问题：
What is the definition error 2.3: PCR_error?
答案：
The formal definition of this error is given in the TR 101 290: Measurement guidelines for DVB systems document that is published by ETSI. A brief summary of this error is given below. For more information about this error or the test that generates it, please consult ETSI TR 101 290.
The PCR error (2.3) is a logical OR of the PCR_repetition_error (2.3a) and the PCR_discontinuity_indicator_error (2.3b).
MPEG transport streams must periodically contain PCR (program clock register) values which are a snapshot of the encoders' program clock at the time that PCR value was generated. These values are then used on the decoder end to reconstruct the 27MHz system clock and ensure that the stream is kept in sync.
A PCR_repetition_error (2.3a) occurs when PCR values are not sent quickly enough. The MPEG standard requires that PCR values be sent no less than every 40 milliseconds. If more than 40ms elapses since the last PCR values then a PCR_repetition_error will be reported.
A PCR_discontinuity_error (2.3b) occurs when two sequential PCR values differ from each other by more than 100 milliseconds without the discontinuity_indicator flag being set in the stream. A difference of more than 100ms is allowed, but only if the discontinuity_indicator flag is set, otherwise this error will be reported.
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