EXFO EXpert IPTV Test Tools (FTB-1 / 2 Pro) - Manual (Page 70)

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A.4 PCR Jitter
PCR (program counter reference) timestamps is generated by the MPEG-2 encoder and
received throughout the subsequent links in the network at least 100 milliseconds apart. The
decoder uses the PCR to lock its own 27 MHz clock to the encoder system time clock (STC).
This 27MHz clock is very sensitive to variations caused primarily by packet transmissions
delays. Therefore, measurement of the PCR timestamp arrival time variation called commonly
“PCR jitter” is essential. The 27 MHz oscillator itself may not exceed 500nsec of internal jitter
per TR 101290.