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 many more use cases are identified and addressed in the Bluetooth specification. In the previous blog, we have seen how multi master connects to a single slave and streams audio. Today we will see how Bluetooth technology is used as an audio broadcast mechanism — streaming data from one master to multiple slaves simultaneously. Audio broadcast over Bluetooth 5.0 enables new categories of public address systems, hearing aids, and multi-listener audio experiences. Embien's cross-domain embedded services include audio device development leveraging Bluetooth 5.0 audio broadcast features for consumer and enterprise products.
Audio Broadcast Introduction
In the Bluetooth spec 5.x, the audio broadcast feature is introduced to support streaming audio from a single master to multiple slaves concurrently. Bluetooth 5.0 brought the extended advertisement capabilities and link layer enhancements that make scalable audio broadcast possible. Bluetooth Auracast LE audio broadcast streaming is built on top of these Bluetooth 5.0 foundations. For audio broadcast, minor updates happened at all layers of Bluetooth. Multimedia on Embedded Systems enabling synchronized audio streaming for multi-listener experiences like Bluetooth Auracast.
Bluetooth Dual Audio
Main changes in Bluetooth 5.0 for audio broadcast are in extended advertisement enhancement, the profile layer, and the link layer. In the Profile layer, a new profile called the Public Broadcast Profile (PBP) is introduced. In the advertisement layer, a secondary advertisement channel is introduced to provide more details about the audio broadcast. In the link layer, dedicated bandwidth is allocated for audio broadcast.
Public Broadcast Profile
The public broadcast profile 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 in Bluetooth 5.0, which supports shareable public audio broadcast transmission. The public broadcast profile allows a Broadcast Source to use extended advertising data (AD) to signal that it is transmitting audio broadcast streams. These audio broadcast streams can be discovered and rendered by Broadcast Sinks that support commonly used audio configurations. The public broadcast profile defines 3 distinct roles:
- Public Broadcast Source
- Public Broadcast Sink
- Public Broadcast Assistant
Public Broadcast Source: It is built on audio 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, and update. A public broadcast profile source 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 audio 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. Audio device development for sink endpoints must handle Bluetooth 5.0 LE Audio codec negotiation.
Public Broadcast Assistant Role: It is built on the features provided by CAP commander. It monitors the audio broadcast on behalf of the Sink. When the audio broadcast is available, PBA commands the sink device to listen to a particular audio broadcast stream.
Earbuds and headphones are common public broadcast profile sink devices. They can delegate the scanning and selection of audio broadcast streams to a PBA device, such as a smartphone. Audio device development for such earbuds must comply with the Bluetooth 5.0 public broadcast profile specification.
Extended Advertisement Data:
Bluetooth Extended Advertisement Data is a feature introduced in Bluetooth 5.0 that allows devices to advertise more data than what is 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 audio broadcast data capacity. Bluetooth Auracast LE audio broadcast streaming relies on extended advertisements to communicate audio broadcast stream parameters to potential listeners.
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 audio broadcast data to be transmitted than would be possible with traditional Legacy Advertising PDUs.
It is worth noting that not all Bluetooth devices support Extended Advertisement Data, so it is important to ensure that both the advertiser and the observer/central device support this Bluetooth 5.0 feature before using it.
Link Layer:
In the physical layer, RF channels are associated into 3 LE physical channels: advertising, periodic, and data. Two devices that wish to communicate use a shared physical channel. To achieve audio broadcast, 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 in Bluetooth 5.0 audio broadcast:
- Framed data traffic
- Unframed 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 audio broadcast or for transport of unicast data. Audio 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, it may avoid the use of L2CAP channels and make direct use of a baseband logical link. SCO logical link reserves physical channel bandwidth and provides 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 worksBluetooth Auracast LE audio broadcast streaming is achieved in 3 stages:
Bluetooth Auracast - Use case
- Transmitter: It includes an extended advertisement packet which includes name, content, and codec for Auracast assistants (Smartphones). The transmitter is the audio broadcast source implementing the public broadcast profile.
- Assistants: It scans the advertisement packet of the audio broadcast transmitter and provides an interactive user interface to the user to select the audio broadcast and connect with it.
- Intimate receiver: The assistant intimates the receiver (Earbuds) about the selected audio broadcast and the way to connect to it. Earbuds supporting Bluetooth 5.0 and the public broadcast profile can then synchronize to the audio broadcast stream.
Conclusion
With Bluetooth 5.0, the capabilities of audio broadcast extend beyond traditional limitations, enabling simultaneous connections with multiple separate audio devices without sacrificing quality or reliability. Bluetooth Auracast LE audio broadcast streaming allows users to stream audio to a group of Bluetooth 5.0 devices — such as headphones, earbuds, or hearing aids — simultaneously. The public broadcast profile provides the standard framework for audio broadcast source and sink implementations, enabling interoperable audio device development across the industry. This Bluetooth 5.0 audio broadcast capability revolutionizes the way we share audio in public spaces, assistive listening environments, and consumer audio products.