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What Is Bluetooth?

Members

Bluetooth Audio Resources

Useful technical resources for Bluetooth audio.

In this section:

• Bluetooth Specification - Bluetooth Audio
• Infotooth Audio Resources
• Other Resources
• Recommended Reading

Bluetooth Specification - Bluetooth Audio

On the Bluetooth air-interface, either a 64 kb/s log PCM format (A-law or μ-law) is used, or a 64 kb/s CVSD (Continuous Variable Slope Delta Modulation) is used. The latter format applies an adaptive delta modulation algorithm with syllabic companding.

The voice coding on the line interface should have a quality equal to or better than the quality of 64 kb/s log PCM.

 

LOG PCM CODEC

Since the voice channels on the air-interface can support a 64 kb/s information stream, a 64 kb/s log PCM traffic can be used for transmission. Either A-law or μ-law compression can be applied. In the event that the line interface uses A-law and the air interface uses μ-law or vice versa, a conversion from A-law to μ-law is performed. The compression method follows ITU-T recommendations G. 711.

 

CVSD CODEC

A more robust format for voice over the air interface is a delta modulation. This modulation scheme follows the waveform where the output bits indicate whether the prediction value is smaller or larger then the input waveform. To reduce slope overload effects, syllabic companding is applied: the step size is adapted according to the average signal slope. The input to the CVSD encoder is 64 ksamples/s linear PCM. The system is clocked at 64 kHz.

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Voice Codecs

Linear	CVSD
8-bit logarithmic	A-law μ-law

Voice coding schemes supported on the air interface.
The appropriate voice coding scheme is selected after negotiations between the Link Managers.

 

Block diagram of CVSD encoder with syllabic companding.

 

Block diagram of CVSD decoder with syllabic companding.

 

Accumulator procedure.

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Error Handling

In the DV and HV3 packet, the voice is not protected by FEC. The quality of the voice in an error-prone environment then depends on the robustness of the voice coding scheme. CVSD, in particular, is rather insensitive to random bit errors, which are experienced as white background noise. However, when a packet is rejected because either the channel access code or the HEC test was unsuccessful, measures have to be taken to “fill” in the lost speech segment.

The voice payload in the HV2 packet is protected by a 2/3 rate FEC. For errors that are detected but cannot be corrected, the receiver should try to minimize the audible effects. For instance, from the 15-bit FEC segment with uncor-rected errors, the 10-bit information part as found before the FEC decoder should be used. The HV1 packet is protected by a 3 bit repetition FEC. For this code, the decoding scheme will always assume zero or one-bit errors. Thus, there exist no detectable but uncorrectable error events for HV1 packets.

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General Audio Requirements

Signal levels

For A-law and µ-law log-PCM encoded signals the requirements on signal levels follows ITU-T G.711.

Full swing at the 16 bit linear PCM interface to the CVSD encoder is defined to be 3 dBm0. A digital CVSD encoded test signal is provided in a Test Signal file available on the website. This signal is generated by a software implementation of a reference CVSD encoder. The digital encoder input signal (1020 Hz, sine-wave) generating the test signal has a nominal power of -15 dBm0. When the CVSD encoded test signal is fed through the CVSD receiver chain, the nominal output power should be -15 ± 1.0 dBm0.

CVSD audio quality

For Bluetooth audio quality the requirements are put on the transmitter side. The 64 ksamples/s linear PCM input signal must have negligible spectral power density above 4 kHz. A set of reference input signals are encoded by the transmitter and sent through a reference decoder (available on the website). The power spectral density in the 4-32 kHz band of the decoded signal at the 64 ksample/s linear PCM output, should be more than 20 dB below the maxi-mum in the 0-4 kHz range.

For more details see the Bluetooth Specification section 12 - Bluetooth Audio.

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Infotooth Audio Resources

• Headset Profile The Headset profile defines the requirements for Bluetooth devices necessary to support the Headset use case.
   
• Fax Profile The Fax profile defines the requirements for Bluetooth devices necessary to support the Fax use case.
   
• Cordless Telephony Profile This profile defines the features and procedures that are required for interoperability between different units active in the ‘3-in-1 phone’ use case.
   
• Intercom Profile This profile defines the requirements for Bluetooth devices necessary for the support of the intercom functionality within the 3-in-1 phone use case.
   
• Dial-Up Networking Profile The Dial-up Networking profile defines the requirements for Bluetooth devices necessary to support the Dial-up networking use case.
   
• Wireless Headset Audio Quality The maximum throughput of BT on the forward path is 723.2 Kbps (DH5 mode). CD-quality audio is 44100 * 16 bits/sample * 2 channels (stereo) -> 1411.2 Kbps. Thus on its own BT headset would only be able to handle mono CD-quality audio. However...
   
• Variable/Fixed CVSD Encoder/Decoder Values? The CVSD encoder/decoder parameters (such as h & Beta) are not variable, as some people might believe, but fixed...
   
• Changing CVSD Encoder/Decoder Values The CVSD values given in the Spec (also see Variable/Fixed CVSD Encoder/Decoder Values? ), are designed for a 16-bit encoder/decoder. However will they change for a 13-bit design?
   
• Service Records for Headset Audio Gateway In the Service Record for Audio Gateway ,UUIDs of Headset (0x1108) are used. However UUIDs for HeadsetAudioGateway (0x1112) exist, but are not used...
   
• Ericsson Development Kit, CVSD/PCM Voice Coding? Ericsson's development tool is using linear PCM for voice coding. But in its BT module specs, it recommends CVSD...
   
• CVSD Encoder Input Issues Section 12.2 of Bluetooth Specification version 1.0 B states; "The input to the CVSD encoder is 64k samples/s linear PCM." However could you create the 64 kbit/s encoder output using just using a 8k samples/s input?
   

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Other Resources

• Bluetooth Headsets and Handsfree Resources and products search.
   
• Bluetooth USB Adapters Resources and products search.
   
• Bluetooth Mobile Phones Resources and products search.
   
• Voice Encoding Methods for Digital Wireless Communications Systems - Bryan Douglas (PDF, 128Kb) Wireless communications operators see phenomenal growth in consumer demand for high quality and low cost services. Since the physical spectrum for wireless services is limited, operators and equipment suppliers continually find ways to optimise bandwidth efficiency. Digital communications technology provides an efficiency advantage over analog wireless communications; multiplexing and filtering is easier, components are cheaper, encryption is more secure and network management is easier. Additionally, digital technology provides more value added services to customers...
   
• Application of Bluetooth Wireless Technology to Guided Audio Tours - University of British Columbia (2001, Zipped PDF, 1.8Mb) This work demonstrates key applications and strategies that many believe will be crucial to Bluetooth’s success: * voice-based services and information access, * multimode operation with flexible delivery options, * proximity-based applications with automatic connection to public access points, * applications with revenue generating potential...