In SoundCheck’s frequency-shift algorithm, a Fast Fourier Transform (FFT) is used to extremely accurately calculate the centroid of a stationary tone. The result of this calculation can then be used to align or synchronize two signals even if they are sampled at different rates. A short, stationary signal is necessary for the frequency shift algorithm to lock on to. This is easily achieved by pre-pending a 1 kHz, 250 ms sine wave to the stimulus signal. Since SoundCheck enables the creation of compound stimuli, this short, single-tone burst can be followed with absolutely any test signal (e.g., a Farina log sweep, noise, speech, or other non-sinusoidal waveforms).
Figure 1: The compound stimulus is made up of a 1 kHz sine wave for 250 ms, followed by a Farina style continuous, exponentially swept sine wave.
The short sine wave also serves as the trigger tone for triggered record, as is necessary for testing Lightning headphones. The trigger tone clearly identifies the start of the signal. Care must be taken to set an appropriate trigger level. If it is too low, ambient noise can cause false triggering; too high and it will never trigger. The trigger level should be set so that it is above the ambient noise and below DUT output level. The Multimeter virtual instrument is an ideal tool for finding the optimal trigger threshold.
A typical stimulus signal for open loop headphone test is shown in Figure 1. This compound stimulus works for both Bluetooth headphones where the sine wave is used for frequency alignment and for Lightning headphones where it additionally serves as a trigger and reference for frequency shift.
