Audiophiles have rightly been sceptical of Bluetooth audio implementations in the past, with slow data transfer speeds and connection drops falling short of the quality offered by high-end wired solutions. Bluetooth technology has improved over recent years though, with the introduction of audio codecs designed specifically to push quality closer and closer to lossless.

Sony’s LDAC and Qualcomm aptX HD are the two options you’ll find inside modern smartphones boasting 24-bit playback capabilities and low latency, that exceed what standard Bluetooth offers. Better yet, both of these technologies are supported as part of Android O’s more transparent Bluetooth codec options, opening the door for audio-buffs to better configure their experiences.

Today we’re going to take a closer look at Qualcomm’s aptX HD technology and what it means for high-end smartphone audio.

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Everything you want to know about Qualcomm’s aptX Bluetooth codec

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How does it work?

Where Qualcomm aptX HD differs from its more common aptX classic version is support for “24-bit music quality” and performance that is “indistinguishable from high resolution audio”. The codec supports 24-bit 48 kHz LPCM audio data, up from 16-bit 48 kHz with classic aptX. That’s not quite the 96 or 192 kHz sample rates that some audiophiles prefer to get a hold of, but it’s a welcome improvement nonetheless.

aptX HD supports 24-bit 48 kHz audio, up from 16-bit 48 kHz with aptX classic. However, Qualcomm's ADPCM technology still compresses data to some extent.

However, the cynical audiophile will probably have spotted the fact that aptX HD isn’t a lossless format, it still relies on compression in order transmit data over limited Bluetooth bandwidth. Qualcomm targets a 4:1 compression ratio with its codecs, so classic offers a set 352 kbps bitrate while HD ups this to 576 kbps – a quarter of the bitrate of an uncompressed 24-bit 48 kHz file.

Just like aptX, the HD version is based on split-band adaptive differential pulse-code modulation (ADPCM) technology, but offers up extra bit-depth and therefore better noise performance in each of the sub-bands. ADPCM differs from common PCM files in that it doesn’t encode each sample at a specific amplitude, but encodes the difference between samples instead. This means that less data has to be sent each sample, helping fit higher quality audio into a small data packet. Hence why bitrates aren’t really comparable between regular PCM and ADPCM codecs.

Furthermore, Qualcomm’s split-band implementation uses different bit-rates (levels of accuracy) across different frequency bands, before recombining them into a complete sound file at the other end. By optimizing these bands around human hearing sensitivity and typical music frequency components, Qualcomm can save on additional data without impacting quality as much.

Compared to aptX classic, the HD version increases the bit-depth of these frequency bands by 2 extra bits, bringing the four bands up to 10, 6, 4, and 4 bits respectively. So as you can see, the highest accuracy is retained in the lower frequencies, where most instrument sounds and harmonics are found in music and film audio. Put simply, it’s a smart codec that not only sounds better than regular Bluetooth SBC, but is also consistently implemented across compatible products.

A question of quality

In a nutshell, aptX HD transfers more data than aptX classic, SBC, and other A2DP Bluetooth profiles. This ensures a closer reproduction of high quality audio files than other codecs. However, making the most of these capabilities requires that you’re using a good quality source file in the first place.

aptx HD increases transmission quality, but won't benefit consumers who primarily stream music from the most popular services. Regular aptX and high quality SBC have these scenarios covered already.

With the technical mumbo-jumbo out of the way, let’s talk practical use cases. Given that digital sales eclipsed physical retail sales last year, following a surge of 60.4 percent in streaming income and some 100 million paid up subscribers signing up, it’s safe to say that most listeners are at best downloading compressed file formats like AAC or MP3, or streaming audio through lower quality free options like Spotify or Pandora.

The bandwidth available with aptX, high-quality SBC, and A2D2 profiles, already cover medium to high quality compressed audio files from these services without many perceivable artifacts. I’ve confirmed this to be the case for myself through a selection of listening tests. So put simply, aptx HD isn’t going to greatly benefit consumers who primarily stream heavily compressed music from the most popular services. That said, those with a decent physical FLAC library or who stream from high quality services, such as Tidal or Qobuz, do stand to benefit.

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With some phones losing the 3.5mm jack, a push for higher quality wireless audio is important to ensure music enthusiasts have an alternative.

Pushing formats forward

aptX HD, much like Sony’s LDAC, is designed for those with the pickiest of ears, who want their wireless audio as close to their lossless physical collection as possible. Many of us probably don’t currently port around the type of audio collection or listening equipment for which aptx HD would make a night and day difference, but there are already niche consumers and services out there who will.

Furthermore, it’s important that hardware companies begin supporting higher quality content if better wireless audio is ever going to trickle down into the mainstream, especially if the 3.5mm jack does end up disappearing. 99 percent of consumers don’t have the benefit of comparing their listening situation to a premium controlled environment, so it’s tough to know what really good audio sounds like. However, as hardware improves, data continues to become faster and less limited by caps, and streaming services up their quality, we’re going to see more and more consumer exposed to better and better wireless audio, which will feed more demand for better quality.

aptX HD may be ahead of the average requirements right now, but it’s an important step in making sure Bluetooth audio is even better tomorrow. Either way, in there here and now you’ll also need to make sure you have compatible headphones as well as a supported phone.

Compatibility

aptX HD is a relatively new technology, it was announced back in January 2016, and therefore isn’t as widely available as its regular aptX counterpart, which can be found inside some 70 percent of Android smartphones. Still, a number of high-profile smartphones support the technology, and more will undoubtedly be on the way in the future.

According to Qualcomm’s list of compatible devices, currently the Oneplus 3, 3T, and 5, along with the LG G5, V20, and G6 are biggest names of notes. The Luna (TG-L900S) and the Vertu Constellation Octane are the only other phones on the list right now.

To use aptX HD, you'll need both a compatible smartphone and headphones or a speaker.

That’s not a very big selection, but fortunately aptX HD is seeing better software support with the rollout of Android Oreo, and developers can gain access to a wide range of configuration options. So, the arrival of Android Oreo should make it simpler than ever for device manufacturers to implement high quality Bluetooth codecs, if they so choose. Although this doesn’t mean that every upcoming phone will ship with these capabilities by default, as there are hardware and licensing issues to account for too.

The LG G6 and OnePlus 5 are the two latest smartphones to support Qualcomm’s aptX HD codec.

You’ll also require compatible playback hardware to use aptX HD, such as wireless headphones or speakers. This is because hardware inside both devices has to support the same standard, wireless handshakes, and speed capabilities to ensure consistent performance. Although Qualcomm’s range of Bluetooth products is pretty extensive, I could only find mention of aptX HD support inside its top-end CSR8675 and CSRA68100 platforms. Clearly the company views this as an enthusiast rather than mainstream technology, but that may change as time progresses.

Again, the current list of supported products isn’t huge, but it is growing. You’ll find LG’s Tone wireless headsets, the Audio-Technica ATH-DSR9 and DRS7 models, and the Astell & Kern XB10 that can make any pair of speakers or headphones wireless, among a few other products listen on Qualcomm’s website.

Is aptX HD worth it?

Qualcomm’s aptX HD technology is certainly a much appreciated improvement to audio quality that makes Bluetooth a more viable platform than ever for those who demand the most from their music collection. The limitations of Bluetooth data speeds still means that wireless can’t offer fully fledged lossless quality regardless of the codec, but that doesn’t mean the quality isn’t good enough. Qualcomm’s intelligent application of split-band ADPCM means that few will be able to make out any meaningful compromises.

In summary, aptX HD is designed for those with a music collection that’s able to make the most of its capabilities, and that certainly doesn’t include everyone. Those who prefer to stream music rather than carry around a large FLAC library, or are only looking to spend sub-$99 on a headset won’t find many benefits here. However for consumers looking to bridge the gap between high fidelity audio and the convenience of wireless, aptX HD is a promising solution. Not to mention that’s helping to drive high-res audio quality into the mainstream and improve wireless audio quality, which is nothing but a good thing.

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Robert Triggs
Lead Technical Writer at Android Authority, covering the latest trends in consumer electronics and hardware. In his spare moments, you'll probably find him tinkering with audio electronics and programming.