Today, many mobile phone manufacturers list Bluetooth as one of the most prominent features in their product descriptions. More and more countries and regions around the world have passed legislation to force users to use hands-free devices or headsets when making or receiving calls while driving. In this respect, Bluetooth is an ideal wireless solution, so more and more mobile phones Both have adopted Bluetooth technology, which has become a trend. Walking around the streets wearing Bluetooth headsets is becoming more and more accepted by the community and increasing the placement rate of Bluetooth on the phone. The placement rate has been increasing year by year, and the combination of headsets and mobile phones ("bundling") has provided more and more, which will help operators and mobile phone manufacturers to differentiate their products. All of these trends point to one point: voice calls are increasingly dependent on Bluetooth.
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As the usage of Bluetooth headsets increases, users are also beginning to demand the quality of their voice signals to reach the quality of the fixed-line calls. Therefore, in the face of pressure to continuously improve the quality of voice signals and reduce power consumption, this paper aims to explore a Bluetooth technology method to solve these problems.
Bluetooth voice challenges
There are two basic forms of information "data" transmitted over a Bluetooth wireless link: asynchronous data and synchronous voice. Asynchronous means that information does not need to be transmitted as a continuous stream of data - as long as all data can reach its destination and be recombined into its original form (eg, images that are not sensitive to transmission), when image data is subdivided into data The block is transmitted while the Bluetooth link is in quiet mode. As for voice traffic, users cannot tolerate that their various parts of the call arrive at the other party at different times, or the content of the call is disassembled. Smooth, real-time information (audio streaming) is important for the available voice connections.
To this end, Bluetooth technology is designed to support two types of air interfaces: the Bluetooth technology specification includes ACL (Asynchronous Connectionless) packets that support data traffic and SCO (Synchronous Connection-Oriented) packets that support voice traffic. SCO is a circuit-switching technology that provides symmetric synchronization services and reserves time slots at fixed time intervals, making it suitable for time-critical data such as voice. The Bluetooth specification also supports three simultaneous voice channels (SCO links) per master.
Label: Voice Channel Variable Slope
The voice channel uses a continuously variable slope incremental modulation (CVSD) speech coding scheme, and the SCO specifies that data packets must not be retransmitted. The CVSD CODEC is chosen for Bluetooth technology because it is very reliable when dealing with lost or corrupted speech samples. The higher the background noise, the higher the interference level, while the CVSD encoded speech can accept speech with a bit error rate of 4%.
When the output bit shows that the predicted value is larger or smaller than the input waveform (in the binary system, 1 or 0 is used to indicate the change in value), this incremental modulation scheme follows the waveform. To reduce the slope overload effect of this basic binary method, syllable companding can be performed: the step size is adjusted according to the average signal slope. Entering the CVSD encoder is a linear PCM signal of 64k samples per second.
The Bluetooth Specification v1.2 seeks to address some of the limitations of SCO connectivity: SCO's uplink and downlink code rates are fixed at 64Kbps and do not allow packet retransmissions. Bluetooth v1.2 introduces extended SCO (eSCO) transmission, which allows retransmission of corrupted data packets; this can improve the transmission quality of SCO, and SCO transmission quality is very important for the connection quality is not necessarily a "binary" connection, such as audio. Or the transmission of video data. However, although Bluetooth currently supports retransmission, Bluetooth voice packets are still encoded by the default CVSD encoding system. This leaves room for improvement in voice traffic processing.
For example, in the sixth generation of Bluetooth technology, CSR solved the limitation of CVSD as a voice traffic coding method, and introduced various improvements of radio frequency (especially scanning technology) to improve sensitivity and transmission power. Another benefit for handset manufacturers is that these radio improvement projects offer significant power savings.
Adding PCM-based encoding to Bluetooth reduces the data rate required for voice and audio traffic, increases potential capacity, or dramatically reduces the power required by Bluetooth systems. CSR, the leader in the Bluetooth chip market, has achieved this in its BlueCore6 product line, which was launched in September 2007.
ADPCM is a well-known and widely accepted CODEC technology that is an ideal complement to CVSD. The latter can handle bit errors, thus overcoming the lack of retransmission of Bluetooth, and can only rely on SCO packets to transmit voice. However, eSCO is able to detect errors and retransmit packets, so there is no need to rely on CVSD.
The difference between ADPCM and CVSD is that it is slow to sample and can detect differences between samples. ADPCM transmits fixed-telephone-quality signals at a lower rate of 32Kbps (CVSD rate of 64Kbps), so Bluetooth transmitters and receivers require only about half the runtime. CSR's experiments have shown that this method can save 40% of the power consumption of the Bluetooth system compared to the CVSD encoding technology used in all other Bluetooth systems.
In assessing audio quality levels, Mean Opinion Score (MOS) is a generally accepted measure of ITU-T audio quality, with 1 representing the lowest quality and 5 representing the highest quality (according to ITU standards, 5 points is almost impossible) of). The MOS score of the 64Kbps CVSD encoded speech signal (retry not allowed) is 2.46; the PCM score of the ADPCM CODEC (allowing 2 retry) of 32Kbps code rate is 4.14. More specifically, if ADPCM is used, the voice call of the Bluetooth connection can be achieved with higher call quality (that is, the call quality of the fixed telephone is reached).
ADPCM coding has other advantages in terms of quality. In a noisy environment, the use of CSR's BlueCore6 headset with AuriStream technology is connected to a mobile phone that also uses AuriStream, which makes voice dialing more accurate and easier to operate.
Because AuriStream on CSR's BlueCore6 supports ADPCM at 32Kbps (half the 64Kbps data rate for CVSD), there is potential to support more voice channels. CVSD traditionally supports three simultaneous SCO voice channels. Although the current Bluetooth specification specifies only three channels, the 32Kbps link ADPCM with Bluetooth Enhanced Data Rate (Bluetooth Specification v2.0 or v2.1, data rate up to 3Mbps) will likely support up to seven. A higher quality eSCO channel. .
CVSD can still provide support when Bluetooth devices need to connect to Bluetooth products that do not use CSR's AuriStream technology. CVSD is still approved in the Bluetooth specification. At present, only products using BlueCore6 technology can benefit from CSR's AuriStream technology using ADPCM. When connected to a non-AuriStream Bluetooth device, BlueCore6 will fall back to CVSD encoding.
The Bluetooth specification provides a solid foundation for voice communication. However, designers who want to get the best voice quality for their products need to consider whether CSR's application of other technologies such as AuriStream encoded by ADPCM can bring them closer to their goals.
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