Bluetooth is a widely used wireless communication technology that enables short-range communication between electronic devices. Bluetooth is known for its low power usage and relatively low cost, making it a popular choice for various consumer applications. Due to which much more use cases are identified and addressed in Bluetooth spec. In the previous blog, we have seen how multi master connected to a single slave and streams audio. Today we will see how Bluetooth technology is used to stream data from one master to multiple slaves simultaneously.

Audio Broadcast Introduction

In the Bluetooth spec 5.x Broadcast Audio feature is introduced to support streaming audio from single master to multiple slaves concurrently. For this minor update happened at all layers of Bluetooth.

Bluetooth Dual Audio

Main changes are in extended advertisement enhancement, Profile layer and link layer. In the Profile layer a new profile is introduced, that is Public Broadcast Profile (PBP), In the advertisement secondary advertisement channel introduced to provide more details about broadcast and in link layer a dedicated bandwidth allocated for broadcast.

Public Broadcast Profile

It is a Profile that enables the transmission of audio streams to unlimited listeners. This profile is designed to work with Bluetooth Low Energy (LE) Audio, which supports shareable public audio transmission. The PBP allows a Broadcast Source to use extended advertising data (AD) to signal that it is transmitting broadcast Audio Streams. These streams can be discovered and rendered by Broadcast Sinks that support commonly used audio configurations. PBS defines 3 distinct roles

1. Public Broadcast Source
2. Public Broadcast Sink
3. Public Broadcast Assistant

Public Broadcast Source: It is built on broadcast features provided by (Common Audio Profile) CAP initiator and (Basic Audio Profile) BAP broadcast source roles. CAP provides features such as broadcast audio start, stop, update. A PBS may use the BAP Broadcast Audio Stream Metadata Update procedure to update metadata information contained in the Broadcast Audio Source Endpoint (BASE), such as language and Streaming_Audio_Contexts.

Public Broadcast Sink: It is built on broadcast features provided by (Common Audio Profile) CAP Acceptor and (Basic Audio Profile) BAP broadcast sink roles. CAP provides features such as broadcast audio receive start and stop.

Public Broadcast Assistant Role: It is built on the features provided by CAP commander, It monitors the broadcast on behalf of PBK. When the broadcast is available, PBA commands the sink device to listen to a particular audio stream.

Earbuds and headphones are common PBK devices. They can delegate the scanning and selection of broadcasts to a PBA device, such as a smartphone.

Extended Advertisement Data:

Bluetooth Extended Advertisement Data is a feature introduced in Bluetooth 5.x that allows devices to advertise more data than what's allowed in a single advertising packet. This is accomplished by offloading some of the data to the secondary advertising channels. Extended Advertisements can be used for all advertising types except Connectable Scannable Undirected, and they are sent on the primary advertising channels. The benefit of the secondary channels is that they allow increased advertising data capacity.

The PDU Payload of the advertising packet includes the information that the advertiser wants to relay to the observer/central device. In the case of Extended Advertisement Data, the payload is divided into multiple segments, with some segments being sent on the primary channels and others on the secondary channels. This allows for a larger amount of data to be transmitted than would be possible with the traditional Legacy Advertising PDUs.

It's worth noting that not all Bluetooth devices support Extended Advertisement Data, so it's important to ensure that both the advertiser and the observer/central device support this feature before using it.

Link Layer:

In Physical layer RF channel are associated in to 3 LE physical channels advertising, periodic and data. Two devices that wish to communicate use a shared physical channel. To achieve this, their transceivers must be tuned to the same RF channel at the same time. The Link Layer uses one physical channel at a given time. Whenever the Link Layer is synchronized to the timing, frequency, and Access Address of a physical channel, it is said to be 'connected' on the data physical channel or ‘synchronized’ to the periodic physical channel.

There are 2 types of data traffic happens

1. Framed data traffic
2. Unframed data traffic

Framed data traffic:

The L2CAP layer services provide frame-oriented transport for asynchronous and isochronous user data. Connection-oriented L2CAP channels may be created for transport of unicast (point-to-point) data between two Bluetooth devices. A connectionless BR/EDR L2CAP channel exists for broadcasting data or for transport of unicast data. Broadcast traffic on the connectionless L2CAP channel is uni-directional.

Un Framed data traffic:

If the application does not require delivery of data in frames, possibly because it includes in-stream framing, or because the data is a pure stream, then it may avoid the use of L2CAP channels and make direct use of a baseband logical link. SCO logical link These logical links reserve physical channel bandwidth and provide a constant rate transport locked to the piconet clock. The Bluetooth core system also supports the direct transport of application data using a Profile Broadcast Data (PBD) logical link.

How it works

Auracast or audio broadcast are achieved in 3 stages.

Bluetooth Auracast - Use case

1. Transmitter: It includes an extended advertisement packet which includes name, content and codec for Auracast assistants (Smartphones).
2. Assistants: It scans the advertisement packet of transmitter and provides an interactive user interface to user to select the broadcast and connect with it.
3. Intimate receiver: Assistant will intimate the receiver (Earbuds) w.r.t selected broadcast and the way to connect to it.

Conclusion

With Bluetooth 5.x, the capabilities of Broadcast Audio extend beyond traditional limitations, enabling simultaneous connections with two separate audio devices without sacrificing quality or reliability. This feature allows users to broadcast content to multiple Bluetooth-connected devices simultaneously, revolutionizing the way we share audio. It allows users to stream audio to a group of Bluetooth devices, such as headphones, earbuds, or hearing aids.