As companies attempt to kill the headphone jack, Bluetooth has become even more prevalent. Though fewer wires in my life has been a relief, the nature of Bluetooth audio introduces some strange new headaches. In order to bring you up to speed in a matter of minutes, we’ve put together this guide for understanding Bluetooth codecs.
The long in the short of it is this: If you’re going wireless, invest in a product that supports a high bitrate. High-quality audio is streamed more efficiently over codecs like aptX than SBC.
Editor’s note: this article was updated on June 6, 2019, to account for research pertaining to AAC’s performance on Android phones and to address Android Bluetooth latency issues.
What you should know
- Basic terms, discussed by Robert Triggs, Android Authority.
- Sample rate (Hz): the number of points of data per second in an audio file. You need two samples to accurately capture any frequency, so audio is sampled at least twice the limits of human hearing (approximately 20 kHz). Higher resolution file formats tend to be exported at 96kHz or greater. A greater sample rate means a greater file size.
- Bit-depth (-bit): the number of bits saved for each audio sample. A higher bit depth records a signal more accurately. CD quality is 16-bit, but high-resolution files extend this to 24-bit. A greater bit depth multiplies the filesize.
- Bit-rate (kbps): usually measured in kbps or Mbps. This is the amount of audio data transferred per second over Bluetooth. For uncompressed files, this is calculated by multiplying the sample rate by the bit-depth.
- Data rates are unstable.
- Bluetooth devices have an assigned range (typically 3 meters for headphones). This is because the further you get from the source, the more interference from physical barriers (e.g. walls, cars, people) and other frequencies (e.g. radios, televisions, WiFi signals).
- Psychoacoustics studies how humans perceive sound. A psychoacoustic model is applied to digital media, and determines what can be deleted to save space without a noticeable loss of sound quality.
- This is how MP3 compression came into the world. Its influence on strategic compression extends to virtually every audio format.
- There are three main types of audio compression formats, uncompressed, lossless, and lossy.
Now that you’ve passed Wireless Audio 101, let’s continue.
In terms of software, a codec determines how Bluetooth is transmitted from the source to your headphones. It encodes and decodes digital audio data into a specific format. Ideally, it transmits a high-fidelity signal at the minimum specified bit-rate. This results in the least amount of space and bandwidth required for storage and playback, respectively. A lower bitrate means better compression and worse sound quality, a high bitrate means better sound quality and worse compression. So how do codecs navigate this compromise?
The low-complexity sub-band codec (SBC) divides the signal into multiple frequency bands and encodes each one independently. Think of SBC as the lowest common denominator among Bluetooth codecs. It’s not the best. It is, however, mandatory among all A2DP-enabled devices, making it virtually universal. Manageable transfer rates (192-320kbps) are delivered at the expense of significant data loss.
Bluetooth's Achilles' Heel is its limited bandwidth. High transfer rates may overload available bandwidth, causing a stutter—or complete crash—of the streaming service.
Now, Qualcomm’s proprietary codecs, aptX, aptX LL, and aptX HD receive frequent recommendations here at SoundGuys, not to mention aptX Adaptive. Though only aptX LL supports a latency of less than 40 milliseconds. What’s more, Android’s wireless efficiency is inconsistent depending on what source device is used.
Why choose aptX over SBC? Greater transfer rates preserve more data. The simpler aptX codec supports 48kHz/16-bit LCPM audio data (352kbps), while aptX HD supports 48kHz/24-bit LCPM audio data (576kbps). Though both are lossy formats, they’re leagues ahead of SBC. Plus, they support a fine enough bit-rate to keep everything running smoothly and sounding phenomenal.
Advanced audio coding (AAC). This is the audio standard for lossy digital audio compression. It also happens to be the license-free standard for YouTube, Sony’s PlayStation 3, and is preferred by Apple. If you have an Android phone, you won’t really benefit from AAC as its performance is unreliable: it’s a power-hungry codec that Android remains unprepared to handle efficiently. iPhone users do benefit from its higher-resolution playback though. It has a transfer rate cap of 250kbps, creating a file similar to that of a mid-quality MP3.
Like Qualcomm, Sony has its own proprietary Bluetooth codec, LDAC. Its variable bit-rate is the defining feature. In theory, it should consistently transfer up to 3x the data compared to SBC. Our studies revealed that its highest bitrates (990 and 660kbps) lose fidelity above 20kHz. What’s more, both aptX and SBC outperform LDAC when it streams at 330kbps, which is the default chosen by many phones. In order to change this, you have to enter developer settings and force a higher bit-rate, but smartphones’ “best efforts” vary greatly.
|Phone||LG V30+||Samsung Galaxy Note 8||Huawei P20 Pro||Huawei P20||Google Pixel 3 XL||Google Pixel 3|
|LDAC 'Best Effort' Setting||990kbps||660kbps||660kbps||660kbps||330kbps||330kbps|
As you can see, it’s easy to get lost in Bluetooth jargon. Though this is a lot of information to keep straight, remember that higher transfer rates are good, but no matter how great the kbps: you need both your phone and headphones to speak the same language. Again, aptX and aptX HD are usually your best bet with consumer-grade headphones. Qualcomm’s codec is becoming increasingly prevalent and low latency is a feature that many of us greatly appreciate.
We’ve barely scratched the surface here. In fact, we’re just beginning to look upon the surface. If you want to learn more, head over to our all-encompassing Bluetooth codec guide.