Lightning Audio

As the name implies, Bluetooth Low Energy is an alternative to Bluetooth Classic which consumes less power. With recent developments to have audio streaming available via BLE, this means that audio streaming now has an alternative protocol that can lead to longer battery life. This is done through the LC3 audio codec, which allows for audio to be transmitted at a much lower bitrate than Bluetooth Classic, with minimal loss in quality. Since there is less data which is needed to be received and processed, the processor needs to do less, leading to power savings.

Audio broadcasting simplifies the use case of sending an audio stream to multiple devices. In Bluetooth Classic, getting this functionality requires support from the peripheral itself. For instance, a pair of earbuds would require one of the earbuds to be the “master” earbud, and have that earbuds relay the audio stream to the other. This adds power usage and software complexity, which can lead to buggy behaviour. If there is a system of speakers using different BTC connections, it could be improved with audio broadcasting. If all peripherals are listening to the same source, as opposed to duplicated sources, the audio will always be in sync. In BLE, all peripherals can listen to the same audio broadcast without any special relay implementation necessary, eliminating all of this additional complexity.

While Bluetooth Low Energy has been around for a while, it has not been able to be used for high quality audio transmission until very recently. Because of this, every consumer audio peripheral currently on the market is still using Bluetooth Classic.

Lightning Audio will likely be the first time the capstone fair attendees will encounter audio being streamed from Bluetooth Low Energy.

Lighting Audio is able to demonstrate the benefits of LE audio in how it improves on Bluetooth Classic, and in what new features it brings.

Results from our audio testing has shown us that our LE Audio stream requires a far smaller bit rate of a Bluetooth Classic stream in order to get a level of quality indistinguishable from an AUX output through mid-range headphones.

We can also use any number of our peripherals to receive audio from a broadcasting source with no delays, synchronization problems, or loss in quality.

Validating the quality of audio quantitatively and objectively isn’t feasible, since audio quality itself is so subjective. We validated the quality of the LE audio stream using a subjective audio quality testing method called MUSHRA (MUltiple Stimuli with Hidden Reference and Anchor). Essentially, MUSHRA tests involve listening to different audio tracks (bass-heavy music, treble-heavy music, podcasts, etc.) by using different audio codecs.

In our tests we tested the following audio codecs using a pair of Steelseries Arctis 3 headphones (mid-range headphones):

• 3.5mm AUX cable audio (Reference)
• SBC @ 384+ kbps (Bluetooth Classic)
• LC3 @ 192 kbps (BLE)
• LC3 @ 96 kbps (BLE)
• LC3 @ 48 kbps (BLE)

During our testing, the reference audio, the Bluetooth classic audio, and the 192 kbps BLE audio were found to be basically indistinguishable from each other for all audio samples. There was a noticeable drop in quality at 96 kbps, and the 48 kbps stream was deemed very low quality.

The operating system used by Lightning Audio is Zephyr RTOS. This is an open-source real-time operating system used for embedded devices. Zephyr supports BLE, so we used the Zephyr libraries for low-level Bluetooth functionality.

The processor used is the nrf5340 from Nordic Semiconductors, which a low-power processor with BLE support, made for embedded devices. We used the nrf5340 development kit, as its I/O allows for effective debugging and testing.

All hardware used by the Lightning Audio peripherals was available “off the shelf”.