Quality issues have caused early optimistic market growth forecasts of Bluetooth stereo to be revised downwards as the reality of real-time, high-quality stereo audio over Bluetooth failed to live up to its initial hype. This article, which was adapted from an APT white paper, examines the key issues and obstacles to achieving high quality Bluetooth stereo and examines just how close we are to getting there.
Wireless audio today
The proliferation of wireless technologies such as Bluetooth and Wi-Fi has given the consumer the ability to receive digital audio wirelessly wherever they may be and however they desire—in the home or on the move, streaming audio over Wi-Fi from a Mac / PC or connecting a dongle to a mobile audio device and listening with wireless headphones.
While the initial novelty of wireless devices enabled early adopters to forgive a multitude of sins, today's consumers are developing much more demanding standards when it comes to issues like audio quality. It could be said that the "Average Joe has grown a set of Golden Ears."
This statement was reinforced in April 2007 when Apple announced that along with 128kbps AAC encoding, iTunes would also be offering tracks at Higher Quality 256kbps AAC encoding, a move signifying the increasing appreciation of quality audio by the mass market.
However, delivering seamless quality audio in real-time using wireless technology is one of the great challenges facing the professional audio engineer. With every technical advance there is often a bottleneck where one aspect of the technology advances beyond the capabilities of another and wireless audio transfer has been hampered by issues of bandwidth constraints, coding delays and the introduction of bit errors.
The convergence of audio with video across the spectrum of consumer devices means wireless audio has an additional issue to overcome. Any delay on delivery results in lip-sync issues, and an unsatisfactory user experience. Wireless headsets for mobile TV and video playback on an iPhone or similar device, wireless headsets for gaming and wireless speakers for stereo and 5.1 all require audio to be delivered in real-time.
To eliminate wires from consumer audio devices, there are many approaches designers can take. Predominantly for personal audio streaming, the most appropriate radio frequency is the license free 2.4GHz spectrum. It can provide sufficient bandwidth, range and power consumption. Bluetooth operates in this space along with other proprietary RF solutions.
To manage the transfer of stereo audio, the Bluetooth Special Interest Group (SIG) has ratified a profile known as A2DP and the market has seen the arrival of A2DP enabled products both on the handset/media player side and on the headset side. Motorola in particular were early to market with products such as the S9 Bluetooth Stereo Active Headphones.
However, to ensure connection to most audio players a dongle will be required for this type of headset device as Bluetooth A2DP is not yet widely used across all mobile media device players or home music systems, be they Mobile Media Devices, Home Theatre or Music System. The reluctance of consumer audio companies to integrate the A2DP profile has predominantly been due to issues of audio quality and coding delay.
What quality is required?
Sixteen bit audio is regarded as the entry level for audio systems now on the market with a minimum sample rate of 44.1 kHz to match that of the venerable audio CD.
16 Bit Digital Audio = 20 Log10 (216) = 96.32 dB
20 Bit Digital Audio = 20 Log10 (220) = 120.4 dB
24 Bit Digital Audio = 20 Log10 (224) = 144.5 dB
Taking CD audio quality as a benchmark, 16-bit, 44.1-kHz audio has a dynamic range of 96dB. To achieve this level of dynamic range in bandwidth limited applications such as Bluetooth stereo headsets, it will be necessary to use at least 16-bit audio as the raw input and then use a compression technology that can reproduce virtually all the original dynamic range at the output.
Uncompressed stereo audio utilizes a bandwidth of 1.411Mbps for CD Quality and, for the vast majority of wireless applications, full bandwidth is simply impractical. Issues of design, efficiency, power optimization and error resilience will all put pressure on available data rates.
Also, in many standards and protocols this bandwidth is simply not available—Bluetooth stipulates a maximum available bandwidth for A2DP at 768kbps. So, for high quality stereo audio, it is necessary to use some form of audio coding to reduce the required data rates.
Click here for Table 1
Table 1: Audio metrics.
The challenge is to find an algorithm that is able to deliver this quality level but utilise very low latency and maintain efficient processing power to remove excessive drain on battery.