You might not realize it, but when you buy Bluetooth headphones, earbuds, or a speaker, you’re also selecting the Bluetooth codec those devices will use. This codec compresses audio for efficient transmission and then decompresses it for playback. However, not all codecs are equal. Bluetooth has limited bandwidth, so the choice of codec is crucial. This is where Qualcomm’s aptX codecs come in. With five versions, each offering different performance qualities to enhance sound, reduce latency, and more. Confused? Let us help you understand aptX to make better audio product decisions.
Why do we have Bluetooth codecs? Because Bluetooth’s wireless connection is narrow compared to Wi-Fi, it can’t always accommodate digital music in its original format. Codecs are employed to shrink audio to fit within that bandwidth. Since 2003, the default codec for stereo Bluetooth sound has been sub-band coding (SBC). Any Bluetooth audio device, from phones to speakers, supports SBC. While SBC delivers good sound quality, manufacturers initially limited its data rate to prevent dropouts. This earned SBC a bad reputation. It also suffered from significant latency. AptX, already used in filmmaking and broadcasting, was recognized as an alternative. Its key benefits over SBC are higher-quality audio and lower latency, meaning better sound and improved synchronization between audio and video.
The AptX suite of technologies was acquired by Qualcomm in 2015. The problem with alternative codecs, including aptX, AAC, and LDAC, is that they require support on both the source (phone, computer, etc.) and the sink (speaker, headphones). If your phone supports aptX but your headphones don’t, the connection will revert to SBC or possibly AAC. Android generally doesn’t have this issue, as aptX has been included since early versions. However, Apple, a major smartphone manufacturer, refuses to include any non-SBC codec except AAC on its devices.
Many early reviewers were impressed by aptX’s improved audio quality. Some still complain when wireless headphones lack aptX because it uses a different form of lossy compression than SBC or AAC, and some prefer its sound. This preference stems from the days when Bluetooth bandwidth was smaller, forcing SBC to run at lower bit rates, while aptX always ran at a fixed 352 kilobits per second (Kbps). AptX’s nondestructive compression was particularly beneficial when most people listened to MP3 music. SBC and AAC use psychoacoustic models of compression, which can worsen already heavily compressed files like MP3, while aptX preserves more details. These benefits are still relevant today, depending on the source. Streaming Spotify on a free account at 160 Kbps, with a lot of compression already applied, won’t yield as much difference as starting with lossless, CD-quality sound. Your phone will have to decode and re-encode the Spotify stream into SBC, AAC, or aptX.
Over time, aptX has evolved into a family of codecs, each with its own strengths and weaknesses. Latency is a challenge in Bluetooth audio due to the numerous links between sound generation and playback. AptX Low Latency (aptX LL) significantly reduces latency, down to about 38 milliseconds, making it suitable for fast-paced games. However, aptX LL is limited to lossy, 16-bit audio and is not the best choice for critical music listening. While finding aptX LL headphones is easy, there are no smartphones or tablets offering aptX LL due to how phones use their antennas. The only way to use aptX LL is through a dedicated transceiver, typically a USB dongle that plugs into a computer or game console.
SBC, AAC, and aptX are the most-supported Bluetooth codecs, but they’re limited to lossy compression at 16-bit/44.1kHz. AptX HD was created to improve quality by supporting audio at up to 24-bit/48kHz and a higher data rate of 576 Kbps. Some consider AptX HD a hi-res audio codec. Its encoding improves signal-to-noise ratio (SNR) and total harmonic distortion (THD+N). AptX HD is backward compatible with classic aptX devices, but it has two drawbacks. Firstly, its fixed data rate can affect audio reliability if the connection can’t maintain 576 Kbps. Secondly, its high data rate requirement makes it unsuitable for true wireless earbuds as they need bandwidth to communicate with each other. While some companies have attempted to use AptX HD in true wireless earbuds, the Bluetooth connection often proves unstable.
Bluetooth wireless connections are known for their unpredictable nature. Interference from various radio frequencies can affect the quality. AptX Adaptive supersedes aptX and AptX HD (while remaining backward compatible) and can adjust its data rate from 110 Kbps to 620 Kbps based on link conditions. It uses the same 24-bit/48kHz resolution as AptX HD but can go higher, up to 24/96. It’s also more efficient, achieving the same quality as AptX HD at a lower data rate. AptX Adaptive can also operate in 16-bit/44.1kHz mode and adjust its settings based on audio activity, switching to a low-latency mode for gaming or a high-quality voice mode for calls. However, AptX Adaptive requires Qualcomm chips on both source and sink devices. It’s not included in all Android phones, like Google Pixel devices. Manufacturers have discretion over which AptX Adaptive features they enable, making it difficult to know which features are available without checking both device specifications.
All the Qualcomm codecs discussed so far use lossy compression. AptX Lossless promises bit-for-bit perfect delivery of CD quality at 16-bit/44.1kHz, a traditional challenge for Bluetooth bandwidth. Qualcomm claims to overcome this limitation by combining AptX Adaptive’s ability to operate at 16-bit/44.1kHz with Bluetooth High-Speed link, delivering the necessary 1 Mbps bit rate for lossless audio. It can scale its data rate based on conditions, going down to 140 Kbps when needed. At the top data rate, users can choose between 16/44.1 lossless or AptX Adaptive’s 24/96 lossy audio. AptX Lossless requires several technologies to work together, so it’s only available on devices with Snapdragon Sound certification.
While expensive headphones and phones should have tools to confirm the benefits of advanced technology, this is not always the case. Tracking sound quality is difficult, and users often rely on phone or headphone makers to provide an app or other means of displaying sound quality information. Some streaming apps, like Amazon Music, provide some details, and external devices like DACs can indicate the codec used. However, nailing down the performance of the entire audio quality chain remains challenging.
AptX Adaptive introduced a new level of codec capabilities, but manufacturers have control over which features they enable. Qualcomm created Snapdragon Sound to address this uncertainty. It’s a certification that guarantees certain AptX Adaptive features, including 24-bit/48kHz, 24-bit/96kHz, low-latency gaming mode, and AptX Voice for calls, when two Snapdragon Sound devices are used together. Any manufacturer using Qualcomm’s S3 or S5 chips can deliver these features if they choose, but the Snapdragon Sound label indicates that all features are supported and have been verified by Qualcomm. The intention of Snapdragon Sound is to give buyers confidence that, as long as they see the label on their devices, everything will work seamlessly. However, the features included in Snapdragon Sound may change over time. Initially, it didn’t include AptX Lossless, but as of 2022, lossless is included. This means that older Snapdragon Sound devices may not support AptX Lossless, even if the phone does. Snapdragon Sound’s features will likely continue to evolve, with newer additions including head-tracked spatial audio, lower latency for gaming, and in-game back-channel voice. Snapdragon Sound is also increasing AptX Lossless’ sampling rates from 44.1 to 48kHz. Buyers will need to research and understand which features are supported by their devices.
While Snapdragon Sound certification indicates tested and certified features and performance, it may not reliably indicate which specific features are usable. It’s important to research both device specifications to ensure compatibility and features.
