US20100138883A1 - Method and apparatus for correcting corrupted digital video transport streams - Google Patents
Method and apparatus for correcting corrupted digital video transport streams Download PDFInfo
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- US20100138883A1 US20100138883A1 US12/698,951 US69895110A US2010138883A1 US 20100138883 A1 US20100138883 A1 US 20100138883A1 US 69895110 A US69895110 A US 69895110A US 2010138883 A1 US2010138883 A1 US 2010138883A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N21/00—Selective content distribution, e.g. interactive television or video on demand [VOD]
- H04N21/20—Servers specifically adapted for the distribution of content, e.g. VOD servers; Operations thereof
- H04N21/21—Server components or server architectures
- H04N21/218—Source of audio or video content, e.g. local disk arrays
- H04N21/2181—Source of audio or video content, e.g. local disk arrays comprising remotely distributed storage units, e.g. when movies are replicated over a plurality of video servers
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/85—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using pre-processing or post-processing specially adapted for video compression
- H04N19/89—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using pre-processing or post-processing specially adapted for video compression involving methods or arrangements for detection of transmission errors at the decoder
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N21/00—Selective content distribution, e.g. interactive television or video on demand [VOD]
- H04N21/20—Servers specifically adapted for the distribution of content, e.g. VOD servers; Operations thereof
- H04N21/21—Server components or server architectures
- H04N21/222—Secondary servers, e.g. proxy server, cable television Head-end
- H04N21/2225—Local VOD servers
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N21/00—Selective content distribution, e.g. interactive television or video on demand [VOD]
- H04N21/20—Servers specifically adapted for the distribution of content, e.g. VOD servers; Operations thereof
- H04N21/23—Processing of content or additional data; Elementary server operations; Server middleware
- H04N21/238—Interfacing the downstream path of the transmission network, e.g. adapting the transmission rate of a video stream to network bandwidth; Processing of multiplex streams
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N21/00—Selective content distribution, e.g. interactive television or video on demand [VOD]
- H04N21/20—Servers specifically adapted for the distribution of content, e.g. VOD servers; Operations thereof
- H04N21/23—Processing of content or additional data; Elementary server operations; Server middleware
- H04N21/239—Interfacing the upstream path of the transmission network, e.g. prioritizing client content requests
- H04N21/2393—Interfacing the upstream path of the transmission network, e.g. prioritizing client content requests involving handling client requests
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N21/00—Selective content distribution, e.g. interactive television or video on demand [VOD]
- H04N21/20—Servers specifically adapted for the distribution of content, e.g. VOD servers; Operations thereof
- H04N21/23—Processing of content or additional data; Elementary server operations; Server middleware
- H04N21/24—Monitoring of processes or resources, e.g. monitoring of server load, available bandwidth, upstream requests
- H04N21/2402—Monitoring of the downstream path of the transmission network, e.g. bandwidth available
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N21/00—Selective content distribution, e.g. interactive television or video on demand [VOD]
- H04N21/20—Servers specifically adapted for the distribution of content, e.g. VOD servers; Operations thereof
- H04N21/23—Processing of content or additional data; Elementary server operations; Server middleware
- H04N21/24—Monitoring of processes or resources, e.g. monitoring of server load, available bandwidth, upstream requests
- H04N21/2404—Monitoring of server processing errors or hardware failure
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- Multimedia (AREA)
- Signal Processing (AREA)
- Databases & Information Systems (AREA)
- Two-Way Televisions, Distribution Of Moving Picture Or The Like (AREA)
- Data Exchanges In Wide-Area Networks (AREA)
Abstract
A digital video system, method, and computer program product for correcting corrupted digital video transport streams is described. The digital video system may include a digital cable, digital broadcast satellite (DBS), high definition television (HDTV), or video-on demand (VOD) system. In a VOD system, a VOD client at each receiver is capable of subscribing to one or more VOD sessions. The headend includes a video server and a control server that is coupled to the video server. The video server transmits one or more VOD sessions to one or more receivers over a transport stream. The control server can also cause the video server to transmit one or more dummy sessions to maintain a predetermined minimum bandwidth of content over the transport stream. Alternatively, the headend can transmit one or more null packets having a randomized payload, as necessary, to maintain a constant bit rate over the transport stream.
Description
- This application is a continuation of U.S. patent application Ser. No. 09/881,609 filed Jun. 14, 2001, which claims priority under 35 USC 119(e) to U.S. Patent Application No. 60/211,881 filed Jun. 14, 2000; both of which are incorporated herein in their entirety by this reference.
- 1. Field of the Invention
- The present invention relates generally to television systems, and specifically, to a method and apparatus for correcting corrupted digital video transport streams using a constant minimum bandwidth allocation, randomized null packet payloads, and/or framing validity technique.
- 2. Background Information
- The advancement of technology has facilitated the development and transmission of digital programming to consumers. One such programming is video on demand (VOD). VOD is a service that is offered by, for example, cable companies which allows customers, for a fee, to order and view movies or other programming at any time, typically by using the consumer's set top box. VOD thus offers flexibility by allowing the user to decide when to watch VOD programs. VOD offers a further advantage of allowing the user to pause the VOD program when the user is interrupted by a telephone call, doorbell, etc.
- However, VOD service may occasionally cause reception problems. When a VOD program is selected for download to a client receiver, the video services received and decoded may be obscured by artifacts. The degree of obscuration may be total yielding the program content unwatchable. The problem is most prevalent when only a single session is being supported by the system on an RF channel and virtually guaranteed to occur when that single session has been paused and the customer temporarily tunes away. The problem occurs immediately after channel change to the VOD transport stream.
- In one embodiment, a digital video system includes a headend that is coupled to a plurality of receivers over a transmission channel. A client at each receiver is capable of subscribing to one or more video sessions. The headend includes a video server and a control server that is coupled to the video server. The video server can transmit one or more video sessions to one or more receivers over a transport stream. The control server can dynamically allocate and terminate video sessions over the transport stream as video clients are added and terminated. The control server can also cause the video server to transmit one or more dummy sessions over the transport stream to maintain a minimum bandwidth of content over the transport stream.
- In another embodiment, a digital video method includes receiving requests from one or more clients for one or more VOD sessions, transmitting one or more video sessions to the one or more clients over a transport stream having a constant bit rate, and transmitting one or more null packets, as necessary, to maintain the constant bit rate over the transport stream. The one or more null packets include a randomized payload.
- Other embodiments are described and claimed herein.
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FIG. 1 illustrates a block diagram of a digital video system, according to one embodiment. -
FIG. 2 illustrates an exemplary block diagram of a receiver. -
FIG. 3 shows ambiguous packet start locations. -
FIG. 4 illustrates an exemplary block diagram of a video-on-demand (VOD) system suitable for implementing one or more embodiments. -
FIG. 5 illustrates another exemplary block diagram of a VOD system suitable for implementation with one or more embodiments. - As discussed herein, various terms are used to describe certain elements or characteristics of the present invention. For example, a “transmission channel” is broadly defined as any communication path between a source and a destination. The communication path may include one or more information-carrying lines (electrical wire, fiber optics, cable, HFC, etc.) or wireless communications through established techniques such as infrared (IR), radio frequency (RF), satellite, etc.
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FIG. 1 illustrates a block diagram of adigital video system 100, according to one embodiment. The transmission format of television signals in thesystem 100 is specified, in one embodiment, by the Digital Multi-Programme Systems for Television, Sound and Data Services for Cable Distribution, ITU-T Recommendation J.83, annex “B”, April 1997 (hereinafter referred to as “ITU-J.83, annex B”). The entire contents of ITU-T Recommendation J.83 including Annexes A, B, C, and D are incorporated herein by reference. For sake of clarity, the present invention will be described with respect to a video-on-demand (VOD) system. However, it is to be understood that the present invention is not limited to VOD systems. That is, embodiments of the present invention may be implemented in digital cable systems, digital broadcast satellite (DBS) systems, high definition television (HDTV) systems, and any other digital video system based on an MPEG transport format. - Referring to
FIG. 1 , thesystem 100 includes aheadend 110 coupled to areceiver 140 over atransmission channel 130. Theheadend 110 may be operated and/or controlled by a broadcaster. The headend includes a video-on-demand (VOD) control server (not shown), one or more VOD servers 112 1-112 N (where “N” is a positive whole number), a multiplexer (mux) 116, andtransmitter 120. EachVOD server 112 may generate one or more VOD programs (or VOD streams). The output(s) of the VOD servers 112 1-112 N are coupled to themux 116, which multiplexes the VOD streams at its input(s) onto an MPEG-2 transport stream (or other MPEG version transport stream) onsignal line 118. Themux 118 is controlled by the VOD control server via signal line(s) 128. - The MPEG-2 transport stream on
signal line 118 includes packets of data having 188 bytes. The MPEG-2 transport stream onsignal line 118 is fed to anMPEG framing block 122, which organizes data into groups of fixed symbols, to provide packet synchronization for the receiver. TheMPEG framing block 122 provides the frames to a Forward Error Correction (FEC)encoder 124. TheFEC encoder 124 uses error correction algorithms and techniques for transporting data reliably over thetransmission channel 130. - Also shown in
FIG. 1 are an embodiment of theFEC encoder 124 intransmitter 120 andFEC decoder 144 inreceiver 140. The FECencoder 124 includes a Reed-Solomonencoder 152,interleaver 154,randomizer 156, andtrellis encoder 158, whereas theFEC decoder 144 includes atrellis decoder 160, de-randomizer 162, de-interleaver 164, and Reed-Solomondecoder 156. The Reed-Solomonencoder 152 provides block encoding for correcting symbols within Reed-Solomon blocks. Theinterleaver 154 evenly disperses symbols to prevent a burst of symbol errors from being transmitted to a Reed-Solomon decoder. Therandomizer 156 randomizes the data to allow effective quadrature amplitude modulation (QAM) demodulator synchronization. Thetrellis encoder 158 allows the introduction of redundancy to improve the threshold signal to noise ratio. Thetransmitter 120 further includes aQAM modulator 126 which modulates the television signals. - The
receiver 140 includes aQAM demodulator 142,FEC decoder 144, andMPEG framing 146. The output of theMPEG framing block 146 is an MPEG-2 transport stream onsignal line 148. -
FIG. 2 illustrates an exemplary block diagram of areceiver 200. Thereceiver 200, in general, receives broadcast signals and performs operations on the broadcast signal to produce digital and/or analog information. For example, thereceiver 200 is responsible for decoding the bit stream, for storing programming data in memory accessible by software executed by thereceiver 200, and for processing the decoded bit stream to produce one or more output signals having appropriate formats. Thereceiver 200 may be incorporated into many systems such as a cable box for a Cable Broadcasting System, an Internet terminal, a digital satellite system (DSS) computer, and the like. - Referring to
FIG. 2 , thereceiver 200 includes adiplexer 210 which receives the broadcast signals transmitted from thetransmitter 110 over thetransmission channel 120. Thediplexer 210 forwards the broadcast signals to both an in-band (IB)tuner 212 and an out of band (OOB)tuner 214. TheIB tuner 212 tunes the in-band broadcast signals (e.g., audio and/or video) to a desired broadcast channel. TheOOB tuner 214 tunes to a private data channel which includes information for billing, encryption, and, in general, remote management of thereceiver 200. - Both the audio/video signals and the private data signals are transmitted to a modulator/demodulator (MOD/DEMOD) 220, which performs, among other things, the functions of the
QAM demodulator 142,FEC decoder 144, and MPEG framing 146 (FIG. 1 ). The MOD/DEMOD 220 converts the analog media signals to digital data and reformats the digital data to an appropriate format to provide a digital media stream (e.g., MPEG-2 transport stream). The bit stream is formatted in accordance with any video compression function and may be encrypted under either a symmetric key cryptographic function or a public-key cryptographic function. Typically, the bit stream includes sensory data (e.g., video and/or audio, or communication data) and control information for a number of shows. The control information for each show includes programming data including, but not limited or restricted to, date of broadcast, broadcast channel number, show start-time, show end-time, show title, show type (e.g., sports, news, etc.), etc. - The MOD/
DEMOD 220 also converts the private signals to private digital data. The MOD/DEMOD 220 then forwards the digital media stream and the digital private data to a transport demultiplexer (demux) and graphics block 228 onsignal lines 222 and 224, respectively. The transport demux 228 extracts the desired audio/video (media) stream intended for thereceiver 200, if any, and forwards the media stream to anMPEG decoder 232 viasignal line 230. The transport demux 228 may also perform decryption of the filtered and/or demultiplexed stream. - The
MPEG decoder 232 decodes the filtered and decrypted stream into its native decompressed audio and video format (e.g., 4:4:4 or 4:2:2 digital video and pulse code modulation “PCM” audio). TheMPEG decoder 232 transmits the digital audio to an audio digital to analog converter (DAC) 240 viasignal line 236. Theaudio DAC 240 converts the digital audio stream to analog signals, and transmits the same to ananalog switch 244. TheMPEG decoder 232 also transmits the digital video stream (e.g., in accordance with the CCIR 656 format) back to the graphics section of thetransport demux 228 onsignal line 234. The graphics section of thetransport demux 228 performs rendering, compositing, superposition of graphics (e.g., menus, icons, etc.) on the video, and other graphics processing. It is to be noted that although the graphics processing hardware is combined with the transport demux hardware in one device (i.e., device 228), the graphics processing hardware may be implemented in a separate stand alone device. - The
IB tuner 212 also provides the traditional television signals to ademodulator 250. In one embodiment, the format of the television signals may be in accordance with a video format established by National Television Systems Committee (NTSC). In other embodiments, the format may be in accordance with Phase Alternating Line (PAL), Sequential Couleur Avec Memoire (SECAM), etc. Thedemodulator 250 demodulates the analog signal and provides audio signals to adigital attenuator 254 and the video signals to a NTSC decoder andADC 252. Thedigital attenuator 254 attenuates the audio signal and provides the same to theanalog switch 244. The NTSC decoder andADC 252 decodes the NTSC signals, converts the decoded signals to a digital video stream in a specified format (e.g., CCIR 656 format), and transmits the same to the graphics portion of thetransport demux 228 on signal line 256. - The
receiver 200 also includes a central processing unit (CPU) 260 which may include a microprocessor, microcontroller, ASIC, etc. TheCPU 260 is coupled to avolatile memory 264 andnon-volatile memory 266, via abus bridge 262. Thevolatile memory 264 may include a random access memory (RAM) and thenon-volatile memory 266 may include read-only memory (ROM), flash memory, etc. Thenon-volatile memory 266 includes interrupt service routines, menus, event data, programming data, software modules and routines, etc. for execution by theCPU 260 to control the operation of thereceiver 200. TheCPU 260 is coupled to aninterface circuit 268 which interfaces with the devices (e.g., MOD/DEMOD 220,transport demux 228,MPEG decoder 232, etc) over asystem bus 272, allowing theCPU 260 to control the devices, check status, etc. - The
receiver 200 further comprises afront panel 270 which includes a display, and keys or switches for receiving user inputs or commands. The display may include a seven-segment, LCD, or other display for displaying the channel number, time, and/or other information such as an event scheduled or programmed in a menu. Thefront panel 270 also receives commands from a remote controller (not shown), and decodes the commands to produce interrupt request signals (IRQs) corresponding to the commands. Each IRQ may be transferred to theCPU 260. - The
interface 268 may be connected to other external peripheral devices. For example, the interface may include circuitry which complies with the IEEE standards document 1394 entitled “Standard for High Performance Serial Bus” (hereinafter referred to as “IEEE 1394”). This enables thereceiver 200 to connect to digital-input peripheral devices such as digital VCRs, digital video disk players, digital laser disk players and the like. - The graphics portion of the
transport demux 228 transmits the traditional digital video signals (from the NTSC decoder and ADC 252) or the digital video signals (from the MPEG decoder 232) to avideo encoder 242 oversignal line 238, based on user input andCPU 260. The graphics portion of thetransport demux 228 may receive data representative of menus from theCPU 260, and overlay or otherwise blend the menus with the digital signal. Thevideo encoder 242 converts the digital video signal to analog form on the Video Out signal line. The Video Out and Audio Out signal lines of thereceiver 200 are coupled to at least one analog-input peripheral device such as a display monitor (e.g., a television receiver having a display screen) and/or a recording device (e.g., a video cassette recorder “VCR”). - The ITU-J.38 annex B specifies two distinct and concurrent types of data synchronization in the
system 100. These include FEC synchronization and MPEG frame synchronization. FEC synchronization is independent of the packet size and is performed by thetransport demux 228 on the incoming stream of symbols from the QAM demodulator of the MOD/DEMOD 220. The frame size for the FEC frame in 256 QAM modulation format is 11,264 symbols (7 bits per symbol) followed by a 40 bit sync word (“trailer”). This structure is used for establishing the correct starting point of a data sequence for the FEC decoder (derandomizer, deinterleaver, and Reed-Solomon decoder). The second type of synchronization is MPEG synchronization, performed by theMPEG decoder 232, which provides 188 byte packet framing compliant with the MPEG2 transport standard, as defined by Generic Coding of Moving Pictures and Associated Audio Information: Systems (ISO/IEC 13818-1:1996). - From the perspective of
receiver 200, the FEC synchronization process is relatively robust with a 2E-10 probability of finding a data sequence that mimics the sync trailer exactly. The probability is reduced further because the FEC frame is further subdivided into blocks of data (122 symbols) with 6 additional parity symbols added per block. In all, a FEC frame of 11,264 symbols includes 88 blocks (128 symbols per block). A count of 88 blocks and a 40 bit frame sync trailer with its characteristic signature would have to be received to truly qualify as a valid frame. Consequently, the chances of false FEC synchronization are remote. The fact that FEC synchronization also transcends packet boundaries further adds to the robustness of the FEC frame protocol. - MPEG frame synchronization, on the other hand, may, in certain situations, cause unrecoverable synchronization errors. MPEG frame synchronization uses a sliding checksum on a bit-by-bit basis to find the start of an MPEG 188 byte packet. The synchronization process involves evaluating the sum of the previous 1496 received bits (187 bytes) and comparing it to the current received byte (byte 188). If the sum of the 187 bytes matches the checksum of the 188th byte, the
MPEG decoder 232 records the match and confirms that the sum calculated over the next 1496 bits (187 bytes) matches the value of the 188th byte. The system keeps statistics regarding how many consecutive matches were obtained over the total number of packets (bytes) evaluated and a threshold can be set to establish a framing “lock” condition. This process has a 1/256 (or 0.4%) chance of yielding a false match on any single 188 byte sample of data. The probabilities of multiple false matches are reduced by the fact that the preceding FEC process is assumed to yield data free of errors due to channel noise and that MPEG data itself has a vary random distribution. The chance of seeing a false checksum match on consecutive packets becomes more improbable as the threshold for matches is increased. If one considers the probability of a random match on the checksum of four consecutive packets, the probability becomes about 2E-10. - Indeed, the correct operation of the MPEG synchronization process is based upon the paradigm that compressed audio and/or video data has a random appearing distribution and that the data payloads of consecutive packets are never repeated. This paradigm is quite valid in high bandwidth video or when statistical multiplexing algorithms are employed on a transport stream to maximize bandwidth utilization of the channel.
- This, however, is not the case for ITU-J.83 annex B based constant bit rate (CBR) systems, especially for VOD applications. In VOD applications, the RF channels are dynamically allocated to clients and as new clients are added, new RF channels are instantiated, and packet identifiers (PIDs) assigned and allocated on a first-come, first-serve basis. In this type of applications, it is quite likely to have as few as a single client on a transport stream at any one time. In all cases, the
VOD system 100 by nature has a constant symbol rate. To maintain the constant symbol rate, theheadend 110 inserts stuffing packets (null packets) implemented as an optional null packet generator 114 (shown by dashed lines) that is input to themux 116, embedded inside themux 116, or embedded inside the transmitter 120 (e.g., at the QAM modulator 118). The MPEG transport specification (Generic Coding of Moving Pictures and Associated Audio Information: Systems (ISO/IEC 13818-1:1996)) defines a unique PID for the purpose of null packets and assigns the ID “0x1FFF” for a null packet. The MPEG standard specifically indicates that the data payload for a null packet may be any value. Typically, the payload bytes are set to 0xFF and it has been said that this is an implementation side effect of an MPEG requirement that the padding bytes used in section data be 0xFF. - The MPEG framing checksum is actually a linear block code and is designed such that when data is correctly recovered the resultant syndrome byte (from the CRC checksum) is equal to 0x47, the traditional MPEG2 synchronization byte value. Because the checksum is based upon a linear feedback shift register, the actual algorithm employed will result in six discrete points (phases) when a consecutive sequence of null packets with a payload of 0xFF are processed as points of MPEG frame synchronization, as illustrated in
FIG. 3 . However, only one point provides the correct MPEG frame synchronization. The other five points are ambiguous and while satisfying the mathematical criteria for the checksum, do not align with the correct MPEG2 transport packet boundary. - As the density of null packets increases, the probability of synchronizing to an ambiguous and incorrect MPEG frame approaches the limit of ⅚ (or 83%). This will result in long periods to tune to a high null packet stream and the potential for never actually synchronizing correctly. Analysis of a cable network implementing a VOD system indicates that as few as 180K bits/second of non-null data might be sent on a 256 QAM transport running at 38M bits/second, a null packet density of 99.53%.
- An additional parameter that may come into play to further compound the MPEG frame synchronization is the type of multiplexer used. Evaluations of certain multiplexers show that there may be a direct correlation between the onset of the synchronization problem and the brand of multiplexer. Further investigation has indicated that the reason for this is due to the algorithm used in the multiplexer itself when creating the MPEG transport stream. The MPEG specification is not clear on how data is multiplexed regarding packet distribution. In at least one type of multiplexer algorithm, data is placed in the transport stream in bursts as opposed to placing date in the transport stream in a uniform and homogeneous distribution. When data is not evenly distributed, the concentration of null packets is locally increased due to the burst nature of the multiplexer algorithm. Consequently, the type of multiplexer algorithm used may exasperate the synchronization problem, making the number of repeated null packets appear to be greater than is actual and therefore reduce the threshold to the onset of the problem.
- In the implementation of a transport decoder based upon a certain brand of demodulator integrated circuit, the problem is further compounded because there is no direct notification to the host of the frame synchronization problem. It is quite easy for the demodulator to detect the condition because it has internal status regarding the condition of FEC synchronization, and can also determine whether the Reed-Solomon module has been able to correct all data errors detected. In the case of false MPEG frame synchronization due to high null packet densities, the FEC system will indicate normal operation, yet any non-null packets passing out of the demodulator will fail the checksum due to the framing error. In this demodulator, the transport error bit in the stream output (TE=1) is set and it is up to the MPEG decoder to determine what action to take. In many cases, this will result in no special action. In addition, upon query of the demodulator, all internal subsystems will report normal conditions, creating yet another ambiguity.
- Based upon the evaluation of the issue and an analysis of the cause, it becomes apparent that the VOD system is operating in a mode that is outside the intent or capability of the ITU-J.83, annex “B” standard. Even demodulators that have no reporting ambiguity, as described above, are prone to the framing error problem and will exhibit high error rates, long tuning times and high probability of onset even with multiple re-acquisitions. The problem in general has been shown both by analysis and through empirical data to be systemic in nature and not unique to any one combination of multiplexer, modulator, or demodulator components. The problem can also be recreated under the correct conditions using live encoders instead of a VOD system, although the conditions that cause the problem in a live encoder environment are highly unlikely to occur in real deployment. The opposite is true however for a VOD implementation using dynamic allocation of PIDs in a CBR environment. The problem is not only likely, but rather, should be expected.
- Disclosed herein are three separate and independent techniques for correcting the MPEG frame synchronization problem. In a first technique, the headend maintains a constant minimum bandwidth allocation to reduce the number of null packets in the transport stream. In constant minimum bandwidth allocation, the headend system controlling assignment of VOD clients to RF carriers and PIDs insures that each RF channel and its associated transport stream has a minimum non-null content at all times.
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FIG. 4 illustrates an exemplary block diagram of a video-on-demand (VOD)system 400 suitable for implementing one or more embodiments. Referring toFIG. 4 , theVOD system 400 includes aheadend 410 that is coupled to aplurality receivers 435 1 to 435 A (where “A” is a positive whole number) over a hybrid fiber coax (HFC)network 430. Each of thereceivers 435 1 to 435 N is capable of subscribing to VOD programming for display on an attached television (or monitor) 440. In one embodiment, thereceivers 435 1 to 435 N may be identical to the receiver 200 (FIG. 2 ). Theheadend 410 generates and/or transmits VOD sessions having VOD programming over one or more transport streams to subscribing clients. Theheadend 410 may dynamically allocate and de-allocate VOD sessions over one or more transport streams assigned to one or more RF channels as subscribers atreceivers 435 are added and deleted. - The
headend 410 includes aVOD control server 415, one or moreVOD video servers 420 1 to 420 N, a multiplexer (mux) 414, andtransmitter 425. Although one transmitter is shown, theheadend 410 may include a plurality of transmitters. TheVOD control server 415 includes one or more processors, memory, and software programs running thereon for processing requests by subscribing clients for VOD programming (via the client'sreceiver 435 or set top box), assigning and allocating VOD sessions on one or more transport streams corresponding to one or more RF channels and PIDs to clients on the fly, and controlling the one or moreVOD video servers 420 1 to 420 N for transmitting VOD programming to subscribers. The one or moreVOD video servers 420 1 to 420 N each includes one or more processors, memory, mass storage, etc. for storing and transmitting VOD programming to subscribing clients in thesystem 400, in response to control signals received from theVOD control server 415. TheVOD video server 415 may include other components and perform other functions. For example, theVOD video server 415 may include a billing database for processing billing for VOD subscribers. In one embodiment, thetransmitter 425 is substantially identical to the transmitter 120 (FIG. 1 ). - In accordance with the first technique, the
VOD control server 415 insures that each RF channel and its associated transport stream has a minimum non-null content at all times. The actual minimum content varies from one system to another and depends upon factors such as video bit rate, modulation constellation, multiplexer homogeneity, downstream bit error rate (BER), etc. To maintain a minimum non-null content for each RF channel at all times, theVOD control server 415 creates dummy copies of purchased content and/or activates “padding streams” (hereinafter referred to as “dummy sessions”). The dummy sessions cause the video server(s) 420 to spool out content to PIDs. However, no client is assigned or authorized to access the content. This would create non-null content on the transport stream that is above the threshold for minimum content. When client demand for channel bandwidth exceeds the minimum bandwidth (e.g., a threshold), the session streams can be terminated and the bandwidth assigned to revenue producing session streams. A software module running on the VOD control sever 415 performs the function of causing the one ormore video servers 420 1 to 420 N to create one or more padding streams or dummy sessions, if necessary, to maintain a minimum bandwidth. This may also be accomplished through hardware. - The second technique involves filling null packets with a random or pseudo-random payload. In this embodiment, a random number generator is utilized for this purpose. Alternatively, the null packets may be replaced with a plurality of predetermined static packets (e.g., a plurality of different and predefined packets). This alternative technique dispenses with the requirement of generating random numbers for the packet payloads on the fly. The use of a random or pseudo random number generator or the use of static packets is generally shown as
block 510. The random number generator may also be implemented as a software routine running on one ormore video server 420. Alternatively, the modifiednull packet generator 510 may be internal to themux 414 or internal to thetransmitter 425. - In one embodiment, the random number generator has a period greater than (188*n) bytes. The robustness of such a system would be directly proportional to the number (n) of packets before the algorithm would repeat the “random” sequence. The greater the period, the more tolerant the system may be to channel noise, while minimizing sensitivity to the null padding issue. This technique may have no impact on the stream session
VOD control server 415, but may significantly affect the implementation of the multiplexer and/or modulator hardware and software. This technique guarantees compliance with the spirit of annex “B” regardless of utilization of the transport stream bandwidth. - The third technique is a countermeasure that does not prevent the conditions leading to MPEG framing synchronization errors, but provides a mechanism for detecting MPEG framing errors and responding to it.
- Referring to
FIG. 2 , the MOD/DEMOD 220 adds a circuit to its internal logic that implements the following condition: -
IRQ=!FEC_SYNC*!UNCORRECTABLE_RS_ERROR_FLAG*MPEG_CS_ERROR, (1) - FEC_SYNC is a variable used to indicate an error condition in one or more of the QAM demodulator, Trellis Decoder, De-Randomizer, and De-Interleaver, which error may be due to data corruption, a synchronization problem, noise, etc.,
- UNCORRECTABLE_RS_ERROR_FLAG is a variable used to indicate an error condition in the Reed-Solomon Decoder due to an unsuccessful attempt to correct errors in the stream, and
- MPEG_CS_ERROR is a variable used to indicate a bad checksum or checksum error in the MPEG framing block.
- If the condition in equation (1) is met (i.e., only the MPEG_CS_ERROR is asserted to indicate a false lock), the MOD/
DEMOD 220 generates an interrupt (e.g., hardware maskable interrupt) to theCPU 260 upon an uncorellated error between the FEC and the MPEG framer, which would occur whenever the system incorrectly synchronized and attempted to process non-null transport data. In response, theCPU 260 may attempt to reinitialize/resynchronize the MOD/DEMOD 220, perhaps with a different combination of register settings to change the MPEG framing criteria(rion). This would allow thereceiver 200 to be optimized for minimum channel change time through reduced demodulator acquisition time, and provide a mechanism to detect framing errors and to have applied more robust synchronization thresholds to attempt to reduce the probability of a false lock. - The first and second techniques may be implemented either individually or together for additional robustness. The third technique, which involves the demodulator, may be implemented with one or both of the first and second techniques.
- It is to be appreciated that embodiments of the present invention may be implemented in various VOD systems.
FIG. 5 illustrates another exemplary block diagram of aVOD system 500 suitable for implementing one or more embodiments. Thesystem 500 includes aheadend 510 that includes aVOD control server 515. TheVOD control server 515 is coupled to one or more remote servers 525 1-525 C (where “C” is a positive whole number) over a packet switchednetwork 520. Each remote server 525 may include one or more VOD video servers, one or more muxs, and one or more transmitters similar to the arrangement shown inFIG. 4 . TheVOD control server 515 may process requests by clients for VOD programming, assign and allocate transport streams on RF channels and PIDs to clients on the fly, transmit media streams to the remote servers 525 1-525 C, and control the remote servers 525 1-525 C over a control channel for transmitting VOD programming to subscribers. TheVOD control server 515 may cause the remote servers 525 1-525 C to create dummy sessions or padding streams and spool out content to PIDs, without assigning clients to the PIDs. In another embodiment, each remote server 525 may separately and locally execute a process for creating dummy sessions and/or padding streams on the fly, according to the first embodiment. This frees up theVOD control server 515 to perform more system-wide tasks. Each remote server 525 may further include a random number generator device or process for filling null packets with a random or pseudo-random payload, according to the second embodiment. - The packet switched
network 520 supports many different protocols such as TCP/IP for transmitting packets and asynchronous transfer mode (ATM) cells between theVOD control server 515 and remote servers 525 1-525 C. In one embodiment, the packet-switchednetwork 520 is the Internet and/or an Intranet. - The remote servers 525 1-525 C may be located at any physical point within the system. For example, the remote servers 525 1-525 C may be located at hubs, and each provides VOD programming to a multitude of clients at receivers 535 1-535 B over the
HFC network 530. In another embodiment, theheadend 510 may be substantially identical to theheadend 410 to also spool out VOD programming to subscribing clients. - Embodiments of the present invention may be implemented as a method, apparatus, system, computer program product, signal, etc. When implemented in software, the elements of the present invention are essentially the code segments to perform the necessary tasks. The program or code segments can be stored in a processor readable medium or transmitted by a computer data signal embodied in a carrier wave over a transmission channel or medium. The “processor readable medium” may include any medium that can store or transfer information. Examples of the processor readable medium include an electronic circuit, a semiconductor memory device, a ROM, a flash memory, an erasable ROM (EROM), a floppy diskette, a CD-ROM, an optical disk, a hard disk, a fiber optic medium, a radio frequency (RF) link, etc. The computer data signal may include any signal that can propagate over a transmission medium such as electronic network channels, optical fibers, air, electromagnetic, RF links, etc.
- While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art.
Claims (23)
1. A video-on-demand (VOD) system, comprising:
a headend coupled to a transmission channel, said headend comprising:
a video server that transmits, over the transmission channel, one or more VOD sessions to one or more receivers, where each receiver comprises a VOD client capable of subscribing to one or more VOD sessions over a transport stream; and
a control server coupled to the video server, the control server is adapted to dynamically allocate and terminate VOD sessions over the transport stream as VOD clients are added and terminated, and to cause the video server to transmit one or more dummy sessions over the transport stream such that a bandwidth over the transport stream is substantially constantly maintained above a predefined minimum bandwidth of content.
2. The VOD system of claim 1 , wherein the control server is further adapted to transmit one or more dummy sessions over the transport stream to maintain the minimum bandwidth of content over the transport stream to ensure that each receiver can synchronize to a subscribed VOD session.
3. The VOD system of claim 1 , wherein the control server is adapted to determine whether the bandwidth of content over the transport stream is below a predetermined threshold, and to cause the video server to transmit the one or more dummy sessions to maintain the bandwidth of content over the transport stream at or above the predetermined threshold.
4. The VOD system of claim 1 wherein each receiver includes a demodulator, decoder, and an MPEG frame synchronizer.
5. The VOD system of claim 1 wherein said headend further comprises a transmitter having an MPEG frame synchronizer, encoder, and modulator.
6. The VOD system of claim 1 wherein the transport stream is transmitted over a radio frequency channel.
7. The VOD system of claim 1 wherein the video server is further configured to transmit the one or more VOD sessions over one or more radio frequency (RF) channels each associated with a different transport stream, and wherein said control server is configured to cause the video server to transmit one or more dummy sessions over each transport stream to substantially constantly maintain the predetermined minimum bandwidth of content over each of the one or more transport streams.
8. A video-on-demand (VOD) server, comprising:
a server that receives requests from one or more VOD clients for one or more VOD sessions, causes transmission of one or more VOD sessions over a transport stream to the one or more VOD clients, determines whether the number of VOD sessions transmitted over the transport stream is below a minimum threshold, and causes transmission of one or more padding sessions over the transport stream if the number of VOD sessions transmitted over the transport stream is below the minimum threshold to maintain the number of VOD sessions at or above the minimum threshold.
9. The VOD server of claim 8 wherein the server causes transmission of VOD sessions over a plurality of radio frequency channels each associated with a transport stream, the server determines, for each transport stream, whether the number of VOD sessions is below the minimum threshold, and, for each transport stream, causes transmission of one or more padding sessions if the number of VOD sessions transmitted over the respective transport stream is below the minimum threshold to substantially constantly maintain the number of VOD sessions at or above the minimum threshold.
10. The VOD server of claim 8 wherein when the server receives a request for a new VOD session from a VOD client, the control server terminates one or more of the one or more padding sessions, and causes transmission of the new VOD session.
11. A video-on-demand (VOD) system, comprising:
a transmission channel;
a plurality of receivers coupled to the transmission channel, a VOD client at each receiver capable of subscribing to one or more VOD sessions over a transport stream; and
a headend coupled to the transmission channel, said headend including a video server than can transmit one or more VOD sessions to one or more receivers, and a control server coupled to the video server, the control server to dynamically allocate and terminate VOD sessions over the transport stream as VOD clients are added and terminated, said headend to transmit one or more stuffing packets to maintain a substantially constant bit rate over the transport stream, said one or more stuffing packets including a randomized payload.
12. The VOD system of claim 11 wherein the headend generates the one or more randomized payloads using one of a random number generator, pseudo-random number generator, and predetermined static payload.
13. The VOD system of claim 11 wherein the video server can transmit one or more VOD sessions over one or more radio frequency (RF) channels each associated with a transport stream, and wherein said headend to transmit one or more stuffing packets for each transport stream to maintain the substantially constant bit rate over the one or more transport streams, said one or more stuffing packets including a randomized payload.
14. The VOD system of claim 13 wherein the headend includes a random number generator that generates a plurality of random numbers, said plurality of random numbers being used to fill the payloads of the one or more stuffing packets.
15. A video on demand (VOD) method, comprising:
receiving requests from one or more VOD clients for one or more VOD sessions;
transmitting one or more VOD sessions to the one or more VOD clients over a transport stream having a substantially constant bit rate; and
transmitting one or more null packets to maintain the substantially constant bit rate over the transport stream, said null packets including a randomized payload.
16. The VOD method of claim 15 , wherein transmitting one or more null packets comprises transmitting one or more null packets to maintain the substantially constant bit rate over the transport stream, said null packets each including a randomized payload populated using one of a pseudo-random number generator, random number generator, and one of a plurality of predetermined static payloads.
17. The VOD method of claim 15 , further comprising dynamically allocating and terminating VOD sessions over the transport stream as VOD clients are added and terminated, respectively.
18. A digital video system, comprising:
a headend coupled to one or more transmission channels, said headend comprising:
a video server that transmits one or more video sessions over a transport streams on a transmission channel to one or more receivers, where the one or more receivers are capable of subscribing to one or more video sessions, and
a control server coupled to the video server, the control server adapted to cause the video server to transmit one or more dummy sessions over the transport stream to substantially constantly maintain a predetermined minimum bandwidth of content over the transport stream.
19. The digital video system of claim 18 , wherein the control server further determines whether the bandwidth of content over the transport stream is below a predetermined threshold, and causes the video server to transmit the one or more dummy sessions to ensure that the bandwidth of content is substantially continuously maintain at or above the predetermined threshold, wherein the dummy sessions comprise non-null data and cannot be accessed by the one or more receivers.
20. The digital video system of claim 18 , further comprising:
the transmission channel; and
the one or more receivers coupled to the transmission channel, where a client at each receiver is configured to subscribe to the one or more video sessions over the transport stream;
wherein the headend transmits digital video programming in accordance to one of a digital broadcast satellite (DBS) system, digital cable system, high definition television (HDTV) system, and video-on-demand (VOD) system.
21. A computer program product, comprising:
a computer usable medium having computer readable program code embodied therein to minimize a possibility of an erroneous MPEG frame synchronization in a digital video receiver, the computer readable program code in said computer program product comprising:
first computer readable program code to transmit one or more video sessions to the one or more clients over a transport stream having a substantially constant bit rate; and
second computer readable program code to transmit one or more null packets to maintain the substantially constant bit rate over the transport stream, said null packets including a randomized payload.
22. The computer program product of claim 21 , wherein the first computer readable program code comprises first computer readable program code to transmit one or more video sessions in accordance to one of a digital broadcast satellite (DBS) system, digital cable system, high definition television (HDTV) system, and video-on-demand (VOD) system.
23. A digital video receiver, comprising:
a demodulator to demodulate analog signals received over a transmission channel;
decoder to decode signals from the demodulator;
an MPEG framer to synchronize to a video session in a transport stream;
a memory including one or more instructions;
a processor, coupled to the memory, demodulator, decoder, and framer, the processor, in response to the one or more instructions, to,
detect a checksum error from the MPEG framer indicative of a false synchronization to the video session, and
responsive to detecting a checksum error, resynchronize the MPEG framer to the transport stream.
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US7020812B2 (en) * | 2002-08-12 | 2006-03-28 | Broadcom Corporation | System and method for detection and recovery of false synchronization using packet header information |
Cited By (2)
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US20060150102A1 (en) * | 2005-01-06 | 2006-07-06 | Thomson Licensing | Method of reproducing documents comprising impaired sequences and, associated reproduction device |
US9043701B2 (en) * | 2005-01-06 | 2015-05-26 | Thomson Licensing | Method and apparatus for indicating the impaired sequences of an audiovisual document |
Also Published As
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US7657913B2 (en) | 2010-02-02 |
US20020087995A1 (en) | 2002-07-04 |
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