer size of a decoder, a maximum value of a coding bit rate, and a current coding bit rate. 2. The method according to claim 1, wherein the determination is made based on the bit rate. 3. The size B of the code buffer of the transmission buffer is represented by B = Bmax × when the reception buffer size of the decoder is Bmax, the maximum value of the coding bit rate is Rmax, and the current coding bit rate is RT. 3. The method according to claim 1, wherein the method is determined by RT / Rmax. 4. The encoded signal transmission method according to claim 3, wherein the switching time of the size of the code buffer of the transmission buffer is a time preceding the time when the encoding bit rate changes by a time τ. 5. The switching time α of the size of the code buffer of the transmission buffer includes a time when a picture is encoded or decoded immediately after a change in the encoding bit rate, a buffer occupancy at the time, and a time before the change. 5. The method according to claim 4, wherein the determination is made based on the old coding bit rate. 6. An encoded signal transmission apparatus for encoding a digital signal at a variable bit rate and transmitting the encoded signal, the size of a code buffer for temporarily storing an encoded signal on an encoder side and controlling a transmission bit rate. A control means for controlling the decoder based on the receiving buffer size of the decoder, the maximum value of the coding bit rate, and the current coding bit rate.