Bluetooth is a short-range, low-power wireless standard for transmission of digital data and has been around since 1998. It has many uses including that of audio streaming.
Bluetooth has evolved a number of specifications and profiles over the years which can be somewhat confusing. We try to give the basics here in this short article.
Bluetooth devices come in all manners of shape, size and function, but as far as audio is concerned there are two basic types: transmitters and receivers. A Bluetooth audio transmitter is any audio source with Bluetooth functionality, for example an iPhone, computer or dongle which receives audio input from another device. A Bluetooth audio receiver can be a wireless headphone or speaker, or a dedicated audio receiver.
The transmitter sends out a digital signal encoded in a way the receiver can recognise. The receiver then decodes this signal and converts it to analogue for audio playback. To establish a link between transmitter and receiver and determine which Bluetooth profiles can be used ,an initial setup step called “pairing” is necessary.
Bluetooth is commonly listed with a specification version, usually ranging from v2.0 to v4.0 (soon there will be v4.1). There are older versions as well, but v2.0 was specified back in 2004 and it is very unlikely that any audio devices on the current market still use previous specifications. Optional components that may follow the Bluetooth version number, such as +EDR (enhanced data rate) and +HS (high speed), do not affect Bluetooth audio streaming.
Bluetooth v2.1 simplified and streamlined the pairing procedure. Later Bluetooth versions improved power handling and power saving functionality which may affect attributes such as range and connectivity, but anything specified v2.1 or newer should be equivalent as far as audio quality is concerned.
Bluetooth audio codecs
What determines how audio is sent to a wireless headphone? That is called the ‘codec’. After the transmitter selects the appropriate profile, it chooses a codec to digitally compress the audio for sending to the receiver. The receiver then decodes the file for playback. Although it is theoretically possible to send MP3s or any other digital format straight to the receiver over Bluetooth, this is not what happens. Instead, the A2DP* profile specifies its own set of audio codecs.
There are several different codecs which can be used including SBC (Sub-band Coding), AAC (Advanced Audio Coding) and aptX. For the purposes of this short introductory article we shall just mention aptX as this is the codec used by Bang & Olufsen’s new portable speaker: the BeoPlay A2.
aptX is a proprietary audio codec developed for demanding audio applications. Its use in Bluetooth devices is entirely optional. In fact, aptX is currently supported by only one manufacturer of Bluetooth chipsets: CSR, which acquired parent company APT back in 2010.
aptX is designed to encode a CD-quality (16-bit / 44.1kHz) audio stream without loss of sound quality through a combination of slightly higher data transfer rate compared to SBC as well as more efficient audio encoding. The caveat is that both the headphone and the source must support aptX; if either one lacks aptX support, the default SBC codec will be used instead. Currently, aptX support is limited to mostly high-end Android smartphones and Hi-Fi wireless devices. Nexus 5, for example, does not support apt-X. And nor do Apple devices such as iPhones and iPods.
aptX encoded audio fits neatly within the available bandwidth of wireless transmission standards to offer an efficient solution for band-width restricted connections.
By incorporating aptX® audio coding technology into the next generation of Bluetooth® stereo products, developers can offer consumers audio quality indistinguishable from wired with an impressive dynamic range.
- Outstanding Bluetooth® Stereo audio quality
- Audio bandwidth matching CD performance
- Flat Frequency Response. Full audio bandwidth faithfully reproduced
- Low audio coding delay. Minimizes latency and ‘lip-sync’ issues
- Non destructive transcoding, means there are no dueling effects with other algorithms
- Uses Time Domain ADPCM principle rather than Psycho-acoustic masking
- Small code / data memory size
- Backward Compatibility: when aptX is not available the target device will pair down to SBC
*A2DP (Advanced Audio Distribution Profile) – designed for multimedia and used for stereo audio transmission over Bluetooth.