aptX Bluetooth Codec

An explanation of the aptX Bluetooth codec and aptX vs SBC

Picture of Sony WH1000XM2 Premium Noise Cancelling Wireless Headphones
aptX HD Sony Wireless Noise Cancelling Headphones (WH1000XM2). © Sony

Different Bluetooth-enabled audio devices can use different codecs that result in various connection and audio quality differences. One codec from Qualcomm that's advertised as having a "better-than-CD" quality is called aptX.

The purpose for aptX (previously spelled apt-X) is to provide audio equipment the means for better sound quality than what other codecs can offer. Devices that might use aptX include headphones, smartphones, tablets, car stereos, or other types of Bluetooth speakers.

The term aptX refers to not only the original technology but also a suite of other variations like Enhanced aptX, aptx Live, aptX Low Latency, and aptx HD - all useful in different scenarios within the audio realm.

How aptX Compares to SBC

By default, all Bluetooth devices have to support the standard low-complexity sub-band coding (SBC) codec. However, other codecs like aptX can be used along with SBC, which was only built to provide reasonable sound quality.

SBC supports sampling frequencies up to 48 kHz and and bit rates up to 198 kb/s for mono streams and 345 kb/s for stereo streams. For comparison, aptX HD transfers audio at up to 576 kb/s for a 24-bit 48 kHz file, which allows for higher quality audio data to be moved more quickly.

Another difference is the compression method used with these two codecs. aptX utilizes what's called adaptive differential pulse-code modulation (ADPCM). "Adaptive differential" refers to how and what audio sample is transmitted.

What happens is that the next signal is predicted based on the prior signal, and the difference between the two is the only data that's moved.

ADPCM also divides the audio into four separate frequency bands that ultimately provide each with their own signal-to-noise ratio (S/N), which is defined by the expected signal to the level of background noise.

aptX has been shown to have a better S/N when dealing with most audio content, which usually falls below 5 kHz.

With aptX Low Latency, you can expect less than 40 ms of latency, which is vastly better than SBC's 100-150 ms. What this means is that you can stream audio that coincides with a video, and expect the sound to match up with the video without as much delay as a device that uses SBC. Having audio that stays in sync with the video is important in areas like video streaming and live gaming.

The other aptX compression algorithms mentioned above have their own uses too. For example, aptX Live is built for low bandwidth scenarios when wireless microphones are being used. Enhanced aptX is designed more for professional applications and supports up to a 1.28 Mb/s bit rate for 16-bit 48 kHz data.

What all of this comes down to when using aptX devices is that you should be able to experience a smooth and crisp sound with a high level of audio detail, and listen to high quality material with less hiccups and delays.

aptX Devices

The very first aptX source device was Samsung's Galaxy Tab 7.0 Plus, but the Qualcomm aptX technology is currently used in millions of consumer electronics from hundreds of brands.

You can find aptX in soundbars, tablets, speakers, and headphones manufactured by companies like Vizio, Panasonic, Samsung, and Sony.

You can find some of these devices on Qualcomm's aptX Products website. From there, you can filter the results to show aptX, aptX HD, and aptX Low Latency devices.

The Codec Isn't All That Matters

Be mindful of the fact that aptX is only a codec and does not mean that the headphones, speakers, etc., will perform well just because the SBC codec isn't being used. The idea is that the Bluetooth technology itself is what serves the benefits.

In other words, even when an aptX device is used, there won't be a massive improvement when listening to a low quality audio file or using broken headphones; the codec can only do so much for the audio quality, and the rest is left up to the actual sound data, frequency interference, device usability, etc.

It's also important to be aware that both the sending and receiving Bluetooth device need to support aptX for the benefits to be seen, else the lesser codec (SBC) is used by default so that both devices can still work.

A simple example can be seen if you're using your phone and some external Bluetooth speakers. Say your phone uses aptX but your speakers don't, or maybe your phone doesn't but your speakers do. Either way, it's the same as not having aptX at all.