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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 dual-sine chirp signals that are in the Apple TV Surround Generator app.

LDT works by analyzing the dual-swept sine signal, which sends one chirp to the reference (left) loudspeaker, and the other to the loudspeaker chosen for testing, offset by a fixed amount. LARSA uses the offset time to pick out which signal comes from which loudspeaker, and run them both through the same analysis, and determine precisely the difference in the initial delay time. 

This process ends up being able to determine the loudspeaker delay difference, including all electronic processing delays, DSP delays, or any other sources of delay, since we are using the actual acoustic waveforms. This process is very repeatable, noise immune, and can resolve differences with a resolution of 0.5ms.

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 processor. Both chirps will be output from the Left channel. 

Now you should see 2 plots on the main screen. 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 very 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 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. In many cases leaving the trim value to 0.0ms will be the correct setting.

Measuring Loudspeaker Delay

Repeat the test for the rest of the loudspeakers, and write down the results.

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, although there are other methods that may be used for sub time alignment.

Now you can adjust all of the delay times in the receiver by adding or subtracting the numbers, and go back and run the test again and observe the results. If needed, go back and again adjust the receiver delay times.

Tips for Best Results

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

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