CN1134170C - 视频传输系统 - Google Patents

视频传输系统 Download PDF

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CN1134170C
CN1134170C CNB971911487A CN97191148A CN1134170C CN 1134170 C CN1134170 C CN 1134170C CN B971911487 A CNB971911487 A CN B971911487A CN 97191148 A CN97191148 A CN 97191148A CN 1134170 C CN1134170 C CN 1134170C
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grade
transmitter
packet switching
switching network
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CN1199532A (zh
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An
A·N·辛哈
J�������ڶ�
T·J·J·登特内尔
P
S·P·P·普容克
J
H·G·J·特尼斯
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Koninklijke Philips NV
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Abstract

在一种视频传输系统中，将视频服务器(4)经发射装置(6)连接到ATM网络(12)，为了以适当的费用获得好的传输质量，发射机(2)包括请求装置(8)，以每个景物为基础重新商定传输协议。

Description

视频传输系统
技术领域
本发明涉及一种视频传输系统，包括具有视频源的发射机，该视频源用于提供编码视频信号，所述发射机包括用于通过分组交换网络向接收机发射所述编码视频信号的发射装置。
本发明还涉及在这种传输系统中所用的一种发射机，以及一种视频传输方法。
背景技术
从公开的欧洲专利申请No.517273A2，可知一种按照前同步信号传输的系统。
为了以数字形式发射视频信号，采用视频编码器对视频信号编码。因为在视频信号中的图象具有不同的性质，对具有预定质量的视频信号的编码所需的位速率不是恒定的，而是可变的。编码视频信号的位速率依赖于图象中的细节量和图象的序列中存在的运动量。
鉴于这种变化的位速率，可以预期，在不久的将来，分组交换网络对于该可变位速率编码视频信号的传送将变得很重要。这些分组交换网络多半按照ATM标准操作。这些分组交换网络对于传送具有可变位速率的用户信号特别适用，因为它们并不建立用于完成连接的电路，而只是通过建立呼叫所确定的路径将一些分组从源传到目的地。如果，比如暂时没有要发射的分组，则可将所述传输需求的资源用于发射自其它连接的分组。
为了避免在分组交换网络中的拥塞，在呼叫建立中，以发射机与分组交换网络之间商定的所谓通信量协议来定义由该网络保证的服务的等级。这种通信量协议使该网络根据统计的基础分配资源，并且它对发射机造成一个在其中能够操作的边界。在发射机与网络之间要商定的连接因素是比如服务种类，像CBR(恒定位速率)、VBR(变化位速率)、ABR(可用位速率)和UBR(未指定的位速率)。服务等级(GOS)参数是比如CLR(信元损失比)、max CTD(最大信元传送延迟)和峰-峰CDV(信元延迟变化)。使用视频编码器的共同性问题是所需求的通信参数不总是预先可用的。结果是，自网络请求的服务等级低，使它不能发射该编码视频信号，或者服务的等级比需求的高，导致了不必要的高费用。
在根据上述本发明的传输系统中，如果其位速率超过从网络请求的服务等级，利用视频编码器减少该编码视频信号的位速率。这种位速率的减少导致了编码图象质量降低。实际上，有时请求的服务等级高于需要的，因此，产生不必要的高化费，也是接受的。
发明内容
本发明的目的是提供一种根据前同步信号的视频传输系统，其中避免了编码质量的降低。
因此，根据本发明的视频传输其特征在于，其发射机包括，景物变化指示装置，用于提供景物变化指示；服务等级指示装置，用于指示按照所述景物变化的景物所需的服务等级；其特征还在于，所述发射机包括请求装置，响应所述的景物变化指示，根据所述需求的服务等级，请求分组变换网络的服务。
本发明是基于一种认识，即所需求的编码视频信号的位速率在景物之间可能明显不同。通过根据新景物所需的服务等级请求网络的服务，将会使请求的网络的服务等级很好地适宜于该编码视频信号所需的服务等级。
本发明的实施例的特征在于安排一请求装置，用于请求至少由峰值位速率测量，平均位速率测量和突发性测量所限定的服务。
清楚的是，峰值位速率，平均位速率和具有峰值位速率的最大突发尺寸的组合是请求视频编码器的输出信号特征的适合方法。
本发明的另一实施例的特征在于，该分组交换网络包括用于向发射机发射答应信息的装置，如果该答应信息指示该分组交换网络不能提供所请求的服务，则所安排的视频源用于提供其需求比请求服务等级低的编码视频信号。
可能突然发生在景物的开始请求的网络的服务等级在所述景物期间不够了。则为了保持在所述景物的通信量协议的边界里向网络发射的数据，安排视频编码器以图象质量的最小畸变为代价来限制它的输出位速率。
附图说明
现在将参照附图描述本发明，其中：
图1为根据本发明第一实施例的视频传输系统；
图2为根据本发明第2实施例的视频传输系统中所用的发射机；
图3为在图2的传输系统中所用的视频编码器的实施例；
图4是表示对于通常的影片材料以时间为函数每帧所需的位速率的图示。
具体实施方式
在图1的传输系统中，视频源的第一输出，这里是服务器4，带有编码视频信号D。所述视频源的第一输出连接到发射装置6的第一输入。提供景物变化指示NS的服务器4的第2输出连接到请求装置8的第一输入。服务器4的第三输出，提供下一个景物所需服务等级的指示GOS，连接到请求装置8的第二输入。请求装置8的第一输出连接到发射装置6的第二输入，而请求装置8的第二输出连接到服务器4。发射装置6的输出连接到ATM网络12。ATM网络12的输出连接到发射机2中的信号接收机10。带有速率指示信号R的信号接收机10的输出连接到请求装置8的第三输入。
ATM网络12的另一输出连接到接收机14。在接收机14中由网络终端16接收自ATM网络的信号，将网络终端16的输出连接到视频译码器18。在视频译码器18的输出可得到译码视频信号。
在图1传输系统中的服务器4里，将要发射的视频信号以压缩形式存储在服务器4中。最好根据MPEG-2标准压缩视频信号，使得有3-15Mbit/s位速率的高视频质量。将该编码视频信号加到发送装置6，用于经ATM网络12进行发射。
每次开始新的景物，将信号NS与信号GOS一起从服务器4传送到请求装置8。将信号NS和GOS与编码视频信号同步地存储在服务器4中。因此，信号NS指示由编码视频信号所表示的视频信号中的景物变化，而信号GOS指示下一个景物所需的服务等级。根据本发明的发明构思，根据由信号GOS指示的所需服务等级，请求装置8设法与ATM网络商定新的通信量协议。ITU-TB-ISDN标准Q.2931包括在信号连接期间修改带宽通信量描述符(PCR、SCR和MBR)的可能性。可以用两种方式进行商定。
第一种可能是，请求装置8经发射机6向网络提交由信号GOS限定的服务等级的请求，该网络经接收机10，以是否能提供请求服务的等级的信息进行回答。如果不能提供请求的服务等级，前面商定的通信量协议保持有效，请求装置8对服务器4供给信号R，以指示服务器4必须在前面商定的通信量协议的边界内保持它的输出。
第二种可能是，请求装置8经发射机6向网络提出由信号GOS限定的服务等级的请求，该网络以可以提供请求的服务等级的信息进行回答，或者给出它可提供的服务等级。在后一情况，请求装置8返回信号给ATM网络，告知是否接受网络的提供。
ATM网络12将自发射机2的数据给到用户14。通过网络终端16接收来自ATM网络12的数据并传送给视频译码器18。在视频译码器18的输出，可得到该译码视频信号。
在图2的发射机中，将未编码视频信号加到视频源20的输入和景物变化检测器24的输入。将视频源20的第一输出连接到发射装置22的输入。将视频源20的第二输出连接到请求装置26的第一输入。将景物变化检测器24的输出连接到请求装置26的第二输入。将请求装置26的第一输出连接到视频源20。将请求装置26的第二输出连接到发射装置22的第二输入。发射装置22的输出构成了发射机2的输出。将该发射机的输入连接到接收机28。将接收机28的输出连接到请求装置26的第三输入。
安排视频源20用于实时编码在它的输入端的未编码视频信号。将编码视频信号D供给发射装置22，以便发送到ATM网络。景物检测器24从未编码视频信号取得指示新景物起始的信号NS。可用几个方法来检测新景物的起始。检测景物变化的第方法是，通过一个象素接一个象素地减去象素值并将整个图象上的象素差求和，来比较两个相继的图象。对于图象之间的差值测量e可写物： e = Σ i = 1 N Σ j = 1 M | p 1 [ i , j ] - p 2 [ i , j ] | - - - ( 1 )
在(1)中，P1[i，j]表示第一图象的象素值，P2[i，j]表示第2图象的象素值。M是在一行中的象素数目，N是在图象中的行的数目。如果值e超过预定的阈值，则报告景物变化的出现。
检测景物变化的第二方法是，产生两个相继图象的亮度值的直方图。在所述直方图中，以所述亮度值为函数存储预定亮度值出现的数目。在景物变化的情况下两个相继图象的直方图将显著不同。可将相继直方图之间的差值测量d写为： d = Σ i = 1 P | H 1 [ i ] - H 2 [ i ] | - - - ( 2 )
在(2)中，H1[i]表示第一图象的直方图，H2[i]表示第2图象的直方图。P是在直方图中斜坡道(bin)的数目。如果该测量d超过预定的阈值则认为景物变化出现。使用直方图的方法的优点是，减少了由于运动产生的两个图象差值的影响，因为在图象中元素的变化位置按照式(1)影响差值e，但所述运动对该直方图完全没有影响。文章“Adaptive Frame Type Seletion for Lon Bit-Rate Video Coding”byJ.Lee和B.W.Dickinson，in SPIE Vol.2308，pp.1411-1421，1994，广泛地讨论了检测景物变化的方法。
如果景物出现变化，请求装置26自视频源20读出下一个景物所需服务等级的估算GOS，请求装置26按照视频源20所需的服务等级，向ATM网络提供出通信量协议的请求。如果该请求被接受，则该视频编码器可以根据新的通信量协议进行发射。如果该请求被否定，请求装置26将信号R供给视频源20，指示该视频编码器必须根据前面商定的服务协议来限定它的输出。如果该网络在实际服务等级与请求服务等级之间建议一个服务等级。则该请求装置向视频源20通知新服务协议的参数。
由发射机请求的服务等级深深地依赖于应用的种类。在实现视频传输的情况需要的服务等级，比如记录到用于后来使用的数字视频记录器上的视频传输所需的服务等级高。第一种选择，必须做的是要用的连接种类。在下表中，是可从网络请求的与GOS参数一起的不同服务种类
服务种类 参数
常位速率(CBR) 峰值位速率(PCR)信元损失比(CLR)最大信元传送延迟(max CTD)峰-峰信元延迟变化(P-PCDV)
可变位速率(VBR) 峰值信元率(PCR)可保持的信元率(SCR)最大突发尺寸(MBS)信元损失比(CLR)最大信元传送延迟(max CTD)峰-峰信元延迟变化(P-PCDV)
在CBR请求连接的情况，只有被规定的描述符是PCR值。这个值必须从来自视频源20的位速率确定。传送的单元可以是图象，但在MPEG视频编码器的情况，传递的单元可以是图象组(GOP)。在MPEG编码中，给出了编码图象的不同种类。有不用来自邻近图象信息进行编码的所谓I图象。还有采用来自邻近图象信息进行编码的所谓B和P图象。在B图象的编码中，采用来自前面和随后图象的信息。在P图象编码中只采用来自前面图象的信息。包括1个I图象的两个I图象之间的图象组叫做图象组。一般，I图象的编码需要最大的位速率。可以说，景物变化检测器向编码器通知景物变化，以便使它开始新的GOP。
如果传输的单元是图象，则必须从网络请求景物的最大I图象所需的PCR。如果传输的单元是GOP，则必须从网络请求最大景物图象组所需的PCR。对于典型的影片材料，以GOP作为传输单元所需的PCR是以图象作为传输单元所需的PCR的大约一半。
在从网络请求：VBR服务的情况，规定的通信量描述符是PCR，SCR和MBS。在PCR、SCR、MBS和(虚拟)缓存器的大小L之间有一关系，对于该缓存器以实际的位速率读入并以恒定速率SCR读出，无溢出。由下式给出这种关系： MBS = 1 + L 1 - SCR PCR - - - ( 3 )
以PCR、SCR和MBS为函数的缓存器的大小L可写为： L = ( 1 - SCR PCR ) · ( MBS - 1 ) - - - ( 4 )
从式(4)中可清楚，当PCR比SCR大很多时L是大的，而如果PCR基本等于SCR则L是小的。因为L是在网络中所需缓存器大小的量度。较大的L值将造成用户的较高化费。较大的PCR会同样导致用户的较高化费。可设计用户设备以选择适当值的SCR、PCR和MBS，从而使其化费最小。
在按照图3的视频源中，将输入连接到MPEG视频编码器30。将带有输出信号CV的MPEG视频编码器30的第一输出连接到缓存器34的输入端。将带有输出信号RATE的MPEG视频编码器30的第2输出连接到服务等级估算装置32的输入。服务级估算装置32的输出提供所需服务等级的输出信号GOS。带有输出信号D的缓存器34的第一输出构成了视频源20的输出信号。将带有输出信号F的缓存器34的第2输出连接到MPEG视频编码器30的控制输入，将输入信号R加到缓存器34的第二输入。
在视频编码器30中，根据众所周知的MPEG-2编码标准对输入视频信号编码。将在MPE62视频编码器30的输出端上的编码视频信号CV暂存在缓存器34中。以由速率信号R确定的位速率从缓存器读出编码视频信号。假设在一个景物期间这个位速率是恒定的。假设每个景物的CBR通信协议。根据由ATM网络12可供给的服务等级，通过请求装置26确定速率R。信号F表示缓存器34的填充速率。被MPEG视频编码器30使用以减少当缓存器趋于溢出时它的输出速率。通过减少量化编码过程中用的所谓DCT(离散余弦变换)系数所用的量化电平的数目，MPEG视频编码器30可减少它的输出速率。
在图4的曲线中，有典型的影片材料的每帧所需的位数。从图4可以清楚地知道，从景物到景物所需位速率强烈变化。在下表中，给出了自影片四个婚礼和一个葬礼“Four Wedding and a Funeral”的30分钟的MPEG2(非可量的(nonscalable)，主图形(main profile)，主电平(main level))编码部分的通信量描述符。给出的这些通信描述符，是对于现有技术的传输系统，其中只在连接的开始商定服务的等级；和对于本发明的传输系统，其中当景物变化时重新商定服务的等级。
服务 通信量 图象平均 图象最大 GOP平均 GOP最大
现有技术 CBR PCR(信元/秒) 31500 31500 16800 16800
VBR PCR(信元/秒) 31500 31500 16800 16800
PCR(信元/秒) 7538 7538 7538 7538
PCR(信元/秒) 683712 683712 973629 973629
本发明 CBR PCR(信元/秒) 18520 31500 8981 16800
VBR PCR(信元/秒) 18520 31500 8981 16800
PCR(信元/秒) 7336 13090 7336 13090
PCR(信元/秒) 4067 34020 11470 83410
从表中可清楚地看出，与本发明所用的传输系统相比，现有技术的传输系统必须从网络请求高得多的服务等级。特别是在现有技术系统中的大值MBS时将导致在网络中的很大缓存器。
在CBR情况，根据现有技术，其L的值对于基于图象的传输是520098，而对于基于GOP的传输是536770。本发明的系统，对于基于图象和GOP传输两者，L的最大值是27000，L的平均值是2500。

Claims (9)

1.一种视频传输系统，包括具有用于提供编码视频信号的视频源的发射机，该发射机包括用于通过分组交换网络向接收机发送所述编码视频信号的发射装置，其特征在于，此发射机还包括：景物变化指示装置，用于提供景物变化指示；服务等级指示装置，用于指示所述景物变化之后的景物所要求的服务等级；并且此发射机还包括请求装置，用于根据所述要求的服务等级向此分组交换网络请求服务以响应所述景物改变指示。
2.根据权利要求1的视频传输系统，其特征在于，所述请求装置用于请求至少利用峰值位速率测量、平均位速率测量和突发性测量定义的服务。
3.根据权利要求1或2的视频传输系统，其特征在于，所述分组交换网络包括用于向发射机发送接受消息的装置，所述视频源包括提供装置，用于在该接受消息指示所述分组交换网络不能提供请求的服务时提供要求比请求的服务等级低的服务等级的编码视频信号。
4.一种用于视频传输系统的发射机，所述发射机包括用于提供编码视频信号的视频源，此发射机还包括用于通过分组交换网络向接收机发送所述编码视频信号的发射装置，其特征在于，所述发射机还包括：景物变化指示装置，用于提供景物变化指示；服务等级指示装置，用于提供所述景物变化之后的景物所要求的服务等级的指示；并且所述发射机还包括请求装置，用于根据所述要求的服务等级向所述分组交换网络请求服务以响应所述景物改变指示。
5.根据权利要求4的发射机，其特征在于，所述请求装置用于请求至少利用峰值位速率测量、平均位速率测量和突发性测量定义的服务。
6.根据权利要求4或5的发射机，其特征在于，所述视频源包括提供装置，用于在所述接受消息指示所述分组交换网络不能提供所述请求的服务时提供要求比请求的服务等级低的服务等级的编码视频信号。
7.一种视频传输方法，包括：提供编码视频信号；通过分组交换网络向接收机发送所述编码视频信号，其特征在于，所述方法还包括：提供景物变化指示；提供所述景物变化之后的景物所要求的服务等级的指示；并且此方法还包括：根据所述要求的服务等级，向所述分组交换网络请求服务，以响应所述景物改变指示。
8.根据权利要求7的视频传输方法，其特征在于，所述方法还包括请求至少利用峰值位速率测量、平均位速率测量和突发性测量定义的服务。
9.根据权利要求7或8的视频传输方法，其特征在于，所述方法还包括以下步骤：如果来自所述分组交换网络的接受消息指示所述分组交换网络不能提供所请求的服务，则提供要求比请求的服务等级低的服务等级的编码视频信号。
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