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 and LDAC than the popular, yet limited, SBC and AAC codecs.
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 at least twice the limits of human hearing (approximately 20 kHz). Higher resolution file formats tend to be exported at 96 kHz or greater. A greater sample rate means a greater filesize.
- 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-bits, but high resolution files extend this to 24-bits. 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 (192-320 kbps) 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. Though only aptX LL supports a latency of less than 40 milliseconds, Qualcomm has retained backwards compatibility with preexisting CSR aptX products, such as Noontec’s Hammo Wireless headphones. This family of codecs is the most stable currently.
Why choose aptX over SBC? Greater transfer rates preserve more data. The simpler aptX codec supports 48 kHz / 16 bit LCPM audio data (352 kbps), while aptX HD supports 48 kHz / 24 bit LCPM audio data (576 kbps). 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. It has a transfer rate cap of 250 kbps, 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, which transfers up to 3x the data compared to SBC. It’s able to maintain a sample rate and bit depth at 96 kHz / 24 bit (maxed at 990 kbps). High transfer rates are great and all, but the source device also has to be compatible with the software. If the device can’t support such high transfer rates, it’ll revert back to a lower one that’s stable. Prior to Android 8.0 Oreo, LDAC was only available on Sony’s Xperia phones—so the codec was only available to someone with all-Sony equipment. Those of us with older phones or iPhones won’t be able to enjoy Oreo’s codec inclusions.
As you can see, it’s easy to become inundated with—and fascinated by—the swathes of information pertaining to Bluetooth technology. 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 sister-site Android Authority to learn more about Bluetooth.