High quality wireless audio in the car: how developments in Bluetooth are making it possible

Traditionally Bluetooth as an audio content transport mechanism has struggled to fulfil all the key requirements for automotive OEM’S. With a specific focus on synchronised audio to video and as wideband stereo content, decision makers on in-vehicle infotainment solutions were driven by certain performance metrics. The 2 key requirements being:
• 16 bit word resolution with a sampling frequency of 44.1 kHz i.e. CD quality (or 10Hz to 20kHz frequency bandwidth)
• Latency performance of under 40 milliseconds and preferably close to 30 milliseconds.

For stereo audio, the A2DP profile mandated codec of SBC was used within the Bluetooth transport layer. Unfortunately, at its most popularly used implementation (Bit Pool 32), SBC was only able to reproduce audio quality closer to FM quality (32kHz Fs or 10Hz to 15kHz frequency bandwidth). SBC’s underlying compression techniques being Frame Based has culminated in an overall Bluetooth and audio codec latency in excess of 100ms. In addition, SBC could only offer a highly variable latency which removed the option of delaying the video signal, making syncing video and audio for playing games or watching films impossible.

Because of these unattractive features, rather than using Bluetooth, automotive manufacturers were forced away from standards-based options to the more inflexible and constraining solutions offered by proprietary vendors such as SCMS’s Kleer. However, as A2DP is set to stay with us for many years OEMs need to think about dealing with current audio-quality issues, rather than looking for alternative solutions.

Given that consumers are changing their habits away from AM/FM radio and CDs and are looking to their smartphones as a hub for music and content to be bought in to the car, it is important for automotive suppliers to take the audio quality of Bluetooth A2DP streaming as seriously as they do the quality of the CD/AM/FM radio.
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Happily for all in the design chain, there is now an alternative which will render Bluetooth “fit for purpose” for the automotive sector. CSR acquired the rights for the aptX codec, specifically to address the commonly acknowledged problems with wideband stereo audio in Bluetooth. The aptX codec is Sample Based and achieves its non-destructive compression process through innovative procedures embedded within a wider ADPCM process. Using a gentle 4:1 compression ratio, aptX can deliver CD-quality audio which is imperceptible from the PCM original and has a coding latency of 1.92ms. It has been integrated into various CSR automotive solutions including BC05MM, CSR8311 as well as aftermarket solutions such as CSR8670, addressing both audio quality and latency.

Additionally the aptX codec now enjoys widespread adoption in the smartphone and consumer space, with brands such as Samsung, Apple, Motorola and HTC all adopting the aptX codec for wireless streaming. With the latest smartphones and CSR in-vehicle connectivity solutions now supporting aptX there is an opportunity to address the SBC based audio issues outlined above in the next generation automotive designs.

Bluetooth is a standard that has been readily adopted by a myriad of consumer electronics manufacturers. All smartphones and the majority of feature phones and portable media players include Bluetooth as a feature and offer the A2DP profile for wideband stereo audio. The ability for a user who has stored audio on a handset to readily move from home to car to office while streaming audio to headsets and speakers without the need for dongles is truly a compelling vision.

By using these standards a car manufacturer can have rear-seat screens which use Wi-Fi to deliver content from the head end to the screens and Bluetooth to deliver the audio; but they can also support video and audio streaming from a brought in device, for example a smartphone or Portable Media Player, because they use the same standards.

Given the investment by the semiconductor companies offering Bluetooth, the design cycles are quick, economy of scale ensures advantageous pricing and R&D effort ensures a rich feature set beyond the Bluetooth radio. CSR has developed two solutions designed specifically for the automotive market that offer all this functionality, helping OEMs to considerably reduce the system BOM in the design phase. CSR8311, the first wideband speech IC qualified for the automotive market, is also the first Bluetooth low energy IC ready for automotive use. It uses an automotive QFN 0.5mm pitch package with availability of both USB and UART for modularity. The CSRBC05MM is a multimedia device incorporating an on-board DSP, known as Kalimba, along with the Bluetooth radio. Using the Kalimba processors, will drive down the BOM costs as this device can be used for post-processing and audio enhancement algorithms.

In terms of future-proofing, Bluetooth has a well-defined roadmap as it is managed by the Special Interest Group (SIG) with options for faster data rates and significantly lower power requirements and can address many different use cases and applications outside audio.

Automotive OEM’s now have access to a standards-based solution, which offers wide-scale adoption, ease of use, enjoys pricing associated with economy of scale and features that have been developed by power houses investing in R&D. The choices should be quite simple for OEM’s who are considering their next design cycle.