** NEW !**

LARSA Subwoofer Delay Test (SDT) provides a solution for obtaining the real-world delay of the sub in a surround system, compared to another loudspeaker. To use it, you will typicall need the LARSA module, the Apple TV Surround Generator app, with the Atmos test signal package installed. SDT is a free feature, once you have the LARSA module installed. 

If your system has analog inputs you can get the LARSA sine chirp test signal into your sound system either from the iOS device running LARSA, or from another iOS device controlled by the device running LARSA, using the standalone iOS Generator app.

The SDT works by doing dual-band analysis and deconvolution of the test signal, and then comparing the initial direct sound times of both calculations. The basic resolution is +/-0.3ms, or about 6”. 

How To Use Subwoofer Delay Mode

Turn on Subwoofer Delay Mode on the LARSA settings page, and select External Chirp

Start the Surround Generator app on Apple TV 4k, and select ATMOS and the LARSA signal. Pick a loudspeaker, perhaps Left, as a starting point.

Tap Measure, then start the test signal on Apple TV. It may need to loop to engage the subwoofer.

Now you should see 2 plots on the main screen, kind of like a crossover plot. Switch to the ETC screen, and you should see the subwoofer delay with respect to the chosen main loudspeaker.

Adjust your sound system delay, either by adding delay time to the main loudspeaker, or to the subwoofer. Repeat the test and observe the results. Adjust the sound system delay as necessary, until you get as close to 0 as possible. Since we are concerned mostly with aligning the low frequencies, getting within a few inches / cms is sufficient to make a big improvement on the sound field coherency.

Tips for Best Results

Use a fairly loud test signal, since we need to be able to clearly find the low frequencies.

If you have multiple subwoofers that have independant delay time controls, disconnect all but one and test them separately.