PES

PES (Packetized Elementary Stream): The PES packet includes a PTS or may have both PTS & DTS. PTS means Presentation Time Stamp and DTS means decoding Time Stamp. The header begins of the PES with a 32 bit and it also give the information like which stream is belongs to which packet. Packet may either have an audio or both audio and video elementary stream. Calculation of PCR: In the program clock reference the PCR_PID is available in the Program map table i.e. PMT_PID is same as the PCR_PID and the PMT_PID is available in the TS adaptation table. In realistic manner the decoder have to repair the system clock with the PCR values and their specific instances of the arrival. It has to be observe that in the transport stream the program’s have a system clock which is represent a PCR values that is available in PCR fields which is used by the decoder whenever it is required. The value of the system clock frequency is measured in Hz and shall meet the subsequent constraints: 27 000 000 – 810 ? system_clock_frequency ? 27 000 000 + 810 rate of change of system_clock_frequency with time ? 75 ? 10-3 Hz/s PCR I = PCR_base I * 300 + PCR_extension I PCR_base I = system_clock_frequency * t I DIV 300 MOD 233 PCR_extension I = system_clock_frequency * t I DIV 1 MOD 300 In the transport stream the PCR value are available in the transport stream packet having a unique PID of each and every program this PID is available in the program Generally the video elementary stream is useful for the program but audio elementary is also there but for PCR we use the video elementary stream. According to the DVB/MPEG each value of the PCR have to be insert in the transport stream packet at a one hundred ms. The PCR is a 42 bit lengthy value which is compressed into two parts that is PCR_base and PCR_extention. The PCR_base is a 33 –bit field and PCR_ext is a 9-bit field. The PCR value has to be incremented with the 27 MHz. PCR_value =(PCR_base * 300 + PCR_extension 27 MHZ) *1000 The bit rate of the encoded transport stream: i is the index of any byte in the transport stream for i”< i < i'. i'' is the index of the byte containing the final bit of the maximum latest program_clock_reference_base field relevant to the program being decoded. PCR (i") is the time encoded within the program clock reference base and extension fields in units of the system clock. i' is the index of the byte containing the final bit of the straight away following program_clock_reference_base field relevant to the program being decoded. PCR CORRECTION: In the PCR correction for the proper decoding the PCR-PTS and the PCR-DTS should be match in decoder as one in the encoder. PTS and DTS stand for “Presentation Time Stamp” and “Decode Time Stamp” respectively. These are the time stamps which are useful for the purpose of the decoding so the audio and the video frames encode as the encoder wants them to be. it is count number running 27Mhz/300 = 90 Khz clock that are purely derived from the 27 Mhz clock which is available in the encoder engine. They decide the pre time like which frame has to be decode and present which is generally planned by the encoder. Furthermore if we say so to the PCR, the PTS and DTS are transfer because both values are useful to the PCR and they can maintain the information which has program clock reference of decoding time stamp shows the PCR value that the particular information is to be decoded. program clock reference presentation time stamp shows the time at which the decoded information is to be output. Because of this reason the PTS value is constantly higher than the PCR value of the actual day and it is also slightly higher as compare to the DTS. The PTS and DTS timestamps are encoded within the starting of the image which will make sure that the complete photograph has entered the buffer earlier than the timestamp takes place, the give up of the coded image is observed and the image length in bytes is added to the timestamp. PCR Re-Stamping: In the program clock reference the encoder clock should be matched exactly on the decoder end so that the decoder display the frame in all over the place which is a design route as encoder supposed them to be. For the better and proper accuracy the PCR is derived from 27 MHz frequency within the range of +/- 30 ppm that is numerically 27 Mhz +/- 810 Hz at the time of encoding and also the time stamp such as PTS, DTS and PCR (base and extension). PCR value are the immediately and the instantaneously value of the counters that are exactly increasing at each and every tick off 27 MHz clock for extension nine bits and ninety kHz clock for base 33 bits as shown below diagram 14. The presentation time and the decoding time is derived from the PTS & DTS consistent with equation beneath. PTS(k) ??((system_clock_ frequency ??tpn (k)) DIV 300) % 233 DTS(j) ??((system_clock_ frequency ??tdn (j)) DIV 300) % 233 tpn(k) is the presentation time of presentation unit. tdn(j) is the decoding time of access unit. The two consecutive Program clock reference (PCR) have not been with the same bitrate because due to this reason the time interval of instantly PCR does no longer produce a particular specific range of bits acquired on the decoder. From the decoder point of view there may be a mismatched of the arrived range and as well as predicted range of the bits. Because of this mismatch the decoder cannot re-create a frequency as that of the encoder. One way to resolve this problem that is insert the null packets and changing it to the audio PID. It is the easiest way to actually exchange the PID value of Audio PID packets to Null PID (0x1fff). Here, we are maintain the equal bit rate as of the encode transport stream. Let suppose we are not exchanging the PID value of audio PID with the Null PID and delete the audio PIDs definitely, so we are able to keep the PCR accuracy of the specific channel through putting same number of Null packets among the consecutive PCR PIDs. In this situation we're keeping the equal encoded bitrate. We also want to higher or lower the bitrate of the encoded TS through remuxing. In such scenario, really changing the PID value of the undesirable PID packet will no longer eliminate PCR accuracy errors. Due to the fact real supposed bitrate isn't equal as encoded bitrate. In this situation, we are able to change the PCR values to take account of the introduced/eliminated Packets inside the TS or to insert the precise range of packets appropriate for required output bitrate in order that current PCR value is in sync with the output bitrate