100 Things #83: Open Loop Measurements Of Smart Devices

Open loop testing enables measurement of smart devices, phones, and any device that cannot play a stimulus and record at exactly the same time. Both speakers and embedded microphones on these devices can be measured with SoundCheck, thanks to it’s module test architecture. With SoundCheck, the stimulus and acquisition are separated, so a stimulus file can easily be loaded onto a smart device and triggered either manually or via network commands. For internet connect devices like smart homes, stimulus files can be uploaded and played directly from the cloud.

Open Loop Measurements Of Smart Devices

Learn more about testing smart devices

Our full Smart Device Testing seminar features test demonstrations of smart homes, Bluetooth headphones, and active noise cancellation. Device design considerations and individual component tests are shown, and how SoundCheck can measure audio quality from R&D to production.

Video Script:

Open loop testing lets us measure a device when it’s not possible to play the stimulus and record the response from SoundCheck at exactly the same time, for example smart devices, phones and more. This is simple in SoundCheck, since its architecture has always allowed analysis to be performed independently of acquisition. In fact, we pioneered open loop testing, originally for testing MP3 players,  back in 2006 – a good 10 to 15 years before others started implementing this capability.

Let’s start by explaining how open loop testing is different from conventional closed loop tests. In conventional audio tests, such as speaker and microphone tests, the stimulus playback and response recording occur simultaneously. The two signals are correlated, and ready to analyze.

Devices such as mobile phones and smart speakers do not have a simultaneous input/output audio path, but we can test both the speakers and the microphone using open loop testing. In open loop testing, the stimulus includes a trigger to tell SoundCheck when to start recording. 

Let’s look at a mobile phone test. For the speaker, a stimulus file is loaded onto the phone and played back either manually or via a command. SoundCheck records the response using a trigger record acquisition. To test the microphone, a stimulus is played through a mouth simulator, and manually or via a command recorded to the phone. The recording is then transferred to SoundCheck for analysis. 

I’ve already uploaded the stimulus file onto my phone, and I’ll manually playback the stimulus file and trigger the record in SoundCheck.  This short “sine chirp” in the stimulus file serves as the trigger and is not analyzed. This is  a sophisticated level and cross-correlation trigger that is more robust and less susceptible to false triggers than simple level and frequency options. The test sequence then uses resampling and  frequency shift post processing steps in the sequence to correct for sample rate mismatches and slight sample clock differences between the device under test and your test interface. This enables perfect alignment of stimulus and response for accurate analysis. I’ll run the sequence…

There you go. The top graph displays the frequency response of the phone’s speaker, to the right is the recorded response superimposed over the stimulus, the bottom graphs include THD, Rub and Buzz and Enhanced Rub and Buzz.

Let’s look at some other examples. These Apple “lightning” wired  earbuds can’t be tested using a standard headphone test interface as these do not support the proprietary connector. However,  the lightning earbuds can be connected to an iPhone, and the phone can playback the test stimulus.

Open loop testing even allows testing via the cloud. With a smart speaker, the stimulus file can be uploaded to the cloud, and accessed and played back from the cloud via voice command through the speaker, using the same triggered record acquisition that I demonstrated earlier.

These are just a few common examples, but with these methods, you can measure just about any device or system that does not have a direct wired connection between signal input and output. Check out our seminar on smart device testing for more open loop demos.