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 synchronization signal is recorded and is later used to find the correct initial time delay.

2.

The MLS signal is now played on left channel output, through the speaker. The signal is recorded on the microphone.

3.

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 in the playback or record. This check is not performed if is set in Measurement->Sound Card Settings....

4.

If no error has occurred, the system impulse response is found using cross correlation. The sound card delay (pipeline delay in sigma-delta converters) is added in order to obtain the whole system response (the default is 40 samples, but should be measured for the sound card as shown above).