Abstract:
A system for controlling a conference that takes input streams from a variety of sources, at least some of the streams having a relatively low-resolution, and composes the input streams into a single output stream of a relatively high-resolution. In doing the conversion the system takes intra Macro Blocks (MBs) of the input stream and produces inter output frames. As part of the conversion process, the system converts the Groups Of Blocks (GOBs) that have GOB headers into slices.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Provisional Patent Application Ser. No. 60/337,121 filed on Dec. 4, 2001, entitled “Method and an Apparatus for Mixing of Compressed Video,” which is incorporated herein by reference. 

   BACKGROUND 
   1. Field of the Invention 
   The present invention relates to video communication and more particularly to a method and an apparatus for mixing bit streams of compressed video from more than one video source. 
   2. Description of the Prior Art 
   Video communication between more than two video terminals often requires a Multipoint Control Unit (MCU), a conference controlling entity that typically is a piece of equipment located in a node of a network or in a terminal which receives several channels from access ports and, according to certain criteria, processes audiovisual signals and distributes them to a set of connected channels. Examples of MCUs include the MGC-100, which is available from Polycom Networks Systems Group, the assignee of the present invention. A terminal (which may be referred to as an endpoint) is a location on the network, capable of providing real-time, two-way audio and/or visual communication with other terminals or the MCU. 
   The MCU may include a bank of decoders, encoders, and bridges. The MCU may use a large amount of processing power to handle video communications between a variable number of participants, using a variety of communication and compression standards and a variety of bit streams, for example. The MCU may need to compose these bit streams into a single output stream that is compatible with a conference participant to which the output stream is being sent. Thus, an MCU may need to convert a variety of compression and communication standards having a variety of resolutions into an output stream of a single resolution, compression standard, and communication standard. Consequently, an MCU is typically an expensive and rather complex product. 
   In more recent years, videoconferencing and other forms of multimedia communications have become more commonplace. The advent of personal computers having videoconferencing capabilities creates a demand for MCUs having the capability of multimedia communication between devices housed on Local Area Networks (LANs). This trend raises the need for low cost MCUs. However, low cost MCUs may only handle a limited multipoint communication (e.g., a limited number of compression standards, a limited number of conferees, and a limited number of layouts). 
   U.S. Pat. No. 5,675,393 discloses an image processing apparatus for composing a plurality of coded images into one image without decoding the plurality of coded images when the images are transmitted using the H.261 standard. 
   Quarter Common Intermediate Format (QCIF) is a videoconferencing format that specifies a video frame containing 144 lines and 176 pixels per line. This is one-fourth the resolution of Common Intermediate Format (CIF). QCIF support is required by some of the International Telecommunications Union (ITU) videoconferencing standards. 
   U.S. patent application Ser. No. 09/768,219, Pub. No. 2001/0019354A1, entitled, “Method and an Apparatus for Video Mixing of Bit Streams,” discloses a method and apparatus for mixing as many as four QCIF H.263 compressed video bit streams into a composite CIF image. 
   However, U.S. patent application Ser. No. 09/768,219 does not teach how an MCU can handle a conference in a real communication environment where the different terminals transmit their intra frames at different times forcing the MCU to multiplex an intra frame from one terminal with an inter frame from other terminals. An inter frame is an example of a referential frame, which is a frame that gives the difference between two frames. An intra frame is a type of non-referential frame. A frame is composed of an array of MBs (Macro Blocks). In H.263 it is difficult to multiplex a low-resolution intra frame as part of a high-resolution inter frame without converting it to an inter frame format, since the encoding of an intra-MB in an intra frame is different from an intra-MB in an inter frame. 
   Furthermore, U.S. patent application Ser. No. 09/768,219 does not teach how the MCU handles a need to request an intra frame from one of or some of the sources. For example, it is not clear how the MCU handles a request for an intra frame, which is due to a packet from one of the sources that is missing or arrives late. 
   In addition, the method that is described in U.S. patent application Ser. No. 09/768,219 requires recalculation of the quantizers and/or the Discrete Cosine Transform (DCT) coefficients in some of the MBs. This recalculation requires time and computing power. Therefore, (although possibly not recognized heretofore) there is a need for a method and apparatus that can handle a conference in a real communication environment by composing low-resolution compressed video frames (such as QCIF and CIF frames) into a higher resolution frame such as a CIF or 4CIF frame, respectively, without recalculating the Motion Vector Data (MVD) and/or the quantization coefficients. 
   SUMMARY OF THE INVENTION 
   In one embodiment, the present invention may include a method and an apparatus for multiplexing low-resolution compressed video frames, QCIF, that have been compressed according to compression standards such as, but not limited to, H.263, and MPEG 4 into a higher resolution frame, such as CIF and 4CIF frames according to another or the same compression standard while using slices instead of Group Of Blocks (GOBs). 
   Moreover, in accordance with an exemplary method of the present invention, the method overcomes the problem of multiplexing a low-resolution intra frame as part of a high-resolution inter frame by converting each received low-resolution intra frame format into a low-resolution inter frame format. When there is a need for the MCU to send a high-resolution intra frame, the appropriate video source or sources are requested to send the low-resolution intra frame. The MCU sends an artificial high-resolution intra background frame as the response for the request for an intra frame. Later, the received low-resolution intra frames from the appropriate video sources are converted to low-resolution inter frames and are delivered by the MCU as part of the consecutive high-resolution inter frames. 
   Furthermore, in accordance with an embodiment of the present invention, the method and the apparatus utilizes linked lists as an editorial tool to overcome the differences in the transmitting parameters of the different terminals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more readily understood from reading the following description and by reference to the accompanying drawings (forming a part of the description), wherein an example of the invention is shown. 
       FIG. 1  is a block diagram illustrating an exemplary embodiment of a conferencing module according to the present invention; 
       FIG. 2  shows several exemplary screen layouts; 
       FIG. 3  is a flowchart showing an exemplary method for setting a conference module at the beginning of a conference or while changing layouts; 
       FIG. 4   a  is a flowchart showing the operation of the MCU of  FIG. 1 ; 
       FIG. 4   b  is a flowchart showing an exemplary method of operations performed by parser module; 
       FIG. 5   a  and  FIG. 5   b  are two portions of the same flowchart showing an exemplary method for parsing an intra frame; 
       FIG. 6   a  and  FIG. 6   b  are two parts of the same flowchart showing an exemplary method for parsing an inter frame; 
       FIG. 7  is a flowchart showing an exemplary method for parsing a MB in an intra frame; 
       FIG. 8   a  and  FIG. 8   b  are two parts of a flowchart showing an exemplary method for transmitting an inter frame; 
       FIG. 9  is a flowchart showing an exemplary method for transmitting an intra frame; and 
       FIG. 10  is a flowchart showing an exemplary method for handling a fast update request. 
   

   DESCRIPTION OF THE INVENTION 
   Referring now to the drawings, in which like numerals refer to like parts throughout the several views, exemplary embodiments of the present invention are described. 
   An embodiment of the invention is described using an example including a detailed description of an MCU that multiplexes up to four compressed video frames, having a QCIF low-resolution format, into a single CIF high-resolution frame using H.263 as the compression standard. However, the example is not intended to limit the scope of the invention. Some embodiments of the present invention may multiplex up to four CIF low-resolution frames into a single 4CIF frame, or up to 16 QCIF frames into a 4CIF frame or combinations of one or more QCIF frames and one or more CIF frames into a 4CIF frame. Moreover, the H.263 compression standard is used only as an example. Other standards may be used such as, but not limited to, MPEG 4. 
     FIG. 1  is a block diagram of an exemplary MCU  100 , according to an exemplary embodiment of the present invention. An MCU  100  may include a Local Area Network (LAN)  105  and a plurality of conference modules  110 , each having a conference manager  115 , a shared memory  130 , a plurality of input modules  140 , and a plurality of output modules  150 . Each input module  140  may include a linked list  120 , a depacketizer  143 , and a parser  146 . Each output module  150  may include a depacketizer  155  and a background buffer  157 . 
   A plurality of endpoints, terminals, (not shown), which are connected over a packet based network (not shown), are connected to LAN  105  via Network Interface Modules (not shown). 
   The conference module  110  may use the H.263 compression standard or any other compression standard, for example. The conference module  110  may be a logical unit and may be a hardware module, a firmware module, a software module or any combination of the three. Each module may be a permanent logical module or a temporary one, which is generated by the host computer or appliance of the MCU  100  according to the current needs. Generating temporary logical modules and allocating permanent modules according to the current needs improves the utilization of the resources of the MCU  100 . 
   The linked list  120  may be a linked list of frames of information. The number of the input modules  140  in each conference module  110  can be a fixed number, such as four (e.g., one input module  140  for each square in the screen layout), or it can be a variable number that is set according to the needs of the conference. For example, there may be one input module  140  for each endpoint (not shown in the drawings) that participates in the conference, or one input module  140  for each currently visible participant in the relevant screen layout. In an embodiment, the number of the input modules  140  in the conference and the screen layout can be dynamically changed during the conference. The terminals (not shown), which are connected to the input modules  140  via LAN  105 , can also be dynamically switched by the host (not shown) during the conference. 
   The depacketizer  143  grabs the appropriate compressed video packets from the LAN  105 . These packets are received from the endpoint that has been assigned by the host, via the conference manager  115 , to the depacketizer  143 . The packets are in QCIF format. The depacketizer  143  processes the packets according to a media transport protocol such as Real time Transport Protocol (RTP) and transfers the compressed video bit stream to the parser  146 , which parses the bit stream and converts it to an output stream according to the methods that are described below in conjunction with  FIGS. 4   b ,  5 ,  6  and  7 . 
   At the end of grabbing, depacketizing, and parsing the packets from the LAN  105 , the parser  146  stores the converted compressed video bit stream and the recommended division points in the shared memory  130 . (Recommended division points are points along the image stream that may be used to divide the image stream into packets. Recommended division points are also discussed below in conjunction with  FIGS. 5   a  and  5   b ). The parser  146  stores the pointers to those buffers in the linked list  120 . Using the linked list  120 , the parser  146  generates a chain of input buffers in the shared memory  130 . Moreover, in some embodiments, the shared memory  130  and the linked list  120  may be replaced by using a First In First Out (FIFO) unit for each input module  140 . 
   Each conference module  110  may include a plurality of the output modules  150 . In an exemplary embodiment of the present invention, one output module  150  is used for each endpoint that participates in the conference. Another embodiment may have one output module  150  for each type of screen layout, and may multicast its output to the endpoints that are using this layout. In another embodiment, an MCU  100  may have one output module  150  for each end point in addition to having one output module  150  for each screen layout. 
   The Output module  150  may include a packetizer  155  and a background buffer  157  for storing the compressed video data of background images that may replace video from the endpoints. Output module  150  may be a logical module including any one or any combination of software, hardware, and firmware. 
   The following are few examples of types of compressed video that may be stored in the background buffer  157 . 
   A high-resolution (CIF) intra background frame that replaces a composed high-resolution intra frame. 
   A group of high-resolution Not Coded Slices (NCSs) in which there is one high-resolution NCS for each input module  140 . An NCS is a slice that is similar to the same slice in the previous frame, and therefore it is not coded. An NCS buffer is compatible with a low-resolution frame (such as a QCIF frame). This NCS is used in case the appropriate linked list  120  is empty when the packetizer  150  needs its data. Each NCS has the same Macro Block Address (MBA) field as its appropriate input module  140 . The MBA field in a slice header is the field that contains the Identification (ID) number of the first MB in the slice. 
   One NCS buffer for each empty screen part (some examples of empty screen parts are discussed in conjunction with  FIG. 2 , below) in the screen layout. 
   The packetizer  155  is responsible for converting the requested screen layout into one of higher resolution image, dividing the high-resolution compressed video frame into packets according to the media transfer protocol (such as RTP), and sending the packets to their destinations. 
   The data in the background buffer  157  is prepared before the starting of the conference. The first MB in each slice has the appropriate ID number, which indicates its location in the screen layout. The background buffer  157  also stores data associated with the empty screen parts. 
   The output module  150  gets, from the conference manager  115 , the type of screen layout to generate, the appropriate input modules  140  that comprise the required screen layout, and the data to be stored in the background buffer  157 . The output module  150  reads the data from the shared memory  130  based on the linked list  120  that belongs to the input modules  140  and the data associated with the empty screen part stored in the background buffer  157 . The output module  150  then composes the data from the different sources (e.g., input modules  140  and possibly other sources), into a single high resolution compressed video output stream according to H.263 Annex K. 
   Then the packetizer  155  divides the compressed video bit stream into packets, according to the RTP, using the recommended division points that have been prepared by the parser  146 . 
   The output module  150  transmits the packet from the packetizer  155  via the LAN  105  to its destination. Some exemplary methods of the operation of the packetizer  155  are described below in conjunction with  FIGS. 8 and 9 . 
   The conference manager  115  manages the conference. The conference manager  115  receives control signals from the host (not shown) that convey the desired conference layouts and the parameters of the appropriate endpoints. The conference manager  115  gives each one of its input modules  140  the source address. Moreover, the conference manager  115  gives the type of the screen layout to the output module  150  and loads the compressed video data of the intra background frame, the NCS, and the empty screen part into the background buffer  157 . The conference manager  115  also gives the destination address for its packets and assigns the appropriate input modules  140  to be used in composing the screen layout. Exemplary methods of the operation of the conference manager  115  are described below in conjunction with  FIGS. 2 ,  3  and  10 . 
   The MCU  100 , during the conference setup, negotiates with each of the endpoints to set an asymmetric conference in which each of the endpoints transmits the compressed video using a compression standard (e.g., H.263) and a first resolution (e.g., QCIF, CIF resolution) and receives the compressed video in a higher resolution (e.g., CIF or 4CIF) using H.263 Annex K, for example. Therefore, the transmitting bit rate of each endpoint (not shown) is smaller than its receiving bit rate. The exemplary embodiment of the present invention may use the “Rectangle Mode” of Annex K, which means that all of the slices are rectangles and/or the “Arbitrary Mode,” which declares that the slices within a frame may be in an arbitrary order. 
   At the end of the conference setup process the host (not shown) of the MCU allocates the resources, such as the amount of input modules  140  and output modules  150 , to the relevant conference module  110  supporting the conference. The resource allocation depends on the conference layout. The host loads the conference manager  115  with the communication parameters of each of the endpoints and the required layout or layouts of the conference. 
   The present invention can support different conference layouts, such as a conference with three conferees in which each of the conferees will see the other two and a background slice in the empty screen parts, as shown in  FIG. 2   a , which illustrates the screen of participant B.  FIG. 2   b  represents an exemplary screen of conferee C in a conference with four participants.  FIG. 2   c  represents an exemplary screen of conferee E in a conference with six participants when the speaker is conferee F. 
     FIG. 3  is a flowchart showing an exemplary method  300  for setting a conference module  110 . In step  305 , method  300  is initiated. Step  305  may be an automatic activation of method  300  that occurs at the beginning of a conference or upon changing the conference layout, for example. A change in the conference layout may occur, for example, when the speaker is changed, thereby automatically starting method  300 . In step  310  the conference manager  115  receives and/or actively grabs the communications parameters of each of the endpoints that participate in the conference. Then the conference manager  115  receives and/or actively grabs the parameters of the conference layouts, such as the type of screen layout for each endpoint and the locations of the different video sources in the conference layout. 
   In step  320 , an input module conference layout routine is performed. The conference manager  115  assigns each input module  140  to an endpoint by loading the depacketizer module  143  with the source address of the packets that it has to grab and/or receive from the LAN  105 . Based on the assigned location of the input module  140  in the conference layout, the conference manager  115  calculates the new ID number of the first MB of the low-resolution picture in the required screen layout and loads this number to the parser  146  of the input module  140 . If one of the endpoints participates in a plurality of layouts at different locations, each location requires a dedicated input module  140  with the appropriate ID number of the first MB. 
   In another exemplary embodiment, the ID numbers of the MBs are supplied by the output module  150 . In such an embodiment, each endpoint may have a single input module  140 , which is not aware of the location of the data in the composed layout. Therefore, the input module  140  does not add the MBs&#39; ID numbers to the data. The MBs&#39; ID numbers are added by the output module  150  while composing the high-resolution frame. The conference manager  115  repeats step  320  for each of the input modules  140 . 
   Then the conference manager  115  performs the output module screen layout routine, step  330 , by loading each output module  150  with the destination address or addresses of its packets. If the layout includes one or more empty screen parts the conference manager  115  transfers the compressed video data of the empty screen part to the output module  150  with the data of the background intra frame and the NCS of the QCIF frames. This compressed video data is stored in the background buffer  157 . The conference manager  115  repeats step  330  for each output module  150 . Process  300  is then terminated in step  350 . 
     FIG. 4   a  is a flowchart showing the operation  4400  of MCU  100 . In step  4402  the input module  140  grabs and/or receives the input video packets coming from the video source (which may be the endpoints (not shown), a recorded video source or other video source, for example) that has been assigned to the input module  140  by the conference manager  115  ( FIG. 1 ). The input video packets are sent to the depacketizer  143 , which converts the packets into a compressed video stream, according to a media transfer protocol, and then forwards the input video stream to the parser  146 . In other exemplary embodiments depacketizer  143  and parser  146  may be one module. 
   Step  4404  represents the operation of parser  146 . Parser  146  searches for GOB headers and replaces them with slice headers placing the appropriate data in the slice header, according to standard H.263 Annex K. Parser  146  also defines and/or adds recommended division points along the stream for indicating a possible location along the stream that may be used for determining where to cut the stream into a packet. A detailed description of the operation of parser  146  is disclosed below in conjunction with  FIGS. 5   a ,  5   b ,  6  and  7 . Parser  146  stores the modified video stream and the appropriate links (which may be pointers, for example) to the stored information in the shared memory  130 . The links are stored in a certain location in the shared memory  130  that is referred as the link list. In cases where the low-resolution input compressed video is based on H.263 with annex K the received compressed video frame is already divided into slices and not GOBs. Therefore, there is no need for parser  146  to replace the GOB headers with slice headers. However, the rest of the operation of the parser is the same as the operation for video streams which are compressed according to H.263. 
   In step  4406 , the output module  150  grabs the input buffers from the shared memory  130 . The input buffers correspond to a set of input modules  140  that are assigned to output module  150  by the conference manager  115 . The output module  150  reads each linked list  120  for each assigned input module  140  to obtain the locations of the input buffers in shared memory  130 . The video streams associated with the input modules  140  are composed into a single output video stream. If the screen layout includes empty screen parts, information related to the empty screen parts read from the background buffer  157  is included in the composed stream. The output video stream is forwarded to or grabbed by the packetizer  155 , which divides the output stream into packets based on the recommended division points, divides the information about the empty screen parts into packets, and sends the packets via the LAN  105  to their destination.  FIGS. 8 and 9  discuss the operation of the packetizer  155  in more detail. 
   Steps  4402 ,  4404 , and  4406  may be performed concurrently and are repeated for different video streams, the input modules  140 , and the output modules  150  until the conference ends. At step  4408 , the conference ends and the process  4400  stops. 
     FIG. 4   b  is a flowchart showing an exemplary method  400  of an operation of a parser  146 , which parses the compressed video stream received and/or grabbed from the depacketizer  143 . The method  400  starts a loop that is entered in step  410 . At a return to step  410  the loop could be ended, if the conference is over, for example. In step  410 , method  400  begins by waiting to identify a Picture Start Code (PSC) along the stream, which is a code that appears at and indicates the beginning of each frame. Upon the receipt of a PSC  410  the method  400  proceeds to step  420 . Next the parser  146  requests a new input buffer and a data structure for the recommended division points in the shared memory  130 . Then, in step  430 , a determination is made as to whether the intra indication (an indication that the system is waiting to receive an intra frame) is on, which means that the parser  146  has to wait to receive an intra frame. If the intra indication is off, then the method  400  proceeds to step  440 , where it is determined if the new frame is an intra frame. If the frame is an intra frame, the parser  146  starts, in step  455 , the intra frame routine, which is described below with respect to  FIG. 5 . If the frame is not an intra frame, then in step  445  the parser  146  starts the inter frame parsing routine, which is described below with respect to  FIG. 6 . 
   Returning to step  430 , if the intra indication is on, which means that the system has to wait to receive an intra frame, then the method proceeds to step  450 , where it is determined whether the new frame is in fact an intra frame (as needed). In an embodiment step  450  is performed by checking for an intra frame header. If the frame is of the intra type, parser  146  starts step  455 , which runs the intra frame parsing routine. If the frame is not an intra frame, then the parser  146  releases the new picture buffer that was allocated in step  420 , above, and returns to step  410  and waits for the next PSC. 
   In any one of the steps of method  400 , if the parser  146  has lost synchronization, then it releases the current buffer, requests an intra frame from the appropriate video source, and may turn on the intra frame indication. Parser  146  may determine that the synchronization, between the transmitted information from the source and the actual received information by the MCU  100 , has been lost by realizing that the GOB number is not the expected one, for example. This indicates that at least one packet was lost on the way from the source (not shown) to the MCU  100 . 
   Upon terminating the intra frame parsing routine of step  455  or the inter frame parsing routine of step  445 , the parser  146  returns to step  410  to synchronize on the next PSC. 
     FIG. 5   a  and  FIG. 5   b  are two parts of a flowchart showing an exemplary method for parsing an intra frame. The flowchart of  FIGS. 5   a  and  5   b  is an elaboration of step  455  in  FIG. 4 . 
   During a video conference call over a packet network, an endpoint (not shown) may request to receive an intra frame from the MCU  100 . Such a request may be initiated upon realizing that at least one packet has been lost. In response, the MCU  100  requests a low-resolution intra frame from the endpoints (not shown) that are the source of the images in the intra frame, the ones whose low-resolution frames compose the high-resolution frame of the receiver endpoint. Generally, the source endpoints may respond at different times. The time difference between the arrival of a low-resolution intra frame from the first source endpoint and the last one may be in the range of hundreds of milliseconds. In order to accelerate the response to the request from the receiver end point, MCU  100 , immediately responds by sending an artificial high-resolution intra background frame. Then, upon receiving a low-resolution intra frame from one of the source endpoints, the step  455  converts the intra MBs of the low-resolution intra frame into intra MBs of an inter frame and sends them to the receiver endpoint as part of a set of consecutive high-resolution inter frames. The receiver endpoint upon receiving those intra MBs of high-resolution inter frames, installs the intra MBs instead of the appropriate MBs of the background frame. By using this method MCU  100  combines the recently received low-resolution intra frame with the rest of the image. This method eliminates the need to compose a high-resolution intra frame from, for example, 4 low-resolution intra frames. Otherwise, an MCU that does not respond with the background intra frame may need to store each one of the low-resolution intra frames and the consecutive inter frames that are coming from the different source endpoints until the last intra frame arrives. Then the other MCU has to combine the high-resolution intra frame from all the sources and continues creating the following Inter frames from the stored low-resolution inter frames. This approach creates a huge delay. 
   Since intra MBs are coded differently in an intra frame than intra MBs in an inter frame, the MCU  100  converts the intra frames into inter frames containing only intra MBs. 
   In step  513 , the parser  146  reads the frame header and removes it. In step  516 , based on the ID number of the first MB that has been assigned to the parser  146 , the parser  146  determines whether the location of this frame is in the top left corner of the composed frame. If it is in the top left corner, the MBA (or the ID of the MB) is zero, and the parser  146  proceeds to step  518 . In step  518 , the parser  146  inserts a new inter frame header. The new inter frame header represents an inter CIF frame, which is compressed according to H.263 with ANNEX K, and the parser  146  writes the new frame header into the input buffer. 
   Then the parser  146  moves to step  525  for parsing the first intra MB in the frame. 
   Returning to step  516 , if the location of the image is not in the top left corner of the composed screen layout, then the parser  146  proceeds to step  520 , where the parser  146  converts the intra frame header to a slice header and writes the slice header in the input buffer. The process of converting the frame header to a slice header may include the steps of:
         1. Installing the MB ID number, which has been assigned to the parser  146  based on its location in the screen layout, in the MBA field of the slice header.   2. Installing the PQUANT value in the SQUANT field. SQUANT is a fixed length codeword of five bits which indicates the quantizer QUANT to be used for that slice until updated by any subsequent DQUANT. The codewords are the natural binary representations of the values of QUANT which, being half the step sizes, range from 1 to 31. SQUANT is not present for the slice that follows the picture start code.   3. Writing the width of the slice in the Slice Width Indication (SWI) in the MBs field in the slice header. If a high-resolution frame is being composed from a compressed QCIF frame, the SWI will always be 11.       

   In step  525 , the parser  146  starts a loop for parsing the intra MBs of the frame. For each MB the parser  146  jumps to the intra frame MB parsing routine of step  525 . The MB parsing routine is described in conjunction with  FIG. 7 . 
   Upon terminating the intra frame MB routine of step  525  the parser proceeds to step  530  and determines whether to write a recommended division point. The decision may be based on different criteria. An exemplary criterion is the number of bytes from the last recommended division point to the end of the current MB. This indication helps the packetizer  155  during the process of dividing the composed stream into packets. An additional exemplary method may involve cooperation between depacketizer  143  and parser  148  ( FIG. 1 ). Depacketizer  143  installs a unique pattern at the end of each received packet along the video stream, and upon identifying this pattern, the parser  146  removes it and sets an indication for recommended division points in the allocated data structure. 
   Then, in step  535 , the parser determines whether this is the last MB in the GOB. The decision may be based on counting the MBs. If this MB is not the last, the parser  146  returns to step  525  for parsing the next MB. 
   If this MB is the last in the GOB, the parser  146  moves to step  540  in  FIG. 5   b  and determines whether this GOB is the last GOB in the intra frame. The decision may be based on counting the GOBs. 
   If it is not the last GOB in the input frame, the parser  146  proceeds to step  543 , where the parser  146  reads the next field and determines whether the field is a GOB SC (GBSC). If the field is not a GBSC, the parser  146  returns to step  525  in  FIG. 5   a  and starts parsing the MBs. 
   Returning to step  543 , if the field is a GBSC, the parser  146  proceeds to step  550 , where the GOB header is converted into a slice header, and the slice header is written to the allocated buffer in the shared memory  130 . Using this conversion eliminates the need for processing the Motion Vector Data (MVD) and the quantizers of the compressed stream, which requires real time processing and generates a delay. The delays caused by processing MVDs and quantizers can be overcome by converting the GOBs into slices. 
   The step  550  process of converting a GOB header into a slice header may include the steps of:
         1. Writing SSTUF bits to the input buffers according to the H.263 standard.   2. Calculating the MBA field of the slice header by adding to the ID number that has been assigned to the first MB of the QCIF frame (based on its location in the screen layout), the ID number of the first MB in the current GOB.   3. Installing the GQUANT value in SQUANT field. GQUANT is a fixed length codeword of 5 bits which indicates the quantizer QUANT to be used for the remaining part of the picture until updated by any subsequent GQUANT or DQUANT. The codewords are the natural binary representations of the values of QUANT which, being half the step sizes, range from 1 to 31. SQUANT is not present for the slice that follows the picture start code.   4. Writing the width of the slice in SWI field.       

   After step  550 , the parser  146  returns to step  525  in  FIG. 5   a  and starts parsing the MBs of the new slice. 
   Returning to step  540 , if the GOB is the last one in the low-resolution frame the parser  146  terminates the processing of the intra frame. The parser  146  proceeds to step  564 , and writes the PSTUF, according to the H.263 standard, to the input buffer. In step  567 , the parser  146  resets the intra frame indication, indicating that the request for an intra frame has been fulfilled. PSTUF is a codeword of variable length consisting of less than 8 zero-bits. Encoders may insert this codeword for byte alignment of the next PSC. The last bit of PSTUF may be the last (or the least significant) bit of a byte, so that the video bitstream including PSTUF is a multiple of 8 bits from the first bit in the H.263 bitstream. Decoders may be designed to discard PSTUF. In an embodiment, if for some reason the encoder stops encoding pictures for a certain time-period and resumes encoding later, PSTUF may be transmitted before the encoder stops, to prevent up to the last 7 bits of the previous picture from being sent. In an embodiment, the last bits may not be sent until the coder resumes coding. 
   Then, in step  570 , the parser  146  adds the pointer of the allocated input buffer to the tail of the linked list  120 , thereby recording the location in linked list  120  of the low-resolution frame in the shared memory  130 . In step  575  the parser  146  returns to its main routine  400 , step  410  in  FIG. 4 , waiting to the next PSC. 
     FIG. 6   a  and  FIG. 6   b  are two parts of a flowchart showing the steps of an exemplary method for parsing an inter frame. Both  FIGS. 6   a  and  6   b  are an elaboration of step  445  in  FIG. 4   b . In step  613 , the parser  146  removes the frame header. In step  616 , based on the ID number of the first MB that has been assigned to the parser  146 , a determination is made as to whether the location of this frame is in the top left corner of the composed frame. If the frame is in the top left corner, the first MB ID number is zero. Accordingly, the parser  146  proceeds to step  618 , where a new inter frame header is installed. The new inter frame header represents an inter frame of higher resolution, which is compressed according to H.263 with ANNEX K, and writes the new frame header into the allocated input buffer. Then the parser  146  moves to step  625  for parsing the first inter MB in the frame. 
   Returning to step  616 , if the location of the image is not in the top left corner of the composed screen layout, the parser  146  proceeds to step  620 , where the inter frame header is converted to a slice header, and writes the slice header in the input buffer. The conversion of the inter frame header to the slice header may include the steps of:
         1. Installing the MB ID number, which has been assigned to the parser  146  based on its location in the layout and in the MBA field of the slice header.   2. Installing the PQAUNT value in the SQUANT field.   3. Writing the width of the slice in SWI field in the slice header. If the high-resolution frame is being composed from compressed QCIF frames the SWI will be always 11.       

   After step  620 , the parser  146  proceeds to step  625 , which starts a loop of parsing the MBs and writing the data in the input buffers of the frame. This parsing is common and based on the H.263 standard. 
   Upon terminating the parsing of the MBs in step  635 , the parser  146  moves to step  630  and determines whether to write a recommended division point. The decision may be based on different criteria. An exemplary criterion is the number of bytes from the last recommended division point to the end of the current MB. This recommended division point helps the packetizer  155  during the process of dividing the composed stream, from different sources, into packets. 
   Then, in step  635 , the parser  146  determines whether this MB is the last MB in the GOB. The decision may be based on counting the MBs. If this is not the last MB, the parser  146  returns to step  625  for parsing the next MB. 
   If this MB is the last in the GOB, the parser  146  moves to step  640  in  FIG. 6   b  and determines whether this GOB is the last GOB in the inter frame. The decision may be based on counting the GOBs. 
   If in step  640  it is determined that this is not the last GOB in the input frame, then the parser  146  proceeds to step  643 , where the parser  643  reads the next field and determines whether the field is a GBSC. If the next field is not a GBSC, then the parser  146  returns to step  625  in  FIG. 6   a  and starts parsing the MBs. 
   If in step  643  it is determined that the field is a GBSC, the process proceeds to step  650 , where the GOB header is converted into a slice header and the slice header is written in the allocated input buffer. Using this conversion eliminates the need for processing the MVD and the quantizers of the compressed stream, which requires real time processing and generates delay. The delay can be avoided by converting the GOBs into slices. 
   In step  650 , converting the GOB header into a slice header may include the steps of:
         1. Writing SSTUF bits to the input buffers according to the H.263 standard.   2. Calculating the MBA field of the slice header by adding the ID (MBA) number of the first MB in the current GOB, to the ID number that has been assigned to the first MB of the frame (based on its location in the screen layout).   3. Installing the GQUANT value in SQUANT field.   4. Writing the width of the slice in SWI field.       

   After step  650 , the parser  146  returns to step  625  in  FIG. 6   a  and starts parsing the MBs of the new slice. 
   Returning to step  640 , if the GOB is the last one in the low-resolution frame, the parser  146  proceeds to step  664  to start terminating the processing of the inter frame. In step  664 , the parser  146  writes the PSTUF, according to the H.263 standard, to the input buffer. 
   Then, in step  670 , the parser  146  adds the pointer of the allocated input buffer to the tail of the linked list  120  and, in step  675 , returns to the parser  146 &#39;s main routine  400 , step  410  in  FIG. 4 . 
     FIG. 7  is a flow diagram illustrating the steps of an exemplary method for parsing an intra MB in an intra frame and converting it into an intra MB in an inter frame.  FIG. 7  is an elaboration of step  525  in  FIG. 5   a , which illustrates an intra frame parsing routine. In step  710 , before the parser  146  ( FIG. 1 ) starts parsing the intra MB, the parser  146  writes the Coded macroblock indication (COD) field (‘0’) into the buffer. The COD field indicates whether the MB is coded or not, where a value of zero means that the MB is coded. This field is not in use in an intra frame since all the MBs are coded. In an inter frame, part of the MBs may not be coded, and therefore there is a need for this field. COD is a bit which when set to “0” signals that the MB is coded. If set to “1,” no further information is transmitted for this MB; in that case the decoder shall treat the MB as an inter MB with one motion vector for the whole block equal to zero and with no coefficient data. In an embodiment, the COD is only present in pictures that are not of intra type, but is present for each MB in these pictures. In the advanced prediction mode (see Annex F), overlapped block motion compensation is also performed if COD is set to “1”; and in the deblocking filter mode (see Annex J), the deblocking filter can also affect the values of some pixels of MBs having COD is set to “1.” 
   Then, in step  715 , the parser  146  reads the Macroblock type and Coded Block Pattern Chrominance (MCBPC) field of an intra frame, and in step  720  writes the corresponding Variable Length Code (VLC) to the MCBPC field of the inter frame. An exemplary conversion method may use the following look-up table, for converting the VLC for MCBPC: 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
                 
                 
               NEW VLC 
             
             
                 
                 
               CBPC 
               VLC for 
               for INTER 
             
             
                 
               MB type 
               5, 6 
               INTRA frame 
               frame 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               3 
               00 
                1 
                 00011 
             
             
                 
               3 
               01 
               001 
                00000100 
             
             
                 
               3 
               10 
               010 
                00000011 
             
             
                 
               3 
               11 
               011 
                0000011 
             
             
                 
               4 
               00 
               0001 
                 000100 
             
             
                 
               4 
               01 
               000001 
               000000100 
             
             
                 
               4 
               10 
               000010 
               000000011 
             
             
                 
               4 
               11 
               000011 
               000000010 
             
             
                 
                 
             
           
        
       
     
   
   After replacing the intra MCBPC field, in step  735 , the parser  146  continues parsing the rest of the MB according to the H.263 standard and writes the information to the input buffers. At the end of the parsing the intra MB, in step  740 , the parser  146  returns to the intra frame routine and proceeds to step  530  in  FIG. 5   a.    
     FIGS. 8   a  and  8   b  are two parts of a flowchart showing an exemplary method  800  that an output module  150  uses for transmitting an inter frame. In step  810 , upon receiving a command from the conference manager  115  to send a new composed high-resolution inter frame, the packetizer  155  gets a packet payload buffer and starts a loop over all the input modules  140  that have been assigned to the packetizer  155 . 
   In step  815 , the packetizer  155  reads the linked list  120  of the first input module  140 , and in step  820 , determines whether there is a pointer to a ready frame buffer in the shared memory  130 . If there is no such pointer, the method  800  proceeds to step  825 , where the packetizer  155  installs the appropriate NCS data from the background buffer  157 . Then the method  800  proceeds to step  830  ( FIG. 8   b ). Similarly, if in step  820 , there is a frame buffer ready in the linked list  120 , the method  800  proceeds to step  828  ( FIG. 8   b ). In step  828 , the packetizer  155  reads the appropriate frame buffer from the shared memory  130 . 
   In step  830 , the packetizer  155  starts a loop ( FIG. 8   b ) involving dividing the frame buffer or the NCS buffer into packets. The packetizer  155  ( FIG. 1 ) determines whether there is enough room in the current packet for the rest of the data that is in the buffer. If there is not enough room, the method  800  proceeds to step  833 , where the packetizer  155  copies part of the buffer to the current packet buffer (a buffer dedicated to the current packet). The current packet buffer may also be referred to as the packet payload buffer. The packetizer module  155  may use the recommended division point that has been prepared by the parser  146  ( FIG. 1 ) to determine how much of the buffer is moved to the current packet buffer. 
   The use of recommended division points eliminates the complexity of having a parser in the output module  150  determine how much of the input buffer is copied into the current packet buffer. 
   Then, in step  836 , the output module  150  sends the packet in the current packet buffer over the LAN  105 , gets a new packet payload buffer and returns to step  830  to continue the loop of dividing the input buffer into packets. 
   Returning to step  830 , if there is enough room in the packet for the rest of the input buffer, the method proceeds to step  839 , where the packetizer  155  copies the rest of the buffer into the current packet buffer and releases the input buffers from the linked list  120 . In step  840 , the output module  150  determines whether there is another input module  140  that is assigned to it. If there is, the method proceeds to step  815  and reads the linked list  120  of the next input module  140 . 
   If there is no other input module  140  assigned to output module  150 , the method proceeds to step  850 , where the packetizer  155  checks if there are empty screen parts in the background buffer  157 . If there are empty screen parts, the packetizer  155  returns to step  830  and starts a loop of dividing the empty screen part buffers into packets. If, in step  850 , it is determined that there are no additional empty screen parts or that the packetizer  155  has divided all of the content of the input buffer into packets, the method proceeds to step  855 , where the packetizer  155  sends the last packet of the current high-resolution composed inter frame to the LAN  105 . Then the packetizer  155  waits to receive a command from the conference manager  115  to send a new high-resolution composed frame. 
     FIG. 9  is a flowchart showing an exemplary method  900  for transmitting an intra frame. Method  900  starts with step  910 , where upon receiving the command to transmit an intra frame, the packetizer  155  cleans the linked list  120  and gets a new current packet buffer. Then the packetizer  155  reads the intra background frame from the background buffer  157  and starts the loop of dividing the background frame buffer into packets. 
   In step  920  the packetizer  155  determines whether there is enough room in the current packet for the rest of the data that is in the input buffer. If there is not enough room, the method proceeds to step  923 , where the packetizer copies part of the buffer to the current packet buffer. The packetizer  155  may use the recommended division point that has been previously prepared for determining how the input buffer is copied to the current packet buffer. 
   Then, in step  926 , the output module  150  sends the packet over LAN  105 , gets a new packet payload, places the new packet payload in the input buffer, and returns to step  920  to continue the loop of dividing the Intra background frame into packets. 
   If, in step  920 , it is determined that there is enough room in the packet for the rest of the Intra background frame, the method proceeds to step  930 , where the packetizer  155  copies the rest of the buffer  930  into the current packet buffer and, in step  935 , sends the packet from the current packet buffer over LAN  105 . 
   Then, in step  940 , since the request for an intra frame has been fulfilled, the packetizer  155  resets the intra frame indication and waits for the next command to send an intra or an inter frame. 
     FIG. 10  is a flow diagram illustrating the steps of an exemplary method  1000  for handling a fast update request (a request for an intra frame) from at least one of the endpoints that receives the output of the packetizer  155 . 
   In step  1020 , upon receiving the fast update request, the conference manager  115  turns on the intra frame request indication. After step  1020 , method  1000  divides into two parallel branches. The first branch, the parser branch  1040 , proceeds to step  1042 , where in response to the intra frame request being turned on, each parser  146  releases its current input buffers and then in step  1044  returns to its main routine  400  to step  410  in  FIG. 4 . 
   In parallel, on the second branch, the packetizer branch  1030 , the method proceeds to step  1033 , where the packetizer  155  releases the linked list  120  in all the input modules  140  that are assigned to the packetizer  155  and then in step  1036  jumps to intra frame transmission routine  900  to step  910  in  FIG. 9 . 
   Those skilled in the art will appreciate that the present invention is not limited to composing one or more QCIF input frames to a CIF output frame. The present invention may be used in composing compressed video data, which is received from low-resolution video sources of types other than QCIF into a higher resolution video frame having a type other than CIF. For example, the present invention may be used in composing up to 16 sources of QCIF frames of compressed video data into a single high-resolution 4CIF frame, or composing up to four sources of CIF frames of compressed video data into a single high-resolution 4CIF frame. The present invention also supports composing different resolutions, such as composing three CIF frames and four QCIF frames into a single 4CIF output frame, for example. 
   Those skilled in the art will appreciate that the present invention can be either in the form of additional software residing in the MCU that performs the methods that have been disclosed in the present application, or in the form of additional hardware, which has been added to the MCU. 
   Furthermore, those skilled in the art will appreciate that the present invention can be used in other compression standards such as, but not limited to, MPEG 4. 
   In the description and claims of the present application, each of the verbs, “comprise,” “include,” and “have,” and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements, or parts of their respective subjects or verb. 
   The present invention has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. 
   The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the present invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments will occur to persons skilled in the art. The scope of the invention is limited only by the following claims.