JP7630293B2 - Rate control server, distribution system and rate control program - Google Patents

Description

The present invention relates to a video distribution system for multiple viewing terminals.

In recent years, with the maturation of virtualized network technology, communication services that use different virtual networks depending on the requirements of the service, such as 5G network slicing technology, have become commercially available. Services using virtualized networks are expected to increase in the future. For example, it is technically possible for content distributors to use communication services that allow them to occupy a virtually constructed bandwidth-guaranteed network. In such a network, the content distributor can control all the traffic flowing on the virtual distribution network because it is not affected by the traffic of other operators. Therefore, it is expected that it will be possible to manage the quality of experience (QoE) of viewers when watching videos more precisely than with conventional Internet distribution.

Research into video quality assessment based on QoE is progressing, for example, in the international standard ITU-T SG12 P1203. These assessment indices estimate a Mean Opinion Score (MOS) value from encoding parameters such as video bit rate, resolution, and frame rate.
However, because these indices calculate evaluation values from encoding parameters, they do not take into account image characteristics such as the intensity of motion in the video, and are merely average evaluation indices for all videos.
On the other hand, although not an international standard, evaluation indices such as VMAF (Video Multimethod Assessment Fusion) and SSIM+ (Structural SIMilarity plus) are commonly used to calculate evaluation values by comparing with a reference image before encoding. In addition, like P1203, these evaluation indices are evaluation values based on MOS values, and it is possible to evaluate QoE with a score of 0 to 100.

Meanwhile, the mainstream OTT streaming video delivery is a delivery method that uses the HTTP ABR (Adaptive-Bit-Rate) method, including MPEG-DASH. ABR is a technology that provides multiple quality streams with different bit rates for the same video content, and divides each stream into units called segments that are separated by fixed time intervals, allowing seamless viewing while switching between qualities. The viewing terminal measures the current network throughput and autonomously selects a segment with an appropriate quality for the throughput, thereby preventing video playback from freezing and allowing viewing of high bitrate videos according to the network conditions.

However, even if encoding parameters such as bit rate, resolution, and frame rate are the same, the actual QoE varies from segment to segment depending on the video characteristics. In a typical ABR algorithm, the quality is selected based only on the bit rate, and the QoE is not taken into consideration when selecting the quality.
Therefore, for example, Patent Document 1 proposes a method of selecting quality taking into consideration QoE in addition to the bit rate of a segment.

JP 2020-80547 A

However, as described above, the conventional ABR algorithm (quality selection algorithm) is autonomously controlled by the terminal based on the throughput and the reception buffer amount measured by each terminal. Such an algorithm is generally implemented to select the highest possible quality rate when there is a margin of throughput, which results in a battle for bandwidth with other viewing terminals. As a result, there is a problem that the viewing quality varies randomly from terminal to terminal, impairing the QoE of the service as a whole.
The method of Patent Document 1 also presupposes autonomous control of the terminal, and therefore cannot resolve the competition for bandwidth with other viewing terminals.

The present invention aims to provide a distribution system that can improve the QoE of the entire video distribution service.

In a system in which video content is divided into segments of a fixed time interval and a distribution server selectively distributes the content to viewing terminals at multiple qualities, the rate control server according to the present invention comprises: a data information management unit that manages the quality evaluation value and size of segment data for each content ID, segment number, and quality identification number; a data receiving unit that receives terminal notification information including the content ID and the segment number from each of the multiple viewing terminals; a quality calculation unit that determines the quality identification number of the segment for the specified period based on the quality evaluation value for each quality identification number for each segment, so as to maximize the quality of the entire segments distributed to the viewing terminal during the specified period, provided that the total sum of the sizes of the segment data for the specified period from the segment number included in each of the multiple received terminal notification information is a size that can be distributed during the specified period within the available bandwidth of the system; and a quality sending unit that sends the determined quality identification number to the viewing terminal that requested the segment.

The rate control server may include a terminal information management unit that notifies the quality calculation unit of terminal notification information received by the data receiving unit at a predetermined time after the quality sending unit sends the quality identification number of the segment for the predetermined period.

When the terminal information management unit receives the next terminal notification information from all of the viewing terminals after the quality sending unit has sent out the quality identification number of the segment for the specified period, the terminal information management unit may notify the quality calculation unit of the next terminal notification information received by the data receiving unit regardless of the elapsed time.

The quality calculation unit may determine the quality identification number for each segment so as to maximize the total sum of the quality evaluation values for the specified period at the multiple viewing terminals.

The quality calculation unit may determine the quality identification number for each segment so as to maximize the minimum value of the quality evaluation values for the specified period among the multiple viewing terminals.

The quality calculation unit may increase the quality identification number for each segment by one step, up to an upper limit until the minimum value exceeds a predetermined target value.

The distribution system according to the present invention includes the rate control server, the distribution server, and a plurality of viewing terminals that request and obtain segments of the quality identification number received from the rate control server from the distribution server.

If the rate control server receives the terminal notification information from a new viewing terminal before it has finished sending the quality identification number for the segment for the specified period, it may send a notification to the new viewing terminal instructing it to use a different line than the available bandwidth, and the viewing terminal may request the segment from the distribution server via the different line in response to the notification.

The rate control program of the present invention is for causing a computer to function as the rate control server.

The present invention can improve the QoE of the entire video distribution service.

1 is a diagram showing an overall configuration of a distribution system and a video distribution flow in an embodiment. FIG. 2 is a diagram illustrating a functional configuration of a rate control server in an embodiment. FIG. 13 is a diagram illustrating a preprocessing algorithm in the embodiment. FIG. 13 is a diagram illustrating an example of an initial state after completion of preprocessing by a quality calculation unit in the embodiment. 11 is a diagram illustrating a processing procedure for determining a quality identification number for each segment in an embodiment. FIG. FIG. 11 is a diagram illustrating a processing flow of a rate control server in the embodiment.

An example of an embodiment of the present invention will now be described.
FIG. 1 is a diagram showing the overall configuration of a distribution system 1 according to the present embodiment and a video distribution flow.

The distribution system 1 is made up of a video distribution server 10 , a switch 21 connected to the distribution server 10 , a switch 22 connected to the switch 21 , a group of viewing terminals 30 connected to the switch 22 , and a rate control server 40 .
Here, it is assumed that the network between the switch 21 and the switch 22 is a network that can be occupied by the content provider, and that the other links have sufficient bandwidth.

In addition, for one video content, the distribution server 10 stores streams of multiple qualities with different bit rates (e.g., 1500 kbps, 1000 kbps, 500 kbps) in segments separated by a certain time period (e.g., 2 seconds), and it is assumed that multiple different video contents exist.
In order to distribute traffic on the distribution server 10 and on the links between the distribution servers 10 and the switches 21, a plurality of distribution servers 10 may be provided.

The rate control server 40 is connected to any network accessible from the audio-visual terminals 30, and in this embodiment, is connected to the switch 21.
The rate control server 40 holds bit rate information (e.g., byte size of a segment) for every segment stored in the distribution server 10, and a predetermined quality evaluation value. For example, VMAF can be used as the quality evaluation value, but other evaluation indexes such as PSNR, SSIM, and SSIM+ may also be used. In addition, the rate control server 40 is aware of the available bandwidth BW between the switch 21 and the switch 22.

First, the audiovisual terminal 30 notifies the rate control server 40 of the content ID and the segment number (1).
When the rate control server 40 receives notifications from all the viewing terminals 30, it calculates the quality to be provided for each notification (2). However, if the quality has already been calculated, this step is omitted. Then, the rate control server 40 notifies all the viewing terminals 30 of the quality at the same time (3).
The viewing terminal 30 requests a segment of the appropriate quality from the distribution server 10 (4), and the distribution server 10 transmits the requested segment (5).

FIG. 2 is a diagram showing the functional configuration of the rate control server 40 in this embodiment.
The rate control server 40 is an information processing device that includes a control unit, a memory unit, and various interfaces, and the various functions of this embodiment are realized by the control unit executing software (rate control program) stored in the memory unit.

The control unit of the rate control server 40 includes a data receiving unit 41, a terminal information management unit 42, a quality calculation unit 43, a data information management unit 44, a quality management unit 45, and a quality transmission unit 46.

The data receiving unit 41 receives terminal notification information (content ID, segment number, terminal identification number) including the terminal identification number generated by the terminal information management unit 42 from the viewing terminal 30, and registers it in the terminal information management unit 42. Note that when accessed for the first time, the terminal notification information does not include the terminal identification number.

When each viewing terminal 30 accesses the device for the first time, the terminal information management unit 42 assigns a terminal identification number to the viewing terminal 30 and manages the terminal notification information obtained from the data receiving unit 41 by the terminal identification number. The terminal information management unit 42 notifies the quality management unit 45 of the terminal notification information and notifies the quality sending unit 46 of the terminal identification number.

When the terminal information management unit 42 detects that the quality information stored in the quality management unit 45 has been notified to all viewing terminals 30, it sets a timer to measure a predetermined time. When the timer times out or when notifications are received from all viewing terminals 30 that were previously notified, it notifies the quality calculation unit 43 of the terminal notification information that it currently holds. At this time, if the timer was set, it is canceled.

The quality calculation unit 43 calculates the quality to be provided based on the terminal notification information notified by the terminal information management unit 42 and the quality evaluation value and size for each segment obtained from the data information management unit 44. The quality calculation unit 43 notifies the quality management unit 45 of the calculation result.

The data information management unit 44 manages the quality evaluation value and size for each segment using the content ID and segment number as keys, and notifies the management information in response to a request from the quality calculation unit 43.

The quality management unit 45 uses the terminal identification number and segment number as keys to manage the quality calculated by the quality calculation unit 43, and notifies the quality transmission unit 46 of the corresponding quality in response to a request from the terminal information management unit 42.

The quality sending unit 46 sends the quality notified by the quality management unit 45 to the viewing terminal 30 identified by the terminal identification number notified by the terminal information management unit 42.

Here, the method of quality calculation by the quality calculation unit 43 will be described in detail.
First, define the following variables:
id_c: Content ID notified by the viewing terminal 30
t'_c: segment number notified by the viewing terminal 30 t': segment number, 1≦t'≦total number of segments for each content q[id][t'][l]: quality evaluation value for each content, segment, and quality ss[id][t'][l]: size for each content, segment, and quality BW: available bandwidth (bps)
SD: Video length per segment (sec)
c: terminal identification number, 1≦c≦Csize
t: Time identification number of the segment, 1≦t≦Tsize
l: quality identification number, 1≦l≦lmax
Tsize: Number of segments to be calculated at one time Csize: Total number of viewing terminals 30 lmax: Number of types of video quality sets prepared in ABR

Content stored in the distribution server 10 is managed by content ID. A quality evaluation value is assigned to each content segment before the service starts. In this embodiment, VMAF is used as the evaluation index. This quality evaluation value and the size of the segment are stored in the arrays q[id][t'][l] and ss[id][t'][l], respectively.

The terminal identification numbers c are assigned to the viewing terminals 30 sequentially, starting from 1, in the order of access to the rate control server 40, for example.
The quality identification number l is assigned sequentially from 1, for example, in the ABR moving image quality set, in order from the smallest bit rate.

When id_c (the content ID to be viewed) and t'_c (the segment number to be viewed) are received from all viewing terminals 30, the quality calculation unit 43 initializes, as preprocessing, the following array to be used in the calculation for the terminal identification number c assigned to each viewing terminal 30.
Q[c][t][l]: Terminal, time, and quality evaluation value for each quality used in the calculation SS[c][t][l]: Terminal, time, and quality size (bits) used in the calculation
R[c][t]: Terminal to be calculated, quality identification number for each time, 1≦R≦lmax

FIG. 3 is a diagram showing a preprocessing algorithm in this embodiment.
In the preprocessing, all quality identification numbers R[c][t] are initialized to 1, which is the minimum value, within the range of the total number Csize of viewing terminals 30 and the number Tsize of segments calculated at one time (in one cycle).
In addition, for all quality identification numbers, the quality evaluation value Q[c][t][l] and size SS[c][t][l] are substituted with the corresponding values of the quality evaluation value q[id][t'][l] and size ss[id][t'][l] that have been prepared in advance.

FIG. 4 is a diagram illustrating an initial state after completion of preprocessing by the quality calculation unit 43 in this embodiment.
This example shows a case where Csize (number of terminals) is 2, Tsize (number of segments to be calculated) is 4, and lmax (quality number) is 3. In this case, the quality calculation unit 43 determines quality identification numbers R[c][t] for four segments for two viewing terminals 30 in one quality calculation.

Once preprocessing is complete, for segments from time t:1 to 4 for each of the two viewing terminals 30, all eight elements of the quality identification number R[c][t] are initially set to 1, and the 24 elements of Q[c][t][l] are set to quality evaluation values obtained from the data information management unit 44.
Although the segment size SS[c][t][l] is not shown, similar to Q[c][t][l], the 24 elements of the array are set to values obtained from the data information management unit 44.

Next, the calculation process for obtaining each quality identification number R[c][t] will be described in detail.
In the initial state, the quality identification numbers R[c][t] calculated by the quality calculation all store the value 1, which indicates the lowest quality. At this time, the quality calculation unit 43 increases any of the elements of R[c][t] by one step at a time within a range that satisfies the following conditional expression (1) to calculate the optimal quality.

By determining R[c][t] within the range of conditional formula (1), it becomes possible to complete one cycle (from receiving terminal notification information from the viewing terminal 30 to completing delivery of segments t:1 to t:4) within (Tsize·SD) seconds, allowing stable viewing without freezing during video playback on the viewing terminal 30.

FIG. 5 is a diagram illustrating a specific processing procedure for determining the quality identification number R[c][t] for each segment in this embodiment.
This example shows a processing procedure when the objective is to "maximize the average QoE of all terminals."

In step S1, the quality calculation unit 43 extracts pairs of c and t that satisfy R[c][t]<lmax and for which R[c][t] has not been determined in step S4.

In step S2, the quality calculation unit 43 determines whether or not a pair (c, t) was extracted in step S1. If the determination is YES, the process proceeds to step S3, and if the determination is NO, the process ends.

In step S3, the quality calculation unit 43 executes the following calculation formula (2) for the extracted (c, t) to obtain a pair (result_c, result_t) that has a high effect of improving the QoE.

In step S4, the quality calculation unit 43 uses the obtained (result_c, result_t) to increase R[result_c][result_t] by one step (R[result_c][result_t]++).

In step S5, the quality calculation unit 43 determines whether or not conditional expression (1) is satisfied by R[c][t] updated in step S2. If the determination is YES, the process proceeds to step S1, and if the determination is NO, the process proceeds to step S6.

In step S6, the quality calculation unit 43 lowers R[result_c][result_t] by one level (R[result_c][result_t]--) and determines R[result_c][result_t].
Then, the process proceeds to step S1.

In this processing procedure, when all R[c][t] are determined or all R[c][t]=lmax, the loop of steps S1 to S6 is exited and the quality calculation ends.
After the quality calculation is completed, the quality identification number stored in R[c][t] (t=1) is notified to each viewing terminal 30 in response to the terminal notification information received from each viewing terminal 30. When the second or subsequent terminal notification information (t:2 to t:4) is received from a viewing terminal 30, the corresponding R[c][t] has already been calculated, so the quality calculation unit 43 omits a new calculation and notifies the already calculated quality identification number stored in R[c][t].

In this processing procedure, if the objective is to "raise the minimum QoE on a segment-by-segment basis," formula (2) in step S3 is changed to the following formula (3).

Furthermore, when a target QoE (Target_QoE) is set, the following conditional formula (4) is added to the loop termination condition after using the calculation formula (3).

The target QoE is a value set by the delivery side and may be determined based on the quality evaluation value used. By setting a target QoE, delivery with excessive quality can be suppressed, and a reduction in communication volume can be expected.

FIG. 6 is a diagram showing a process flow of the rate control server 40 in this embodiment.
This process (a cycle of steps S11 to S19) is repeatedly executed when the service of the distribution system 1 is started, and the system waits for terminal notification information from the viewing terminal 30 until the service is ended.
At this time, the data receiving unit 41 receives terminal notification information including the content ID (id_c) and the segment number (t'_c) from the viewing terminal 30. If the viewing terminal 30 already has a terminal identification number c assigned by the terminal information management unit 42, this terminal identification number c is also notified.

In step S11, the terminal information management unit 42 sets a timer (e.g., 100 ms).

In step S12, the terminal information management unit 42 waits until the timer times out or until it receives notifications from all the viewing terminals 30 that sent data in the previous cycle. Whether or not a terminal sent data in the previous cycle is a terminal can be determined by whether or not it has received a notification of the terminal identification number c from the terminal.
If there has been no previous notification and this is the first notification received, the process may immediately proceed to step S3.

In step S13, the terminal information management unit 42 determines whether or not terminal notification information has been received. If the determination is YES, the process proceeds to step S14, and if the determination is NO, the process returns to step S11.

In step S14, the terminal information management unit 42 registers information about new viewing terminals 30 that have been notified so far. At this time, the terminal information management unit 42 assigns a terminal identification number c to each viewing terminal 30.

In step S15, the quality calculation unit 43 performs quality calculations for the registered viewing terminal 30 for each piece of content for several future segments (e.g., four segments) including the notified segment number, using the procedure described above (Figure 5).

In step S16, the quality sending unit 46 simultaneously sends to the registered viewing terminals 30 the segment corresponding to the segment number in the terminal notification information, i.e., the quality identification number of the first segment for which the quality has been calculated. At this time, the quality sending unit 46 also simultaneously sends the terminal identification number c to the newly registered viewing terminal 30.

In step S17, the system waits until the next terminal notification information is received from the viewing terminal 30 that sent the quality identification number.

In step S18, the quality sending unit 46 sends the already calculated quality identification number of the segment corresponding to the segment number of the received terminal notification information to the corresponding viewing terminal 30.

In step S19, the quality control unit 45 determines whether or not the quality identification number of the last segment for which the quality has been calculated has been sent. If the determination is YES, one cycle ends. On the other hand, if the determination is NO, the process returns to step S17.

By this process, the viewing terminal 30 requests the segment with the quality identification number received from the rate control server 40 from the distribution server 10, and plays back the acquired segment data while storing it in a buffer.

Note that, in step S13 and thereafter, if terminal notification information is received from an unregistered viewing terminal 30, it may be processed in the next cycle.
Alternatively, in order to shorten the waiting time at the viewing terminal 30, an initial buffer may be started using a different line, such as a best-effort line with a low bandwidth usage fee, and synchronization may be performed in the next and subsequent cycles. In this case, for example, a notification indicating the use of a different line instead of the quality identification number is sent to the viewing terminal 30, and the viewing terminal 30 and the distribution server 10 are connected by a different line.

According to this embodiment, the rate control server 40 of the distribution system 1, which divides video content into segments of fixed time intervals and selectively distributes the content at multiple qualities, manages the quality evaluation value and size of the segment data for each content ID, segment number, and quality identification number, and when it receives terminal notification information including the content ID and segment number from each of multiple viewing terminals 30, it determines a quality identification number based on the quality evaluation value for each quality identification number for each segment so as to maximize the quality of the entire segments distributed to the viewing terminal 30 during the specified period, provided that the total sum of the sizes of the segment data for a specified period from this segment number onwards is a size that can be distributed during the specified period within the available bandwidth of the distribution system 1, and sends the determined quality identification number to the viewing terminal 30 that requested the segment.

As a result, the rate control server 40 determines the delivery rate on a segment-by-segment basis based on the QoE evaluation value, thereby efficiently utilizing the available bandwidth and cooperatively controlling multiple viewing terminals 30. At this time, the rate control server 40 determines the quality identification number taking into account the quality for a specific future period at all viewing terminals 30, which widens the range of quality options and makes it possible to more efficiently select the desired quality. As a result, the QoE of the entire video distribution service can be improved.

After sending out the quality identification numbers of the segments for a predetermined period, the rate control server 40 determines the quality for the corresponding segments based on the terminal notification information received at the predetermined time.
Therefore, the rate control server 40 can appropriately control the quality provided to the multiple viewing terminals 30 in a synchronized manner.

Furthermore, after the rate control server 40 has sent out quality identification numbers for segments for a specified period, when it receives the next terminal notification information from all of the viewing terminals 30, it determines the quality for the corresponding segments based on the received next terminal notification information, regardless of the amount of time that has elapsed.
Therefore, the rate control server 40 can reduce the waiting time due to the timer and perform efficient processing.

The rate control server 40 determines a quality identification number for each segment so as to maximize the total sum of the quality evaluation values for a given period of time for multiple viewing terminals 30, thereby maximizing the average QoE of all viewing terminals 30 and raising the QoE evaluation value of each terminal as much as possible within a limited bandwidth.

In addition, the rate control server 40 determines a quality identification number for each segment so as to maximize the minimum value of the quality evaluation values for a specified period of time at multiple viewing terminals 30, thereby raising the minimum QoE on a segment-by-segment basis and minimizing the viewing time during which the viewer feels uncomfortable as much as possible within the limited bandwidth.

Furthermore, the rate control server 40 increases the quality identification number for each segment by one step at a time, with the upper limit set to the point where the minimum quality evaluation value exceeds a predetermined target value, thereby ensuring viewing at a sufficient target QoE while suppressing delivery at excessive quality and reducing communication volume.

If the rate control server 40 receives terminal notification information from a new viewing terminal 30 before finishing sending the quality identification numbers of the segments for a specified period, it sends a notification to the new viewing terminal instructing it to use a different line, and in response to this notification, the viewing terminal 30 may request the segments from the distribution server 10 via the different line.
This allows the viewing terminal 30, when making a new request for video distribution, to immediately start playing the video without waiting for the rate control server to finish its processing cycle.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments. Furthermore, the effects described in the above-described embodiments are merely a list of the most favorable effects resulting from the present invention, and the effects of the present invention are not limited to those described in the embodiments.

In this embodiment, the configuration and operation of the distribution system 1 have been mainly described, but the present invention is not limited to this, and may be configured as a method or program for distributing videos, including each component.

Furthermore, the functions of the distribution system 1 may be realized by recording a program for realizing the functions on a computer-readable recording medium, and reading and executing the program recorded on the recording medium into a computer system.

The term "computer system" here includes hardware such as the OS and peripheral devices. Additionally, "computer-readable recording media" refers to portable media such as flexible disks, optical magnetic disks, ROMs, and CD-ROMs, as well as storage devices such as hard disks built into computer systems.

Furthermore, "computer-readable recording medium" may include a recording medium that dynamically holds a program for a short period of time, such as a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line, or a recording medium that holds a program for a fixed period of time, such as a volatile memory inside a computer system that serves as a server or client in such a case. Furthermore, the above program may be one that realizes part of the functions described above, or may be one that can realize the functions described above in combination with a program already recorded in the computer system.

REFERENCE SIGNS LIST 1 Distribution system 10 Distribution server 21 Switch 22 Switch 30 Viewing terminal 40 Rate control server 41 Data receiving unit 42 Terminal information management unit 43 Quality calculation unit 44 Data information management unit 45 Quality management unit 46 Quality transmission unit

Claims (9)

In a system in which video content is divided into segments of a fixed time interval and a distribution server selectively distributes the content to a viewing terminal at multiple qualities,
a data information management unit that manages a quality evaluation value and a size of segment data for each content ID, segment number, and quality identification number;
a data receiving unit that receives terminal notification information including the content ID and the segment number from each of the plurality of viewing terminals;
a quality calculation unit which determines the quality identification numbers of the segments for a predetermined period based on the quality evaluation value for each quality identification number for each segment, so as to maximize the quality of the entire segments delivered to the viewing terminal during the predetermined period, on the condition that the total sum of the sizes of segment data for the predetermined period following the segment number included in each of the multiple received terminal notification information is a size that can be delivered during the predetermined period within the available bandwidth of the system;
a quality sending unit that sends the determined quality identification number to the viewing terminal that has requested the corresponding segment. The rate control server according to claim 1, further comprising a terminal information management unit that notifies the quality calculation unit of terminal notification information received by the data receiving unit at a predetermined time after the quality sending unit sends out the quality identification number of the segment for the predetermined period. The rate control server according to claim 2, wherein, when the terminal information management unit receives the next terminal notification information from all of the viewing terminals after the quality sending unit has sent out the quality identification number of the segment for the predetermined period, the terminal information management unit notifies the quality calculation unit of the next terminal notification information received by the data receiving unit regardless of the elapsed time. A rate control server according to any one of claims 1 to 3, wherein the quality calculation unit determines the quality identification number for each segment so as to maximize the total sum of the quality evaluation values for the predetermined period at the multiple viewing terminals. A rate control server according to any one of claims 1 to 3, wherein the quality calculation unit determines the quality identification number for each segment so as to maximize the minimum value of the quality evaluation values for the predetermined period at the multiple viewing terminals. The rate control server according to claim 5, wherein the quality calculation unit increases the quality identification number for each segment by one step up to an upper limit until the minimum value exceeds a predetermined target value. A rate control server according to any one of claims 1 to 6;
The distribution server;
a plurality of viewing terminals that request and acquire segments of the quality identification number received from the rate control server from the distribution server. when the rate control server receives the terminal notification information from a new viewing terminal before finishing sending the quality identification number of the segment of the predetermined period, it sends a notification to the new viewing terminal instructing the new viewing terminal to use a different line from the available bandwidth;
The distribution system according to claim 7 , wherein the viewing terminal, in response to the notification, requests the segment from the distribution server via the separate line. A rate control program for causing a computer to function as a rate control server according to any one of claims 1 to 6.

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