They require a variable amount of data and are subject to variable delay due to multiplexing and transmission. In order to keep the audio and video locked together, time stamps are periodically incorporated in each picture. This clock is obtained by dividing the 27-MHz program clock by 300. Since presentation times are evenly spaced, it is not essential to include a time stamp in every presentation unit. Instead, time stamps can be interpolated by the decoder, but they must not be more than 700 ms apart in either program streams or transport streams.
Time stamps indicate where a particular access unit belongs in time.
Lip sync is obtained by incorporating time stamps into the headers in both video and audio PES packets. When the time line count reaches the value of the time stamp, the RAM is read out. This operation has two desirable results. First, effective timebase correction is obtained in each elementary stream. Second, the video and audio elementary streams can be synchronized together to make a program.