Abstract: A method and a test instrument are provided for measuring program clock reference (PCR) jitter in a data stream, such as an MPEG-2 transport stream. The test instrument includes a line interface circuit for receiving the data stream, a digital computer for processing the data stream and a local clock for generating timestamps. The digital computer extracts from the data stream a PCR sample value which represents a count of a system time clock used to generate the data stream. The timestamp is obtained from the local clock and is associated with the PCR sample value. The digital computer determines a reconstructed system time clock from a set of values including the PCR sample value and the timestamp, and previously-received PCR sample values and associated timestamps within a window representative of a predetermined time interval. The window is shifted as new PCR sample values are received.