High-res Bluetooth?

There are now two systems claiming to deliver high-resolution audio via Bluetooth. Sony’s LDAC appeared last year on a number of products, including the company’s multi-room audio range, while at CES 2016 Qualcomm announced the extension into 24-bit territory of CSR’s widely-adopted aptX Bluetooth codec.

aptX HD claims 24-bit audio transmission up to 48kHz by Bluetooth, while Sony’s LDAC claims it can achieve 24-bit/96kHz transmission.

Each system requires both the sending device and receiving device to carry the required software — just as currently it’s of no benefit having a Bluetooth speaker equipped with the aptX if you don’t also have an aptX-compatible phone to deliver it. Android is by far the platform on which aptX has been most widely adopted, and aptX HD has been initially launched for Android devices only, though Qualcomm intriguingly says that during the course of “the next year or so, it will be deployed more widely” — perhaps a hint that Apple may be reconsidering aptX inclusion after employing it only briefly in a limited number of Macbook models some years ago.

aptX HD will retain its current maximum sampling rate of 44.1/48kHz but will accept a 24-bit input, Qualcomm claiming that “24-bit audio resolution is maintained by using an extra two bits in each of the four sub-bands of processing”. It is immediately available as part of the CSR8675 Bluetooth audio system-on-chip (SoC) platform which supports 24-bit audio end-to-end.

How the existing Bluetooth specifications can handle either high-res audio system without drastic lossy compression is a mystery to the mathematicians here at S+I HQ. When Sony’s LDAC was announced, we entered correspondence with both Sony Australia and the Japanese head office, during which, after some confusion, they confirmed LDAC to require a bit-rate of 990kbps to achieve 24/96 delivery, far higher than the specification of the whole stereo A2DP profile of Bluetooth. Even then, a 24/96 file has a bit-rate higher than 4.5Mbps. To get that losslessly into 990kbps would require a lossless compression factor down to around 20%, more than double what we’ve ever seen before in lossless transmission systems, which usually achieve around 50% data reduction. This includes the currently aptX codec, which is often advertised as 'CD quality' but isn't, since it achieves around 50% lossless compression but also includes an additional level of lossy compression to fit within the constraints of Bluetooth. We have contacted Qualcomm to establish how extra 'room' has been discovered to push aptX to 24-bit transmission.

Our full piece on Sony's LDAC can be read at www.avhub.com.au/LDAC

We should emphasise that we are thoroughly delighted by any improvement in Bluetooth transmission quality of audio, for which the fundamental audio specs have remained unchanged for a decade. There are promising developments in extending the mesh capabilities of Bluetooth, which have existed since its earliest days, while a fascinating Bluetooth 'router' was shown at CES by Cassia Networks, which can interconnect up to 22 devices, extending both range and speed (still currently on pre-sale only, so exercise due diligence). But these solutions require multiple Bluetooth devices to act as a network, which is not how either HD audio system purports to operate.

So are these new HD audio systems really ‘Bluetooth’, or do they use proprietary forms of wireless transmission to achieve their promised 'HD' delivery? We'll report further as we untangle the tech behind the announcements.