Patent Publication Number: US-8996713-B2

Title: Video streaming

Description:
This application is the U.S. national phase of International Application No. PCT/GB2011/000821 filed 27 May 2011 which designated the U.S. and claims priority to GB 1011047.6 filed 30 Jun. 2010, and EP 10251246.4 filed 12 Jul. 2010, the entire contents of each of which are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     This invention relates to video streaming, in particular where the streaming is over a network with a non-deterministic bandwidth availability. It is particularly, though not exclusively, applicable to the delivery of video that has been encoded at a number of different constant quality levels. 
     BACKGROUND TO THE INVENTION 
     Traditionally a video on demand service, such as the applicant&#39;s BT Vision service, is supported by encoding video at a constant bit rate and delivering it over a network at the same constant bit rate. This generally requires bandwidth reservation on the network, which can be expensive to provide. 
     Video encoded using compression techniques naturally has variable bit rate, as the number of bits produced when encoding a picture depends on the picture content: how similar it is to previously encoded pictures and how much detail it contains. Some video scenes can be coded to a given quality with a small number of bits, whereas other scenes may require significantly more bits to achieve the same quality. When constant bit rate encoding is used, video has to be coded at time varying quality to meet the bit rate constraint. This has been shown to be sub-optimal to the user, who would prefer to see constant quality. Also, by fixing the bit rate independent of the genre of the video content, some genres of content can be encoded well, such as news and drama, whereas others, such as fast moving sport and music videos and concerts, can only be coded quite poorly. Adaptive video delivery using variable bit rate encoding can be used to overcome these problems. 
     With an adaptive delivery system, the need for bandwidth reservation is removed, with the video delivery system adapting the bit rate of video delivered according to the available network throughput. Content can be encoded at a number bit rates corresponding to a number of quality levels, and delivered over the network without bandwidth reservation. Generally the video data would be delivered as fast as possible; while the quality level (encoded bit rate) is adapted according to the network throughput achieved so as to maximise the quality of the video delivered whilst ensuring that all video data is delivered over the network in time for it to be decoded and displayed without interruption. 
     International patent application WO 2009/112801 describes a variable bit rate encoding method that maintains a constant perceptual quality. Use is made of a perceptual quality metric (one that achieves a good correlation with actual viewer perception by taking into account masking effects) in a video encoder to encode with constant perceptual quality. Coding parameters, specifically the quantisation parameter, is set separately for each frame taking into account masking effects based on relative contrast levels in each frame. The resulting encoded bitstream has a variable bit rate. 
     International patent application WO 2005/093995 describes a network with a video server connected to a number of client devices over a shared backhaul. Video content is encoded at a number of constant quality levels and the encoded bitstreams stored on a network based server. In response to requests from the clients, the encoded bitstreams are selected by the server and delivered over the shared network to the clients. Switching between the different bitstreams, and hence qualities, can be done depending on the actual network throughput, with an aim to maximise the quality of the stream. 
     However, when delivering video content that has been encoded at two or more quality levels, it is necessary to determine the minimum bit rate required to deliver the remainder of the video content at each of the available quality levels, so that a decision can be made as to whether to switch to a different quality bitstream depending on the actual network delivery rate. 
     One way to determine the minimum delivery bit rate for a given video stream is to analyse the statistics of the encoded video streams prior to commencing delivery. Thus, for a plurality of positions within each video stream, pairs of data can be pre-calculated, each pair containing a delivery bit rate and the minimum start-up delay that would be required if that delivery rate were to be used for timely delivery of the remainder of the given video stream. This data is then used during the subsequent streaming process to determine whether a switch can be made to a different quality stream, based on the amount of data already buffered at the receiver and the actual network delivery rate. Preferably, the quality of the stream selected is as high as the network delivery rate can support. 
     SUMMARY OF THE INVENTION 
     It is the aim of embodiments of the present invention to provide an improved method of streaming a video sequence over a network. In particular where the video sequence is encoded at a number of different quality levels, and embodiments of the present invention aim to provide an improved method of determining the particular quality level of video sequence to switch to, whilst ensuring timely delivery of the video sequence. 
     According to one aspect of the present invention, there is provided a method of transmitting a media stream from a server to a receiver over a transmission link in a network, comprising:
         (a) encoding a media sequence at a plurality of quality levels;   (b) determining for each encoded media sequence a set of pre-calculated data comprising, for each of a plurality of temporal positions in the sequence, i) for each of a plurality of transmission rates, the minimum preload required at the receiver for timely delivery of the sequence from said temporal position to the end of the sequence, and ii) the transmission rate required for timely delivery of the sequence from said temporal position to the end of the sequence where no preload is present at the receiver, where the preload is the playout duration of the data buffered at the receiver;   (c) transmitting one of h encoded media sequences to the receiver; and during transmission,   (d) determining the current preload at the receiver, and for a media sequence encoded at one of the plurality of quality levels;
           selecting from the set of pre-calculated data associated with the respective quality level a first preload above and a second preload below the current preload, and the respective first and second transmission rates;   calculating an intersection time as the time when the total number of bits delivered by the first and second transmission rates are equal when taking into account the respective first and second preloads;   calculating the number of bits delivered by the intersection time;   calculating an acceptable delivery rate required for timely delivery of the sequence as the number of bits delivered by the intersection time divided by the sum of the current preload and intersection time; and   
           after an elapsed time following step (d)   (e) calculating a sufficient delivery rate required as equal to the difference between the number of bits delivered by the intersection time and the actual number of bits delivered during the elapsed time divided by the sum of the current preload and intersection time minus the elapsed time; and calculating a new acceptable delivery rate by repeating step (d); and   (f) determining the minimum required delivery rate to ensure timely delivery of the remainder of the sequence as the minimum of the new acceptable delivery rate and the sufficient delivery rate.       

     Preferably, the media stream is a video stream. The quality levels may be fixed for e each encoded sequence. The quality levels may be perceptual quality levels. 
     The receiver preferably performs steps (d) to (f). 
     The calculating of the intersection time may be determined as the difference between the product of the second preload and second transmission rate and the product of the first preload and the first transmission rate, wherein said first intersection time is equal to the difference between the products divided by the difference between the first and the second transmission rates. 
     The calculating of the number of bits delivered by the intersection time may be determined as the product of the second transmission rate and the sum of the second preload and the intersection time, or the product of the first transmission rate and the sum of the first preload and the intersection time 
     The method may further comprise (g) determining the actual transmission rate of the sequence over the transmission link, and deciding whether to switch to a sequence encoded at a different quality level based on the minimum required delivery rate compared to the actual transmission rate. 
     The temporal position is a position of a group of pictures in an encoded video stream. 
     According to a further aspect of the present invention, here is provided a method of handling a media stream at a receiver, comprising:
         (a) receiving a set of pre-calculated data for a media sequence encoded at a plurality of quality levels, said set comprising, for each of a plurality of temporal positions in the sequence, i) for each of a plurality of transmission rates, the minimum preload required at the receiver for timely delivery of the sequence from said temporal position to the end of the sequence, and the transmission rate required for timely delivery of the sequence from said temporal position to the end of the sequence where no preload is present at the receiver, where the preload is the playout duration of the data buffered at the receiver;   (b) receiving an encoded media sequence; and during reception,   (c) determining the current preload at the receiver, and for a media sequence encoded at one of the plurality of quality levels;
           selecting from the set of pre-calculated data associated with the respective quality level a first preload above and a second preload below the current preload, and the respective first and second transmission rates;   calculating an intersection time as the time when the total number of bits delivered by the first and second transmission rates are equal when taking into account the respective first and second preloads;   calculating the number of bits delivered by the intersection time; calculating an acceptable delivery rate required for timely delivery of the sequence as the number of bits delivered by the intersection time divided by the sum of the current preload and intersection time; and   
           after an elapsed time following step (c)   (d) calculating a sufficient delivery rate required as equal to the difference between the number of bits delivered by the intersection time and the actual number of bits delivered during the elapsed time divided by the sum of the current preload and intersection time minus the elapsed time; and calculating a new acceptable delivery rate by repeating step (c); and   (e) determining the minimum required delivery rate to ensure timely delivery of the remainder of the sequence as the minimum of the new acceptable delivery rate and the sufficient delivery rate.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention reference will now be made by way of example only to the accompanying drawings, in which: 
         FIG. 1  is shows a video server capable of adaptive video delivery; 
         FIG. 2  is shows a receiver capable of receiving adaptive video; 
         FIG. 3  is a graph showing a cumulative bit curve for a video sequence encoded at variable bit rate and its piecewise constant, monotonic decreasing, delivery schedule, referred to as the “downstairs delivery schedule”; 
         FIG. 4  is a flow chart showing the encoding of video content at various quality levels and the pre-calculation of parameters associated with each of those bitstreams; 
         FIG. 5  is a graph showing how he preload time can be calculated for a given delivery rate; 
         FIG. 6  is table showing examples of pre-calculated data associated with a bitstream; 
         FIG. 7  is a flow chart showing how a receiver can request and receive a video stream, including the pre-calculated data; 
         FIG. 8  is a flow chart showing how the receiver can use the pre-calculated data to determine the minimum delivery rate required to stream each quality of video. 
         FIG. 9  is a graph showing the delivery rate required when a given amount of data is already buffered at the client can be estimated from two sets of pre-calculated data associated with a smaller and larger delivery rate; 
         FIG. 10  is a graph showing a delivery schedule where the delivery rate is calculated by interpolation using pre-calculated data, and where delivery is at the calculated rate, and where a new rate is calculated after delivering each group of pictures; 
         FIG. 11  a graph showing how the calculated required delivery rate of  FIG. 8  varies with time; 
         FIG. 12  is a flow chart showing the steps performed to determine the minimum required delivery rate using an improved interpolation method; and 
         FIG. 13  is a graph showing how both the interpolated required delivery rate and the improved calculated required delivery varies with time when delivery is at a fixed rate greater than the actual required delivery rate; 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention is described herein with reference to particular examples. The invention is not, however, limited to such examples. 
     The network contains one or more network based video servers connected to one or more video receivers over a shared network. Multiple pieces of video content are each encoded at a number of constant quality levels and stored on one or more of the network based servers. In response to requests from a receiver, video content is selected on a server and delivered over the shared network to the receiver. The system selects the quality level of the stream to deliver based on characteristics of the video stream currently being delivered, and the number of bits already buffered at the video receiver and the available network throughput. The aim is to select the video quality being delivered according to the available network throughput so as maximise the quality while ensuring that all video data is delivered over the network in time for it to be decoded and displayed without interruption. 
     The server  100  shown in  FIG. 1  comprises a video input interface  102  that receives digitally coded video signals  104 , in uncompressed form. A processor  106  operates under control of program modules stored in local storage  108 , for example a hard disk, and has access to memory  110  and a video output interface  112  for connection, via a telecommunications network to plural client devices. The video output interface  112  may include a buffer to temporarily store video encoded by the processor  106  before outputting the encoded bit stream  114 . The memory includes a memory area  150  for storage of the encoded video and parameters associated with the encoded video. 
     The program modules include a general purpose operating system  120  and various video coding software modules which implement one of more of the video coding methods shortly to be described. The software modules comprise several program types, namely:
         a control module  122 ;   a compression module  124 : in this example the compression software implements a coding algorithm compliant with the ITU H.264 standard;   a perceptual quality evaluation module  126 ;   a bitstream evaluation module  128 ;       

     It is understood that plural video streams can be received, encoded and stored in storage  110 , for later streaming via the output interface  112 . 
     The server  100  transmits encoded video to a receiver, which includes a video decoder.  FIG. 2  illustrates an example receiver  200 . The receiver  200  is connected to the video server  100  over a suitable network and is capable of receiving and decoding encoded video streams transmitted by the server  100 . 
     Specifically, the receiver  200  comprises a video input interface  202  that receives encoded video signals  204 , such as encoded bitstreams transmitted by the server  100 . A processor  206  operates under control of program modules stored in local storage  208 , for example a hard disk, and has access to memory  210  and a video output interface  212  for connection to a suitable output device such as a monitor or television. The video input interface  202  includes a buffer  203  to temporarily store received encoded video until the encoded video is ready for decoding by the processor  206 . 
     The program modules include a general purpose operating system  220  and various video decoding software modules which implement a decoding method for decoding the received encoded video bitstreams. The software modules comprise several program types, namely:
         a control module  222 ;   a decoding module  224 : in this example the decoding software implements a decoding algorithm compliant with the ITU H.264 standard;   an interpolation module  226 ;   a bitstream switching module  228 ;       

     A video sequence that has been encoded at variable bit rate can be delivered over a network at piecewise constant bit rate, with the rate of each piece decreasing monotonically. This is believed to have first been noted by Professor Mohammed Ghanbari. He referred to the resulting bit rate profile as a “downstairs” function. 
     The applicant&#39;s international patent application WO2004/047455 describes one method of analysing a video sequence that has been encoded at variable bit rate to determine a video delivery schedule for that sequence consisting of one or more contiguous periods of constant bit rate delivery in which the rates are monotonically decreasing. 
     In an alternative visualisation the cumulative number of bits in the encoded video sequence is plotted against time. An example of such a cumulative bit curve is shown in  FIG. 3 .  FIG. 3  shows the cumulative bit curve, which is a plot of the total number of bits in the encoded video sequence to date against time. The “downstairs” delivery rate required for initial delivery of this stream is the slope of the line of lowest slope that passes through the origin and is never to the right of the cumulative bit curve, that is, ensuring all bits are delivered before they are needed for decoding. Such a delivery line (or delivery schedule) will touch the cumulative bit curve in one or more places, but typically only once. When delivery to that point (or the last of those points) in the video sequence has occurred, all video bits delivered will be decoded and no bits will be buffered awaiting subsequent decoding. From this point onwards, a new line of lowest slope could be calculated, and the next coded video bits delivered at this lower rate. 
     Video that is encoded at constant bit rate will have a cumulative bit curve near to a straight line. Video that is not coded at constant bit rate, for example, video coded with constant quantisation index or with constant perceptual quality will have a cumulative bit curve that in general will not be a straight line. Such video could be transmitted over a network with a delivery schedule similar to the cumulative bit curve, and thus require little buffering in the receiver, or, with unlimited buffering available in the receiver, could be delivered with any delivery schedule subject to every bit being received before it is needed for decoding. When plotted on a graph, with time along the horizontal axis and cumulative bits on the vertical axis, this requirement can be expressed as the delivery schedule must never be to the right of the cumulative bit curve. When there is unlimited buffering at the receiver, it is always possible to deliver any bitstream using constant bit rate delivery at any constant rate: it is just necessary to ensure decoding is delayed sufficiently to keep the delivery schedule to the left of the cumulative bit curve. In the extreme case, the whole of the video sequence could be delivered very slowly and decoding started only as the delivery is about to finish. 
     In the examples considered here, we consider discrete “chunks” each comprising one or more frames. The choice of chunks is subject to the consideration that, in order to keep to a minimum any interdependence of quality between chunks, a chunk can be one or more frames coded independently of any other chunk, typically starting with an I-frame (one coded without prediction from any other frame). In all the examples here, the chunk chosen was a group of pictures (GOP) of IBBP format and all the plots shown are of cumulative bits per group of pictures, Σb, where b is the number of bits in a group of pictures, plotted against group of pictures index. 
     In the streaming system envisioned, each video sequence is encoded multiple times by the video server  100 , each time with a different level of perceptual quality. Encoding at fixed levels of perceptual quality is preferably performed as described in the applicant&#39;s International application WO2009/112801. However, other methods of encoding could be used, such as encoding with a constant quantisation index. 
     We assume the receiver has sufficient buffering to be able to store a whole video stream. This allows the timing of delivery of the video data to be decoupled from the decoding of it, provided we ensure data is delivered before it is needed for decoding. 
     The delivery bit rate required to deliver a video stream in timely fashion depends on how much data is already buffered at the receiver, and the bit rate profile of the bits yet to be decoded. By analyzing the encoded video stream at the video server  100  prior to delivery, and by monitoring the amount of data buffered at the receiver  200  following delivery by the server  100 , we can determine, for each possible quality level that could be delivered, the minimum bit rate required for timely delivery. 
     The encoding and pre-calculations performed by the video server  100  are summarised in the flow chart of  FIG. 4 . In step  400  of  FIG. 4 , the server  100  receives a video sequence. In step  402 , the compression module  124  in the server  100  encodes the video sequence at a number of fixed quality levels L q  (where q=1 . . . Q). The amount of data (bits) b qg  generated for each group of pictures g (where g=1 . . . G) is also determined, by the bitstream evaluation module, and stored in memory  150  together with each of the encoded bitstreams. The quality level may for example be set by selecting a suitable quantisation parameter for the encoding process. 
     In step  404 , each of the encoded bitstreams are analysed by the bitstream evaluation module  128  to create a file of pre-calculated data giving the following information for each quality at which the content was encoded for each group of pictures.
     1) Firstly we calculate the minimum delivery rate D qg  that would be needed for delivery (transmission) of that group of pictures and all subsequent groups of pictures without decoder buffer underflow occurring at the receiver  200  if no video is initially buffered at the decoder. This rate can be referred to as the zero start-up delay delivery rate, or zero preload delivery rate, for the given group of pictures. The term “preload” and start-up delay will be used interchangeably to refer to the duration of the video buffered at the receiver ready for playback.   

     So, we calculate the delivery rate a qgk  that would be needed to deliver quality q, from group of pictures g to group of pictures k using the earlier calculated amount of bits, b qg : 
     
       
         
           
             
               
                 
                   
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     The minimum delivery rate D qg  can then be calculated as he maximum value of a qgk  for all k satisfying g≦k≦G: 
     
       
         
           
             
               
                 
                   
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     This minimum delivery rate D qg  is calculated for each group of pictures g in the sequence, and for all sequences encoded at each of the qualities L q .
     2) Secondly we calculate the minimum amount of time, or preload, T qgr  that must be buffered at the receiver for delivery (transmission) of that group of pictures and all subsequent groups of pictures without decoder buffer underflow occurring at each of a set of delivery rates D qr  (r=1 . . . r max ) where the delivery rates are measured in units of bits per group of pictures period. For convenience, and so that each sequence will have the same range of relative delivery rates, the rates D qr  may defined as fixed multiples, C r , of the average coded bit rate at that quality:   

     
       
         
           
             
               
                 
                   
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     This time T qgr  may be visualised as being obtained by drawing, on  FIG. 5 , a line of slope D qr  having origin, X, on the cumulative bit curve at group of pictures g−1. For each subsequent group of pictures, the vertical distance between the line and the cumulative bit curve is measured, and the largest distance, P qgr , of the line below the cumulative bit curve is determined. This represents the required preload of bits for delivery at rate D qr  starting with delivery of the group of pictures with index g. This largest distance, P qgr , is divided by the slope D qr  to obtain the preload T qgr . 
     Thus 
     
       
         
           
             
               
                 
                   
                     
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     Note, if the line in  FIG. 5  is always above the cumulative bit curve and all distances are negative, then the required preload is zero. 
     Thus, Equation 5 can be rewritten as: 
     
       
         
           
             
               
                 
                   
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     In step  406  of  FIG. 4 , these pre-calculated parameters from step  404  can be stored as a suitable file in memory  150 . Thus, we will effectively have a table of data for each encoded video stream (at each quality L q ), with the following data per GOP g—the delivery rate required for zero start-up delay, and the pre-load required for each of a number of delivery rates (based on fixed multiples C r  of the average bit rate of the entire stream). 
       FIG. 6  shows an example of a table of pre-calculated data with a video sequence of 10 GOPs, and encoded at two different quality levels L q =2.7 and 3.7. Rate multiples C r =0.6, 0.8, 1.0 and 1.2 are used in this example. 
     The pre-calculated data can be used to determine the rate required for each quality stream to be transmitted given the amount of preload at the receiver. For example, if the amount of preload at the receiver is measured to be 10 seconds, or 10000 ms, and the next GOP to be delivered is g=6, then we can see from  FIG. 6  that the rate required for delivery at L q =2.7 is between C r =1.0 and 1.2. Similarly, for L q =3.7, the rate required is between C r =0.8 and 1.0. Thus, an actual streaming rate of (C r =)1.2 times the mean bit rate for L q =2.7 would be sufficient to support the L q =2.7 stream. 
     However, an actual streaming rate of (C r =)1.0 times the mean bit rate for L q =2.7 would not be sufficient to support the L q =2.7 stream. Similarly, an actual streaming rate of (C r =)1.0 times the mean bit rate for L q =3.7 would be sufficient to support the L q =3.7 stream, However, an actual streaming rate of (C r =)0.8 times the mean bit rate for L q =3.7 would not be sufficient to support the L q =3.7 stream. 
     The amount of data that can realistically be pre-calculated is limited, and so it is not possible to have pre-calculated data for all possible values of preload. Thus, in order to determine more accurately the streaming rate required to support a given quality level of video when a certain preload is present at the receiver, an interpolation method is proposed as described below. 
     Now, once the server has, finished pre-calculating the data as described, the server  100  can start sending the encoded video to the receiver  200 .  FIG. 7  shows the processes at the receiver  200  when requesting video content from the server  100 . Starting at step  700 , the receiver  200  makes a request to the server  100  for some video content. The server  100  sends to the receiver  200  the pre-calculated data associated with the requested video, which is received by the receiver  200  in step  702 . The receiver  200  stores this pre-calculated data in memory  210 . The server  100  then starts streaming the requested video by selecting a suitable quality L q  bitstream to transmit to the receiver  200 . Thus, in step  704 , the receiver  200  starts receiving the video stream encoded at quality L q . 
     The selection of the initial quality of the bitstream to be transmitted to the receiver  200  may be done according to any suitable method by the server  100  or can be determined by the receiver  200 , such as starting with the lowest quality available, or selecting the quality that is most appropriate for the available network bandwidth at that time. 
     Once the receiver  200  has started to receive the encoded video stream, the receiver can perform the steps shown in  FIG. 8  using the stored pre-calculated data. 
     In step  800  of  FIG. 8 , the next group of pictures to be delivered is that with index g. The receiver calculates how much data τ g  is buffered in the receiver&#39;s buffer  203 , in terms of time. τ g  is the preload or put another way, the start-up delay for the next data to be delivered. This (assuming that the units of time are group of pictures periods) is simply a count of h number of groups of pictures received minus the duration of video played out. 
     Then for each video quality level L q , starting at the highest quality in step  802  and working downwards, calculate using the interpolation module  226  in step  804  a delivery rate A qg  that would be sufficient to deliver the group of pictures with index g and all subsequent groups of pictures at this quality without decoder buffer underflow occurring using an interpolation method and based on the pre-calculated data stored at the receiver, as follows. 
     We need to know he delivery rate required given the amount of data already buffered at the receiver, which if measured in terms of the play-out time it represents, corresponds to the preload or start-up delay for the next data to be delivered. As it is not feasible to generate pre-calculated data for all possible values of start-up delay, we can estimate the delivery rate required by interpolation using the information that we have pre-calculated. 
       FIG. 9  shows how the delivery rate required when a given amount of data is already buffered at the client can be safely estimated from two sets of pre-calculated data, one giving the amount of preload required for a smaller delivery rate and the other giving the amount of preload required for a larger delivery rate. 
       FIG. 9  shows the cumulative bit curve and delivery schedules for three delivery bit rates, A 1 , A 2  and A 3 . The minimum amounts of preload required, expressed as start-up delays for the next data to be delivered, S 1  and S 3 , for the delivery rates A 1  and A 3  respectively, have been pre-calculated. The optimum rate for the actual start-up delay S 2  present at the receiver (measured as τ g ), where S 2  is between S 1  and S 3 , would be A 2  and would give the dot-dash line shown. However, we do not have any pre-calculated data for preload S 2  and thus the required delivery rate A 2 . The problem then is to provide a safe estimated value A 2 ′ for the delivery rate A 2  required for start-up delay S 2 , i.e. A 2 ′≧A 2 . The solution is to select pre-calculated data that we do have, specifically a preload S 1  that is greater than S 2 , and preload S 3  that is less than S 2  and their respective delivery rates A 1  and A 3 , and apply the interpolation method set out below. 
     To ensure timely delivery of bits, the delivery schedule A 2 ′ must be on or to the left of the cumulative bit curve. This is ensured if the delivery schedule A 2 ′ passes through the intersection of the delivery schedules, A 1  and A 3 , which occurs at time T. The intersection of the delivery schedules A 1  and A 3  occurs when the total bits B delivered by each delivery schedule are equal (taking into account the respective preloads). Thus, we can say:
 
 A 1·( T+S 1)= A 3·( T+S 3)   [7]
 
     Equation 7 can be rewritten as Equation 8 below to give the intersection time T. 
     
       
         
           
             
               
                 
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                       - 
                       
                         A 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         3 
                       
                     
                   
                 
               
               
                 
                   [ 
                   8 
                   ] 
                 
               
             
           
         
       
     
     An acceptable delivery rate for start-up delay S 2  is the straight line of slope A 2 ′ passing through this point, given by Equation 9 below. As shown in  FIG. 9 , this may not optimal. 
     
       
         
           
             
               
                 
                   
                     A 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       2 
                       ′ 
                     
                   
                   = 
                   
                     
                       
                         
                           ( 
                           
                             
                               S 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               1 
                             
                             + 
                             T 
                           
                           ) 
                         
                         · 
                         A 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                     
                       
                         S 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         2 
                       
                       + 
                       T 
                     
                   
                 
               
               
                 
                   [ 
                   9 
                   ] 
                 
               
             
           
         
       
     
     Thus, Equation 9 gives a safe estimate of the minimum delivery rate A qg , that is, the rate is sufficient deliver the group of pictures with index g and all subsequent groups of pictures without decoder buffer underflow at the receiver  200 , but may be more than is absolutely necessary. 
     Referring to the example described above in relation to  FIG. 6  where the next GOP g=6, and the measured preload is 10000 ms. Let&#39;s assume the average bit rate for L q =2.7 is 262 kbit/s. For L q =2.7 stream, set S 1 =11799 ms, S 3 =1127 ms, A 1 =262 (1.0×262), and A 3 =314.4 (1.2×262). Thus, applying equation 8, we get T=52233. Now, using equation 9, this calculated value of T and S 2 =10000 ms, we get A 2 ′=269.6 kbit/s. Thus, the minimum delivery rate A qg =269.6 kbit/s. If the network throughput is at least 269.6 k/bit/s, then the L q =2.7 stream can be supported from GOP g=6 onwards. 
     In step  806 , A qg  can be compared to the actual network throughput D to determine if the required minimum rate is met. If the D is at least equal to A qg , then stream of quality L q  under consideration can be streamed by the network. Thus, in step  810 , a decision can be made to switch to, or remain on, this L q  quality level stream. The decision as to exactly when to switch may be based on other factors, such as tolerances and thresholds relative to the calculated minimum A qg . If D is not at least A qg , then processing passes to step  808 , where the pre-calculated data for the next lowest L q  stream can be analysed, and steps  804  onwards repeated to determine the minimum delivery rate A qg  required for the this next stream L q . 
     Steps  804  and  806  can be repeated for all quality levels if required to get picture of the rates required for each quality level of stream. 
     It can be seen from  FIG. 9  that when the intersection of he delivery schedules A 1  and A 3  is on the cumulative bit curve, the interpolation method will give the exact required rate, and otherwise will provide an over estimate of the required rate. 
     Now consider repeating the calculation of required delivery rate after the delivery of the group of pictures with index g and zero or more subsequent groups of pictures. If the delivery rate achieved is exactly A qg , then the delivery schedule follows the line A 2 ′, and each time the interpolation is repeated, the same intercept time T will be calculated, and the same required delivery rate A qg  will be calculated. If the delivery rate achieved is different from A qg , then the delivery schedule will diverge from the line A 2 ′. But during the period that the delivery schedule remains between the lines A 1  and A 3 , it can be seen from  FIG. 9  that each time the interpolation is repeated, the same intercept time T will be calculated, and a different, but consistent, required delivery rate A qg  will be calculated. 
     Now consider the case where A 3  is the highest delivery rate for which pre-calculations have been performed, and where the current delivery point is to the right of the line A 3 , and to the left of the cumulative bit curve, as otherwise it would represent an invalid delivery point as the bits would not have been delivered in time for decoding. In this case interpolation is performed using the rate A 3  and the associated pre-calculated start up delay, S 3 , as one set of data, and the pre-calculated zero start-up delay delivery rate as the other.  FIG. 10  shows such a delivery schedule where delivery is at the calculated rate, and a new rate is calculated after delivering each group of pictures.  FIG. 11  shows how this calculated delivery rate varies with time. 
       FIG. 11  shows that even when delivery is at the rate calculated by interpolation, which is known to produce a rate greater than or equal to that actually needed, the rate when calculated again after delivering a group of pictures may be higher. And when this higher rate is used to deliver the next group of pictures, the next calculated rate may be higher still. This behaviour of calculating ever increasing rates does not continue indefinitely, but stops when the pre-calculated rate for delivering with zero start up delay decreases from its previous value. This happens at the cusps of the cumulative bit curve of  FIG. 10 . When the first such point is reached, the newly calculated rate is actually lower than the first calculated value, as shown in  FIG. 11 , as the average delivery rate to this time has been higher than needed, and hence a lower rate from this time is possible. But as more groups of pictures are delivered, the calculated delivery rates increase again due to the increasing rates pre-calculated for delivering with zero start up delay. 
     This is a problem with the combination of interpolation with the pre-calculation of the required start up delays for a finite number of delivery rates. Although the use of pre-calculated delivery rates for delivering with zero start up delay is useful in that it allows a delivery rate to be calculated for low values of start up delay, the fact that this rate can change rapidly from one group of pictures to another leads to this problem of rapidly varying calculated delivery rates, when there is actually no change in the underlying required rate. In a real video delivery system, such variation in the calculated delivery rate may lead to unnecessary switching between video streams encoded at different qualities. This would cause unnecessary changes in video quality to the viewer, and unnecessary effort in the video delivery system to perform such switches. We can overcome this problem by adapting our interpolation method in the following way. 
     When performing the interpolation calculation using the pre-calculated data with the highest bit rate and the pre-calculated rate required for zero start up delay using Equations 8 and 9, the number of bits, B, that would be delivered at the interception point T, is also calculated as in Equation 10 as follows:
 
 B =( S 3 +T )· A 3=( S 1+ T )· A 1   [10]
 
     As the next and subsequent groups of pictures are delivered, the number of subsequent bits delivered, b, are counted, and the elapsed time, t, is measured. The ratio between these is calculated, to give an actual delivery rate achieved, r. When this value is greater than or equal to the previously calculated interpolated required delivery rate, A 2 ′, we can calculate a sufficient delivery rate, a, as the rate required to reach the previously calculated interception point (T, B) from the current delivery point (t, b), as in Equation 10. 
     
       
         
           
             
               
                 
                   a 
                   = 
                   
                     
                       ( 
                       
                         B 
                         - 
                         b 
                       
                       ) 
                     
                     
                       ( 
                       
                         
                           S 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                         + 
                         T 
                         - 
                         t 
                       
                       ) 
                     
                   
                 
               
               
                 
                   [ 
                   11 
                   ] 
                 
               
             
           
         
       
     
     This calculation can be applied repeatedly until the quantity B bits have been delivered since B was calculated, and while r is calculated to be greater than or equal to the interpolated required delivery rate, A 2 ′. In addition, after delivering each group of pictures, the interpolation calculation using the pre-calculated data with the highest bit rate and the pre-calculated rate required for zero start up delay can be performed to give a new interpolated required delivery rate, A 2 ′, and the lower, A 2 ″, of the two calculated required delivery rates used:
 
 A 2″=min( a, A 2′)   [12]
 
     Use of equations 10, 11 and 12 are included as part of step  804  in this improved interpolation method to determine A 2 ″, which is a more accurate measure of the minimum required delivery rate A qg . This improved method is also set out in  FIG. 12 , which effectively expands upon step  804  in  FIG. 8 . 
     In step  1200  of  FIG. 12 , the minimum required delivery rate is estimated according to the earlier methods using equations 8 and 9. In step  1202  the sufficient delivery rate is estimated using equations 10 and 11. And in step  1204 , the minimum delivery rate is estimated using equation 12. 
     An example is shown in  FIG. 13 , where the cumulative bit curve is as shown in  FIG. 10 . Actual delivery is at a constant rate of 1.4 MBits per group of pictures period, as shown by the line labelled ‘r’. The curve labelled ‘A 2 ′’ shows the delivery rate required as calculated by interpolation using the pre-calculated data with the highest bit rate and the pre-calculated rate required for zero start up delay using Equations 7 and 8. This curve is different to that of  FIG. 11  because in this case actual delivery is at a fixed rate whereas in the previous case it was at the calculated required delivery rate. The curve labelled ‘A 2 ″’ shows the delivery rate required when using Equations 10 and 11. It can be seen that this is a smooth curve, accurately reflecting the fact the delivery is consistently at a rate faster than necessary. It is also always below the actual delivery rate r, hence indicating that the required rate is always less than the actual delivery rate, and hence there is no need to switch to a lower bit rate, lower quality video stream. Whereas if the required rate were to be deduced simply from interpolation, with the result being A 2 ′, it would be concluded at times that the actual delivery rate r were insufficient for delivery of the stream, and that a switch to a lower bit rate, lower quality video stream should be carried out. Such a switch would be unnecessary, and could be avoided by the use of the improved calculations of Equations 11 and 12. 
     Note that the curves labelled A 2 ′ and A 2 ″ in  FIG. 13  stop when the calculated rates fall to the level of A 3 , the highest rate for which start up delay values have been pre-calculated. In practice start up delay values would have been pre-calculated for some lower rate A 1 , and from this point onwards, interpolation would be performed between the pre-calculated data for rates A 3  and A 1 , and the delivery rate for zero start up delay would not be used. 
     The method for calculating the minimum required delivery rate can be summarised as follows. Firstly we encode a video sequence at multiple levels of quality as in step  402 . Then in step  404 , we pre-calculate for each group of pictures the delivery rate that would be needed for timely delivery of that group of pictures and all subsequent groups of pictures if no time is initially buffered at the decoder. Also in step  404 , we select a set of delivery bit rates, and for each of these rates we pre-calculate the minimum amount of time (pre-load) that must be buffered at the decoder for timely delivery of that group of pictures and all subsequent groups of pictures. The pre-calculated data is provided to the decoder in step  702 . Then during video delivery, interpolation is performed using these pre-calculated data according to Equations 8 and 9, and where applicable using Equations 11 and 12, to calculate a safe estimate of the minimum required delivery rate as shown in  FIGS. 8 and 12 . 
     After such calculation of the minimum delivery rate required for each quality of video encoding, the video streaming system (in a preferred example, the receiver) would determine, in consideration of the expected network throughput, which quality of video to transmit next. The expected network throughput may be estimated from past measurements of actual throughput or by other means. Other factors, such as the amount of data buffered at the receiver, may also be taken into account when making the decision of which quality of video to transmit next. For example, it may be decided not to switch to a higher quality if less than a threshold of data, such as that which would take five seconds to display, were buffered at the receiver, or it may be decided to switch to a lower quality if less than a threshold of data were buffered, regardless of how the expected network throughput compares to the delivery rate required for each quality of video encoding. The decision of which quality of video to switch to could be made as often as every group of pictures, as it is possible to change between the different encodings at the start of any group of pictures. 
     The decision to switch to a different quality stream is done by the bitstream switching module  228 . The receiver  200  can send a request to the server  100  to ask it to start transmitting a different quality video stream selected (as chosen by the receiver). The server may immediately switch and transmit the new stream, which may be delayed until a suitable switching point such as at the start of a new GOP. The server can then send data to the receiver to indicate that a switch has been made, and also start transmitting the new video stream. 
     Whilst the above examples have been described with reference to the receiver  200  performing the calculations that determine the required minimum bit rates and when to switch streams, the server could alternatively perform the same. The requirement is that the server would need to know the preload at the receiver&#39;s buffer in order to perform the required calculations. 
     Furthermore, whilst reference is made to determining the pre-calculated data at intervals of a GOP, and determining the minimum bit rate required at various GOP positions, the invention can equally be based on any temporal position within the stream instead of GOP positions, such as some other suitable frame interval. 
     Exemplary embodiments of the invention are realised, at least in part, by executable computer program code which may be embodied in application program data provided for by the program modules at he server and receiver. When such computer program code is loaded into the memory of the server or receiver for execution by the respective processor, it provides a computer program code structure which is capable of performing at least part of the methods in accordance with the above described exemplary embodiments of the invention. 
     Furthermore, a person skilled in the art will appreciate that the computer program structure referred can correspond to the process flow charts shown in  FIGS. 4 ,  7 ,  8  and  12  where each step of the flow chart can correspond to at least one line of computer program code and that such, in combination with the processor in the respective server or receiver, provides apparatus for effecting the described process. 
     In general, t is noted herein that while the above describes examples of the invention, there are several variations and modifications which may be made to the described examples without departing from the scope of the present invention as defined in the appended claims. For example, the invention could also be applied to the audio streams or other media streams instead of video. One skilled in the art will recognise modifications to the described examples.