Abstract:
Techniques for determining an output rate for a bit stream, the output rate being determined by applying information read from the bit stream to available bandwidths. The techniques are employed to construct a statistical multiplexer that multiplexes varying bit-rate bit streams onto a satellite up-link. Minimum and maximum output rates for each bit stream are determined such that neither a queue for the bit stream in the multiplexer nor the bit stream&#39;s decoder will underflow or overflow. The multiplexer looks at the fullness of the memory buffers and correlates the fullness with the likelihood of a condition in which output will exceed bandwidth. If there is not enough bandwidth to give every bit stream its minimum rate, glue frames may be inserted, or low priority channels dropped.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation in part application of pending U.S. application Ser. No. 08/823,007, filed Mar. 21, 1997, by Huang, et al, entitled “Using a Receiver Model to Multiplex Variable-Rate Bit Streams Having Timing Constraints,” and assigned to Scientific-Atlanta, Inc. now U.S. Pat No. 6,052,384. 
    
    
     FIELD OF THE INVENTION 
     The invention relates in general to the transmission of variable-rate bit streams and more particularly to detecting and preventing imminent bandwidth overflow. 
     BACKGROUND OF THE INVENTION 
     A new problem in data transmission is the transmission of data that requires a high bandwidth, is bursty, and has temporal constraints. Traditionally, data transmission has been done on the public switched networks provided by the telephone companies or on packet networks. The public switched networks are designed for interactive voice applications and so provide relatively low-bandwidth circuits that satisfy stringent temporal constraints. The packet networks are designed for the transfer of data between computer systems. The only constraint is that the data eventually arrive at its destination. The amount of bandwidth available for a transfer depends on the degree of congestion in the network. The packet networks thus typically make no guarantees about when or even in what order the data in a burst of data will arrive at its destination. 
     It may thus be appreciated that neither the telephone network nor the packet network is well-adapted to handle high-bandwidth, bursty data with time constraints. An example of such data is digital television which has been compressed according to the Motion Picture Experts Group (“MPEG”) MPEG-2 standard, otherwise set forth in ISO/IEC 13818-1 and 13818-2. 
     Referring now to FIG. 1 there is illustrated therein those details of the MPEG-2 standard that are required for an understanding of the present invention. The MPEG-2 standard defines an encoding scheme for compressing digital representations of video. The encoding scheme takes advantage of the fact that video images generally have large amounts of spatial and temporal redundancy. There is spatial redundancy because a given video picture has sections where the entire area has the same appearance; the larger the areas and the more of them there are, the greater amount of spatial redundancy in the image. There is temporal redundancy because there is often not much change between a given video image and the ones that precede and follow it in a sequence. The less the amount of change between two video images, the greater the amount of temporal redundancy. The more spatial redundancy there is in an image and the more temporal redundancy there is in the sequence of images to which the image belongs, the fewer the bits of information that will be needed to represent the image. 
     Maximum advantage for the transmission of images encoded using the MPEG-2 standard is obtained if the images can be transmitted at variable bit rates. The bit rates can vary because the rate at which a receiving device receives images is constant, while the images have a varying number of bits. A large image therefore requires a higher bit rate than a small image, and a sequence of MPEG images transmitted at variable bit rates is a variable-rate bit stream with time constraints. For example, a sequence of images that shows a news anchorperson in front of a solid color background will have much more spatial and temporal redundancy than a sequence of images for a commercial or MTV song presentation, and the bit rate for the images showing the news anchor will be far lower than the bit rate for the images of the MTV song presentation. 
     The MPEG-2 compression scheme presents a sequence of video images as a sequence of compressed pictures, each of which must be decoded at a specific time. There are three ways in which pictures may be compressed. One way is intra-coding, in which the compression is done without reference to any other picture. This encoding technique reduces spatial redundancy but not time redundancy, and the pictures resulting from it are generally larger than those in which the encoding reduces both spatial redundancy and temporal redundancy. Pictures encoded in this way are called I-pictures. A certain number of I-pictures are required in a sequence, first, because the initial picture of a sequence is necessarily an I-picture, and second, because I-pictures permit recovery from transmission errors. 
     Time redundancy is reduced by encoding pictures as a set of changes from earlier or later pictures or both. In MPEG-2, this is done using motion compensated forward and backward predictions. When a picture uses only forward motion compensated prediction, it is called a Predictive-coded picture, or P picture. When a picture uses both forward and backward motion compensated predictions, it is called a bi-directional predictive-coded picture, or a B picture for short. P pictures generally have fewer bits than I-pictures and B pictures have the smallest number of bits. The number of bits required to encode a given sequence of pictures in MPEG-2 format is thus dependent on the distribution of picture coding types mentioned above, as well as the picture content itself. As will be apparent from the foregoing discussion, the sequence of pictures required to encode the images of the news anchorperson will have fewer and smaller I-pictures and smaller B and P pictures than the sequence required for the MTV song presentation, and consequently, the MPEG-2 representation of the images of the news anchorperson will be much smaller than the MPEG-2 representation of the images of the MTV sequence. 
     The MPEG-2 pictures are being received by a low-cost consumer electronics device such as a digital television set or a set-top box provided by a cable television (“CATV”) service provider. The low cost of the device strictly limits the amount of memory available to store the MPEG-2 pictures. Moreover, the pictures are being used to produce moving images. The MPEG-2 pictures must consequently arrive in the receiver in the right order and with time intervals between them such that the next MPEG-2 picture is available when needed and there is room in the memory for the picture which is currently being sent. In the art, a memory which has run out of data is said to have underflowed, while a memory which has received more data than it can hold is said to have overflowed. In the case of underflow, the motion in the TV picture must stop until the next MPEG-2 picture arrives, and in the case of overflow, the data which did not fit into memory is simply lost. 
     FIG. 1 is a representation of a system  10  including digital picture source  12  and a television  14  that are connected by a channel  16  that is carrying a MPEG-2 bit stream representation of a sequence of TV images. The digital picture source  12  generates uncompressed digital representations (“UDR”) of images  18 , which go to variable bit rate (“VBR”) encoder  20 . Encoder  20  encodes the uncompressed digital representations to produce a variable rate bit stream (“VRBS”)  22 . Variable rate bit stream  22  is a sequence of compressed digital pictures  24 ( a . . . n ) of variable length. As indicated above, when the encoding is done according to the MPEG-2 standard, the length of a picture depends on the complexity of the image it represents and whether it is an I-picture, a P picture, or a B picture. Additionally, the length of the picture depends on the encoding rate of VBR encoder  20 . That rate can be varied. In general, the more bits used to encode a picture, the better the picture quality. 
     The variable rate bit stream  22  is transferred via channel  16  to VBR decoder  26 , which decodes the compressed digital pictures  24 ( a . . . n ) to produce uncompressed digital pictures  28 . These in turn are provided to television  14 . If television  14  is a digital television, they will be provided directly; otherwise, there will be another element which converts uncompressed digital pictures (“UDP”)  28  into standard analog television signals and then provides those signals to television  14 . There may of course be any number of VBR decoders  26  receiving the output of a single encoder  20 . 
     In FIG. 1, channel  16  transfers bit stream  22  as a sequence of packets  30 . The compressed digital pictures  24  thus appear in FIG. 1 as varying-length sequences of packets  30 . Thus, picture  24 ( a ) may have “n” packets while picture  24 ( n ) has “k” packets. Included in each picture  24  is timing information  32 . Timing information contains two kinds of information: clock information and time stamps. Clock information is used to synchronize decoder  26  with encoder  20 . The MPEG-2 specification refers to this clock information as the Program Clock Reference (PCR). The time stamps include the Decoding Time Stamp (“DTS”) which specifies when a picture is to be decoded and the Presentation Time Stamp (“PTS”) which specifies when it is actually to be displayed. The times specified in the time stamps are specified in terms of the clock information. As indicated above, VBR decoder  26  contains a relatively small amount of memory for storing pictures  24  until they are decoded and provided to TV  14 . This memory is shown at  34  in FIG.  1  and will be referred to hereinafter as the decoder&#39;s bit buffer. Bit buffer  34  must be at least large enough to hold the largest possible MPEG-2 picture. Further, channel  16  must provide the pictures  24  to bit buffer  34  in such fashion that decoder  26  can make them available at the proper times to TV  14  and that bit buffer  34  never overflows or underflows. Bit buffer  34  underflows if not all of the bits in a picture  24  have arrived in bit buffer by the time specified in the picture&#39;s time stamp for decoder to begin decoding the picture. 
     Providing pictures  24  to VBR decoder  26  in the proper order and at the proper times is made more complicated by the fact that a number of channels  16  may share a single very high bandwidth data link. For example, a CATV provider may use a satellite link to provide a large number of TV programs from a central location to a number of CATV network head ends, from which they are transmitted via coaxial or fiber optic cable to individual subscribers or may even use the satellite link to provide the TV programs directly to the subscribers. When a number of channels share a medium such as a satellite link, the medium is said to be multiplexed among the channels. 
     FIG. 2 shows such a multiplexed medium. A number of channels  16 ( 0 ) through  16 ( n ) which are carrying packets containing bits from variable rate bit streams  22  are received in multiplexer  40 , which processes the packets as required to multiplex them onto high bandwidth (“HBW”) medium  42 . The packets then go via medium  42  to demultiplexer  44 , which separates the packets into the packet streams for the individual channels  16 ( 0  . . .  n ). A simple way of sharing a high bandwidth medium among a number of channels that are carrying digital data is to repeatedly give each individual channel  16  access to the high bandwidth medium for a short period of time, referred to hereinafter as a slot. 
     One way of doing this is shown at  50  in FIG.  2 . The short period of time appears at  50  as a slot  52 ( n . . . o ); during a slot such as slot  52 ( o ) a fixed number of packets  30 ( n . . . o ) belonging to a channel  16  may be output to medium  42 . Each channel  16  in turn has a slot  52 , and all of the slots taken together make up a time slice  54 . When medium  42  is carrying channels like channel  16  that have varying bit rates and time constraints, slot  52  for each of the channels  16  must output enough packets to provide bits at the rate necessary to send the largest pictures to channel  16  within channel  16 &#39;s time, overflow, and underflow constraints. Of course, most of the time, a channel&#39;s slot  52  will be outputting fewer packets than the maximum to medium  42 , and sometimes may not be carrying any packets at all. Since each slot  52  represents a fixed portion of medium  42 &#39;s total bandwidth, any time a slot  52  is not full, a part of medium  42 &#39;s bandwidth is being wasted. 
     In order to avoid wasting the medium bandwidth, a technique is used which ensures that each time slice is generally almost full of packets. This technique is termed statistical multiplexing. It takes advantage of the fact that at a given moment of time, each of the channels in a set of channels will be carrying bits at a different bit rate, and the medium bandwidth need only be large enough at that moment of time to transmit what the channels are presently carrying, not large enough to transmit what all of the channels could carry if they were transmitting at the maximum rate. The output of the channels is analyzed statistically to determine what the actual maximum rate of output for the entire set of channels will be and the medium bandwidth is sized to satisfy that actual peak rate. Typically, the bandwidth that is determined in this fashion will be far less than is required for multiplexing in the manner shown at  55  in FIG.  2 . As a result, more channels can be sent in a given amount of bandwidth. At the level of slots, what statistical multiplexing requires is a mechanism which in effect permits a channel to have a slot in time slice  54  which varies in length to suit the actual needs of channel  16  during that time slice  54 . Such a time slice  54  with varying-length slots  56  is shown at  55 . 
     Methods of statistically multiplexing bit streams are disclosed in, for example, U.S. Pat. No. 5,506,844, entitled,  Method for Configuring a Statistical Multiplexer to Dynamically Allocate Communication Channel Bandwidth , to Rao, issued Apr. 9, 1996; and U.S. patent application Ser. No. 08/823,007 entitled  Using a Receiver Model to Multiplex Variable Rate Bit Streams Having Timing Constraints , filed Mar. 21, 1997, now U.S. Pat. No. 6,052,384 the disclosures of each of which are incorporated herein by reference. 
     While the methods of statistically multiplexing bits streams disclosed in the referenced patents and applications all perform in an adequate manner, there are certain limitations that have come to light. For example, while the Rao patent does disclose a way to maximize the degree to which the Medium&#39;s bandwidth is used, it has a number of shortcomings, perhaps the most important of which relates to the fact that it adjusts the multiplexing by changing picture quality. A second shortcoming relates to the fact that it teaches encoders that encode digital images as piece-wise constant bit streams. These bit streams have a lower degree of compression than variable-rate bit streams. 
     The limitations inherent in the &#39;007 application relate to the fact that the high bandwidth medium is of constant bandwidth and therefore statistical multiplexing can only accommodate input channels of limited peak bit rates. Each time slice, each of the channels is allocated a portion of the high bandwith medium output of the multiplexer. Accordingly, the bit rate allocation is essentially static, and cannot respond to changing conditions, such as instantaneous peak bit rate changes from one or more channels. Likewise, in the &#39;007 application the problems associated with overflow are dealt with by replacing non-anchor pictures in the MPEG-2 bit stream (e.g., B picture in an I-B-P sequence, or any I-picture in an all I-picture sequence) with smaller “glue” pictures. While this is an important innovation, it does not alone guarantee the total bandwidth requirements of all channels at a given instant will be less that the available output of the multiplexer (i.e., does not guarantee prevention of a bandwidth overflow condition). Nor does it provide any means of detecting when overflow is imminent, and which pictures in which channels will be effected. 
     Accordingly, there exists a need for an improved method of handling potential bandwidth overflow situations. Such an improved method should not only improve allocation of bandwidth, but should also better anticipate imminent bandwidth overflow, and the channels likely effected. All this should also be accomplished without compromising overall effectiveness of the multiplexer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram illustrating how digital television pictures are encoded, transmitted, and decoded; 
     FIG. 2 is a block diagram showing multiplexing of variable-rate bit streams onto a high band width medium; 
     FIG. 3 is a block diagram of a statistical multiplexer which implements a preferred embodiment of the invention; 
     FIG. 4 is a more detailed block diagram of a part of the statistical multiplexer of FIG. 3; 
     FIG. 5 is a diagram illustrating the identification of an aggregate panic condition in accordance with the instant invention; and 
     FIG. 6 is a diagram illustrating the aggregate panic condition in greater detail in accordance with the instant invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward. 
     At its simplest, the invention relates to the ability to look ahead to see whether or not the multiplexer system will have sufficient bandwidth to accommodate the video information that will need to be output. The process described hereinbelow looks at the relative space needs per channel and allocates bits (or MPEG packets) as required. By looking at the fullness of the statistical memory buffer (SMB) described in greater detail hereinbelow, panic conditions, i.e., a condition in which bandwidth requirements will exceed bandwidth availability, can be identified. Once identified, such conditions may be avoided. 
     Referring now to FIG. 3, there is illustrated therein an overview of a statistical multiplexer  80  for MPEG-2 bit streams which is implemented according to the principles of the invention. The main components of multiplexer  80  are packet collection controller  82 , a transmission controller (“TC”)  84 ( 0 ) for each variable-rate bit stream  22 ( 0 ), a packet delivery controller  86 , and a modulator  88 , which receives the output of packet delivery controller  86  and outputs it in the proper form for transmission medium  42 . Packet collection controller  82  collects packets from variable-rate bit streams  22 ( 0  . . .  n ) and distributes the packets that carry a given bit stream  22 ( 0 ) to the bit stream&#39;s corresponding transmission controller  84 ( i ). In the preferred embodiment, the packets for all of the bit streams  22 ( 0  . . .  n ) are output to bus  90 . Each packet contains an indication of which bit stream it belongs to, and packet collection controller responds to the indication contained in a packet by routing it to the proper transmission controller  84 ( i ). It should be noted here that the packets in each bit stream  22 ( i ) arrive in transmission controller  84 ( i ) in the order in which they were sent by encoder  20 ( i ). 
     Transmission controller  84  determines the rate at which packets from its corresponding bit stream  22  is output to medium  42 . The actual rate determination is made by transmission rate controller (“TRC”)  92 , which at a minimum, bases its determination on the following information: for at least a current picture in bit stream  22 ( 0 ), the timing information  32  and the size of the current picture. A Video Buffer Verifier (VBV) model  94 , which is a model of a hypothetical bit buffer  34 . VBV model  94  uses the timing information and picture size information to determine a range of rates at which bit stream  22  must be provided to the decoder&#39;s bit buffer  34  if bit buffer  34  is to neither overflow nor underflow. Transmission rate controller  92  provides the rate information to packet delivery controller  86 , which uses the information from all of the transmission controllers  84  ( 0  . . .  n ) to determine during each time slice how the bandwidth of transmission medium  42  should be allocated among the bit streams  22  during the next time slice. The more packets a bit stream  22 ( i ) needs to output during a time slice, the more bandwidth it receives for that time slice. 
     Continuing in more detail, transmission controller  84  obtains the timing and picture size information by means of bit stream analyzer  96 , which reads bit stream  22  as it enters transmission controller  84  and recovers the timing information  32  and the picture size  98  from bit stream  22 . Bit stream analyzer  96  can do so because the MPEG-2 standard requires that the beginning of each picture  24  be marked and that the timing information  32  occupy predetermined locations in each picture  24 . As previously explained, timing information  32  for each picture  24  includes a clock value and a decoding time stamp (“DTS”). Transmission controller  84  and later decoder  26  use the clock value to synchronize themselves with encoder  20 . The timing information is found in the header of the Packetized Elementary Stream (“PES”) packet that encapsulates the compressed video data. The information is contained in the PTS and DTS time stamp parameters of the PES header. The MPEG-2 standard requires that a time stamp be sent at least every 700 milliseconds (msec). If a DTS is not explicitly sent with a compressed picture, then the decoding time can be determined from parameters in the Sequence and Picture headers, or extrapolated from the DTS value of a previously transmitted picture. For details, see Annex C of ISO/IEC 13818-1. Bit stream analyzer  96  determines the size of a picture simply by counting the bits (or packets) from the beginning of one picture to the beginning of the next picture. 
     The timing information and the picture size are used in VBV model  94 . VBV model  94  requires the timing information and picture size information for each picture in bit stream  22  from the time the picture enters multiplexer  80  until the time the picture is decoded in decoder  26 . The DTS buffer  100  must be large enough to hold the timing information for all of the pictures required for the model. It should be noted here that VBV model  94  behavior is defined solely by the semantics of the MPEG-2 standard, not by any concrete bit buffer  34 ( i ). Any bit buffer for a working MPEG-2 decoder must be able to provide the decoder with the complete next picture at the time indicated by the picture&#39;s timing information; that means that the bit buffer  34 ( i ) for any working MPEG-2 decoder must be at a minimum large enough for the largest possible MPEG-2 picture. Given this minimum buffer size, the timing information for the pictures, and the sizes of the individual pictures, VBV model  94 ( i ) can determine a rate of output for bit stream  22 ( i ) which will guarantee for bit buffers  34 ( i ) of any working MPEG-2 decoder that each picture arrives in the bit buffer  34 ( i ) before the time it is to be decoded and that there will be no overflow of bit buffer  34 ( i ). 
     FIG. 4 shows the details of a preferred embodiment of transmission controller  84  and packet delivery controller  86 . The figure shows three of the n transmission controllers, namely transmission controllers  84 ( i ), ( j ), ( k ), and the two major components of packet delivery controller  86 , namely central bit rate controller  102  and switch  104 . Beginning with transmission controller  84 ( i ), in addition to transmission rate controller  92 , analyzer  96 , and VBV model  94 , transmission controller includes statistical multiplexer buffer (SMB)  106 , a meter  108  for buffer  106 , and throttle  110 . It is to be noted that while only three transmission controllers (i, j and k) are illustrated, the invention is not so limited. Any number of transmission controllers may be employed. Likewise, as each transmission controller is essentially the same, only one,  84 ( i ) will be described in detail. 
     SMB  106  is a first-in-first-out pipe buffer which holds the bits of bit stream  22 ( i ) while they are in transmission controller  84 ( i ). In the preferred embodiment, SMB  106  receives pictures  24  (in FIG. 1) in bursts that contain all or almost all of the bits in the picture, depending on the picture size and maximal bit rate specified by the encoder. Such bursts are termed herein picture pulses, and the time period represented by such a picture pulse is denoted as T p , which is the inverse of video frame rate. For example, T p =1/29.97=3 milliseconds (msec) for NTSC video coding. As previously stated, packet delivery controller  86  provides packets in time slices  54 . The length of time of one of these slices is denoted herein as T c . In one preferred embodiment, T c  is 10 msec. 
     SMB  106  must of course be large enough to be able to accept picture pulses of any size during the time it takes to read out the largest expected picture pulse. SMB  106  further must be emptied at a rate that ensures that it cannot overflow, since that would result in the loss of bits from bit stream  22 ( i ). It also should not underflow, since that would result in the insertion of null packets in the bit stream, resulting in the waste of a portion of the multiplexed medium. Meter  108  monitors the fullness of SMB  106  and provides information concerning the degree of fullness to TRC  92 . TRC  92  then uses this information to vary the range of bit rates that it provides to packet delivery controller  86  as required to keep SMB  106  from overflowing or underflowing. Throttle  110 , finally, is set by TRC  92  on the basis of information  112 ( i ) that it has received from packet delivery controller  86  to indicate the number of packets  30  that bit stream  22 ( i ) is to provide to medium  42  in time slice  54 . 
     In determining the range, TRC  92  sets the minimum rate for a given time slice  54  to the maximum of the rate required to keep SMB  106  from overflowing and the rate required to keep VBV model  94 ( i ) from underflowing and the maximum rate for the time slice to the minimum of the rate required to keep SMB  106  from underflowing and the rate required to keep VBV model  94  from overflowing. Continuing with packet delivery controller  86 , packet delivery controller  86  allocates the packets  30  that can be output during the time slice  54  T c  to bit streams  22 ( 1  . . .  n ) as required to simultaneously satisfy the ranges of rates and priorities provided by TRC  92  for each transmission controller  84  and maximize the number of packets  30  output during time slice  54 . In the preferred embodiment, controller  86  has two components, central bit rate controller  102 , which is an algorithm which executes on a processor that analyzes the information received from each of the transmission rate controllers  92  in order to determine how many packets from each bit stream are to be output in the next time slice  54 , and switch  104 , which takes the number of packets  30  permitted by throttle  110  for each bit stream during the time slice  54 . Switch  104  is implemented so as to deliver packets from each throttle such that the packets are evenly distributed across time slice  54 . Implementing switch in this way reduces the burstiness of the stream of packets to decoder and thereby reduces the amount of transport packet buffer needed in decoder  26 . Such implementations of switch  104  are well-known in the art. Accordingly, central bit rate controller  102  executes an algorithm which will be referred to hereinafter as “the bit rate allocation algorithm.” The bit rate allocation algorithm is prescribed pursuant to the algorithm described in the commonly assigned U.S. Ser. No. 08/823,007 new U.S. Pat. No. 6,052,384 application incorporated herein by reference. An algorithm which will be referred to hereinafter as “the aggregate panic algorithm” provides additional capability to the bit rate allocation algorithm. Specifically, the capability of detecting when the high bandwidth medium output of the multiplexer is insufficient to deliver the entirety of data (or pictures) buffered in the SMBs of the multiplexer is provided. Additionally, capability to remedy this situation is also provided. 
     Referring now to FIG. 5 there&#39;s illustrated therein a diagram illustrating the identification of an aggregate panic condition in accordance with the instant invention. The diagram  150  illustrates essentially a snapshot of the state of all channel SMBs at the start of a particular time slice for the multiplexer illustrated in FIGS. 3 and 4. As may be appreciated, FIG. 5 illustrates  3  channels (channel  1 , channel  2 , channel M) but the invention is not so limited. Any number of channels may be present. Each SMB of each channel has a currently outgoing picture or picture  0 ,  152 ( 1 ),  152 ( 2 ) and  152 ( m ), each with a certain number of packets remaining in the picture, followed by the next outgoing picture or picture  1 ,  154 ( 1 ),  154 ( 2 ) and  154 ( m ) which are themselves followed by the next outgoing pictures represented in FIG. 5 by picture  2  and picture  3  in the SMBs  156 ( 2 ),  156 ( m ) and  158 ( 2 ). It is to be understood that each SMB may have a different amount or number of pictures buffered in the SMBs themselves, as is illustrated in FIG.  5 . 
     Since the algorithm requires processing time on a processing unit, the results of the bit-rate allocation algorithm must be applied to a future time slice. That is, there is processing delay from when the bit-rate allocation algorithm begins execution to when the results may be applied. This delay, which is a fixed delay, will be denoted in terms of the number of time slices ahead and identified by the term “N_TC_AHEAD.” N_TC AHEAD is the number of time slices processing delay of the bit-rate allocation algorithm of a variable rate multiplexer. In one embodiment, N_TC_AHEAD is one or two time slices ahead so that the processing delay is at most, two time slices. 
     When the bit-rate allocation algorithm begins execution of a particular time slice, the algorithm must adjust its knowledge of the contents of all SMBs to create a snapshot of the SMBs for N_TC_AHEAD time slices ahead of the current time. Thus, if “time_now” represents the current system time at which the bit rate allocation algorithm begins executing, the bit-rate allocation algorithm adjusts its knowledge of the SMBs to a snapshot at time: 
     
       
         time_now+N_TC_AHEAD* T   c   
       
     
     Returning to FIG. 5, the aggregate panic method algorithm of the instant invention may execute each and every time slice, and operates on the same snapshot of all the channel SMBs as does the bit-rate allocation algorithm. This implies a snapshot at system time equal to time_now+N_TC AHEAD*T c  where time_now is the system time at the beginning of the time slice when the aggregate panic algorithm is initiated. The aggregate panic method computes the number of packets “num_packets” buffered in the entire variable rate multiplexer by summing over all m channels the number of packets in the associated channels SMB: 
     
       
         num_packets=;  P   i (1 ≦i≦M ) 
       
     
     where i is the channel index, P i  is equal to the total number of transport packets in SMB i  and SMB i  is the SMB corresponding to channel “i.” Next, the latest decoding time stamp (“DTS”) hereinafter referred to as latest_DTS of all pictures of all SMBs is determined. The picture with the latest DTS can be viewed as the last picture to leave the variable rate multiplexer of the entirety of pictures that are then currently stored in the multiplexer. The previous_DTS is the DTS of the immediately preceding picture of the picture with the latest_DTS, both of which are illustrated in FIG. 5 by pictures  156 ( 2 ) and  158 ( 2 ) which are the previous_DTS and latest_DTS respectively. Determining latest_DTS in this fashion (e.g., by simply comparing DTSs of all pictures in the SMBs) requires that all channel bit streams be on the same system time base. This further implies that all encoders inputting data to the variable rate multiplexer are locked to the same system clock. This system clock is designated the system clock reference (“SCR”), and when all encoders are locked to the same SCR, the DTS of all the channel bit streams will be referenced to the same clock. This therefore is one way that it is possible to determine the latest picture by inspecting the DTS only. Conversely, if all encoders are not locked to the same SCR, then the latest picture would be given by the following formula: 
     
       
         latest_DTS=DTS of picture j for channel i which maximizes (DTS( i,j )−SCR( i )) 
       
     
     where i is the channel index which ranges from (1≦i≦m) for m channels, and j ranges from (0≦j≦number of entire pictures in SMB (i)), and picture j is the jth outgoing picture for channel i. In this case, the latest picture is that picture, the DTS value of which minus the SCR of the associated channel is greatest. 
     In the known art of the variable rate multiplexer, each picture must arrive at the decoder by the system time given by the DTS of the immediately proceeding picture. This will guarantee that there is at least one picture in the decoder at all times since it guarantees that by the time the picture is decoded, the next picture will have arrived at the decoder. Thus, the entirety of data in the variable rate multiplexer must be transmitted out of the multiplexer in an amount of time given by −t which is itself expressed by: 
     
       
         − t =previous_DTS−(time_now+ N   —   TC _AHEAD* T   c ) 
       
     
     where, time_now is the current system time (or current value of the SCR) and T c  is the time period of the time slice of the bit-rate allocation algorithm (e.g. 10 msec). Since it is required that num_packets be transmitted from the multiplexer in an amount of time given by −t, it is possible to determine whether or not the multiplexer has sufficient bandwidth to achieve this by comparing num_packets with the maximum number of packets which the multiplexer can transmit in this amount of time. The maximum number of packets that the multiplexer can transmit in −t will be referred to hereinafter as num_packets_max and is computed as follows:                num_packets      _max     =     Rmux   *       -   t     /   TP_size                   =       (     Rmux   *     T   c     *     -   t       )     /     (     TP_size   *     T   c       )                   =       (     Rmux   *       T   c     /   TP_size       )     *     (       -   t     /     T   c       )                   =     Np_tc   *       -   t     /   Tc                                    
     Where Rmux is equal to the output bandwidth of the multiplexer in bits per second (a constant), TP_SIZE is the number of bits in a transport packet, Np_tc is equal to the number of packets which the multiplexer can transmit over a time slice, and T c is equal to the period in the time slice. 
     An aggregate panic condition therefore exists if num_packets is greater than num_packets_max. In this event, the total number of packets buffered in the entire multiplexer cannot possibly be transmitted in a timely enough manner such that the last picture arrives at its respective decoder by the DTS of the immediately preceding picture. The output bandwidth of the multiplexer is insufficient to guarantee this. To remedy this situation, non-anchor pictures are replaced with smaller glue pictures. As is readily understood in the art, a glue picture is a prestored minimally coded picture, with no discrete cosine transform (“DCT”) coefficients and a 0 motion vector so that they are essentially repeat pictures. Beginning with the lowest priority channel, all non-anchor pictures buffered in that channel&#39;s SMB are replaced with glue pictures. For each picture replaced by a glue picture, num_packets will need to be adjusted to account for the fact that there is a smaller picture in one of the SMBs of the multiplexer. Therefore, for each glue frame insertion, num_packets will be adjusted as follows: 
     
       
         k_packets_saved=p( i,j )−g_packets num_packets=num_packets−k_packets_saved 
       
     
     where, p(i,j) is equal to the number of packets in outgoing picture number j for channel i, and the range of j is (1≦j≦number of full pictures in SMB i ), the range of i is (1≦i≦M) for M channels, g_packets is the number of packets in the prestored glue picture (a constant, which, in the preferred embodiment, is equal to two for standard resolution digital video) and k_packets_saved is equal to the number of packets difference between the original non-anchor picture and the glue picture. 
     This process continues for each channel, where glue pictures replace the non-anchor pictures and the num_packets is recomputed until num_packets becomes ≦num_packets_max. When this occurs, the aggregate panic condition has been removed and therefore the bandwidth of the multiplexer should be sufficient to transmit the entirety of all packets in the multiplexer by the time required for the final picture. Accordingly, when num_packets has been adjusted to the point that num_packets is≦num_packets_max, the aggregate panic algorithm is completed and no additional glue pictures are inserted. 
     Referring now to FIG. 6 there is illustrated therein a diagram of an aggregate panic condition in greater detail, in accordance with the instant invention. In particular, the method illustrated with respect to FIG. 5 hereinabove examines the entirety of the data in the multiplexer, and uses the DTS of the latest picture of that entirety of data to determine whether or not there exists sufficient bandwidth to transmit the entirety of data from the multiplexer. As is illustrated in FIG.  6  and will be described in greater detail hereinbelow, this method may be extended to analyze the entirety of data up to outgoing picture “k” where k will vary from  1  to number of pictures in the SMB with the largest number of pictures. FIG. 6 illustrates the pictures buffered in the SMBs for channels  1 ,  2 , . . . m. Each channel has a currently outgoing picture  160 ( 1 ),  160 ( 2 ),  160 ( m ), which is outgoing picture  0 , and the next outgoing picture immediately following picture  0  is referred to as picture  1 ,  162 ( 1 ),  162 ( 2 )  162 ( m ), which are followed by additional pictures in each SMB as illustrated. Each of the SMBs may have a different number of pictures buffered as is illustrated in FIG. 6, where SMB  2  and SMB m include picture  2 ,  164 ( 2 ) and  164 ( m ) respectively, and SMB  2  further includes picture  3 ,  166 . Note that each currently outgoing picture in all of the SMBs, (or each of the pictures  0 ) may or may not be an entire picture, since part of the outgoing picture has already been transmitted from multiplexer. 
     Referring to picture  1 ,  162 ( 1 ),  162 ( 2 ),  162 ( m ), the term latest_DTS( 1 ) is defined as the latest DTS of all picture  1   s  of all SMBs. Similarly, the term previous_DTS( 1 ) will be understood to be the DTS of the immediately preceding picture with latest DTS( 1 ). Num_packets( 1 ) is the total number of packets in all SMBs up to picture  1 . This includes all packets in the currently outgoing pictures of all SMBs and all packets in the next outgoing pictures. Num_packets( 1 ) must be transmitted from the multiplexer in an amount of time given by −t( 1 ) where: 
     
       
         −t( 1 )=previous_DTS( 1 )−(time_now+ N   —   TC _AHEAD* T   c ) 
       
     
     The maximum number of packets which can be transmitted from the multiplexer in this amount of time is given similarly as above in which: 
     
       
         num_packets_max( 1 )= Np _tc*−t(1)/ T   c   
       
     
     if num_packets( 1 ) is≦num_packets_max( 1 ), this implies that the output bandwidth of the multiplexer is sufficient to transmit the entirety of all picture is from the multiplexer in time for the latest picture to arrive by the DTS of the immediately proceeding picture. In this case, there is no aggregate panic condition for picture  1 , and the method described herein moves on to the next picture, all as described hereinbelow. 
     An aggregate panic condition exists for picture  1  if num_packets( 1 ) is greater than num_packets_max( 1 ). In this event, the bandwidth of the multiplexer is not sufficient to transmit the entirety of data up to picture  1  to guarantee the latest picture of all picture  1 s will be transmitted to its respective decoder by the DTS of the immediately preceding picture. To remedy this situation, non-anchor pictures are replaced by smaller glue pictures, until this condition is removed. 
     Accordingly, beginning with the lowest priority channel, if picture  1  is a non-anchor picture, it is replaced by a glue picture as described above. Then, num_packets( 1 ) is adjusted according to the following formula: 
     
       
         k_packets_saved=p( i , 1 )−g_packets num_packets( 1 )=num_packets( 1 )−k_packets_saved 
       
     
     where p(i,  1 ) is equal to the number of packets in outgoing picture  1  for channel i, where i is the channel index which increments from lowest priority to highest priority channel, and range for i is (1≦i≦m) for m channels, and k_packets_saved and g_packets are defined as set forth hereinabove. 
     The process of glue frame insertion continues until all channel picture  1 s have been examined for glue frame insertion opportunities or num_packet( 1 ) has been adjusted to the point where it is less than num packet_max( 1 ). When num_packets( 1 ) is≦num_packets_max( 1 ), this means that the bandwidth of the mux is sufficient to transmit the entirety of data for all picture  1   s  of all SMBs in a timely manner as set forth hereinabove. 
     After picture  1  has been processed as discussed hereinabove, the aggregate panic method moves on to picture  2  and performs an identical computation. The algorithm then moves from picture to picture in order to identify those situations in which a panic condition exists, dealing with each subsequently occurring panic condition as described hereinabove with respect to picture  1 . The algorithm continues iterating up to outgoing picture N p max where N p max is the number of pictures in the SMB having the greatest number of pictures, illustrated in FIG. 6 as picture  166 . Typically, the algorithms described hereinabove with respect to FIGS. 5 and 6 have performed each and every time slice, since in general, each of the SMBs will have received new data in each time slice. 
     In order to more easily understand the invention, it is necessary to understand that there is a correlation between SMB fullness and the likelihood of a panic condition existing. As is described hereinabove, panic condition exists when the number of packets to be outputted from the multiplexer exceeds the bandwidth available to the multiplexer for a given time slice. The methods described hereinabove with respect to FIGS. 5 and 6 look at the fullness of the SMBs and from this measure of fullness, is able to detect up to 90% of the panic conditions which are likely to occur. For the additional 10% of panic conditions, a more refined method such as that described in commonly assigned application Ser. No. 08/823,007, now U.S. Pat. No. 6,052,384 the disclosure of which is incorporated herein by reference, may be employed. 
     While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.