Sometimes when running an acquisition with an AmpConnect and AmpConnect message steps, the recorded time waveform will be missing data at the beginning. This can manifest in test results by showing very low frequency response in the affected area, or very high THD because the waveform in that frequency range is just noise.
This example shows a stepped sine sweep from 20-20kHz. Zooming into the waveform, a jump can be seen where the recorded time waveform begins in the middle of a sine wave missing the beginning of the stweep. This is missing data in the 20-22Hz range. The frequency response at the low end has a visible dip, and the THD has a spike upwards at the same frequency.
The cause of this is changing the state of the Bias or gain on an input channel on the AmpConnect. The input is muted on an input channel when either of these are changed to avoid an impulse passing to the headphone jack or into SoundCheck. When the gain is changed, the input is muted for 20 milliseconds and when Bias is turned on or off, the input is muted for 2 seconds.
If a sequence is using AmpConnect message steps to change either of these, a pause step must be added before the acquisition to insure that the entire waveform is properly recorded.