This happens during a loop-back measurement

1.  When the measurement is started a synchronization signal is first sent through the right channel output to the loop-back. The delay of the synchronization signal is stored and is later used to find the correct system delay.

2.  The output signal is now played on left channel output through the loudspeaker. The signal is recorded by the microphone.

3.  After the left channel output signal is finished, the synchronization signal is played once more on the right channel output. This synchronization signal is compared against the first synchronization signal and the time difference is computed. The purpose of this is to check that there has been no gaps or glitches in the playback or record. This final check can be turned off by selecting as shown below

4.  If no error has occurred, the system impulse response is calculated. The sound card delay (pipeline delay in sigma-delta converters) is added in order to include the whole system response (the default is 40 samples, but should be measured for the sound card as explained in the Measurement 1 section).