LTV Structures — How Codec Parameters Are Encoded

All codec parameters in BAP are carried in LTV (Length-Type-Value) structures. This is a compact binary format:

For example, the Sampling_Frequency LTV structure for 16 kHz looks like this in raw bytes:

/* LTV for Sampling_Frequency = 16000 Hz */
0x02  /* Length = 2 (1 byte Type + 1 byte Value) */
0x01  /* Type = Sampling_Frequency */
0x03  /* Value = 0x03 = 16000 Hz (see Bluetooth Assigned Numbers) */

/* LTV for Frame_Duration = 10 ms */
0x02  /* Length = 2 */
0x02  /* Type = Frame_Duration */
0x01  /* Value = 0x01 = 10 ms */

/* LTV for Octets_Per_Codec_Frame = 40 octets */
0x03  /* Length = 3 (1 byte Type + 2 bytes Value) */
0x04  /* Type = Octets_Per_Codec_Frame */
0x28 0x00  /* Value = 0x0028 = 40 (little-endian) */

These LTV structures are packed one after another in the Codec_Specific_Configuration field. This field appears inside PAC records (for capabilities) and in ASE characteristics (for active configuration).

Decoding the Codec Setting Name — What Does “16_2_1” Mean?

BAP uses a standardized naming convention for codec and QoS settings. Once you understand it, you can read any setting name at a glance.

Codec Capability Settings at a Glance

The table below shows the main codec settings defined by BAP. 16_2 and 24_2 are the mandatory settings. Everything else is optional.

QoS Parameters — What They Mean

Once codec settings are agreed upon, the Unicast Client submits QoS parameters to the Unicast Server. These parameters control how the isochronous channel behaves in the Link Layer.

QoS Set 1 vs Set 2 — Low Latency vs High Reliability

QoS Settings Reference (selected configurations)

Presentation Delay — Synchronizing Left and Right Earbuds

Presentation Delay is one of the most important concepts in BLE Audio. It solves the problem of left and right earbuds playing audio at slightly different times.

The Unicast Client picks a single Presentation_Delay value that falls within the supported range of all Unicast Servers in the same CIG. Both left and right earbuds wait until the same point in time before rendering audio — achieving synchronization without a wired connection.

How Stereo Audio Works — Multi-channel LC3

Because LC3 is mono, stereo is handled by running two LC3 encoders in parallel. BAP defines an Audio_Channel_Allocation LTV to assign each channel to a spatial location (Front Left, Front Right, etc.).

For a stereo earbud scenario, the phone sends one SDU with two codec frames inside it. The Audio_Channel_Allocation LTV tells the receiver which frame is FL and which is FR. Codec_Frame_Blocks_Per_SDU can be set to >1 if you want to bundle multiple time-periods into one SDU (useful for reducing packet overhead).

BlueZ Code — Setting Up a CIS with QoS 16_2_1

In BlueZ, isochronous streams are set up using ISO sockets introduced in Linux kernel 5.10+. Below is how you configure a CIG with the 16_2_1 settings in C using the BlueZ socket API.

#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <bluetooth/bluetooth.h>
#include <bluetooth/hci.h>
#include <bluetooth/iso.h>   /* BlueZ ISO socket header */

int create_cis_socket_16_2_1(void)
{
    int fd;
    struct sockaddr_iso src_addr = {
        .sa_family = AF_BLUETOOTH,
        .v.cis.dev_id = 0,   /* hci0 */
    };

    /* BT_ISO_QOS for QoS configuration set 16_2_1 */
    struct bt_iso_qos qos = {
        .cig = BT_ISO_QOS_CIG_UNSET,
        .cis = BT_ISO_QOS_CIS_UNSET,
        .in  = {
            .interval    = 10000,   /* SDU interval: 10 ms in µs */
            .latency     = 10,      /* Max transport latency: 10 ms */
            .sdu         = 40,      /* Max SDU size: 40 octets (16_2 codec) */
            .phy         = BT_ISO_PHY_2M,
            .rtn         = 2,       /* Retransmission number: 2 (low latency set _1) */
        },
        .out = {
            .interval    = 10000,
            .latency     = 10,
            .sdu         = 40,
            .phy         = BT_ISO_PHY_2M,
            .rtn         = 2,
        },
    };

    /* Create ISO socket (AF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_ISO) */
    fd = socket(AF_BLUETOOTH, SOCK_SEQPACKET, BTPROTO_ISO);
    if (fd < 0) {
        perror("socket");
        return -1;
    }

    /* Set QoS options before bind/connect */
    if (setsockopt(fd, SOL_BLUETOOTH, BT_ISO_QOS, &qos, sizeof(qos)) < 0) {
        perror("setsockopt BT_ISO_QOS");
        return -1;
    }

    /* Bind to local adapter */
    if (bind(fd, (struct sockaddr *)&src_addr, sizeof(src_addr)) < 0) {
        perror("bind");
        return -1;
    }

    return fd;  /* ready to connect() to the remote device */
}