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LARSA Loudspeaker Delay Test (LDT) provides a solution for obtaining the real-world delay time of the loudspeakers in a surround system, compared to the reference loudspeaker. To use it, you will need the LARSA module and the Apple TV Surround Generator app, with either the 5.1 signals or the Atmos test signal package installed. LDT is a free feature, once you have the LARSA module installed. 

LDT requires the special LARSA sine chirp signals that are in the Apple TV Surround Generator app.

LDT 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”. The Left channel loudspeaker is the used as the reference channel for LDT.

Setting up and Calibrating Loudspeaker Delay Mode

Turn on Delay Mode on the LARSA settings page, and select External Chirp, and 0.5s Chirp.

We recommend setting the Left channel delay in your receiver to the largest distance that you expect to see, based on the distance to each of the other loudspeakers. This will usually give you the best results, as then the other channels will be delayed relative to the Left channel.

Make sure that the sub will not be engaged while you are testing the main speakers. If you cannot turn off bass management, or are unsure, just unplug any subs during the main speaker tests.

Start the Surround Generator app on Apple TV 4k, and select ATMOS or 5.1 and the Delay signal. Pick a the Left loudspeaker so that we can check the delay calibration.

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 (tap the > button on the screen to switch plots), and you should see the delay time, which should be 0ms, or close to it. The two plots on the screen are showing the two deconvolutions which have been computed. If the time is not very close to 0, open the settings page page and scroll down to the +/- control that shows the Delay Trim Samples. Adjust this value to increase or decrease the calibration time, and go back to the ETC plot and check the delay time. Repeat this until you get a value close to 0ms.

Measuring Loudspeaker Delay

Repeat the test and observe the results. Adjust the sound system delay as necessary, until you get as close to 0 as possible. Getting within a few inches / cms is sufficient to make a big improvement on the sound field coherency.

When you get to the subwoofer, follow the same procedure. The sub should be able to reproduce enough of the sine chirp to get good results.

Now you can enter all the delay times into the receiver, and go back and run the test again and observe the results.

Tips for Best Results

Use a fairly loud test signal, since we need to be able to clearly find the both sine chirp signals.

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