The presentation timestamp (PTS) tells the player exactly when a decoded frame should be shown or heard, and it is the timestamp that actually drives lip-sync. Every audio and video frame in a modern container or elementary stream carries one, expressed on a common clock — 90 kHz in MPEG systems — so the player can line them up on the same timeline. PTS is distinct from the decode timestamp (DTS): for video that uses B-frames, frames are decoded in one order and presented in another, so the two differ. By comparing each stream's PTS against a shared reference, the player renders audio and video together; mismatched or drifting PTS is a classic source of sync trouble.
