In-depth analysis of the sound transmission principle of Bluetooth headsets

I. Introduction

In today’s digital age, Bluetooth headsets have become an indispensable audio device in people’s daily lives. It has freed users from the constraints of traditional wired headsets and brought great convenience and freedom to users. Whether in sports and fitness, commuting, work and study, leisure and entertainment, Bluetooth headsets allow people to enjoy high-quality music, clear calls and rich audio content anytime and anywhere. However, many people may not know very well how Bluetooth headsets achieve sound transmission. This article will deeply analyze the principle of Bluetooth headset sound transmission, from its basic structure, pairing connection, audio signal processing to the final sound playback, and give a comprehensive and detailed explanation.

II. Basic structure of Bluetooth headsets

(I) Bluetooth module

The Bluetooth module is one of the core components of Bluetooth headsets, which is responsible for wireless signal transmission. The module has a built-in Bluetooth chip and related circuits, which can realize Bluetooth communication with audio source devices (such as smartphones, computers, tablets, etc.). Through the Bluetooth protocol, it can receive, decode and forward digital audio signals sent by the audio source device to ensure that the audio data can be accurately transmitted to other components of the headset.

(II) Driver unit

The driver unit is a key component of Bluetooth headsets that converts electrical signals into sound waves, and is usually called a speaker. It consists of a magnet, a voice coil, and a diaphragm. When the audio signal is transmitted to the driver unit in the form of an electrical signal, the voice coil will vibrate under the action of the magnetic field, which in turn drives the diaphragm to vibrate. The vibration of the diaphragm causes the surrounding air to produce pressure changes, thereby forming sound waves, which are eventually transmitted to the human ear, allowing people to hear the sound. The quality and performance of the driver unit directly affect the sound quality of the Bluetooth headset. A high-quality driver unit can provide clearer, fuller, and more layered sound.

(III) Battery

The battery provides power support for each component of the Bluetooth headset and is an important component to ensure the normal operation of the headset. The battery capacity and type used by different types of Bluetooth headsets are also different. Common ones include lithium batteries. Battery life is an important indicator that users pay attention to. It determines how long users can use the headset after a single charge. In order to extend the battery life and improve the battery life, Bluetooth headsets usually adopt a low-power design and are equipped with corresponding power management chips to optimize the battery’s energy consumption.

(IV) Control Panel

The control panel is the interface for users to interact with Bluetooth headsets. It usually contains function buttons such as volume adjustment button, play/pause button, answer/hang up call button, etc. By operating these buttons, users can easily control the playback status of the headset, adjust the volume, answer or hang up the phone, etc. Some high-end Bluetooth headsets are also equipped with a touch control panel, and users can use gestures such as touch and slide to achieve more operation functions, providing a more convenient and intelligent user experience.

III. Pairing and connection of Bluetooth headsets with audio source devices

(I) Turn on pairing mode

When users use Bluetooth headsets for the first time, they need to put them in pairing mode. The method of turning on pairing mode for Bluetooth headsets of different brands and models may be different, but usually it is necessary to press and hold a specific button on the headset (such as the power button, multi-function button, etc.) for a few seconds until the indicator light of the headset starts flashing, indicating that the headset has entered the pairing state and is waiting for the audio source device to connect.

(II) Search for devices

Users need to turn on the Bluetooth function in the Bluetooth settings of the audio source device and search for available Bluetooth devices. At this time, the audio source device will scan the Bluetooth signals in the surrounding environment and display the searched Bluetooth device names in the list of connectable devices. The user can find the name of his Bluetooth headset in the list.

(III) Establishing a connection

The user selects the Bluetooth headset for pairing, and the audio source device and the headset will establish a connection through the Bluetooth protocol. During this process, you may need to enter a pairing code or confirm the connection request. The pairing code is usually a simple combination of numbers or letters. Some Bluetooth headsets have preset pairing codes (such as “0000” and “1234”) when they leave the factory, and users can enter them directly; while some Bluetooth headsets do not need to enter a pairing code, and only need to confirm the connection request on the audio source device. Once the connection is successfully established, audio data can be transmitted between the audio source device and the Bluetooth headset.

IV. Processing and transmission of audio signals

(I) Generation of audio signals

When the user plays music, videos or answers a call on the audio source device, the audio source device converts the audio data into digital signals. For example, for music files, the decoding chip inside the audio source device will decode music files in MP3, AAC, FLAC and other formats and restore them to the original digital audio signals. These digital audio signals contain information such as the frequency and amplitude of the audio, and are the basis for subsequent audio transmission and processing.

(II) Signal Coding

In order to transmit wirelessly via Bluetooth, the audio source device will encode the digital audio signal. Common audio coding formats include SBC, AAC, aptX, LDAC, LHDC, etc. These coding formats use different audio compression and decompression algorithms, which can reduce the amount of audio data while ensuring a certain sound quality, thereby adapting to the bandwidth limitations of Bluetooth wireless transmission.

• SBC (Sub-band Coding): This is a mandatory coding format specified by the Bluetooth Audio Transfer Protocol (A2DP), and all Bluetooth audio chips will support it. The bit rate of SBC coding during transmission is relatively low, usually around 328Kbps, and the sound quality is relatively average, but it has advantages in compatibility.
• AAC (Advanced Audio Coding): AAC is a high-compression coding algorithm provided by Dolby Laboratories for the music community. At the same bit rate, AAC usually sounds better than SBC, and it can provide richer audio details and higher sound quality. Many Apple devices use AAC as the audio transmission format.
• aptX: aptX was originally called apt-X. It originated from the research results of Queen’s University Belfast. It was later acquired by CSR and renamed aptX. CSR was acquired by Qualcomm. aptX is based on the principle of adaptive differential pulse code modulation (ADPCM) and does not use psychoacoustic or masking effect technology. It has multiple versions, such as aptX (basic version), aptX Low Latency (low latency version), aptX HD (high-definition audio version) and aptX Adaptive (adaptive version), which can meet the needs of different users for sound quality and latency.
• LDAC: LDAC is an audio codec developed by Sony. It realizes the transmission of 24bit/96kHz high-resolution audio (Hi-Res Audio) via Bluetooth at a bit rate of up to 990Kbps. The high transmission bit rate prevents high-resolution audio files from being over-compressed, thereby ensuring sound quality. LDAC can transmit sound quality close to Hi-Res Audio, but currently only a small number of devices are compatible.
• LHDC (Low-Latency Hi-Definition Audio Codec): LHDC is currently recognized as one of the best Bluetooth audio coding technologies, which can support the transmission of 24bit/96KHz high-resolution audio, with a maximum data transmission rate of up to 900Kbps and a latency as low as 80ms. It is the second Hi-Res Audio Wireless Bluetooth high-quality standard solution certified by the Japan Music Association after Sony LDAC.

(III) Wireless transmission

The encoded audio signal is sent to the Bluetooth headset via the Bluetooth protocol. Bluetooth technology uses the 2.4GHz frequency band for wireless communication and adopts frequency hopping spread spectrum (FHSS) technology to maintain connection stability and low latency in complex electromagnetic environments. During the transmission process, the audio signal is divided into data packets for transmission. After receiving these data packets, the Bluetooth headset will reorganize and parse them in a certain order.

(IV) Signal reception and decoding

The Bluetooth module of the Bluetooth headset is responsible for receiving the audio signal from the audio source device. The received signal is an encoded digital signal, and the Bluetooth module will decode it and restore the compressed data to the original digital audio signal. The decoding process corresponds to the encoding process of the audio source device, and different encoding formats require the use of corresponding decoding algorithms.

(V) Digital-to-analog conversion

The decoded digital audio signal is still a discrete digital signal and cannot directly drive the driver unit to produce sound. Therefore, it is necessary to convert the digital signal into an analog sound signal through a digital-to-analog converter (DAC). The DAC converts the discrete values ​​in the digital signal into a continuous voltage or current signal. The amplitude and frequency changes of these analog signals correspond to the sound wave characteristics of the audio.

(VI) Signal amplification

The converted analog sound signal is usually weak and cannot directly drive the driver unit to produce a loud enough sound. Therefore, the signal amplifier chip inside the headset needs to be used to amplify the analog signal. The signal amplifier chip will adjust the signal gain appropriately according to the characteristics of the audio signal so that it has enough power to drive the driver unit.

V. Sound playback

(I) The driver unit vibrates and makes sound

The amplified analog sound signal is transmitted to the driver unit. The voice coil in the driver generates an alternating current under the action of an analog signal, which causes it to vibrate under the action of a force in a magnetic field. The vibration of the voice coil drives the diaphragm to vibrate together, and the vibration of the diaphragm causes the surrounding air to produce pressure changes, forming sound waves. These sound waves are transmitted to the human ear through the air. The eardrum of the human ear senses the pressure changes of the sound waves and converts them into neural signals, which are transmitted to the brain for analysis, and finally people can hear the sound.

(II) Sound transmission characteristics of different types of Bluetooth headsets

• Air conduction headset: Air conduction headset is a type of headset that transmits sound through air vibration. It uses beamforming technology for directional sound transmission, and the sound is transmitted to the ear canal through air as a medium, and then transmitted to the inner ear. Unlike traditional headphones, air conduction headphones do not block the ear canal, allowing users to maintain awareness of the surrounding environment while listening to music, thereby improving safety. Its sound transmission process is similar to the way humans hear sound in a natural environment, but because the sound is transmitted through the air, the open design may cause the low-frequency response to be not as strong as the in-ear headphones with closed ear canals, and there may also be a certain degree of sound leakage. However, this design also provides users with a more comfortable wearing experience and better awareness of ambient sound.