Multiplex communications – Communication techniques for information carried in plural... – Adaptive
Reexamination Certificate
2000-05-12
2004-10-05
Nguyen, Chau (Department: 2667)
Multiplex communications
Communication techniques for information carried in plural...
Adaptive
C370S535000, C348S512000
Reexamination Certificate
active
06801544
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of converting a packetized stream of information signals representing information arranged in separate, consecutive data packets of digital format, into a stream of information signals with time stamps, the method comprising receiving the serial stream of information signals, detecting the data packets in the serial stream of information signals while establishing a time of arrival of the data packet, and generating time stamp data related to the time of arrival for each detected data packet.
The invention also relates to a method of converting such a stream of information signals with time stamps into a packetized stream. The invention further relates to converting means for performing both methods, as well to a system for storage and retrieval or transmission of such a stream of information signals.
2. Description of the Related Art
The subject matter, as described in the opening paragraph, is known from International Patent Application WO 96/30905, corresponding to U.S. Pat. No. 5,953,483, Document (D1) in the List Of Documents. More specifically, this document discloses the recording and reproduction of an information signal comprising packets that may arrive irregularly as a function of time in the serial data stream of an MPEG information signal.
Such an MPEG information signal is used for serial transmission of a digital data stream representing a compressed digital video signal and a corresponding compressed audio signal. For instance, the draft Grand Alliance HDTV System Specification dated Feb. 22, 1994, Document (D2) in the List Of Documents, more specifically, chapters V and VI of this specification, contain a description of a transport system for transmitting a MPEG information signal for broadcasting purposes or for transmission via a cable network.
The format of an MPEG information signal has been developed by the Moving Pictures Experts Group (MPEG). This group was established to develop standards for coded representation and compression of moving pictures, audio and their combination. It operates in the framework of the Joint ISO/IEC Technical Committee. Currently produced standards are MPEG-1 (ISO 11172), MPEG-2 (ISO 13818) and MPEG-4. Industries, as well as several international bodies, have adopted these standards. The standards enable interoperability in digital video and audio applications and services.
Within an MPEG serial data stream, the video and/or audio signals may be transmitted via transport stream packets having a fixed amount of bytes (188), the first byte being a synchronization byte. A transport stream packet comprises information of only one of the video signals, or one of the audio signals or one of the data signals transmitted via the MPEG serial data stream.
Synchronization of the decoding and presentation at a receiver is important for a real time digital data delivery system. This is to ensure that a video signal is presented at the proper speed, that video and audio remain synchronized, and that the decoder can properly manage its buffers. A loss of synchronization leads to either buffer overflow or underflow at a decoder, and, as a consequence, loss of information. This is different from analog information, such as, e.g., in NTSC where information for pictures is transmitted in a synchronous manner, so that one can derive a clock directly from the picture synchronization. However, in a digital compression system, the amount of data generated for each picture is variable as dependent on the picture encoding approach and complexity. Thus, timing cannot be derived directly from the start of the picture data. There is no natural concept of synchronization pulses in a digital bit stream. Therefore, the time base at the decoder side must be locked to that of the encoder. The solution is to transmit timing information with selected transport stream packets, to serve as a reference for timing comparison at the decoder.
This is done by transmitting, at regular periods, a sample of a reference clock called program clock reference (PCR). This clock reference (PCR) indicates the expected time of completion of the reading of that timestamp from the bit stream at a decoder. The phase of a local clock running at the decoder is compared to the PCR value at the instant at which it is obtained, in order to adjust the clock rate, if necessary, to determine whether the decoding process is synchronized. A MPEG transport stream can therefore be regarded as a real-time transport stream.
With the use of a second type of time stamp, called decoding time stamp (DTS) or presentation time stamp (PTS), the exact moment, relative to the above-described locked decoder clock, is indicated where a video frame or an audio frame has to be decoded or presented, respectively.
The transport stream packets will be transported across a medium. If the delay of this medium is not equal for each transport stream packet, then it is possible to corrupt the decoding time base. An extra transmission delay between two succeeding transport stream packets that contain a PCR, will cause jitter on a decoder clock. Therefore, only a specific amount of jitter is allowed by the respective standards.
A storage or recording device can also be treated as a transmission channel with infinitesimal delay. At play back, the timing between successive transport stream packets must be in such a way reconstructed that it becomes equal to the timing between succeeding transport stream packets as they arrived at the input of the recording device during recording. Further, it can generally be said that the recording or storage of a complete MPEG information signal, comprising multiple program streams, is not well possible, because of the too high data rate of the MPEG information signal. Therefore, only one or a few of the video signals and their corresponding audio signals, corresponding to a selected program stream, will, in practice, be selected for recording. However, as a consequence, transport stream packets corresponding to a specific program stream will generally be selected on an irregular basis as a function of time.
In order to preserve the timing relation between the subsequent transport stream packets selected, the previously mentioned Document (D1) describes the measure to insert time stamps in each of the transport stream packets recorded. After selection and storage, the timing relation between the subsequent packets with reproduction can be recovered using the time stamps.
According to Document (D1), the combination of time stamps and transport stream packets is incorporated into a specific data format (D-VHS MPEG-2 STD) in which data is recorded with a magnetic playback/recording system, such as a VHS-based digital videocassette recorder. This format is not a standard MPEG format representing a real-time data stream, but allows representing a non-real-time data stream. According to the D-VHS MPEG-2 STD format, recording of signal blocks representing a fixed amount of 112 bytes is allowed. Within two signal blocks of 112 bytes each, one transport stream packet of 188 bytes can be stored together with an additional corresponding time stamp of 4 bytes. (The other 32 bytes are used for other purposes, such as, for example, synchronization, identification, and parity information). This format is only used within a VHS-based digital recorder.
In applications such as can be found in digital home networks, several video/audio and data devices may be interconnected to each other. In a digital home network, digital services may deliver digital content, such as digital video broadcast (DVB), to the home via digital networks using cable, satellite, ether or telephone. Other sources of digital content may be within the home, such as digital camcorders, still cameras and pre-recorded digital media, such as CD and DVD. A digital home network may allow this content to be transported to and between not only an already mentioned D-VHS recorder, but also between set-top boxes, personal computers, television devices,
Denissen Adrianus Johannes Maria
Lambert Nicolaas
Rijckaert Albert Maria Arnold
Lam Daniel K.
Nguyen Chau
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