Abstract:
There is provided according to embodiments of the invention a method of changing channels in a hybrid multicast video distribution environment, which is carried out by receiving a plurality of multicast video streams concurrently in a server, buffering first segments of the video streams respectively, wherein the first segments comprise an independently decodable frame and zero or more predicted frames, encoding the first segments to form new segments having new frame quality parameters that are less than the frame quality parameters of the first segments, assembling the first segments and the new segments into respective channel packs of the video streams, and responsively to a request, identifying one of the video streams and transmitting at least a portion of the oldest stored channel pack of the one video stream from the server to a receiving device. Related apparatus and methods are also described.

Description:
RELATED APPLICATION INFORMATION 
     The present application claims the benefit of priority from GB Patent Application GB 1107386.3 of NDS Limited, filed 4 May 2011. 
     FIELD OF THE INVENTION 
     The present invention relates in general to communication systems, and particularly, but not exclusively, to communication systems based on satellite transmission. 
     BACKGROUND TO THE INVENTION 
     The meanings of certain acronyms and abbreviations used herein are given in Table 1. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Acronyms and Abbreviations 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 AAA 
                 Authentication, Authorization, and Accounting 
               
               
                   
                 FCCHD 
                 Fast Channel Change High Definition 
               
               
                   
                 GOP 
                 Group of Pictures 
               
               
                   
                 HD 
                 High Definition 
               
               
                   
                 IP 
                 Internet Protocol 
               
               
                   
                 MPEG-2 
                 Motion Picture Experts Group 2 
               
               
                   
                 MPEG2-TS 
                 Motion Picture Experts Group 2 Transport Stream 
               
               
                   
                 onid 
                 Original Network Identifier 
               
               
                   
                 PCR 
                 Program Clock Reference 
               
               
                   
                 SD 
                 Standard Definition 
               
               
                   
                 sid 
                 Station Identifier 
               
               
                   
                 STB 
                 Set-Top Box 
               
               
                   
                 SVG 
                 Scalable Vector Graphics 
               
               
                   
                 tsid 
                 Transport Stream Identifier 
               
               
                   
                   
               
             
          
         
       
     
     As digital television transmission has replaced analog broadcasts, viewers have found that the advantages of the newer mode are offset by a relatively long latency period when changing channels, as compared to the nearly instant response of a traditional analog tuner. This is a consequence of motion-compensated video encoding schemes, of which the well-known MPEG-2 standard is exemplary. The following international standards and specifications are particularly relevant to the understanding of the disclosure of the present invention: ITU-T Specification H.264 (March 2005), Advanced Video Coding for Generic Audiovisual Services; ISO/IEC 13818-1, Information Technology—Generic Coding of Moving Pictures and Associated Audio Information: Systems (Second Edition, 2000). 
     A video sequence known as a “group of pictures” (GOP) begins with an I-Frame, which is an independently decodable frame. The I-frame is followed by zero or more “predicted frames” known as P-frames and B-frames, which are encoded relative to the I-frame and/or one another, and generally cannot be decoded unless the I-frame is known. In other words, the GOP is decodable only once the I-frame has become available to the decoder. 
     A channel change request may occur randomly in the course of an MPEG-2 stream. It therefore is generally unaligned with the current I-frame. Hence, decoding of a new MPEG-2 stream is delayed until arrival of a new I-frame. This delay can be many seconds in duration, particularly when there is little motion occurring in the image being transmitted. The delay in video presentation may be increased even more by inherent latencies within the decoder. The result, if not compensated in some manner, is user dissatisfaction. 
     U.S. Pat. No. 7,562,375 proposes to change channels in a digital video environment by configuring a server to retain at least one independent frame for each video channel of multiple video channels that are being distributed using multicast communications. The server is adapted to respond to channel change requests from clients by transmitting the retained independent frame of a requested video channel to a requesting client using unicast communication. 
     U.S. Pat. No. 7,430,222 discloses facilitating a fast channel change by receiving and decoding a unicast acquisition media stream that corresponds to a soon-to-be requested target multicast media stream. This occurs before requesting the target multicast media stream of the new channel. Once the transmission of the unicast stream is caught up with the multicast stream, the unicast acquisition stream is spliced to the target multicast media stream. 
     SUMMARY OF THE INVENTION 
     The present invention, in certain embodiments thereof, seeks to provide an improved communication system for receiving multicast video streams in a hybrid environment. 
     There is provided according to embodiments of the invention a method of changing channels in a hybrid multicast video distribution environment, which is carried out by receiving a plurality of multicast video streams concurrently in a server, buffering first segments of the video streams respectively, wherein the first segments comprise an independently decodable frame and zero or more predicted frames, encoding the first segments to form new segments having new frame quality parameters that are less than the frame quality parameters of the first segments, assembling the first segments and the new segments into respective channel packs of the video streams, and responsively to a request, identifying one of the video streams and transmitting at least a portion of an oldest stored channel pack of the one video stream from the server to a receiving device. 
     According to an aspect of the method, the video streams may have a plurality of bit rates and a plurality of video compression formats. 
     Still another aspect of the method includes down-sampling the first segments, such that at least a portion of the new segments have a smaller bit-rate than the first segments, respectively. 
     An additional aspect of the method includes reducing a framing rate of the first segments. 
     One aspect of the method includes maintaining a profile of an issuer of the request, the profile including an available bandwidth of a communication channel between the server and the issuer of the request. The profile may include a description of a delivery path to the issuer of the request through a data network. 
     A further aspect of the method includes selecting the portion of the oldest stored channel pack responsively to an analysis of the profile. 
     According to yet another aspect of the method, the channel packs have different numbers of the new segments. 
     There is further provided according to embodiments of the invention an apparatus for changing channels in a hybrid multicast video distribution environment, including a server that is operative for receiving a plurality of multicast video streams concurrently, a channel pack module for buffering first segments of the video streams respectively, wherein the first segments comprise an independently decodable frame and zero or more predicted frames. The channel pack module is operative for encoding the first segments to form new segments having new frame quality parameters that are less than the frame quality parameters of the first segments, and for assembling the first segments and the new segments into respective channel packs of the video streams. The apparatus includes a cache storing recently assembled channel packs of the video streams and a network interface, through which the server is operative for transmitting, responsively to a request identifying one of the video streams, at least a portion of the oldest stored channel pack of the one video stream from the server to a receiving device. 
     According to an aspect of the apparatus, the video streams may have a plurality of bit rates and a plurality of video compression formats. 
     According to an additional aspect of the apparatus, encoding the first segments includes down-sampling, such that at least a portion of the new segments have a smaller bit-rate than the first segments, respectively. 
     According to still another aspect of the apparatus, encoding the first segments includes reducing a framing rate thereof. 
     In yet another aspect, the apparatus includes a data storage unit for maintaining a profile of an issuer of the request, the profile including an available bandwidth of a communication channel between the server and the issuer of the request. The profile may also include a description of a delivery path to the issuer of the request through a data network. 
     According to aspect of the apparatus, the server is operative for selecting the portion of the oldest stored channel pack responsively to an analysis of the profile. 
     According to one aspect of the apparatus, the channel packs have different numbers of the new segments. 
     There is further provided according to embodiments of the invention a method of changing channels in a hybrid multicast video distribution environment, which is carried out by receiving, decoding and rendering a first multicast video stream on a display using a receiving device having a synchronization module, and receiving an order for a channel change operation from the first multicast video stream to a second multicast video stream. The method is further carried out, responsively to the order and while receiving the second multicast video stream, by requesting a channel pack from a fast channel change server, the channel pack including an encoded segment of the second multicast video stream that has an independently decodable frame and zero or more predicted frames of the second multicast video stream. Frame quality parameters of the encoded segment are less than frame quality parameters of a corresponding segment of the second multicast video stream. The method is further carried out by accepting the requested channel pack from the fast channel change server, decoding and rendering the encoded segment of the accepted channel pack on the display, and determining in the synchronization module a time shift between a frame of the accepted channel pack and a corresponding frame of the second multicast video stream. The method is further carried out by iteratively requesting a new channel pack from the fast channel change server, and with the new channel pack again accepting the requested channel pack, decoding and rendering the encoded segment and determining a time shift at intervals that are less than a playback duration of corresponding segments of the second multicast video stream until the time shift is less than a predetermined value, and thereafter rendering the second multicast video stream on the display. 
     According to an aspect of the method, the encoded segment is a down-sampled version of the corresponding segment of the second multicast video stream, such that the encoded segment has a smaller bit-rate than the corresponding segment of the second multicast video stream. 
     According to a further aspect of the method, the encoded segment has a smaller framing rate than the corresponding segment of the second multicast video stream. 
     According to still another aspect of the method, the first multicast video stream has a different video compression format than the second multicast video stream. 
     According to an additional aspect of the method, the first multicast video stream is received at a different bit rate than the second multicast video stream. 
     There is further provided according to embodiments of the invention an apparatus for changing channels in a hybrid multicast video distribution environment, including a receiver operative for decoding and rendering a first multicast video stream on a display and having a user interface and a communications interface linked to a fast channel change server, a synchronization module cooperative with the receiver. The receiver and the synchronization module are co-operative for receiving an order via the user interface for a channel change operation from the first multicast video stream to a second multicast video stream; responsively to the order and while receiving the second multicast video stream requesting a channel pack via the communications interface from the fast channel change server, the channel pack including an encoded segment of the second multicast video stream having an independently decodable frame and zero or more predicted frames of the second multicast video stream. Frame quality parameters of the encoded segment are less than frame quality parameters of a corresponding segment of the second multicast video stream. The receiver and the synchronization module are further co-operative for accepting the requested channel pack from the fast channel change server, decoding and rendering the encoded segment of the accepted channel pack on the display. The synchronization module is operative for determining a time shift between a frame of the accepted channel pack and a corresponding frame of the second multicast video stream. The receiver and the synchronization module are further co-operative for iteratively requesting a new channel pack from the fast channel change server, and again accepting the new channel pack, decoding and rendering the encoded segment and determining a time shift at intervals that are less than a playback duration of corresponding segments of the second multicast video stream until the time shift is less than a predetermined value, and thereafter decoding and rendering the second multicast video stream on the display. 
     According to aspect of the apparatus, the encoded segment is a down-sampled version of the corresponding segment of the second multicast video stream, such that the encoded segment has a smaller bit-rate than the corresponding segment of the second multicast video stream. 
     According to one aspect of the apparatus, the encoded segment has a smaller framing rate than the corresponding segment of the second multicast video stream. 
     According to an additional aspect of the apparatus, the first multicast video stream has a different video compression format than the second multicast video stream. 
     According to still another aspect of the apparatus, the first multicast video stream is received at a different bit rate than the second multicast video stream. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, reference is made to the de-tailed description of embodiments, by way of example, which is to be read in conjunction with the following drawings, wherein like elements are given like reference numerals, and wherein: 
         FIG. 1  is a schematic diagram of a system, which is constructed and operative in accordance with an embodiment of the invention; 
         FIG. 2  is a diagram explaining the function of a channel pack module in the system shown in  FIG. 1 , in accordance with an embodiment of the invention; 
         FIG. 3  is a flow chart illustrating an exemplary operation of the channel pack module shown in  FIG. 1  in accordance with an embodiment of the invention; and 
         FIG. 4  is a series of diagrams illustrating the time relationships of frames of a conventional video stream of the new channel and channel packs during a fast channel change operation in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the various principles of the present invention. It will be apparent to one skilled in the art, however, that not all these details are necessarily always needed for practicing the present invention. In this instance, well-known circuits, control logic, and the details of computer program instructions for conventional algorithms and processes have not been shown in detail in order not to obscure the general concepts unnecessarily. 
     Embodiments of the present invention may include software programming code, which is typically maintained in permanent storage, such as a computer readable medium. In a client/server environment, such software programming code may be stored on a client or a server. The software programming code may be embodied on any of a variety of known non-transitory tangible media for use with a data processing system, such as a diskette, hard drive, or CD-ROM. The code may be distributed on such media, or may be distributed to consumers from the memory or storage of one computer system over a network of some type to storage devices on other computer systems for use by consumers of such other systems. 
     DEFINITIONS AND TERMINOLOGY 
     The term “encoded” is used throughout the present specification and claims, in all of its grammatical forms, to refer to any type of data stream encoding including, for example and without limiting the scope of the definition, well known types of encoding such as, but not limited to, MPEG-2 encoding, H.264 encoding, VC-1 encoding, and synthetic encodings such as Scalable Vector Graphics (SVG) and LASER (ISO/IEC 14496-20), and so forth. It is appreciated that an encoded data stream generally requires more processing and typically more time to read than a data stream, which is not encoded. Any recipient of encoded data, whether or not the recipient of the encoded data is the intended recipient, is, at least in potential, able to read encoded data without requiring cryptanalysis. It is appreciated that encoding may be performed in several stages and may include a number of different processes, including, but not necessarily limited to: compressing the data; transforming the data into other forms; and making the data more robust (for instance replicating the data or using error correction mechanisms). 
     The term “compressed” is used throughout the present specification and claims, in all of its grammatical forms, to refer to any type of data stream compression. Compression is typically a part of encoding and may include image compression and motion compensation. Typically, compression of data reduces the number of bits comprising the data. In that compression is a subset of encoding, the terms “encoded” and “compressed”, in all of their grammatical forms, are often used interchangeably throughout the present specification and claims. 
     Similarly, the terms “decoded” and “decompressed” are used throughout the present specification and claims, in all their grammatical forms, to refer to the reverse of “encoded” and “compressed” in all their grammatical forms. 
     The terms “scrambled” and “encrypted”, in all of their grammatical forms, are used interchangeably throughout the present specification and claims to refer to any appropriate scrambling and/or encryption methods for scrambling and/or encrypting a data stream, and/or any other appropriate method for intending to make a data stream unintelligible except to an intended recipient(s) thereof. Well known types of scrambling or encrypting include, but are not limited to DES, 3DES, and AES. Similarly, the terms “descrambled” and “decrypted” are used throughout the present specification and claims, in all their grammatical forms, to refer to the reverse of “scrambled” and “encrypted” in all their grammatical forms. 
     Pursuant to the above definitions, the terms “encoded”; “compressed”; and the terms “scrambled” and “encrypted” are used to refer to different and exclusive types of processing. Thus, a particular data stream may be, for example: encoded, but neither scrambled nor encrypted; compressed, but neither scrambled nor encrypted; scrambled or encrypted, but not encoded; scrambled or encrypted, but not compressed; encoded, and scrambled or encrypted; or compressed, and scrambled or encrypted. 
     Likewise, the terms “decoded” and “decompressed” one the one hand, and the terms “descrambled” and “decrypted” on the other hand, are used to refer to different and exclusive types of processing. 
     The terms “decimated” and “decimated slices” generally indicates a reduction in the frame quality of the stream by eliminating data in the frame, e.g., by reducing sampling rates, bit rates, or quantizing frame data, but not necessarily by a factor of 10. 
     System Architecture 
     Turning now to the drawings, reference is initially made to  FIG. 1 , which is a schematic diagram of a system  10 , which is constructed and operative in accordance with an embodiment of the invention. The system  10  includes a hybrid head-end component  12 , which includes a fast channel change server  14  coupled to a channel pack module  16  and a conventional broadcast head-end unit  18 . The head-end component  12  is adapted to a hybrid environment in which the fast channel change server  14  processes multiple video streams concurrently. While shown as separate entities in  FIG. 1 , the fast channel change server  14  may be integral with the fast channel change server  14 . It may be realized as a hardware device, implemented as executable program code, or as a combination of hardware and software. 
     An exemplary mix of digital services received by the fast channel change server  14  comprises any number of video and audio channels CH- 1  . . . CH-N, shown as digital television streams  20 ,  22 ,  24 , a video-on-demand stream  26 , and a digital simulcast stream  28 . These and other types of digital video streams may be received in many combinations and constitute a hybrid multicast video distribution environment for the system  10 . The video streams may arrive at different bit rates, in different video compression formats. They may be encrypted differently or may be unencrypted. The arriving streams are multiplexed in multiplexor  30  Under control of a processor  32 , the streams  20 ,  22 ,  24 ,  26 ,  28  are demultiplexed in demultiplexor  34  and segments of the streams assembled in the channel pack module  16  into respective units referred to herein as “channel packs” or sometimes as “packs”, of which channel packs  36 ,  38 ,  40  are shown. The structure of the channel packs is described below. Assembled channel packs are returned from the channel pack module  16  for storage in channel database  42 , which may be integral with the fast channel change server  14  as shown or embodied as a separate unit or included in the channel pack module  16 . The channel database  42  caches a number of the most recently assembled channel packs. The number may be fixed, or adaptive to the characteristics of individual video streams and the capabilities of communication channels between the fast channel change server  14  and a receiving device  44 . 
     The fast channel change server  14  operates as a unicast server that delivers on a per request basis recently assembled channel packs for a given channel to the receiving device  44 , which is typically a set-top box. Delivery of the channel packs may be achieved by any convenient method. For example the fast channel change server  14  may be provided with a network interface  46  to a data network  48 . Alternatively terrestrial radio or cable transmission or via a satellite receiving station  50  may be used. 
     The receiving device  44  may be a set-top box and typically includes a tuner  52 , a transport demultiplexing and decryption module  54 , which operates under control of a microcontroller  56 , which has associated memory, a source decoder  58  and communicatively coupled random access memory  60 . A user I/O device  62  provides a user interface for accepting subscriber commands such as channel change requests, and for providing output information. The receiving device  44  optionally is provided with a module  64  for performing authentication, authorization, and accounting (AAA) functions. A synchronization component  66  deals with certain events that occur during a channel change sequence. External interfaces  68  deliver suitably decoded and descrambled video to a display  70 . 
     The receiving device  44  processes conventional broadcast signals from the broadcast head-end unit  18  at all times. However, during a channel change operation from an old to a new channel, the receiving device  44  additionally requests and receives channel packs from the fast channel change server  14 . In order to execute the requested channel change operation according to embodiments of the invention, selected slices of the channel packs are decoded and rendered, while concurrently monitoring PCRs from the conventional broadcast stream of the new channel. At the conclusion of the channel change operation, the receiving device  44  ceases requesting channel packs and resumes processing the conventional broadcast stream of the new channel. 
     Channel Packing Component 
     Reference is now made to  FIG. 2 , which is a diagram explaining the function of the channel pack module  16  ( FIG. 1 ) in accordance with an embodiment of the invention. The channel pack module  16 , as noted above, is responsible for packing the channel streams into decodable audio and video elements or packs. A channel pack at least contains one stream slice, e.g., a segment  72  of stream  20  that includes at least an MPEG2-TS I-frame, and therefore is self-contained and decodable. Alternatively, the slice could include an IDR-frame in the case of an H264 AVC video stream. Other compressed formats may also be processed, mutatis mutandis. 
     An original slice  74  , including the segment  72  of the video stream  20 , is aligned with GOP boundary  76 . The slice  74  is re-encoded on-the-fly to generate several new slices having reduced frame quality parameters, using well-known constrained average bit rate encoding techniques. The new slices may be encoded at lower resolution. Additionally or alternatively the new slices may be encoded at a lower framing rate. Typically, several re-encoded slices are generated having different quality parameters. An exemplary channel pack  78  comprises the original slice  74  and three re-encoded slices  80 ,  82 ,  84 . The output parameters of channel pack  78  are shown in Table 2. Each slice in the channel pack  78  is also identified by a unique channel pack identifier  86  , referred to herein as a “pack_Id ”, which comprises the classic DVB triplet “onid.tsid.sid” (original network identifier; transport stream identifier; station identifier) and the program clock reference (PCR) value of the first packet of the slice in the transport stream. Thus, the exemplary channel pack  78  is composed of (1) the channel pack identifier  86 ; (2) the original slice taken from the audio/visual stream (slice  74  ); and (3) one of the re-encoded sub-slices (slices  80 ,  82 ,  84 ). While slices  80 ,  82  are both transmitted at 25 fps, slice  80  has B frames, and slice  82  has none because it has been encoded at a higher compression rate than slice  80 . Higher compression rates are of course, associated with progressively reduced frame quality. 
     It will be evident that appending original slices of successive channel packs (respective slices corresponding to slice  74 ) would reconstitute the stream  20 . 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
             
               
                   
                 Slice 74 
                 Transmission: 12 Frames @ 12 Mbps 
               
               
                   
                   
                 Size: 6 Mb 
               
               
                   
                   
                 Framing Rate 25 fps 
               
               
                   
                   
                 Pack_id 
               
               
                   
                 Slice 80 
                 Transmission: 12 Frames @ 6 Mbps 
               
               
                   
                   
                 Size 3 Mb 
               
               
                   
                   
                 Framing Rate 25 fps 
               
               
                   
                   
                 Pack_id 
               
               
                   
                 Slice 82 
                 Transmission: 12 Frames @ 3 Mbps 
               
               
                   
                   
                 Size 1.5 Mb 
               
               
                   
                   
                 Framing Rate 25 fps 
               
               
                   
                   
                 Pack_id 
               
               
                   
                 Slice 84 
                 Transmission: 1 Frame @ 1 Mbps (I-Frame Only) 
               
               
                   
                   
                 Size 0.5 Mb 
               
               
                   
                   
                 Framing Rate 1 fps 
               
               
                   
                   
                 Pack_id 
               
               
                   
                   
               
             
          
         
       
     
     The slices  80 ,  82 ,  84  may be re-encoded in the channel pack module  16  with decimated encoding quality parameters on-the-fly and in parallel. Indeed, the channel pack module  16  typically processes all the streams  20 ,  22 ,  24 ,  26 ,  28  ( FIG. 1 ) concurrently in this manner. However, in view of different characteristics of the streams  20 ,  22 ,  24 ,  26 ,  28 , their respective channel packs may be generated with different numbers of encoded sub-slices and different degrees of decimation. 
     Table 3 shows output parameters of a channel pack assembled from another concurrently received digital stream (not shown in  FIG. 2 ), in which the output parameters are optimized differently from those of Table 2, reflecting the different characteristics of the two digital video streams. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
             
               
                   
                 Slice A 
                 Transmission: 6 Frames @ 6 Mbps 
               
               
                   
                   
                 Size: 3 Mb 
               
               
                   
                   
                 Framing Rate 25 fps 
               
               
                   
                   
                 Pack_id 
               
               
                   
                 Slice B 
                 Transmission: 6 Frames @ 3 Mbps 
               
               
                   
                   
                 Size 1.5 Mb 
               
               
                   
                   
                 Framing Rate 25 fps 
               
               
                   
                   
                 Pack_id 
               
               
                   
                 Slice C 
                 Transmission: 6 Frames @ 1.5 Mbps 
               
               
                   
                   
                 Size 0.75 Mb 
               
               
                   
                   
                 Framing Rate 25 fps 
               
               
                   
                   
                 Pack_id 
               
               
                   
                 Slice D 
                 Transmission: 1 Frame @ 0.5 Mbps (I-Frame Only) 
               
               
                   
                   
                 Size 0.25 Mb 
               
               
                   
                   
                 Framing Rate 1 fps 
               
               
                   
                   
                 Pack_id 
               
               
                   
                   
               
             
          
         
       
     
     Reference is now made to  FIG. 3 , which is a flow chart illustrating an exemplary operation of the channel pack module  16  ( FIG. 1 ) in accordance with an embodiment of the invention. The method is explained for clarity with respect to the example of  FIG. 2 . At initial step  88  original slice  74  is extracted from stream  20 . 
     Next, at step  90 , at time t 0  a current pack is assigned its pack_ID. 
     Next, the slice  74  is passed to parallel re-encoding tasks (steps  92 ,  94 ,  96 ), in which the re-encoded slices  80 ,  82 ,  84  are derived from the slice  74 . The parameters are generally given in Table 2. For purposes of this example, it is assumed that original slice  74  has a HD format. In step  92 , slice  80  retains the HD format of slice  74 , but has half its bit rate. In step  94 , slice  82  has ¼ the bit rate of slice  74  and is reduced to SD resolution. In step  96  the framing rate is reduced to one frame per second. Additionally or alternatively, any number of re-encoded slices having many combinations of decimated parameters and non-decimated parameters other than those shown in  FIG. 2  and  FIG. 3  may be included in a channel pack. At final step  98  the channel pack  78  is assembled and transmitted to a storage resource, e.g., the channel database  42  ( FIG. 1 ). 
     Returning to  FIG. 1 , a profile  100  of the receiving device  44  may be maintained in a data store holding a profile database  102 . The profile  100  includes the bit rate and framing rate information for each slice in a channel pack. The profile database  102  may optionally include such data as an identifier of the receiving device  44 , the available bandwidth of a communication channel between the fast channel change server  14  and the receiving device  44 , and may include the location of the receiving device  44  in a network topology and a description of a delivery path to the receiving device  44  through the data network  48 . The fast channel change server  14  uses the profile  100  to optimize the delivery time of the channel packs transmitted to the receiving device  44 . This is done by minimizing the size of the channel packs as much as is practical. In the case of delivery via a network, delivery time is further optimized by judicious choice of the delivery path between the fast channel change server  14  and the receiving device  44 . The channel packs stored in the channel database  42  include a relatively extensive series of progressively decimated slices. However, the fast channel change server  14  typically generally does not transmit an entire stored channel pack to the receiving device  44 , but rather is adaptive to a desired bit rate by choosing an appropriate slice of a stored channel pack to generate a revised channel pack that is optimized according to the current or predetermined capabilities of the requestor. The fast channel change server  14  is responsible to determine the optimum slice to transmit to the receiving device  44 . The fast channel change server  14  may determine or estimate the path latency between the fast channel change server  14  and the receiving device  44  and vary the slice that is included in the revised channel pack sent in response to the current get_pack request or a subsequent get_pack request. Thus, if the path latency increases, a slice having a greater reduction of frame quality parameters is selected, and vice versa. 
     Additionally or alternatively to maintaining the profile  100 , the maximum acceptable bit rate and other capabilities of the receiving device  44  may be incorporated in a get_pack request received by the fast channel change server  14  from the receiving device  44 . 
     Channel packs are stored in the channel database  42 . However when servicing a get_pack request, the fast channel change server  14  assembles a new, generally smaller pack, taking into consideration the information in the profile  100  (or the get_pack request) in order to determine which component of the channel pack to transmit to the receiving device  44 . If, for instance, the bandwidth between the fast channel change server  14  and the receiving device  44  is about 4 Mb/s, the optimum version of the smaller pack  104  would include slice  82  (4 Mb/s*0.5 s&gt;size of (slice  82 )=1.5 Mb). In this example, as shown in  FIG. 2 , pack  104  includes slice  82  of the channel pack  78 , but does not include slices  74 ,  80 ,  84 . The size-optimized slices in the packs  78 ,  104  are all referenced to the same channel pack identifier, channel pack identifier  86 . 
     In some cases optimization of the transmission of channel packs to the receiving device  44  may require adjusting the network topology so as to position the fast channel change server  14  as “close” as possible to the destination, i.e., requiring a minimum number of hops between the fast channel change server  14  and the receiving device  44 . This may be accomplished by nesting the fast channel change server  14  into a managed IP network, e.g., by establishing a preferred or mandatory path through the network, which may include bypassing some of the network elements. 
     Optimum Slice 
     As has been noted above, the fast channel change server  14  is responsible to determine the optimum slice to be transmitted to the receiving device  44 . One method involves a first order linear estimator. For example, assume a channel packing arrangement having three profiles, as shown in Table 4 
     
       
         
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Profile ID 
                 Frame Quality 
               
               
                   
               
             
             
               
                 0 
                 Low 
               
               
                 1 
                 Medium 
               
               
                 2 
                 High 
               
               
                   
               
             
          
         
       
     
     One slice, denoted by slice(Profile), is sent to a receiver in response to a get_pack request. 
     Let Playback_time_slice be the duration of playback of a slice, e.g., 1 GOP=500 ms. 
     Let SizeOf(slice(profile)) denote the size in bytes of the slice of a given profile. 
     Let packId be a monotonic sequence 0, 1, . . . , 
     Let Delivery_time_slice(PackId) be the elapsed time for delivery from the server to the receiver of a channel pack having a packId with an optimum ProfileId(Packld=n). 
     A function, simple_func( ), returns an optimum profile when the previous optimum profile is established as shown in the pseudocode of Listing 1: 
     
       
         
               
             
               
             
           
               
                   
               
               
                 Listing 1 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 simple_func(packid) 
               
               
                 { 
               
               
                 On established condition simple_func(packid&gt;1) is: 
               
               
                 { 
               
               
                 - Compute epsilon = sizeof(slice(Profile(packid))) / 
               
               
                 sizeof(slice(1+Profile(packid))) 
               
               
                   (note: epsilon &lt; 1 by construction) 
               
               
                 If Delivery_time_slice(packId−1) &gt; Playback_time_slice   then 
               
               
                  profileId(packid) = profile(packId−1) − 1 //go to lower profile 
               
               
                 Else If Delivery_time_slice(packId−1) &gt; epsilon * Playback_time_slice 
               
               
                  then profileId(packid) = profile(packId−1) //stay at the same profile 
               
               
                 Else 
               
               
                  profileId(packid) = profile(packId) + 1 //go to the higher profile) 
               
               
                 } 
               
               
                 On start condition simple_func(packId&gt;1) is: 
               
               
                 { 
               
               
                 PacKid=0 
               
               
                 profileId(packId=0) = “low”, 
               
               
                 // optimize delivery time 
               
               
                 // because fast channel change is the most important. 
               
               
                 } 
               
               
                 } 
               
               
                   
               
             
          
         
       
     
     Alternatively, the optimum profile can be estimated using higher order kalman filters with hysteresis. 
     A get_pack request has the format: get_pack (channel_id), where “channel_id” identifies the desired video stream. The fast channel change server  14  responds with a channel pack: pack(slice(profile(packId+n))). 
     Fast Channel Change Synchronization 
     Continuing to refer to  FIG. 1 , the tuner  52  and the synchronization component  66  are responsible for: (1) starting a delayed broadcast channel change sequence by tuning to and monitoring the transport stream pipeline of the new channel; and (2) starting an immediate broadband channel change sequence by submitting a burst of consecutive get_pack requests for the new channel to the fast channel change server  14 . The fast channel change server  14  responds to the requests in a manner that insures that the receiving device  44  and the synchronization component  66  receive channel packs at intervals that are less than the playback duration of corresponding slices of the conventional video stream of the new channel. The composition of the channel pack (e.g., pack  104  ( FIG. 2 )) is optimized by the fast channel change server  14 , taking into consideration information embedded in the get_pack request or stored in the profile  100  as the case may be. 
     In the following discussion, it is assumed that the first decodable element of a new channel is included in a channel pack produced by the channel pack module  16 , and not from the conventional video stream of the new channel. While the receiving device  44  also receives the stream of the new channel conventionally, it is usually not immediately decodable and at this point is useful mainly for its PCR values. PCR values of the conventional stream and channel packs are monitored throughout the channel change operation by the synchronization component  66 . It will be appreciated that the channel packs and the conventional video stream of the new channel are transmitted via different communication channels. The PCRs of both of these are monitored, and a current PCR value is maintained by the synchronization component  66  for purposes of synchronizing the channel packs with the conventional video stream of the new channel. This can be achieved by the by relating the PCRs as shown in Listing 2: 
     
       
         
               
             
               
             
           
               
                   
               
               
                 Listing 2 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 PCR(last_pack_played)  &lt;  PCR(last_conventional_received) 
               
               
                 such  as timeshifting = PCR(last_conventionnal_received) − 
               
               
                 PCR(last_pack_played). 
               
               
                 When  PCR(any_pack_buffered)  ≈  PCR(conventional)  then 
               
               
                 switch  to conventional stream. 
               
               
                   
               
             
          
         
       
     
     Once decoding of slices of received channel packs is begun by the source decoder  58 , the synchronization component  66  updates the current PCR value to the PCR value of the most recently received channel pack, optionally verifying that its pack_ID of the corresponds to the new channel. 
     It will be evident that the process of requesting, generating, dispatching and receiving channel packs introduces a time shift or delay relative to the realtime conventional video stream. The value of the delay is dependent on the time required for assembling, transmitting, and unpacking the channel packs. A typical value is about 3.5 s for an average GOP at 25 frames per second. Consequently, PCR values received by the transport demultiplexing and decryption module  54  immediately following the inception of decoding during the channel change operation correspond to times that are subsequent to PCR values of received channel packs. As the channel change operation progresses, the time shift decreases because the channel packs, previously cached in the fast channel change server  14 , are dispatched and processed more rapidly than the playback time of corresponding frames of the conventional video stream of the new channel. The synchronization component  66  determines when the time shift is below a threshold value, termed a “crossing value”, and it then switches to the conventional broadcast stream to complete the channel change operation, as explained above with reference to Listing 2. At that point no further get_pack requests are issued by the synchronization component  66  until the next user request for a channel change. 
     EXAMPLE 
     Reference is now made to  FIG. 4 , which is a series of four diagrams, which are snapshots illustrating the time relationships of corresponding frames of the conventional video stream of the new channel and channel packs from the perspective of the synchronization component  66  during a fast channel change operation in accordance with an embodiment of the invention. Some of the dimensions in  FIG. 4  are exaggerated for purposes of explication. In each of the diagrams, GOPs of the conventional transport stream are presented in a row, numbered in italic font. Channel packs containing corresponding GOPs are presented in another row underneath the conventional frames, numbered in regular font. The magnitudes of respective time shifts  106 ,  108 ,  110 ,  112  between corresponding GOPs of the channel packs and conventional video stream  114  of the new channel appearing at the synchronization component  66  ( FIG. 1 ) are measured by the lengths of horizontal lines located between the two rows of frame numbers in each diagram. The synchronization component  66  is represented in  FIG. 4  by triangles  116 . 
     At a time t −1  a user request for a fast channel change, indicated by arrow  118 , is issued. Responsively, the synchronization component  66  issues a get_pack request for the new channel. Typically the communication channel for the get_pack request is via a data network, for example the Internet, employing HTTP commands. However, any suitable network communications protocol may be used with an IP connection. The IP connection may rely on any physical layer. Alternatively, it is possible to employ many other communication techniques for transmission of the get_pack request, e.g., data transmission using telephone lines, terrestrial or satellite radio transmitters and combinations thereof. 
     At time t 0  the fast channel change server  14  has responded. It will be recalled that several channel packs are typically buffered in the fast channel change server  14 . Channel pack- 0   120 , the oldest available channel pack in the channel database  42  ( FIG. 1 ) is transmitted, and becomes available to the synchronization component  66 . However, at this point in time the synchronization component  66  sees intermediate frames of GOP  2  of the conventional stream, which cannot be decoded as the I-frame of GOP  2  is no longer available. Channel Pack- 0   120  is decoded and rendered on the display  70  ( FIG. 1 ). The viewer thus sees a nearly immediate response to the channel change request. It will be noted that time shift  106  is large relative to time shifts  108 ,  110 ,  112 . As the synchronization component  66  is now processing channel packs, it issues another get_pack request. 
     At time t 1  channel pack- 1   122  arrives at the synchronization component  66  and is processed in the same manner as channel pack- 0   120 . It will be noted that the time interval between arrivals of channel pack- 0   120  and channel pack- 1   122  is shorter than that between passages of consecutive GOPs of the conventional video stream through the synchronization component  66 . This is possible as the channel packs are decimated, as described above. Thus, the channel packs can be handled by a communication channel having restricted bandwidth more easily than the conventional video stream, and in the case of decreased framing rate, can be rendered more rapidly in the receiving device  44  ( FIG. 1 ). 
     At time t 2  channel pack- 2   124  is received and decoded as above. The time shift  110  is now approximately half as large as the time shift  106 . 
     At time t 3  channel pack- 3   126  is received. The time shift  112  is minimal, and a crossing-point has been reached. The occurrence of a crossing point may be defined as a time shift that is less than a threshold value  128 . Alternatively, a crossing point may be defined as a zero or negative value of the time shift between the channel pack and the corresponding segment of the conventional video stream of the new channel. In any case, once the crossing point has been identified, the synchronization component  66  ceases to issue get_pack requests and the receiving device  44  begins to process the conventional video stream of the new channel, thereby completing the channel change operation. 
     Reverting to  FIG. 1 , in order to adequately service the get_pack requests, the channel pack module  16  may cache different numbers of channel packs for the various channels available to the receiving device  44 . Caching the three most recent GOPs is typical. In general, there is a trade-off between the resources required to cache the data in the channel pack module  16  and the channel database  42  and the quality of service ultimately provided to the receiving device  44 . If too few channel packs are cached, there may be a delay in the video presentation of the new channel, or dropped frames may occur. If too many are cached, the expense of the hardware may become a limiting factor. Nevertheless, in one embodiment, intentional frame dropping may he used in order to reduce the expense of channel pack assembly and storage in the channel pack module  16  and channel database  42 . While the user&#39;s perception of seamless channel change is somewhat impaired because of the reduced frame quality of the channel packs, this variant satisfies an immediate need to view at least some image data of the new channel. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.