Past and present lives:
The history of Bluetooth technology from 1.0 to 5.0
At present, Bluetooth has become one of the largest wireless communication technologies, which are used in smart homes, consumer electronics, smart wearable devices, instrumentation, smart transportation, smart medical, security equipment, automotive equipment, remote control and other products, penetrating into various industries and fields. Bluetooth has been integrated into all aspects of our lives, quietly changing our living habits and making our daily lives more convenient.
Perhaps few people know that the term Bluetooth is taken from Harald Bluetooth, King of Denmark in the tenth century. Linking "Bluetooth" to later wireless communication technology standards was an engineer Jim Kardach from Intel. At a wireless communications industry conference, he proposed "Bluetooth" as the name of the wireless communication technology standard.
What is Bluetooth
In short, Bluetooth technology enables wireless communication between various digital devices, making the dotted desktop a thing of the past. With Bluetooth wireless technology, you can easily connect your computer and portable devices, mobile phones, and other peripherals wirelessly to each other within 9 meters (30 feet).
Compared with other wireless technologies infrared, wireless 2.4G and WiFi, Bluetooth has many advantages such as perfect encryption measures, stable transmission process and rich compatible devices. Especially today, when the threshold for authorization is gradually lowering, Bluetooth technology has begun to truly popularize all digital devices. However, Bluetooth has not been perfect along the way, from 1.0 to 4.2 to now 5.0, is an extraordinary journey.
A brief history of Bluetooth
The history of Bluetooth actually dates back to World War II. At the heart of Bluetooth is short-range radio communication, which is based on frequency hopping spread spectrum (FHSS) technology, which was patented by Hollywood actress Hedy Lamarr and pianist George Antheil in August 1942. Inspired by the number of keys on the piano, they controlled torpedoes by radio using 88 different carrier frequencies, which had a certain degree of secrecy and anti-jamming ability because the transmission frequency was constantly jumping.
At first, this technology did not attract the attention of the US military, and it was not used by the military for wireless communication systems on the battlefield until the 80s of the 20th century, and frequency hopping spread spectrum (FHSS) technology later played a key role in solving the problem of wireless data transmission and reception including Bluetooth, WiFi, and 3G mobile communication systems.
Bluetooth technology began with a program created by Ericsson in 1994 to study low-power, low-cost wireless communication connections between mobile phones and other accessories. The inventor hopes to create a set of uniform rules (standardized protocols) for wireless communication between devices to solve the communication problem of incompatible mobile electronic devices between users, and replace the RS-232 serial communication standard.
Second, the development history of Bluetooth
Bluetooth 1.0 (1999) Early Bluetooth version, which had the risk of data leakage and was not widely used;
Bluetooth 1.2 Bluetooth technology has been iterating, and Bluetooth 1.2, released in 2003, is a sign that Bluetooth is truly moving towards security and availability;
Bluetooth 2.0 (2004) focuses on improving multitasking capabilities, while supporting duplex mode, which can transfer files while communicating by voice;
Bluetooth 3.0 (2009) introduces high-speed transmission, increases bandwidth, objectively crowds the location of WiFi, and increases power consumption and cost;
Bluetooth 4.0 (2010) repositioned and introduced the BLE concept, Bluetooth Low Energy Low Energy
Bluetooth 5.0 (2016) has gradually become the mainstream of Bluetooth with more powerful transmission capabilities under low energy and improved security and accessibility.
Bluetooth 5.2 (2020) focuses on LE Audio with the release of multiple LE audio specifications and a new LC3 high-quality low-latency encoder to enhance the Bluetooth audio experience.
Third, Bluetooth audio specifications
The cornerstone of the Bluetooth protocol is a specification (Profile), there are currently more than twenty specifications in the Bluetooth standard, a specification is actually a type of communication or protocol, different specifications cover different applications.
Three of these specifications focus on the transmission of audio data. Traditional A2DP and HFP as well as the latest BAP
1. HFP specification
Among them, HFP is based on SCO (Synchronous Connection Oriented) link for two-way transmission of call voice. SCO link has synchronous transmission characteristics, but there is a risk of error in wireless transmission, and in order to ensure the integrity and correctness of synchronous data, it cannot be guaranteed, so the encoder used by HFP needs to be fault-tolerant.
2. A2DP specification
A2DP (Advanced Audio Distribution Profile), Bluetooth stereo audio transmission specification, typical application for Bluetooth headsets. The position of the Bluetooth A2DP specification in the protocol stack is as follows:
Audio data of the A2DP protocol is transmitted on ACL Link, which is fundamentally different from voice data transmitted on SCO. A2DP does not include the function of remote control, which relies on the AVRCP protocol specification. A2DP is a high-layer protocol built on top of the AVDTP protocol, which defines the parameter negotiation, establishment and transmission process of data flow handles between Bluetooth devices and the signaling entity form of mutual exchange, which is the basic protocol of the A2DP framework.
Bluetooth A2DP packets are transmitted based on the AVDTP protocol, and their hierarchical relationships are as follows: The packets have their own header at each layer.
Audio data of the A2DP protocol is transmitted on ACL Link, which is fundamentally different from voice data transmitted on SCO. A2DP does not include the function of remote control, which relies on the AVRCP protocol specification. A2DP is a high-layer protocol built on top of the AVDTP protocol, which defines the parameter negotiation, establishment and transmission process of data flow handles between Bluetooth devices and the signaling entity form of mutual exchange, which is the basic protocol of the A2DP framework.
Bluetooth A2DP packets are transmitted based on the AVDTP protocol, and their hierarchical relationships are as follows: The packets have their own header at each layer.
Before data transmission, the sender (SRC) and receiver (SNK) need to perform a series of signaling interactions to confirm the parameters of the data transmitted by both parties before starting data transmission, and the signaling interaction process is as follows:
Bluetooth audio encoder
The same as short-range wireless communication Bluetooth compared with WIFI advantage is power consumption and its portable plug-inability, only in accordance with the specification to implement the corresponding protocol can achieve quick plugging, but its disadvantages transmission bandwidth is also obvious, now WIFI transmission rate has reached Gbps level, and Bluetooth transmission bandwidth is only 2Mbps, and the early Bluetooth transmission rate is only a few hundred Kbps.
For music audio, the basic CD quality also needs 1.4Mbpsr, if the original audio is transmitted directly, it may exhaust the bandwidth, the anti-interference performance is poor, and the easy card sound is broken, so the transmitted audio data must be compressed and encoded at the source end and decompressed at the sink end.
And how to encode and decode the Bluetooth Alliance standard stipulates that SBC is mandatory support, AAC/MP3 optional support (Optional Codec), in addition to these, can also have the manufacturer's own encoding form (Vendor Specific A2DP Codecs), such as Qualcomm's APT-X, Sony's LDAC, and LHDC.
SBC is an encoding format mandated by the A2DP protocol. Using the different characteristics of the human ear's perception sensitivity to different frequency signals, the coarser quantification is used in the insensitive parts of human hearing and the finer quantification is used in the sensitive parts to obtain better subjective hearing effects, which is an encoder with the worst sound quality. AAC is a high-quality encoder that can be optionally supported by the protocol specification, and later realized that the manufacturer proposed apt-X, LDAC, LHDC and other high-bandwidth high-quality audio encoders, of which LDAC/LHDC supports Hi-RES standard certification 96K high-resolution audio. LC3 is a high-quality low-bandwidth coding technology that is forced to support LE audio with the release of a new generation of LE audio. In general, higher bitrates mean better sound quality, but the emergence of the latest generation LC3 may break this rule, LC3 and SBC subjective comparison scores are as follows, at a bitrate of 345 kbps, SBC scores slightly higher than 4.0, but LC3 can achieve a higher subjective score than SBC at a bitrate of 160 kbps.
As mentioned earlier, Bluetooth audio transmission must be encoded first, and encoding is a certain length of data according to the frame for encoding and compression, where enough data is accumulated and then encoded, and the process of receiving data and then decoding and playback on the device side will cause delay. Most of the posts on the Internet blame the Bluetooth audio delay for this, in fact, the longest encoder encoding frame mentioned above is AAC A frame length of 23ms, Bluetooth transmission of a frame of data can also (usually) be completed in a few ms, but the actual delay is several times this data to more than 100ms, SBC a frame is only less than 2ms, but its delay reaches more than 200.
In fact, the main factor that affects latency is the stability of Bluetooth transmission. Ideally, the sending Bluetooth audio sending end sends packets with uniform time spacing, and the receiving end receives data with uniform time spacing, in this case, the receiving end can be slightly delayed to send the received data through the speaker, and the next packet data can be received before the speaker plays this packet of data and then can be continuously played, at this time the delay of Bluetooth audio depends on the packet interval and transmission time.
However, the reality is always unsatisfactory, Bluetooth as a wireless transmission protocol to transmit data may be interfered by peripheral wireless devices or strong electromagnetic equipment such as induction cookers, microwave ovens and other equipment resulting in data error retransmission, in fact, even if there is no interference due to the performance of the Bluetooth device itself, or because of the characteristics of wireless transmission are likely to be transmitted errors. Transmission error retransmission causes some of these packets to be retransmitted more often, and the transmission time becomes longer, and the packets received by the receiving end are not uniform at equal time. In this case, if the receiving end wants to play smoothly, it must first cache a certain length of data before playing, in order to prevent the late arrival of the packet and lead to playback discontinuity, in this case, the real main factor is to resist the network transmission instability and artificially added delay (cache), and the higher the transmission bitrate, the greater the impact of network fluctuations, and it takes longer delay to prevent lag.
5.BLE Audio
The LE Audio core framework is as follows
LE Audio is a new generation of Bluetooth audio transmission protocol, based on low energy Bluetooth BLE wireless communication, its main features are:
1. Ultra-low power consumption, LE itself is Bluetooth low energy, LE Audio is the first to independently transmit single-channel data for each headset, greatly reducing the amount of data transmission of each headset and reducing the power consumption of transmission and decoding.
2, the new high sound quality, low power audio decoder LC3 (Low Complexity Communications Codec)
LE Synchronization Channel (ISOC: Low Energy Isochronous Channels)
4, support multi-stream audio (Multi-Stream Audio)
5. Support broadcast audio technology (Broadcast Audio)
LE Audio is a next-generation Bluetooth audio solution proposed to solve the shortcomings of traditional Bluetooth audio above: sound quality, latency, and binaural wireless headphones and multi-connection problems. LE Audio defines the new audio transmission mechanism LE Isochronous Channels at the data flow level starting from the Bluetooth underlying protocol. The synchronous channel supports both connected and non-connected modes.
The one-to-one connection mode adopts LE-CIS (LE Connected Isochronous Stream) logical transmission channel, and the clearing mechanism flushes out the delayed data to ensure that the receiving end receives data regularly, thereby reducing the buffer of the receiving end and greatly reducing the delay. Different LE-CIS form CIG (Connected Isochronous Groups), CIS within the same CIG has the same timestamp to ensure that the data in the same group is synchronized, the most obvious application is headphones, left ear and right ear are a CIS, they form a CIG, the time between each other is synchronized.
One-to-many non-connected broadcast mode, called BIS (Broadcast Isochronous Stream), multiple BIS can form a BIG. Synchronization of broadcast mode is achieved through a periodic broadcast PDU that contains a timestamp for each BIS, and once synchronized, the reception convenience can be called the synchronous receiver. Similarly, if for the same broadcaster, each person in a group can simultaneously receive the broadcast audio signal.
Mobile communication technology has banned the telephone lines of every household, and WIFI communication technology has replaced the omnipresent network cables. Since 1998, Bluetooth and Bluetooth audio protocols have been updated many times, from supporting only mono to today's true wireless (TWS) headphones, from supporting only very low sound quality to today's LHDC Hi-Res, Bluetooth audio has grown rapidly and is gradually entering everyone's side to replace people's last half-meter cable (headphone cable). With the maturity of Bluetooth 5.2 BLE Audio technology, Bluetooth broadcast audio may enter public places, and low-power audio streaming between multiple devices provides more possibilities for TWS true wireless stereo sound, bringing greater imagination space for future Bluetooth audio.
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