Detailed Introduction to LE Audio Profile for Low Energy Bluetooth

1. Introduction to LE Audio

1.1. LE Audio Transmission Protocol

At the end of 2019, the Bluetooth Special Interest Group (SIG) released the core protocol for Bluetooth 5.2, which introduced an important feature—LE Audio (Low Energy Bluetooth Audio). Bluetooth application protocols encompass everything from the application layer to the physical layer, and LE Audio is no exception. However, the Bluetooth 5.2 core protocol only defines the link layer transmission method for audio over Bluetooth LE, while the upper layer protocols and the complete LE Audio specifications have been delayed. Recently, the Bluetooth SIG released a more complete specification document for LE Audio.

1.2. Complete Applications of LE Audio

The SIG has defined the following specifications and protocols, which, along with the core protocol, constitute the complete application of LE Audio:

• Basic Audio Profile (BAP)

The Basic Audio Profile is a key specification of LE Audio, defining various roles and the capabilities each role must support, as well as how to use the following services to complete audio application transmission. LE Audio supports both point-to-point audio mode and broadcast audio mode, using different services in each mode.

• Published Audio Capabilities Service (PACS)

The Published Audio Capabilities Service defines the audio capabilities supported by the device, including but not limited to the number of supported codecs and their capabilities. This service allows retrieval of the device’s audio capabilities.

• Audio Stream Control Service (ASCS)

The Audio Stream Control Service defines a set of operation commands for establishing, configuring, and terminating audio streams.

• Broadcast Audio Scan Service (BASS)

The Broadcast Audio Scan Service is used by broadcast audio publishers to inform nearby receivers of broadcast audio parameters. This service is only applicable in broadcast audio scenarios.

• Low Complexity Communication Codec (LC3)

The LC3 codec is used for audio encoding and decoding in LE Audio. As the name suggests, this codec is a low-complexity audio encoder. The LC3 codec supports a wide range of optional parameters, applicable from 8 kHz mono speech to 48 kHz multichannel music, and significantly improves audio quality at the same bit rate compared to the SBC codec used in classic Bluetooth audio specifications.

2. Detailed Explanation of LE Audio

2.1. BAP Specification

The BAP specification serves as the foundational audio specification for LE Audio, located at the following Bluetooth protocol layer.

The BAP specification defines the following roles based on support for point-to-point audio and broadcast audio:

Unicast Role	Unicast Server	Point-to-point audio sink device
Unicast Client	Point-to-point audio source device
Broadcast Role	Broadcast Source	Broadcast audio transmitter
Broadcast Sink	Broadcast audio receiver
Broadcast Assistant	Broadcast audio assisting device
Scan Delegator	Broadcast audio scanning device

The services supported by each role are as follows (Note: X represents unsupported, M represents must support, O represents optional support):

When two devices are in corresponding roles, they can complete service connection and audio transmission service through the operational steps defined by BAP. For example, the steps for Unicast Server and Unicast Client are as follows:

• The Unicast Client discovers the Unicast Server’s PACS service through GATT service discovery and learns the audio parameters.
• The Unicast Client discovers the Unicast Server’s ASCS service through GATT service discovery and learns the current status.
• If the Unicast Client finds that its audio parameters match and the Server’s status is IDLE, it can connect to the audio service.
• The Unicast Client configures audio codec parameters and audio transmission parameters using operation codes defined by ASCS, and then starts audio.
• The Unicast Client initiates the CIS audio transmission stream at the link layer according to the configuration parameters defined by the core protocol 5.2.
• The Unicast Sink notifies the Unicast Source that it is ready to receive audio using ASCS operation codes.
• The Unicast Source activates the LC3 codec and transmits the encoded audio stream to the Unicast Sink via CIS.
• The Unicast Sink receives the audio stream, decodes it, and plays it.

2.2. PACS Service

The PACS service is used for point-to-point audio and defines the audio capabilities of the device. Its service definition is as follows:

Characteristic Name	Requirement	Mandatory Properties	Optional Properties	Security Permissions
Sink PAC	C.1	Read	Notify	Encryption required
Sink Audio Locations	C.2	Read	Notify, Write	Encryption required
Source PAC	C.1	Read	Notify	Encryption required
Source Audio Locations	C.3	Read	Notify, Write	Encryption required
Available Audio Contexts	M	Read, Notify	None	Encryption required
Supported Audio Contexts	M	Read	Notify	Encryption required

Among them, the Source PAC is the audio sending capability attribute, which needs to be defined only when the device supports audio sending. Its format is as follows:

Parameter	Size (Octets)	Description
Number_of_PAC_records	1	Number of PAC records, [i], for this characteristic
Codec_ID[i]	5	Octet 0: Coding_Format value of the [ith] PAC record. Coding_Format values are defined in Bluetooth Assigned Numbers. Octet 1–2: Company_ID value of the [ith] PAC record. Shall be 0x0000 if octet 0 is not 0xFF. Company_ID values are defined in Bluetooth Assigned Numbers. Octet 3–4: Vendor-specific codec_ID value of the [ith] PAC record. Shall be 0x0000 if octet 0 is not 0xFF.
Codec_Specific_Capabilities_Length[i]	1	Length of the Codec_Specific_Capabilities value of the [ith] PAC record. Shall be 0x00 if the Codec_Specific_Capabilities value of the [ith] PAC record is empty.
Codec_Specific_Capabilities[i]	Varies	Codec_Specific_Capabilities value of the [ith] PAC record.
Metadata_Length[i]	1	Length of the Metadata field of the [ith] PAC record. Shall be 0x00 if the Metadata value of the [ith] PAC record value is empty.
Metadata[i]	Varies	LTV-formatted Metadata applicable to the [ith] PAC record. Shall exist only if the value of the Metadata_Length field is not 0x00.

The Sink PAC is the audio receiving capability attribute, which needs to be defined only when the device supports audio receiving. Its format is as follows:

Parameter	Size (Octets)	Description
Number_of_PAC_records	1	Number of PAC records, [i], in this characteristic.
Codec_ID[i]	5	Octet 0: Coding Format of the [ith] PAC record. Coding_Format values are defined in Bluetooth Assigned Numbers. Octet 1–2: Company_ID value of the [ith] PAC record. Company_ID values are defined in Bluetooth Assigned Numbers. Shall be 0x0000 if octet 0 is not 0xFF. Octet 3–4: Vendor-specific codec_ID value of the [ith] PAC record. Shall be 0x0000 if octet 0 is not 0xFF.
Codec_Specific_Capabilities_Length[i]	1	Length, in octets, of the Codec_Specific_Capabilities value of the [ith] PAC record. Shall be 0x00 if the Codec_Specific_Capabilities value of the [ith] PAC record is empty.
Codec_Specific_Capabilities[i]	Varies	Codec_Specific_Capabilities value of the [ith] PAC record.
Metadata_Length[i]	1	Length of the Metadata field of the [ith] PAC record. Shall be 0x00 if the Metadata value of the [ith] PAC record value is empty.
Metadata[i]	Varies	LTV-formatted Metadata applicable to the [ith] PAC record. Shall exist only if the value of the Metadata_Length field is not 0x00.

2.3. ASCS Service

The ASCS service is used for audio control, interacting through a defined set of operation codes to achieve control over audio state transitions.

The ASCS service is defined as follows:

Characteristic Name	Requirement	Mandatory Properties	Optional Properties	Security Permissions
Sink ASE	C.1	Read, Notify	None	Encryption required
Source ASE	C.1	Read, Notify	None	Encryption required
ASE Control Point	M	Write, WriteWithoutResponse, Notify	None	Encryption required

2.4. BASS Service

BASS is the Broadcast Audio Scan Service, used to inform about some parameters of broadcast audio, and is an auxiliary service defined as follows:

Characteristic Name	Requirement	Mandatory Properties	Optional Properties	Security Permissions
Broadcast Audio Scan Control Point	M	Write, Write Without Response	None	Encryption required
Broadcast Receive State	M	Read, Notify	None	Encryption required

2.5. LC3 Codec

The LC3 codec is used for audio encoding and decoding in LE Audio, similar to MP3 and AAC, belonging to frequency domain encoding. Its encoding efficiency is significantly higher than that of the SBC subband codec, and the short frame structures of 10 ms and 7.5 ms greatly improve audio latency. LC3 introduces technologies such as SNS frequency domain noise shaping, TNS time domain noise shaping, and entropy coding, some of which have been used in AAC, enhancing audio quality. The entire encoding process is as follows:

Decoding is the reverse process of encoding, as shown below:

3. Prospects for LE Audio Support

As a very important feature, LE Audio has yet to see widespread application since its release, aside from a few demos. Its large-scale adoption still depends on whether operating system vendors support it, such as Apple’s iOS and Google’s Android. From the current progress (iOS 14 and Android 11), there is still no support for LE Audio. However, looking at the AOSP code, traces of LE Audio can be found in Android 12 (android-s-beta version), as shown below:

From the current completeness of the code, it does not yet represent full functional support. It is expected that Android 12 will improve and begin to apply it. Once smartphones support it, it is believed that manufacturers will launch headphones that support LE Audio. However, audio applications are general-purpose applications, and support from Apple and other operating systems such as Microsoft’s Windows is also extremely important. From this perspective, it may still take time for LE Audio to achieve widespread adoption.————————————————

References

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