The audio loopback latency test measures "round-trip" latency, from the start of signal playback to the receipt of the captured audio. While this test doesn't allow for the separation of the latency measurements in the playback and capture of the system, it gives a good indication of overall latency performance of the system over several device pairs.
Loopback latency is measured by generating a signal on an output device and capturing that signal on a paired input device. The generated signal is a set of noise bursts that can be recognized by the test and is used to accurately determine the start of the signal.
Latency paths
The audio loopback latency test measures the latency of three routes, as shown in Figure 1:
Speaker/Mic: The built-in speaker signal is captured by the built-in microphone. Testing this path is required on handheld devices, such as phones and tablets, and isn't required for nonhandheld devices, such as watches, TVs, automobiles, and refrigerators.
Headset Jack: The signal is routed with a loopback plug from the output to the input of the 3.5 mm analog headset jack.
- USB: The signal is routed through either a USB-to-analog headset adapter with a loopback plug, or a USB audio interface with output jacks connected to input jacks. Obtaining a passing value for either of the USB routes is sufficient to pass. This route might not be available for some Android devices, such as TV and automobile.
Test panel
The Audio Loopback Latency Test panel consists of the following sections, as shown in Figure 1.
Figure 1. Audio loopback latency test panel.
Support utilities
The user needs to run the following utilities before starting the test:
Calibrate Audio: To effectively analyze the captured audio, it's important to generate an output signal at an adequate level. Calibrate Audio invokes the Audio Loopback Calibration panel, which is used to ensure an appropriate signal level.
Audio Devices: Starts a dialog that displays all of the audio devices that are supported by and connected to the DUT. See Audio Devices panel for more information.
Figure 2. Audio loopback latency test support utilities.
DUT configuration section
This section displays information about the configuration of the DUT as shown in Figure 1. The following configuration attributes determine, in part, the pass criteria:
Pro Audio: The DUT declares the Pro Audio flag specifying that the DUT supports a Professional level of audio performance and features such as MIDI support.
Low Latency: The DUT declares the Low Latency flag specifying that the DUT supports a low maximum latency for audio playback and capture.
MPC: The Media Performance Class level declared by the DUT.
MMAP: The DUT supports MMAP mode audio playback.
MMAP Exclusive: The DUT supports MMAP exclusive mode playback.
Watch: The DUT has been determined to be a watch.
TV: The DUT has been determined to be a television.
Automobile: The DUT has been determined to be an automobile.
Handheld: The DUT has been determined to be a handheld device, that is, either a phone or a tablet.
Test sections
For each route, the test section contains two elements:
- Start: Executes the test section. The button is enabled when it's determined that the DUT supports a given available route.
- Instruction/process/status text: Displays instructions for enabling the test section, the status of the test section while it's running, and the results of a test section when complete.
| Section | Description |
|---|---|
| Speaker and mic test | Start is enabled unless the DUT doesn't support either an internal speaker or internal microphone, as in a TV or automobile device. |
| Headset jack test | Start is enabled if the DUT supports an analog headset jack AND an Audio loopback plug is plugged in to an Analog 3.5 mm headset jack on the DUT. |
| USB test | Start is enabled if the DUT supports a USB audio connection AND a USB audio device is connected. |
| Results | Displays the results for the overall test. |
Test process
For each supported route, do the following:
- Connect the appropriate loopback peripheral for analog headset and USB tests. For the speaker and Mic test, place the DUT on a flat surface in a quiet room. The required peripherals are listed in CTS Verifier audio peripherals.
- Run the Support utilites.
- Run the test section by tapping Start.
As the test for each test section completes, the Results section updates to reflect the overall test status.
When all required routes are tested, and each test meets the latency criteria for that route as specified in CDD, PASS is enabled.