Pulse or digital communications – Synchronizers – Frequency or phase control using synchronizing signal
Reexamination Certificate
1997-05-19
2001-07-24
Pham, Chi (Department: 2631)
Pulse or digital communications
Synchronizers
Frequency or phase control using synchronizing signal
C375S356000, C370S517000
Reexamination Certificate
active
06266384
ABSTRACT:
The invention relates to communication systems in general, and more particularly, the invention relates to a method and apparatus for retiming packetized information including timing information.
BACKGROUND OF THE DISCLOSURE
Synchronization of a decoding and presentation process for received bitstreams is a particularly important aspect of real-time digital data delivery systems such as digital television systems. Because received data is expected to be processed at a particular rate (to match the rate at which the data is generated and transmitted), loss of synchronization leads to either buffer overflow or underflow at the decoder, and as a consequence, loss of presentation and/or displaced synchronization.
In a compressed digital video system, such as the system described in the Advanced Television Systems Committee (ATSC) digital television standard document A/53, the amount of data generated to encode each picture is variable (based on the picture coding approach and complexity) and, therefore, timing cannot be derived directly from the start of picture data.
The ATSC system addresses the timing and synchronization issue by transmitting timing information in so-called adaptation fields of selected transport stream packets to serve as a reference for the timing comparison function of a decoder. This is accomplished by transmitting a sample of a 27 MHz clock in a program clock reference (PCR) field that is used to recover the encoder clock at a decoder. The frequency of a local clock running at the decoder is compared to the PCR value in the bit stream at the instant at which the PCR value is obtained to determine whether the decoding process is synchronized. In general, the PCR from the transport bit stream does not directly change the phase of the local clock, but only serves as an input to adjust the clock rate. Thus, the system clock frequency of the decoder or receiver is adjusted and locked to the 27 MHz clock of the encoder or transmitter via the transmitter clock sample included in the PCR field.
To form a program transport stream, a transport encoder multiplexes individual elementary streams which share a common time base (i.e., the same 27 MHz clock source). The elementary streams comprise encoded video, audio or other bit streams. The elementary streams may be, but don't have to be, in a packetized elementary stream (PES) format prior to transport multiplexing. A PES consists of a packet header followed by a packet payload. As the elementary streams are multiplexed, the transport encoder forms the elementary streams into transport packets and adds program identification bits to the transport packet headers that describe the program.
At the decoder, the coordination between the elements of the program is primarily controlled at the presentation display stage based on the use of the common time base. This common time base is imposed by having all the elementary bit streams in a program derive timing information from a single clock, and then by transmitting this timing information via the PCR in an adaptation field of a designated reference transport packet. The data for timing of decoding and presentation is present as decode time stamp (DTS) and presentation time stamp (PTS), respectively, in the packetized elementary bit stream for each individual application.
The above-described timing arrangement is somewhat inflexible in certain applications. For example, it may be desirable to process an elementary stream included within a transport stream. Therefore, a need exists in the art for a time base recovery and insertion method which allows the timing relationships of an input stream to be recovered, modified, and inserted into an output stream.
SUMMARY OF THE INVENTION
The disadvantages heretofore associated with the prior art are overcome by the present invention of a time base recovery and processing method and apparatus that extracts timing information from a bitstream containing timing information and respective program information. The program information may be subjected to further processing steps prior to being associated with locally generated timing information to form an output bitstream. The extracted timing information is used to correct the local timing information such that the timing relationships of the received program information are preserved even after the program information is processed.
Additional corrections may be made to the locally generated timing information. For example, a time delay representing the amount of time required to process the program information may be added to the corrected timing information. In addition, a time shift representing a positive or negative temporal offset may be included in the corrected timing information to control program information temporal alignment.
REFERENCES:
patent: 5467342 (1995-11-01), Logston et al.
patent: 5568403 (1996-10-01), Deiss et al.
patent: 5841987 (1998-11-01), Blatter et al.
patent: 5844636 (1998-12-01), Joseph et al.
patent: 5966387 (1999-10-01), Cloutier
Acampora Alfonse Anthony
D'Alessandro Victor Vincent
Wine Charles Martin
Burke William J.
Pham Chi
Phu Phuong
Sarnoff Corporation
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