If you've ever experienced differing performance from your wireless earbuds, with one dropping in volume, muting, or discharging more quickly, you've likely experienced some of the limitations of True Wireless Stereo (TWS), a workaround to allow a pair of earbuds to operate with the constraint of Bluetooth Classic Audio operating over a unidirectional point-to-point connection.
In the relay (or forwarding) method employed by TWS implementations in Classic Audio, the mobile device source first connects with the primary sink device using the standard Bluetooth Advanced Audio Distribution Profile (A2DP), which then relays the transmission to the secondary sink device for the other channel of the stereo pair. As well as the additional transmitter for the relay link, the primary sink device needs to provide data buffering to maintain synchronisation with the secondary sink, which in turn increases the overall link latency.
This asymmetry in operation means that the primary sink device will draw more current than the secondary and so for a given battery capacity will discharge faster. And obviously, having a considerable density of bone and water in the way of the otherwise line-of-sight connection between the two earbuds will also attenuate the streamed signal from primary to secondary.
As well as being proprietary solutions and not part of the Bluetooth Core Specification, the added functional requirements for TWS sink device implementations represent increased material cost, weight and power consumption compared with the far more efficient and lower latency solution offered by LE Audio Unicast isochronous streaming.
Isochronous data transmission is time-sensitive and ensures a fixed phase relationship between two or more streams received by physically separate sink devices, such as wireless earbuds or speakers operating in stereo pairs.
Bluetooth LE Audio Unicast Connected Isochronous Streams (CIS) enable the transfer of audio data between two devices with an acknowledgement scheme to provide flow control. An Asynchronous Connection-Oriented Link (ACL) control channel is used to set up the CIS stream and maintains the flow control until the stream is terminated.
A Connected Isochronous Group (CIG) contains one or more CIS streams, and in the case of multiple streams, these may flow in the same or opposite directions. The left and right channels of a stereo pair are each implemented as a CIS and contained within a CIG, each stream connecting directly to the wireless earbud, or other sink device, for that channel. The respective ACL flow controls ensure that both streams, being in the same direction, remain time-synchronised for the duration of the connection to maintain the stereo image. This allows the sink devices for the left and right channels of a stereo pair to operate independently and without any need to know about the existence of one another.
As we can see, CIS streaming offers a far superior solution for wireless earbud implementations on multiple counts compared with TWS. With the major smartphone manufacturers adopting the required Bluetooth Core Specification 5.2 this year, combined with the superior performance of the LE Audio LC3 audio codec, we can look forward to the next generation of earbuds providing an enhanced user experience with greater sonic clarity, lower latency, and both longer and equal operation from a battery charge.