JPH10190746A - 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: An encoder buffer 73 which temporarily accumulates the encoded signal of an encoder system-side in encoding a digital signal at a variable bit rate has a code buffer 731 used for rate control by an encoder system. The output bit rate Rout from the encoder buffer 73 is changed to the value Rcur of the new encoding bit rate after prescribed delay time τ from the time when the encoding bit rate Rcur is changed by setting the size of the code buffer 731 to be constant.

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. 8 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. 8, 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 shown 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. 9 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. 10 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. 10, the right side from the line cd indicates a change in the bit occupancy of the decoder buffer, and the left side from the line cd indicates 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 an I picture, P picture as shown in FIG.
It is encoded with one of three types of pictures, a picture and a B picture. 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. 10, the trajectory of the bit occupancy of the decoder buffer is obtained by moving 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. 12, 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. 8, 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 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. 13 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. 13, when the output bit rate R is the same as the encoding bit rate R1 and when the output bit rate R is the same as the encoding bit rate R2, the time from when the picture is encoded to when it is decoded is determined. This is because the time interval of the time has changed. That is, assuming that a time interval when R = R1 is T1 and a time interval when R = R2 is T2, B = Oe (1) + Od (1) = Oe (n) + Od (n) (Oe in FIG. 13) (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.

As shown in FIG. 13, on the decoder system side, from the 0th picture A (0) to the (n-1) th picture A (n-1), the time from when the picture is encoded until when the picture is decoded is obtained. Decoding can be performed stably at a constant time interval T1.

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.

In the first means for this purpose, the output bit rate from the transmission buffer is changed after a predetermined delay time after the encoding bit rate is changed, while keeping the size of the code buffer usable by the encoder system constant. Change to a new encoding bit rate value.

In this specific means, the delay time is determined by the receiving buffer size of the decoder system and the minimum value of the encoding bit rate. The size of the transmission buffer required for the encoder system is determined based on the reception buffer size of the decoder system and the minimum and maximum values of the encoding bit rate.

Specifically, when the receiving buffer size of the decoder is B and the minimum value of the encoding bit rate is RMIN, the delay time τ is determined by τ = B / RMIN. When the maximum value of the encoding bit rate is RMAX, the buffer size BBMAX required for the transmission buffer is determined by BBMAX = B × RMAX / RMIN.

As a second means, the minimum value of the encoding bit rate when the code buffer size used by the encoder for rate control is made equal to the size B required for the reception buffer of the decoder is RMIN, When the maximum value is RMAX, when the encoding bit rate is equal to or more than the minimum value RMIN, the code buffer size is set to a predetermined constant value. When the encoding bit rate is smaller than the minimum value RMIN, the code buffer size is set. Is changed to be smaller than the above-mentioned fixed value.

As a concrete means for this, the size of the code buffer is set to Bcur, and the current encoding bit rate is set to Rcur.
When the encoding bit rate is smaller than the minimum value RMIN, the code buffer size Bcur is calculated based on the reception buffer size B, the minimum value RMIN, and the current encoding bit rate Rcur as follows: Bcur = Rcur × B / RMIN = Rcur × τ.

Specifically, the current encoding bit rate R
cur is from the value Rprev equal to or greater than the minimum value RMIN to the minimum value R
When the value changes to a value smaller than MIN, or changes from a value Rprev smaller than the minimum value RMIN to a value equal to or larger than the minimum value RMIN, the switching time of the size of the code buffer of the transmission buffer becomes δ = (B−Bcur ) / (Rprev-Rcur) is a time preceding the time when the coding bit rate changes by the time δ. Further, after the encoding bit rate Rcur is changed to the current value, the output bit rate from the transmission buffer is changed to the encoding bit rate Rcur at least after a delay time (τ-δ).

If the decoder system on the receiving side is an ISO / I
EC117172-2 or ISO / IEC13818-
2 (MPEG1 or MPEG2), on the encoder system side, the receiving buffer of the decoder system is set to ISO / IEC117172-2 An.
next C or ISO / IEC13818-2 An
VBV (Video Buffering) defined in nex C
Verifier). In this case, when encoding a certain picture, the code buffer size that can be used for rate control is equal to the VBV buffer size required when decoding the picture, and the output bit rate from the encoder buffer of a certain coded picture. Is the V of the picture
Equal to the input bit rate to the BV buffer.

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.

[0061]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a coded signal transmission apparatus for realizing the coded signal transmission method according to the present invention will be described.
This 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.

FIG. 1 shows an example of the configuration of an encoder system to which the first embodiment of the coded signal transmission method of the present invention is applied.
Shown in

In the encoder shown in FIG. 1 (hereinafter referred to as an encoder system), an input video sequence is encoded at a variable bit rate, and is output from a transmission buffer (hereinafter referred to as an encoder buffer) 73 at a variable bit rate. To control this, a major difference from the encoder system of FIG. 8 described as the related art is the function of the output bit rate controller 76 and the size required for the encoder buffer 73.

The encoder system shown in FIG. 8 described as the prior art uses the bit rate R indicated by the terminal 16.
Becomes the output bit rate R from the encoder buffer 13 as it is. In FIG. 8, the encoder buffer 13
It is sufficient that the size B is equal to the size of the decoder buffer 27.

On the other hand, in the encoder system of the first embodiment shown in FIG. 1, the output bit rate controller 76 controls the output bit rate Rout from the encoder buffer 73. The size of the encoder buffer 73 must be larger than the VBV buffer size of the decoder system.

That is, in the first embodiment shown in FIG. 1, the encoder buffer 73 has a code buffer 731 used for rate control by the encoder system. After a predetermined delay time after the coding bit rate is changed, the output bit rate from the encoder buffer 73 is changed to a new coding bit rate value.

The delay time is determined by the receiving buffer size (the size of the decoder buffer) of the decoder system and the minimum value of the encoding bit rate. The size of the encoder buffer 73 is determined by the decoder buffer size of the decoder system and the encoding. The bit rate is determined by the minimum value and the maximum value. Specifically, the delay time τ is determined by setting the decoder buffer size of the decoder system to B.
When the minimum value of the encoding bit rate is RMIN, τ = B / RMIN is determined, and the buffer size BBMAX required for the encoder buffer 73 is B, where the decoder buffer size of the decoder system is B, When the minimum value of the bit rate is RMIN and the maximum value is RMAX, BBMAX = B × RMAX / RMIN.

This will be described with reference to FIG. 1. In the encoder system of FIG. 1, a video signal is input from a terminal 71, and a user can freely set a target value Rcur of the encoding bit rate for the current input video from a terminal 70. It can be specified. The encoding bit rate can be changed.

The video encoder 72 encodes the current input picture so as to approach the allocated bit amount S81 specified by the rate controller 75, and encodes the obtained encoded picture bit stream into the encoder buffer 7.
Supply to 3.

The rate controller 75 calculates the bit occupation amount b0 of the decoder buffer at the start of decoding, the generated bit amount S82 of the coded picture, and the coding bit rate R
With cur and the VBV buffer size B of the decoder system as inputs, the VBV buffer of size B is prevented from overflowing or underflowing by the method described below.
Next, the allocated bit amount S81 of the picture to be encoded is calculated and specified to the video encoder 72.

The start controller 86 instructs the start of the output of the bit stream from the encoder buffer in the same manner as in FIG. 8 described as the prior art.
That is, the instruction to start the output of the bit stream from the encoder buffer 73 is issued by the start controller 86.
Do. FIG. 1 shows a configuration in which the start controller 86 controls a switch 87 provided on the output side of the encoder buffer 73. This start time is
As described later, the encoding bit rate Rcur and the terminal 8
5 from the information of the bit occupancy b0 at the start of decoding of the decoder buffer.

The bit stream output from the encoder buffer 73 at the output bit rate Rout is input to the multiplexer 74. Although not shown, an encoded bit stream of an audio signal and the like are also input to the multiplexer 74. The multiplexer 74 performs system encoding and multiplexing of a plurality of input bit streams, and outputs a multiplexed stream from a terminal 79.

The output bit rate controller 76, encoder buffer 73, and rate controller 75 will be described in detail below.

FIG. 2 shows a change in the bit occupancy of each buffer of the encoder system and the decoder system in the present embodiment. Note that the decoder system at this time is the same as that of FIG. 9 described as a conventional technique.

In FIG. 2, a broken line efijk
To the left is the encoder buffer 7 of the encoder system
3 shows the change of the bit occupancy, and the right side from the same line shows the change of the bit occupancy of the decoder buffer 27 of the decoder system.

The slope at the time t of the polygonal line efijk represents the output bit rate Rout from the encoder buffer 73 at the time t when viewed from the encoder system side. This represents a change in the input bit rate to the encoder buffer 27 at time t.

Although there is no distinction in the encoder buffer 73 in the buffer, for convenience of explanation,
As shown in FIG. 1, the code buffer 731 and the buffer 73
Think in two. The size of the code buffer 731 is
It is the same as the size B of the decoder buffer 27.

The slope of the broken line efgh at time t is
Apparently, it represents the bit rate RV output from the code buffer 731 at the time t.

The broken line efijk and the broken line mn
The vertical axis width of -pq is the VBV of the decoder system.
Represents the upper limit of buffer size B, where B is constant. The broken lines efgh and the broken lines abcd represent the upper limit of the size B of the code buffer 731 of the encoder system, and B is constant.

D0, A (n), ETS (n), DTS (n), T
The same symbols as those in FIG. 10 such as (n) 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 encoded picture, and its size represents the bit amount of the n-th encoded picture A (n). ETS (n) is the n-th coded picture A
(n) is encoded, and DTS (n) is the time at which the n-th encoded picture A (n) is decoded. T (n) is the time from the time ETS (n) at which the n-th encoded picture A (n) was encoded to the time DTS (n) at which the n-th encoded picture A (n) is decoded. The interval.

Here, for the input video sequence, the encoding bit rate Rcur is indicated as follows. Rcur = RX: from the 0th picture A (0) to the (n-1) th picture
Encoding bit rate until the A-th picture A (n-1) (ETS (0) ≦ t <ETS (n)) Rcur = RY: From the n-th picture A (n) to the m−1-th picture
Encoding bit rate until the A-th picture A (m-1) (ETS (n) ≦ t <ETS (m)) Rcur = RZ: Encoding bit rate after the m-th picture A (m) ( ETS (m) ≦ t) These bit rates RX, RY, and RZ are specified from the terminal 70 in accordance with the input video signal input from the terminal 71. In this example, it is assumed that RX is the minimum bit rate, RY is the maximum bit rate, and RX <RZ <RY.

Here, T (i) is the i-th picture A (i)
From the picture A (i) from the time ETS (i) at which
Is a time interval up to the time DTS (i) at which is decoded.

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 all pictures (A (i)). That is, T = T (0) = T (1) =... = T (n) (2-1)

Therefore, as shown in FIG. 2, the trajectory of the bit occupancy of the decoder buffer is
It is necessary that the trajectory of the bit occupancy of No. 3 is advanced to the future by time T (translated horizontally to the right). This (2
How to determine the output bit rate Rout to satisfy the condition of the expression -1) will be described.

Code buffer 73 of encoder system
Output bit rate RV from 1 (line efgh)
Changes in the encoding bit rate Rcur
Synchronize with changes.

Do = (B−b0) / RX: Delay time from ETS (0) to the start of output of a bit stream from the code buffer 731. RV = RX: 0 ≦ t <ETS (n) RV = RY: ETS (n) ≦ t <ETS (m) RV = RZ: ETS (m) ≦ t At this time, the output bit rate RV of the code buffer 731
Are RX, RY, and RZ, respectively, the time required for changing the bit occupancy of the encoder buffer 73 from B to 0 (the maximum delay time of the encoder buffer) is τ1, τ
2, τ3, τ1 = B / RX τ2 = B / RY τ3 = B / RZ, and since RX <RZ <RY, τ1>τ3> τ2. The size of the code buffer 731 is constant,
If the output bit rate RV is a variable bit rate,
The maximum delay time of the encoder buffer 73 changes. In this case, the maximum value is τ1.

Therefore, apparently, the code buffer 731
Is provided at the subsequent stage, and the maximum delay time of the buffer 732 is added to the maximum delay time of the code buffer 731 at the time t so that the maximum delay time τ of the encoder buffer 73 becomes constant with respect to the time t. I do.
Let τ = τ1 based on the maximum delay time τ1 of the code buffer 731. The required delay time of the buffer 732 in this case is a shadow portion sandwiched between the polygonal lines fgh and fijk. That is, the polygonal line efij-
k is a polygonal line obtained by horizontally translating the polygonal line abcd to the right by a time τ1 (= τ). Broken line efi
The slope at time t of −jk represents the bit rate Rout output from the encoder buffer 73 at time t. Rout = RX: 0 ≦ t <ETS (n) + τ Rout = RY: ETS (n) + τ ≦ t <ETS (m) + τ Rout = RZ: ETS (m) + τ ≦ t That is, the encoding bit rate Rcur (or code) After a time τ from the change of the output bit rate RV from the buffer 731, the output bit rate Rout of the encoder buffer 73 is changed. The output bit rate controller 76 in FIG. 1 controls this.

The time τ and the buffer size BBMAX required for the encoder buffer 73 are as follows: the decoder VBV buffer size is B, and the minimum encoding bit rate Rcur is RM.
Assuming that IN and the maximum value are RMAX, τ = B / RMIN = Bcur / RMAX BBMAX = B × RMAX / RMIN In the specific example of FIG. 2, RMIN = RX, RMAX = RY
It is.

If the above condition is satisfied, T = T (n) = τ + D0 (constant) D0: Transmission path delay amount (constant)

The encoder system must perform encoding with care given to the above buffer model so as not to cause the decoder buffer to overflow and underflow. In other words, the broken line efijk and the broken line mnp are set so that the step-like trajectory on the decoder system side does not exceed the decoder buffer size B.
The rate controller 75 must be controlled to fall between −q.

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 (picture A
(size of (k)) must satisfy the following condition.

[0092]

(Equation 2)

When the underflow of the encoder buffer 73 does not matter, instead of the equation (2-4), (2-
5) may be used.

A (k) ≦ Od (k) (2-5) where A (k) ≧ 0, 0 ≦ Od (k) ≦ B When the equation (2-5) is used, the encoder buffer shown in FIG. 73
If the output bit rate from is less than the specified Rout, the multiplexer 74 performs bit stuffing to increase the output bit rate to Rout.

Here, Rout is the input bit rate to the decoder buffer 27 at time t, and B is the size of the decoder buffer 27, which is constant.

The input bit rate (Rout) to the decoder buffer 27 at the time t is equal to the output bit rate from the encoder buffer 73 at the time in the past by the time D0, and is as follows.

Rout = RX: D0 ≦ t <ETS (n) + D0 + τ Rout = RY: ETS (n) + D0 + τ ≦ t <ETS (m) + D0 + τ Rout = RZ: ETS (m) + D0 + τ ≦ t It may be considered that D0 = 0. Then, since the bit stream input to the decoder buffer 27 can be considered from t = 0, the handling of time becomes easy.

In the encoder system of FIG. 1, picture A (i) for i <k corresponds to S82. The rate controller 75 indicates a value (the value of the picture A (k)) that satisfies the condition of the expression (2-4) as the allocated bit amount S81 of the k-th picture A (k). By controlling in this manner, encoding can be performed so that the decoder buffer does not overflow or underflow.

A comparison between the VBV buffer model and the actual encoder system will be described below.

FIG. 3 shows the difference between the VBV buffer model and the locus of the bit occupancy of each buffer in the actual encoder system.

In the above-described first embodiment, since the description has been made assuming the VBV buffer model, the locus of the ideal bit occupancy as shown in FIG. 3A is obtained. On the other hand, in actual encoder systems and decoder systems, generally, the trajectory of the bit occupancy of each buffer is represented by (b) in FIG.
It is as shown in That is, encoding and decoding of each picture is performed at the picture display interval (for example, 1/2
(97.97 seconds, 1/25 seconds), so that the trajectory is a curve as shown in FIG. The trajectory of such a curve is affected by the bias of the coded bit amount in the picture.

In both the VBV model and the actual encoder system and decoder system, a time interval T from the time ETS (i) at which the picture A (i) was encoded to the time DTS (n) at which the picture A (i) is decoded. Is constant for all i.

The difference here is that the trajectory of the bit occupancy of the actual decoder buffer is a trajectory having a larger bit amount by the maximum Δd than the trajectory of the bit occupancy of the VBV model. This Δd is Δd = Rout / P Rout: input bit rate to the decoder buffer P: picture display interval

Therefore, in an actual encoder system, the overflow of the decoder buffer can be avoided by making the usable buffer size smaller by Δd than the VBV buffer size.

For this purpose, the encoder system must control the rate controller 75 so that the locus of the bit occupancy of the VBV buffer of the decoder system falls between the straight line cd and the straight line ij in FIG. Must.

In this case, even if the actual locus of the bit occupancy of the decoder buffer passes through a locus larger by Δd than the bit occupancy of the VBV buffer, it is guaranteed that no overflow of the decoder buffer will occur.

By considering this in the encoder system of the first embodiment, more stable image reproduction on the decoder system side can be guaranteed.

That is, equation (2-4) may be changed as follows. Od (k) + Rout × (dts (k + 1) −DTS (k)) − B + Δd ≦ A (k) ≦ Od (k) (3-1) As the allocated bit amount S71 of the k-th picture, (3 -
A value (the size of the picture A (k)) that satisfies the condition of the expression (1) is indicated. By controlling in this manner, encoding can be performed so that the decoder buffer does not overflow or underflow.

The encoder system according to the first embodiment has the following configuration.
Even if the encoding bit rate changes, a buffer having the same size as the VBV buffer size of the decoder system can always be used without limiting the buffer size that can be used by the encoder system. Therefore, when the delay amount between the encoder system and the decoder system is not so much taken into consideration, it is better to use the encoder system of the first embodiment.

Next, a second embodiment of the present invention will be described with reference to the drawings.

The minimum bit rate RMIN set in the encoder system of FIG. 1 showing the first embodiment is determined by the code buffer size used for rate control by the encoder system in the decoder buffer (VBV buffer). This is the minimum bit rate that can be the same as the size.

The second embodiment to be described here is an improvement of the first embodiment. When encoding at a variable bit rate, the encoded signal when the encoding bit rate is smaller than RMIN The transmission method will be described.

FIG. 4 shows a configuration example of an encoder system to which the second embodiment of the coded signal transmission method of the present invention is applied.
Shown in

In the encoder system shown in FIG. 4, an input video sequence is encoded at a variable bit rate, and is output from the encoder buffer 110 at a variable bit rate. In the encoder system of FIG. 4, the difference between the encoder system of FIG. 8 described in the related art for controlling the encoder buffer 110 is the function of the output bit rate controller 109 and the buffer size controller 108. And the size required for the encoder buffer 110.

The encoder system shown in FIG. 8 described as the prior art uses the bit rate R indicated by the terminal 16.
Becomes the output bit rate R from the encoder buffer 13 as it is. In FIG. 8, the encoder buffer 13
It is sufficient that the size B is equal to the size of the decoder buffer 27.

On the other hand, in the encoder system of FIG. 4, the output bit rate controller 109 controls the output bit rate Rout from the encoder buffer 110. In the encoder system of FIG. 4, the buffer size controller 108 controls a code buffer size Bcur that can be used for rate control by the encoder system. In addition, the size of the encoder buffer 110 needs to be larger than the size of the decoder buffer (VBV buffer).

In the encoder system shown in FIG.
1, a video signal is input, and Rcur which is a target value of the encoding bit rate for the current input video signal is Rcur.
Can be freely specified by the user from the terminal 103. The unit for changing the current encoding bit rate Rcur is, for example, a GOP (Group Of Pi
ctures).

The video encoder 106 encodes the current input picture so as to approach the allocated bit amount S91 specified by the rate controller 107, and converts the bit stream of the encoded picture into an encoder buffer 110.
And supplies the generated bit amount S92 of the coded picture to the rate controller 107.

The rate controller 107 determines the initial bit occupation amount b0 of the decoder buffer at the start of decoding, the generated bit amount S92 of an encoded picture, the current encoding bit rate Rcur, the buffer size controller 10
8 with the buffer size Bcur specified by
The allocated bit amount S91 of the picture to be encoded next is calculated by the method described later, and is specified to the video encoder 106.

The bit stream output from the encoder buffer 110 at the output bit rate Rout is input to the multiplexer 111. Although not shown, an encoded bit stream of an audio signal and the like are also input to the multiplexer 111. The multiplexer 111 performs system encoding and multiplexing of a plurality of input bit streams, and outputs a multiplexed stream from a terminal 112.

The output bit rate controller 109, buffer size controller 108, rate controller 107 and encoder buffer 110 are shown in FIG.
This will be described with reference to an example.

FIG. 5 shows a change in the bit occupancy of each buffer of the encoder system and the decoder system in the present embodiment. The decoder system at this time is
This is shown in the same manner as FIG. 9 described as a conventional technique. In FIG. 5, the same symbols as those in FIG. 2, such as D0, ETS (n), DTS (n), and T, have the same meaning.

The left side from the polygonal line fkmnp represents the change in the bit occupancy of the encoder buffer 110 of the encoder system, and the right side from the same line represents the change in the bit occupancy of the decoder buffer 27 of the decoder system. .

The slope at the time t of the polygonal line fkmnp represents the output bit rate Rout from the encoder buffer 110 at the time t when viewed from the encoder system. This represents a change in the input bit rate to the decoder buffer 27 at time t.

The slope of the broken line fghhij at time t represents the encoding bit rate Rcur (t) at time t. The width in the vertical axis direction at the time t of the polygonal lines fghhij and the polygonal lines abcde is the code buffer size that the encoder system can use for the rate control at the time t. Represents Also, the broken line fkmnp
And the width of the polygonal line qrsuv at the time t in the vertical axis direction represents the VBV buffer size required for the decoder at the time t. The buffer size that the encoder system can use at t = ETS (i) and the decoder system is t =
The buffer sizes required for DTS (i) are equal.

In this example, it is assumed that the encoding bit rate Rcur (t) at time t is specified as follows.

0 ≦ t <ETS (a): Rcur (t) = RMIN ETS (a) ≦ t <ETS (n): Rcur (t) = R1 (R1 ≧ RMIN) ETS (n) ≦ t <ETS ( m): Rcur (t) = Ra (Ra <RMIN) ETS (m) ≦ t: Rcur (t) = R2 (R2 ≧ RMIN) FIG. 2 described in the example of FIG. 5 and the above-described first embodiment. 5 is that ETS (n) ≦ t <ETS in FIG.
That is, a value for the encoding bit rate Rcur (t) of (m) that is smaller than the minimum bit rate RMIN described in the first embodiment is specified.

A method of controlling the output bit rate Rout from the encoder buffer 110 under the control of the output bit rate controller 109 in this case will be described.

The encoding bit rate Rcur in the section of 0 ≦ t <ETS (n) and t ≧ ETS (m) is the minimum bit rate RMIN
This is the above value. The coding bit rate R in these sections
The output bit rate Rout from the encoder buffer 110 instructed by the output bit rate controller 109 for cur is the same as the method described in the first embodiment (FIGS. 1 and 2). That is, after the current encoding bit rate Rcur changes, the encoder buffer 110
Is the minimum delay time until the output bit rate Rout changes in the equation (a-1).

Τ = B / RMIN (a-1) where B is the size of the decoder buffer (VBV buffer).

For example, at t = ETS (a), after the encoding bit rate Rcur changes from the minimum bit rate RMIN to R1, t = ETS (a) + τ (point k
At the time (), the output bit rate Rout changes from the minimum bit rate RMIN to R1.

Also, from the time ETS (i) at which the i-th (i = a, n, m, etc.) picture A (i) is encoded,
The time interval (delay) up to the time DTS (i) at which it is decoded and the minimum delay time of T are calculated by equation (a-2).

T = τ + D0 (a-2) where D0 is a channel delay (constant transmission path delay time).

Next, the encoding bit rate Rcur (t) of ETS (n) ≦ t <ETS (m) is a value Ra smaller than the minimum bit rate RMIN. FIG. 6 shows an enlarged view of the vicinity of this section. In this case, after the encoding bit rate Rcur changes from R1 to Ra at t = ETS (n) (time at the point h),
T = ETS (n) + (τ−δ) after a delay time of (τ−δ1)
1) At (time of point m), the output bit rate Rout changes from R1 to Ra. Further, after the encoding bit rate Rcur changes from Ra to R2 at t = ETS (m) (time at the point i), t = ETS (n) after a delay time of (τ−δ2).
At + (τ−δ2) (time of point n), the output bit rate Rout changes from Ra to R2.

Here, δ1 and δ2 are respectively expressed by equation (a-3) and
It is calculated by the equation (a-4).

Δ1 = (B−Ba) / (R1−Ra) (a-3) δ2 = (B−Ba) / (R2−Ra) (a-4) The output bit rate controller 10 in the example of FIG.
The output bit rate Rout from the encoder buffer 110 indicated by 9 is summarized as follows.

0 ≦ t <ETS (a) + τ: Rout = RMIN (d−1) ETS (a) + τ ≦ t <ETS (n) + (τ−δ1): Rout = R1 (d−2) ETS ( n) + (τ−δ1) ≦ t <ETS (m) + (τ−δ2): Rout = Ra (d-3) ETS (m) + (τ−δ2) ≦ t: Rout = R2 (d-4) Next, a method of controlling the code buffer size Bcur that can be used by the encoder system at the time t controlled by the buffer size controller 108 will be described.

As the code buffer size Bcur in the interval between t ≦ ETS (n) −δ1 and t ≧ ETS (m), a buffer having the same size B as the decoder buffer (VBV buffer) can be used.

T ≦ ETS (n) −δ1, t ≧ ETS (m): Bcur (t) = B On the other hand, when ETS (n) ≦ t ≦ ETS (m) + δ2, the code buffer size Bcur usable by the encoder system is , (A-
It is smaller than the size calculated by the formula 5).

ETS (n) ≦ t ≦ ETS (m) + δ2: Bcur (t) = Ba = Ra * B / RMIN = Ra * τ (a-5) Further, ETS (n) −δ1 ≦ t ≦ ETS ( In (n), the code buffer size Bcur is calculated by the following equation (a-6).
It changes from B to Ba with time.

ETS (n) −δ1 ≦ t ≦ ETS (n): a = t− (ETS (n) −δ1) Bcur (t) = B * (δ1−a) / δ1 + Ba * a / δ1 (a− 6) Similarly, in ETS (m) −δ2 ≦ t ≦ ETS (m), (a-7)
As calculated by the formula, the code buffer size Bcur
Changes from Ba to B with time.

ETS (m) −δ2 ≦ t ≦ ETS (m): a = t− (ETS (m) −δ2) Bcur (t) = B * (δ2-a) / δ2 + Ba * a / δ2 (a− 7) As described above, the buffer size controller 108
Specifies a code buffer size Bcur that can be used by the encoder system at time t.

By the way, the encoding bit rate Rcur is
The output bit rate Rout and the code buffer size Bcur when continuously changing at a value smaller than the minimum bit rate RMIN will be described with reference to FIG.

FIG. 7 is a graph showing the relation of t ≧ ETS in FIG. 5 (or FIG. 6).
The encoding bit rate Rcur of (m) is rewritten to the encoding bit rate Rcur of t ≧ ETS (j), and FIG.
Then, from t = ETS (j), the encoding bit rate Rcur is
An example in which Rb (<RMIN) is changed is shown. In this case, the encoding bit rate Rcur continuously changes from Ra to Rb at a value smaller than the minimum bit rate RMIN.

ETS (n) ≦ t <ETS (j): Rcur = Ra (Ra <RMIN) ETS (j) ≦ t: Rcur = Rb (Rb <RMIN) The output bit rate Rout and the code buffer size Bcur at this time Will be described.

First, the output bit rate Rout is t
If ≧ ETS (n) + (τ−δ1), the encoding bit rate Rc
Change synchronously with ur.

ETS (n) + (τ−δ1) ≦ t <ETS (j): Rout = Ra (d-5) ETS (j) ≦ t: Rout = Rb (d-6) Next, the code buffer size Bcur Will be described. When t ≧ ETS (j), the code buffer size Bcur is (a−
It becomes the value Bb calculated by the expression 8).

T ≧ ETS (j): Bcur (t) = Bb = Rb * B / RMIN = Rb * τ (a-8) Also, a code buffer size of ETS (j) −τ ≦ t <ETS (j) Bcur is calculated from Ba to Bb as calculated by (a-9).
Changes with time.

ETS (j) −τ ≦ t <ETS (j): a = t− (ETS (j) −τ) Bcur (t) = Ba * (τ−a) / τ + Bb * a / τ (a−
9) When the encoding bit rate Rcur changes to a value equal to or higher than the minimum bit rate RMIN next to Rb, as described above in the example of FIG. 6, t ≧ ETS (m) −δ2. Bit rate Rout and code buffer size Bc
ur changes. Note that the coding bit rate Rcur is R
When the bit rate changes from b to the minimum bit rate RMIN, δ2
= Τ.

As described above, the encoding bit rate Rcu
When r continuously changes at a value smaller than the minimum bit rate RMIN, the output bit rate controller 109 controls the output bit rate Rout, and the buffer size controller 108 controls the code buffer size Bcur.

The encoder system must encode the decoder buffer in such a manner as not to cause the overflow and underflow of the decoder buffer, assuming the above buffer model. That is, the rate controller 107 must be controlled so that the step-like trajectory on the decoder system side in FIG. 5 falls between the polygonal line fkmnp and the polygonal line prsuv. Must. In order to satisfy this, the step-like trajectory on the encoder system side is divided into a broken line abcde and a broken line fghhi.
What is necessary is just to control the rate controller 107 so as to fall within −j.

In the first embodiment, the state of the bit occupancy of the buffer on the decoder system side is assumed,
The method of controlling the rate has been described. Here, a method of controlling the rate assuming the state of the bit occupancy of the encoder buffer on the encoder system side will be described.

When encoding the k-th picture A (k), the encoder system encodes the picture by assuming the state of the bit occupancy of the encoder buffer at that time. 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.

[0154]

(Equation 3)

Here, b0: initial bit occupancy of the decoder buffer at the start of decoding.

Oe (i): Bit occupancy of the encoder buffer immediately before encoding the i-th picture A (i).

ETS (i): encoding time of the i-th picture A (i).

A (i): coding bit amount of the i-th picture A (i).

AA (i): the allocated bit amount of the i-th picture A (i).

Bcur (i): Code buffer size Bcur (buffer size controller 1) usable by the encoder system at the time of encoding the i-th picture A (i)
08 indicates. Rcur (i): coding bit rate Rcur at the interval between the i-th picture A (i) and the (i + 1) -th picture A (i + 1).

Bcur (i) and Rcur (i) are specified as described in the above example.

When the underflow of the encoder buffer does not matter, the equation (b-4) may be used instead of the equation (b-2).

AA (k) ≦ Bcur (k) −Oe (k) Equation (b-4) where AA (k) ≧ 0, 0 ≦ Oe (k) ≦ B (b-4) Encoder buffer 11 of FIG.
If the output bit rate from 0 is less than the specified Rout, the multiplexer 111 performs bit stuffing to increase the output bit rate to Rout.

The rate controller 107 instructs the video encoder 106 on the value AA (i) satisfying the condition of the expression (b-2) as the allocated bit amount S91 of the i-th picture A (i). The coded bit amount (the size of A (i)) of the i-th picture A (i) is supplied to the rate control as the bit amount S92 actually generated by the video encoder 106. By controlling in this manner, encoding can be performed so that the decoder buffer does not overflow or underflow.

The buffer size BBMAX required for the encoder buffer 110 is determined in the same manner as in the first embodiment.
It is calculated by the equation (b-3).

BBMAX = RMAX × B / RMIN = RMAX × τ (b-3) where: B: VBV buffer size of the decoder system.

RMIN: The minimum bit rate of the encoding bit rate Rcur (t) that can make the code buffer size usable by the encoder system equal to the decoder buffer size B.

RMAX: the maximum value of the encoding bit rate Rcur (t).

In the description of the time interval, T in the above equation (a-2), as described above as the minimum delay time, as shown in the equation (c-1), the delay time is set to Δτ rather than τ. May be larger. Δτ is a value of 0 or more independent of τ. this is,
In the example of FIG. 5, the diagram of the bit occupancy of the decoder buffer sandwiched between the broken line fkmnp and the broken line qrsuuv is translated in the future (to the right) by Δτ. It can be represented by the figure that was made

T = (τ + Δτ) + D0 (c-1) When Δτ is larger than 0, the encoder buffer 11
The buffer size BBMAX required for 0 requires a buffer having a size of ΔB calculated by equation (c-2) in addition to the value calculated by equation (b-3).

ΔB = RMAX × Δτ (c-2) When Δτ is larger than 0, the output bit rate Rout from the encoder buffer 110 shown in the above equations (d-1) to (d-4) is calculated. The timing is delayed by Δτ into the future.
That is, the expressions (d-1) to (d-4) are the expressions (e-1) to (e-
It changes like equation 4).

Δτ ≦ t <ETS (a) + τ + Δτ: Rout = RMIN (e-1) ETS (a) + τ + Δτ ≦ t <ETS (n) + (τ−δ1) + Δτ: Rout = R1 (e-2) ETS (n) + (τ−δ1) + τ ≦ t <ETS (m) + (τ−δ2) + Δτ: Rout = Ra (e-3) ETS (m) + (τ− δ2) + Δτ ≦ t: Rout = R2 (e-4) Further, the expressions (d-5) and (d-6) are changed to the expressions (e-5) and (e-6), respectively. .

ETS (n) + (τ−δ1) + Δτ ≦ t <ETS (j) + Δτ: Rout = Ra (e-5) ETS (j) + Δτ ≦ t: Rout = Rb (e−
6) As described above, in the second embodiment of the present invention, the code buffer size Bcur used by the encoder system for rate control is determined by the decoder buffer (V
(BV buffer) Set the minimum value RMIN of the encoding bit rate Rcur when it is the same as the size B. When Rcur is greater than RMIN, Bcur = B. When Rcur is smaller than RMIN, Bcur <B. This makes it possible to obtain the same effect as in the first embodiment.

The present invention is not limited to only 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.

[0175]

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 side is controlled according to the encoding bit rate. When the digital signal is encoded at a variable bit rate from the (transmission) side and transmitted in real time to the decoder system (reception) side at a variable bit rate, the buffer on the decoder system side does not overflow or underflow. Stable signal reproduction becomes possible.

As a first specific example of the control of the size of the transmission buffer, assuming that the size of the code buffer that can be used by the encoder is fixed, after a predetermined delay time after the encoding bit rate is changed, the size of the transmission buffer is changed. Is changed to a new encoding bit rate value. In this case, the above-mentioned delay time is determined by the reception buffer size of the decoder system and the minimum value of the encoding bit rate. The size of the transmission buffer required for the encoder is determined based on the reception buffer size of the decoder system and the minimum and maximum values of the encoding bit rate.

As a second specific example, the minimum value RMIN and the maximum value of the encoding bit rate when the code buffer size used for the rate control by the encoder is the same as the size required for the reception buffer of the decoder. Is set to RMAX, when the encoding bit rate is equal to or more than the minimum value RMIN, the code buffer size is set to a predetermined constant value,
When the encoding bit rate is smaller than the minimum value RMIN, the code buffer size is changed to be smaller than the above-mentioned fixed value.

According to these specific examples, stable signal reproduction corresponding to a variable bit rate can be specifically realized. Also, according to these specific examples, even if the encoding bit rate changes, the buffer size that can be used by the encoder is not limited, and a buffer having the same size as the VBV buffer size of the decoder system can always be used. Become. Therefore, it is desirable to use this specific example when the amount of delay between the encoder and the decoder system is not much considered.

[Brief description of the drawings]

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

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

FIG. 3 is a diagram illustrating a difference in a locus of a bit occupancy of a buffer between a VBV buffer model and an actual encoder system.

FIG. 4 is a block circuit diagram of an encoder system according to a second embodiment of the present invention.

FIG. 5 is a diagram illustrating a change in a bit occupancy of a buffer of an encoder system and a decoder system according to a second embodiment.

FIG. 6 is an enlarged view of FIG. 5;

FIG. 7 is a diagram illustrating another example of a change in the bit occupancy of a buffer of the encoder system and the decoder system according to the second embodiment.

FIG. 8 is a block circuit diagram of a conventional encoder system when an output bit rate is constant.

FIG. 9 is a diagram showing a conventional decoder system.

FIG. 10 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. 11 is a diagram illustrating types of pictures.

FIG. 12 is a diagram illustrating the relationship between the bit occupancy of each buffer of the encoder system and the decoder system.

FIG. 13 is a diagram illustrating a problem that occurs when the output bit rate is made variable in the encoder system of FIG. 8;

[Explanation of symbols]

52,72 video encoder, 53,73 video buffer, 55,75 rate controller, 5
6,76 output bit rate controller, 57 buffer size controller, 66,86 start controller, 731 code buffer, 732 buffer

Claims (7)

[Claims] 1. An encoded signal transmission method for encoding a digital signal at a variable bit rate and transmitting the encoded signal, wherein a transmission buffer for temporarily storing an encoded signal on an encoder side comprises a code used by the encoder for rate control. Changing the output bit rate from the transmission buffer to a new value of the encoding bit rate after a predetermined delay time after the encoding bit rate is changed, with the size of the code buffer being constant. A coded signal transmission method characterized by the above-mentioned. 2. The delay time is determined by a size of a reception buffer of the decoder and a minimum value of the coding bit rate. The size of the transmission buffer is determined by a size of a reception buffer size of the decoder and a coding bit rate. 2. The method according to claim 1, wherein the method is determined by a minimum value and a maximum value. 3. When the delay time is τ, the receiving buffer size of the decoder is B, and the minimum value of the encoding bit rate is RMIN, the delay time τ is determined by τ = B / RMIN. The buffer size required for the transmission buffer is BB
When the maximum value of the encoding bit rate is RMAX, the buffer size BBMA required for the transmission buffer
2. The method according to claim 1, wherein X is determined by BBMAX = B * RMAX / RMIN. 4. A coded bit when a size required for a receiving buffer of a decoder is B and a code buffer size used for rate control by the encoder is the same as a size B required for a receiving buffer of the decoder. When the minimum value of the rate is RMIN and the maximum value is RMAX, when the encoding bit rate is equal to or more than the minimum value RMIN, the code buffer size is set to a predetermined constant value, and the encoding bit rate is smaller than the minimum value RMIN. 2. The method according to claim 1, wherein the code buffer size is changed to be smaller than the fixed value. 5. The constant value is the size B of the receiving buffer of the decoder, the delay time is τ, the size of the code buffer is Bcur, the current encoding bit rate is Rcur, and the encoding bit rate is Is smaller than the minimum value RMIN, the size Bcur of the code buffer is calculated based on the reception buffer size B of the decoder, the minimum value RMIN, and the current encoding bit rate Rcur, as follows: Bcur = Rcur × B / 5. The method according to claim 4, wherein RMIN = Rcur.times..tau .. 6. A value Rprev equal to or greater than the minimum value RMIN, τ = B / RMIN, a current encoding bit rate Rcur, and a value Rprev equal to or greater than the minimum value RMIN. From the minimum value RMIN to the value smaller than the minimum value RMIN, or from the value Rprev smaller than the minimum value RMIN to a value not less than the minimum value RMIN, the switching time of the code buffer size of the transmission buffer is represented by δ = (B−Bcur) / (Rprev−Rcur) A time preceding the time when the encoding bit rate changes by the time δ, and at least a delay time after the encoding bit rate Rcur is changed to the current value. After (τ−δ), the output bit rate from the transmission buffer is changed to the above-mentioned encoding bit rate Rcu.
The method according to claim 4, wherein the method is changed to r. 7. An encoded signal transmitting apparatus for encoding a digital signal at a variable bit rate and transmitting the encoded signal, wherein a transmission buffer for temporarily storing the encoded signal on the encoder side comprises a code used by the encoder for rate control. A control for changing the output bit rate from the transmission buffer to a new value of the encoding bit rate after a predetermined delay time after the encoding bit rate is changed while keeping the size of the code buffer constant. A coded signal transmission device comprising a means.