A frame skip can occur when data is received later than its PTS denotes. For purposes of this description, "synchronization" between the encoder clock and the decoder clock implies that the clocks are operating at the same frequency, but with a possible phase offset between them.Īssuming an error-free environment, synchronizing the encoder and the decoder clocks eliminates frame skips or frame holds mat occur when displaying the transmitted data. The decoder clock in the MPEG-2 decoder must therefore operate at the same frequency as the encoder clock if the data streams are to be properly presented to a user. When encoding the data, the MPEG-2 encoder inserts the PTS into the transport stream based on samples of the STC. The value of each PTS is ultimately dictated by the frequency of the encoder clock in the encoder, which clocks an STC in the MPEG-2 encoder. The transport stream is provided with presentation time-stamps (PTS) to indicate to the MPEG-2 decoder when to present the individual frames of video and audio data to the user. A difference in the encoder and decoder clock frequencies causes buffer underflow or overflow. It is necessary to maintain accurate synchronization between the encoder clock used to encode the data and the decoder clock used to decode the data in order to properly demultiplex and decode the audio and video data. The PCR values within the MPEG-2 transport stream are used to accurately recover the encoder clock in the MPEG-2 decoder. The MPEG-2 standard requires that the PCR values must be provided at intervals of no more than 100 ms in the transport stream. Value are referred to as the PCR extension, and express a value of the system time clocK. Of the 42 Dits, the first 33 bits of the PCR value are referred to as the PCR base, and express a value of he encoder system time clock in 90 kHz-time base units. The PCR is a 42-bit value that represents time stamps from a relative STC that is clocked by an encoder clock within an MPEG-2 encoder. The header carries information to identify the type of data that is carried within the payload and information required for decoding the transport packet stream. Each transport packet is subdivided into a header and a payload. All audio, video and auxiliary information to be carried within a given data channel is divided into 188-byte long transport packets. The packets are then multiplexed onto a single data channel, allowing a relatively large number of users to transmit and receive data over a common data channel. The MPEG standards provide for the transmission of digital information from multiple sources by dividing the digital data into a number of packets. The control signal is applied to the voltage-controlled oscillator to adjust the oscillation frequency of the oscillator. To produce a control signal, the error signal is filtered, scaled, and added to a control variable within a low-pass filter and processor. As subsequent PCR values are received, a subtracter subtracts the value of the PCR from the value of the local STC to produce an error signal. The counter is initially loaded with a PCR value from the digital data stream. The decoder clock is fed back to a counter to produce a local system time clock (STC). A voltage-controlled oscillator produces a decoder clock frequency of 27 MHz.
The frequency of a decoder clock is maintained at approximately the same frequency as an encoder clock, based on program clock reference (PCR) values contained in a digital information stream. US-A 5,699,392 discloses a system for the recovery of an encoder clock from an MPEG-2 transport stream. The invention further relates to a method of locking a clock frequency to a time base of a data stream by comparing the clock frequency to time stamps representing the time base. The invention relates to a device for processing a data stream, the device comprising a clock recovery system for locking a clock frequency to a time base of the data stream by comparing the clock frequency to time stamps representing the time base.
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simple multiples (22,22') of the free running frequency (FREF).
Other sub-systems (23,23') run on frequencies that are not locked to the time base, e.g. MPEG audio and video processing clock frequencies are synthesized (25,35) from a free running reference frequency (FREF) and locked to the MPEG time base on basis of time stamps (PCR) provided in the MPEG data stream. MPEG, The exemplary device uses a free running clock (20) that generates a reference frequency (FREF) from which a desired locked clock frequency is synthesized (25,35) under control of a processing unit (24,34) that compares (241,341) the locked clock frequency to the time stamps (PCR).
THE DEVICE FOR PROCESSING A DATA STREAM The present invention relates to an exemplary data processing device includes a clock recovery system for locking a clock frequency to time stamps (PCR) of an incoming data stream e.g.
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