Background

Recently, the amount of Bluetooth devices that support A2DP source/ HFP capability has grown dramatically. Nowadays, most users might have multiple devices, such as mobile phones, tablets, soundbars, TV sets, game consoles, etc. (Please refer to Figure 1 / Figure 2.) Simultaneously, the TWS earbud is now the most popular audio/voice sink device. If solution vendors can provide an easier way of creating/maintaining connections and smoothly switching between source devices, it will end up helping users significantly. Thus, Multilink, called Multi-connection, became a mandatory capability in Earbud.

Figure 1. Bluetooth? Enabled Device Shipments by Radio Version. (Picture published by Bluetooth SIG, 2023 March 28, https://reurl.cc/XLagq3)

Figure 2. Bluetooth Audio & Entertainment Device Shipments (Picture published by Bluetooth SIG, 2023 March 28, https://reurl.cc/XLagq3)

1 What is multilink, and why do customers need it?

Multilink technology allows for earbuds to be able to be connected with two source devices concurrently. The Earbud can depend on the source device's audio/voice status to automatically provide ways to intelligently switch sound sources, helping users have a seamless audio/voice handover experience. Before Earbud supported multilink functionality, when the user wanted to switch audio sources, they must complete several steps, like disconnecting from the original device and reconnecting to the desired one (Figure 3). Here are two examples to show how multilink makes things easier.

Figure 3. Manually switch earbud connection

1.1 Usage scenario at home

The user connects the Earbud with a music player and mobile phone at the same time. When the phone is ringing, the Earbud will automatically switch the audio to the mobile phone even if the user is listening to music from a music player. (Figure 4)

Figure 4. Automatically switching audio/ voice

1.2 Usage scenario at the office

In the post-pandemic era, online meetings have become the most popular communication methodology for daily work. During the weekday, users can connect their Earbuds to their notebook and their mobile phone at the same time. The user can easily switch to join discussions with their colleagues on their notebook or answer calls from the mobile phone without reconnecting between the notebook and mobile phone. (Figure 5)

Figure 5. Earbud switch source by call status

1.3 Usage scenario with iOS accessories

iOS users, in addition to iPhones, usually buy several accessories like Apple Watch, iPad, etc. With multilink capability, they can go out to exercise while bringing only their Apple Watch and Earbud and when they get home, resume their Earbud connection with their iPhone. Users can easily change the links, depending on their requirements.

2 What advantages does Realtek's multilink solution provide?

Realtek's solution provides the best audio/voice transmission balance while maintaining a stable connection with two devices. At the same time, Realtek also provides friendly usage scenarios for customers to switch audio/voice between two connected devices. Below are what we provide to customers.

2.1 Audio switch (A2DP)

Earbud provides A2DP preempt/non-preempt ways for different types of products. If the product support preemption, the Earbud can automatically select the active audio source based on the device’s status. All the user needs to do is to push the play button from the source device and then they will be able to enjoy the music. (Figure 6). In the non-preempt scenario, the audio source will not be allowed to switch to the new one until the current streaming status is idle.

Figure 6. A2DP preempt scenario

Here is how RTK provides a flexible solution for deciding the active device.

2.1.1 Control source device

1. Whenever two source devices are idle (No A2DP/ SCO/ Voice assistant), the headset will take the first connected device as the active one.

2. Whenever any source device has triggered any operation, it will become the user's preferred device. And the headset will take it as the primary controlled device.

For example:

Step 1: Play music through SRC_A

Step 2: Headset will output audio data from SRC_A

Step 3: When the user plays music through SRC_B

Step 4: The headset will pause SRC_A and switch the active media stream to SRC_B

The Earbud will take voice as the highest priority; it will automatically preempt the current audio stream and switch to voice stream. If Earbud receives two voice calls at the same time, only the first call is accepted.

2.1.2 Voice call preempt scenario

Once the headset is already in a call, to avoid the user getting interrupted, the headset will disconnect the upcoming SCO to keep the current call. After the user ends the active call, the headset will try to connect the inactive one automatically.

3 Reference

2023 Bluetooth® Market Update