Using high quality wireless audio in auto infotainment systems - Embedded.com

Using high quality wireless audio in auto infotainment systems

As the home environment marches towards the reality of wireless connectivity, car drivers and passengers are expecting the same functionality and freedom associated with a wire-free environment.

There are two applications where consumers expect to enjoy high-quality audio: The first is streaming content from their smart devices to the head unit, and the second is listening to high-quality audio from their rear-seat entertainment system while watching a movie or playing a game.

Bluetooth, the most ubiquitous of wireless protocols, has the ability to address both these applications but only recently has had the potential to make this possible.

Car drivers and passengers are expecting the same functionality and freedom associated with a wire-free environment as they are now experiencing with their home environment.

Traditionally Bluetooth as a audio content transport mechanism has struggled to fulfill all the key requirements for automotive OEM'S. With a specific focus on synchronized audio to video and as wideband stereo content, decision makers on in-vehicle infotainment solutions were driven by certain performance metrics. The two 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 (Low Complexity Subband Coding) 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.

As well as this consumers are changing their habits away from AM and FM radio and CDs and are looking to their smartphones as a hub for music and content to be bought in to the car. As a result of this new demand for smartphone integration new car audio designs are increasingly replacing the CD and offering Bluetooth wireless audio connectivity instead.

Smartphones can stream content, including music, podcasts and audiobooks, as well as using the cloud to access internet radio. Given that consumers are changing their habits 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.

Happily for all in the design chain, there is now an alternative which will render Bluetooth “fit for purpose” for the automotive sector. The aptX codec has been integrated into various CSR automotive solutions including BC05MM, CSR8311 as well as aftermarket solutions such as CSR8670 and all options readily address 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, 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.

aptX
The aptX codec has been the mainstay of the professional audio industry for many years for applications that require bit rate reduction techniques with low latency properties. It was originally adopted by the post production, cinema, TV and music sectors for both storage and transport applications.

The broadcast industry has used aptX extensively for transport of audio from studio to transmitter towers. It has been mandated by the European Broadcast Union for all public service broadcasters in Europe and affiliate members across the globe and as such it is implemented in the vast majority of the 44,000 public and commercial radio stations world-wide. In July 2010, CSR acquired the rights of 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.

Audio Quality
There are two ways to judge the performance of an audio codec – subjective and objective. In the case of the former, referencing the wide ranging, highly acclaimed individuals and organisations, which have adopted aptX is a source of great comfort.

aptX has been used by many brands that rely on high-quality audio including Sennheiser, Samsung, BBC, IRT (representing the regional German broadcasters), SR in Sweden, NHK in Japan, KBS in Korea, Pixar and Disney. The aptX codec has also been widely adopted in the handset market by manufacturers including Samsung, HTC, Motorola and Sharp.

In terms of the latter empirical data has indicated that aptX offers an average figure of -0.067 using ITU-R BS-1116-1 test standards for judging the performance of high quality audio codec's. The scale goes from 0 to -5, where -1 is deemed to be imperceptible from the original source content. Multiple tracks are listened to and scored by a panel of 30 trained acousticians and details on the sum and average are collected.

Latency
As previously stated, the aptX codec has a compression/decompression time of 1.92ms. However, the more important feature is the sample based nature of the codec. As the codec has a 4:1 compression ratio, granularity in terms of editing points can be down to four samples.

The result of being able to slice the codec so thin is that the Bluetooth packets can be populated in an extremely efficient manner. This efficiency then allows the buffers to be reduced to a level that overall latency (both codec and Bluetooth transport) can be 32ms, while retaining RF robustness.

A figure of less than 40ms is required for true audio synchronisation or “lip sync,” which is a function of the size of the video frame. For any OEM planning on using Bluetooth as part of their rear-seat entertainment solution, making sure they have a solution that contends with SBC latencies and provides true “lip sync” for watching video content or gaming on the rear-seat screen is vital to ensure the best customer experience.

What's next for the automotive industry?
At the beginning of this article, it was outlined that due to the restrictive nature of SBC in Bluetooth, the automotive industry was forced into other proprietary options. However, now that CSR has addressed these frailties, the industry will need to heavily consider connectivity in terms of the ecosystem, ease of use, cost (in terms of silicon and system) and how to future-proof their offering.

Bluetooth is a standard that has been readily adopted by a myriad of consumer electronics manufacturers. Over three billion Bluetooth chips have been sold in the past 10 years. 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.

Other proprietary solutions limit the flexibility an OEM can offer to allow users to consume media in the way that they want to, because they are incompatible with other widely-adopted standards including Wi-Fi and Bluetooth. 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.

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.

Summary
CSR has addressed the problems associated with Bluetooth for all applications which require CD-quality audio and low latency by implementing aptX. Kleer developed a proprietary, one-off solution to fill the vacuum that arose before Bluetooth got its house in order. Automotive OEMs 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.

Jonny McClintock is a graduate of Electronics from the University of Ulster and has over 25 years of experience in the audio industry. He was Director of Sales and Marketing for Audio Processing Technology and then for APT Licensing Ltd, before the latter was acquired by CSR in July 2010. He is now Director of aptX Sales and Marketing for CSR.

This article originally appeared on EE Times Europe.

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