Sync_ byte: The sync_ byte is a set eight-bit field whose value is ‘0100 0111’ (0x47). Sync_ byte emulation in the desire of values for different frequently happening fields, including PID, must be prevented.
Transport_ error_ indicator: The transport_ error_ indicator is a 1-bit flag. When set to ‘1’ it shows that as a minimum 1 uncorrectable bit mistakes exists inside the related transport stream packet. This bit can be set to ‘1’ by means of entities outside to the transport layer. While set to ‘1’ this bit shall not be reset to ‘0’ except the bit value in errors have been corrected.
Payload_ unit_ start_ indicator: The payload_ unit_ start_ indicator is a 1 bit flag which means that transport stream packets that carry PES packets and ‘zero’ indicates no PES packet start.
Transport_ priority: The transport_ priority is a 1-bit indicator. While set to ‘1’ it shows that the associated packet is of more priority than different packets having the identical PID which do no longer have the bit set to ‘1’. The transport mechanism can use this to prioritize its records inside an elementary stream relying on the utility the transport_ priority field can be coded irrespective of the PID or inside one PID most effective. This discipline can be modified by means of channel particular encoders or decoders.
PID: The PID is a 13-bit field, indicating the kind of the records saved inside the packet payload. PID value 0x0000 is reserved for the program association table. PID value 0x0001 is reserved for the Conditional access table. PID values 0x0002 – 0x000F are reserved. PID value 0x1FFF is reserved for null packets.
Transport_ scrambling_ control: This 2-bit area shows the scrambling mode of the transport flow packet payload. The transport stream packet header, and the version field while present, shall now not be scrambled. Inside the case of a null packet the value of the transport_ scrambling_ control area will be set to ’00’.
Adaptation_ field_ control: This 2-bit field shows whether or not this transport stream packet header is observed by means of an adaptation field or payload.
PROGRAM CLOCK REFERENCE (PCR): While MPEG-2 transport streams are transmitted, the program clock reference (PCR) is transmitted as a time reference inside the receiver. A sufficiently correct and successfully obtained PCR is a precondition for proper information display inside the receiver. This program may have reference clock even as encoding is taking location. Transport stream is supposed for lengthy distance journey. Additionally, despite the fact that the normally 27 Mhz clock is used for encoding, it’s miles pretty feasible that the encoding clock frequency might not be precisely identical as decoding clock frequency. That is executed through PCR insertion.
PCR explanation and use: The PCR is a time reference this is constantly transmitted with every program of a transport stream. The PCR values of a program are completely allocate to a packet identity (PID) within the transport stream.
With a purpose to synchronize the playback rate with the broadcast rate, a program Clock Reference (PCR) timestamp is embedded inside the transport stream packets. This timestamp allows to maintain the receiver device Time Clock in sync with the transmitting one. That is had to make sure that a image is displayed at the appropriate time within the receiver’s show unit. If those clocks get out of sync, both the images are shown too speedy and the image buffer will underflow, or they may be shown too sluggish and the buffer will overflow, each inflicting interference with the playback.
Figure indicates how the PCR is utilized by the decoder to recreate a remote version of the 27-MHz clock for every program. The encoder clocks power a continuously strolling binary counter, and the value of those counters are sampled periodically and located inside the header adaptation fields as the PCR. Every encoder produces packets having a unique PID. The decoder acknowledges the packets with the proper PID for the chosen program and ignores others. On the decoder, a VCO generates a nominal 27-MHz clock and this drives a nearby PCR counter. The local PCR is in comparison with the PCR from the packet header, and the difference is the PCR section errors. this mistake is filtered to manipulate the VCO that finally will carry the local PCR count number into step with the header PCRs. Substantial VCO filtering guarantees that jitter in PCR transmission does not modulate the clock. The discontinuity indicator will reset the nearby PCR be counted and, optionally, can be used to lessen the filtering to assist the device fast lock to the brand new timing.
PCR Monitoring: The measurements defined underneath are to be achieved constantly or periodically, i.e. as a part of a monitoring method. The subsequent parameters are measured:
PCR repetition error: The PCR values obtained on a abundant normal basis as a minimum each forty/hundred Ms.
PCR discontinuity indicator error: If the difference between two consecutive PCR values is outside the range of 100ms, this error can arise.
PCR accuracy error: This measurement define the distinction between the present day PCR value and the value described by means of its function inside the TS.
PCR Transmission: A PCR is dispatch with every program of a transport stream. The PCR values of a program are simplest transmitted in transport stream packets having a unique PID, that is particular within the program map table (PMT), that is commonly the video elementary stream of the program.
Not each TS packet containing the particular PID always consists of a PCR value it’s miles enough to insert a value right into a TS packet every forty/a hundred ms. because of this, the PCR value is transmitted in an elective field of the extendable header (adaptation field) inside the transport stream packet.
The PCR value is 42 bits lengthy and addition with a frequency of 27 MHz. the PCR is made from a 33 bit field (program clock reference base) and a 9 bit field (program clock reference extension).