Unexpected Phase Change Event
The Unexpected Phase Change event provides a fine-grained indication of lost or repeated energy. In Referenced Mode, the system knows the Reference Audio file that is being played on the Source DUT; therefore, the system knows when a specific tone should be expected. During this interval, the system checks that the measured average frequency is the same as the expected frequency. If this is correct, the system will continue to monitor the instantaneous frequency. If the instantaneous frequency deviates sufficiently from the current average frequency, the frequency measurement state machine will reset and begin re-measuring.
Typically, the outcome is the discovery of the next scripted (expected) frequency (see "Unexpected Frequency Event" and "Frequency Detection"). However, another outcome can be that the same frequency as the previous average frequency is rediscovered, and this is reported as an Unexpected Phase Change event. Such phase changes are an indicator of losses of signal that do not result in amplitude dropouts, or signal substitution (repetition) of previous audio energy due to things such as “packet loss concealment” tactics.
The information reported with the Unexpected Phase Change event is listed below:
Info1: The phase jump expressed in degrees.
Info2: Not used
Msg: Not used