Editing MPEG transport stream (section)

=== PCR ===<!-- This section is linked from [[PCR (disambiguation)]] -->
To enable a decoder to present synchronized content, such as audio tracks matching the associated video, at least once each 100&nbsp;ms, a ''program clock reference'' (PCR) is transmitted in the adaptation field of an MPEG-2 transport stream packet. The PID with the PCR for an MPEG-2 program is identified by the ''pcr_pid'' value in the associated PMT. The value of the PCR, when properly used, is employed to generate a ''system_timing_clock'' in the decoder. The system time clock (STC) decoder, when properly implemented, provides a highly accurate time base that is used to synchronize audio and video elementary streams. Timing in MPEG-2 references this clock. For example, the [[presentation time stamp]] (PTS) is intended to be relative to the PCR. The first 33 bits are based on a 90&nbsp;kHz clock. The last 9 bits are based on a 27&nbsp;MHz clock. The maximum jitter permitted for the PCR is {{no wrap|+/- 500 ns}}.