Available for digital download, this new book from Nick Hunn, Chair of the Bluetooth SIG Hearing Aid Working Group, Vice Chair of the Generic Audio Working Group, and a key contributor to the LE Audio specifications, provides an in-depth, technical overview of the LE Audio specifications, illustrates how they work together, and shows you how to use them to develop innovative new applications.

The following is an excerpt from Hunn’s book that looks at how to set up and use Broadcast Audio Streams.

In this chapter we’ll look at how to configure Broadcast Audio Streams. Broadcast is the major new feature of Bluetooth® LE Audio and has the potential to change the way that everybody uses audio. The exciting use cases it introduces include the ability to share audio, along with the ability to set up ad-hoc private connections. The latter is enabled by the Broadcast Assistant features which are defined in the Broadcast Audio Scan Service (BASS) and bring totally new control and user experiences.

Once again, the main specification is BAP – the Basic Audio Profile, but we now need to add another one of the GAF specifications called BASS. BASS defines how an additional device can be used to help find broadcast Audio Streams and instruct one or more Acceptors to synchronise with them.

The Common Audio Profile (CAP) once again comes into play. As well as providing procedures to set up and receive broadcast streams, it also defines the role of a Commander. A Commander can be a physical device, or an application on a phone or a TV. We’ll look at the Commander role in more detail once we’ve gone through the basics of sending and receiving Broadcast Audio Streams. CAP’s main task is to lay down rules about associating Context Types and Content Control IDs and repeat the order in which things need to be done.

We’ll start with the basics of setting up and receiving a Broadcast Audio Stream, then look at what the Commander brings to the picture. In Chapter 12, we’ll delve further into the possibilities within broadcast audio, examining some of the new use cases that it can enable.

Although the broadcast topology originated with a desire to improve the quality provided by inductive hearing loops, Bluetooth LE Audio provides far more versatility than an inductive loop. In many cases there will be an ACL connection between the Broadcast Source and the Broadcast Receiver in addition to the one between the Broadcast Source and a Commander. Devices can even support both broadcast and unicast at the same time, acting as a relay between broadcast and unicast connections. But before we get into those complications, we’ll start with the basics of broadcast by itself.

The broadcast specifications fall into three separate functions:

    • Transmitting a Broadcast Audio Stream: At its simplest, a Broadcaster (which is an easier name to use for a Broadcast Source) acts independently – it generally has no idea whether any receivers are present and listening to it.
    • Finding a Broadcast Audio Stream: This will generally use a Broadcast Assistant, often referred to as a Commander, (although technically that’s just a role, of which the Broadcast Assistant is a sub-role). A Broadcast Assistant can find Broadcast Audio Streams for a Broadcast Receiver. Broadcast Assistants elevate broadcast from being a simple telecoil replacement into a very powerful new topology, which allows encrypted streams to be used for private audio, both at a personal and an infrastructure level. They also make it much easier to select amongst multiple Broadcast Audio Streams. Broadcast Assistants can be designed as stand-alone devices, or can be collocated with any Broadcast Source.
    • Receiving a Broadcast Audio Stream: A broadcast receiver, (which is essentially the same as a Broadcast Sink), can scan for the presence of a Broadcast Audio Stream and synchronise to it. At this basic level, it also acts independently. A Broadcast Receiver can synchronise to encrypted or unencrypted Broadcast Audio Streams, but needs to obtain the Broadcast_Code to decrypt an encrypted Audio Stream. This can be done out of band, or with the help of a Broadcast Assistant.

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8.1 Setting Up a Broadcast Source

In Chapter 4 we covered the basics of Broadcast Isochronous Streams (BIS) and Broadcast Isochronous Groups (BIG). Unlike unicast streams, a Broadcast Source and Broadcast Sink operate independently. This makes the Broadcast Source very different from any other Bluetooth Central device, as it acts unilaterally. Unlike the unicast case, which we explored in the previous chapter, no commands, requests or notifications take place between devices. Instead, the Broadcast Source is driven entirely by its specific application.

One result of this is that a Broadcast Source has a very simple state machine, shown in Figure 8.1. As there are no interactions with any Acceptors, the procedures are very straightforward, consisting of commands from the Host to the Controller within the Broadcast Source.

Figure 8.1 Broadcast Source State Machine and Configuration procedures

To configure a Broadcast Source, the Host needs to provide the Controller with details of the BIG configuration. This is used by the Controller to schedule the BISes. It also needs to provide the information to populate the Broadcast Audio Source Endpoint (BASE), which describes the configuration of each BIS and its content. The configuration data is provided by the application running the Broadcast Source. In some applications, the metadata for inclusion in the BASE may be part of an application; in others it might be supplied externally, either from a control input, or extracted from an incoming audio stream, such as a TV’s electronic program guide data.

The Broadcast Audio Profile (BAP) defines six procedures for transitions and configuration updates of a Broadcast stream:

    • The Broadcast Audio Stream configuration procedure
    • The Broadcast Audio Stream establishment procedure
    • The Broadcast Audio Stream disable procedure
    • The Broadcast Audio Stream Metadata update procedure
    • The Broadcast Audio Stream release procedure, and the
    • The Broadcast Audio Stream reconfiguration procedure

Because there are no connections between devices, these procedures are mostly confined to HCI commands, sent when the Initiator is ready to start broadcasting. CAP bundles them together into just five procedures, combining configuration and establishment into its Broadcast Audio Start procedure:

    • Broadcast Audio Start procedure
    • Broadcast Audio Update procedure
    • Broadcast Audio Stop procedure
    • Broadcast Audio Reception Start procedure
    • Broadcast Audio Reception Stop procedure

For detailed information on starting and receiving a Broadcast Audio Stream, read the free digital download, Introducing Bluetooth® LE Audio.

FEATURED DOWNLOAD

Introducing Bluetooth® LE Audio

Download your free digital copy and find out how LE Audio will change the way we design and use audio.

FREE DOWNLOAD

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