The presentation timestamp (PTS) is a timestamp metadata field in an MPEG transport stream or MPEG program stream that is used to achieve synchronization of programs' separate elementary streams (for example Video, Audio, Subtitles) when presented to the viewer.
The PTS is given in units related to a program's overall clock reference, either Program Clock Reference (PCR) or System Clock Reference (SCR), which is also transmitted in the transport stream or program stream.
The PCR or SCR has a resolution of 27MHz which is suitable for synchronization of a decoder's overall clock with that of the usual remote encoder, including driving TV signals such as frame and line sync timing, colour sub carrier, etc.
The time bases of different programs within a transport stream may be different.
In some cases it may be possible for programs to share PCR fields.