We explore the technology and performance of aptX audio.

Bluetooth has come a long way since the technology was introduced in 1998. The addition of the Advanced Audio Distribution Profile (A2DP) in 2003 brought support for high-quality audio streaming, but Bluetooth still didn’t offer anywhere near the quality of a wired connection. This unfortunate fact is often overlooked in favor of the technology's convenience factor, but what if we could have the best of both worlds? This is where Qualcomm's aptX comes in, and it is a departure from the methods in place since the introduction of Bluetooth audio.

What is aptX audio? It's actually a codec that compresses audio in a very different manner than that of the standard Bluetooth codec, and the result is as close to uncompressed audio as the bandwidth-constrained Bluetooth technology can possibly allow. Qualcomm describes aptX audio as "a bit-rate efficiency technology that ensures you receive the highest possible sound quality from your Bluetooth audio device," and there is actual science to back up this claim. After doing quite a bit of reading on the subject as I prepared for this review, I found that the technology behind aptX audio, and its history, is very interesting.

A Brief History of aptX Audio

The aptX codec has actually been around since long before Bluetooth, with its invention in the 1980s and first commercial applications beginning in the 1990s. The version now found in compatible Bluetooth devices is 4th-generation aptX, and in the very beginning it was actually a hardware product (the APTX100ED chip). The technology has had a continued presence in pro audio for three decades now, with a wider reach than I had ever imagined when I started researching the topic. For example, aptX is used for ISDN line connections for remote voice work (voice over, ADR, foreign language dubs, etc.) in movie production, and even for mix approvals on film soundtracks. In fact, aptX was also the compression technology behind DTS theater sound, which had its introduction in 1993 with Jurassic Park. It is in use in over 30,000 radio stations around the world, where it has long been used for digital music playback.

So, while it is clear that aptX is a respected technology with a long history in the audio industry, how exactly does this translate into improvements for someone who just wants to listen to music over a bandwidth-constrained Bluetooth connection? The nature of the codec and its differences/advantages vs. A2DP is a complex topic, but I will attempt to explain in plain language how it actually can make Bluetooth audio sound better. Having science behind the claim of better sound goes a long way in legitimizing perceptual improvements in audio quality, particularly as the high-end audio industry is full of dubious – and often ridiculous – claims. There is no snake-oil to be sold here, as we are simply talking about a different way to compress and uncompress an audio signal – which is the purpose of a codec (code, decode) to begin with.

How aptX can improve wireless sound quality

Standard Bluetooth audio streams are handled with a codec called SBC (low complexity subband codec), and it uses a lossy form of compression as it encodes the signal, to be decoded at the receiving end. What do I mean by "lossy"? With this type of compression, a significant amount of the original signal is removed from the audio source, and this is done using something called psychoacoustic auditory masking, a technique in which signals can be eliminated based on the idea that certain frequencies would otherwise be “masked” by other frequencies, and can therefore be omitted. This is the method used by an MP3 encoder to reduce file size without causing significant noticeable quality loss. I will not attempt to convince anyone who claims to hear no audible difference when comparing MP3 to a CD source that there is anything wrong with this, as I am only pointing out that such compression results in a significant enough loss of the original signal to be considered “lossy” compression.

To understand how aptX is different, I'll begin with a look at standard pulse code modulation, which is the simplest form of digital audio (Compact Disc audio, Blu-ray soundtracks, and WAV/AIF files are common examples of PCM). The process begins by capturing the original analog sound wave at intervals defined by the sample rate. For 44.1 KHz audio the waveform is sampled 44,100 times per second, with each sample’s amplitude being recorded using a particular bit-depth (16-bit in the case of CD audio). The higher the sample rate and bit depth, the more accurate the resulting representation of the original analog wave will be.

A quantized waveform (image credit: Digital Sound & Music)

That’s it. It is an extremely accurate method of digitizing sound, but it requires a significant amount of data for high resolution. The terminology might be a little daunting, with words like “quantization” used to describe not only PCM, but aptX streaming as well, but these are not especially difficult concepts. With PCM each sample of the analog signal taken must be be quantized, or simply, turned into the ones and zeros that your device will store to represent the audio signal (for example, 16-bit is the CD bit depth for quantization). In the 'high-resolution' music world many PCM files are losslessly compressed using formats such as FLAC or ALAC to achieve a smaller file size, but to go beyond what these codecs can provide in bandwidth reduction – a necessity when streaming audio over Bluetooth – more is required.

So how can aptX make a difference, considering it is still operating within the restriction of the 345 Kbps Bluetooth audio streaming specification? The difference is in the way the signal is compressed before being transmitted, as aptX does not use psychoacoustic techniques to achieve its compression. It is still considered “lossy”, but aptX uses time domain-based, not frequency domain-based, compression using ADPCM (adaptive differential pulse-code modulation).

"As it's name implies ADPCM is a technique which re-codes the difference between two digital audio samples, using quantisation step-sizes that adapt to the energy of the input audio signal. In this way ADPCM can provide a similar audio quality to linear PCM but at a much reduced bit-rate."

With the ADPCM compression used in aptX Bluetooth streams, the difference between the quantized samples can be transmitted and used to reconstruct the original signal on the receiving end, saving data, and therefore valuable bandwidth.

Without going into any more detail from my research into this subject (those links are all PDF files, just to warn you!), I will simply say that there is a lot of dense reading to be found on the subject of ADPCM. And anyway, that's enough technical mumbo-jumbo! None of this means much if aptX doesn't improve the sound of a Bluetooth stream. So, how does aptX sound? I will discuss my findings on the next page.

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