Bluetooth is specifically designed to provide low-cost, robust, efficient, high capacity, ad hoc voice and data networking with the following characteristics:|
- 1 Mb/sec. transmission/reception rate exploits maximum available channel bandwidth.
- Fast frequency hopping avoids interference.
- Adaptive output power minimizes interference.
- Short data packets maximize capacity during interference.
- Fast acknowledge allows low coding overhead for links.
- CVSD (Continuous Variable Slope Delta Modulation) voice coding enables operation at high bit-error rates.
- Flexible packet types supports a wide application range.
- Relaxed link budget supports low-cost single chip integration.
- Transmission/reception interface tailored to minimize electric current consumption.
The Bluetooth technology was not planned to be just a physical wireless medium offering merely a platform for high-level protocols and applications. The aim is to provide something more, with immediate device-interoperability as soon as the first Bluetooth products hit the market. But this can only be achieved if all the communication blocks, including radios, protocols and applications, are accurately defined and can interoperate.
Bluetooth uses either a 64 kb/s log PCM format (A-law or m-law) or a 64 kb/s CVSD (Continuous Variable Slope Delta Modulation). The CVSD-format uses an adaptive delta modulation algorithm with syl-labic 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, using either A-law or m-law compression. If the line interface uses A-law and the air interface uses m-law or vice versa, a conversion from A-law to m-law is performed. The compression method follows ITU-T recommendations G. 711.
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/second linear PCM.