Question: What are the differences between Bluetooth wireless codecs?

Answer: To understand the differences between the Bluetooth wireless codecs we first need to understand a few key terms: bit depth, sample rate, bit rate, latency, and compression. Bit depth is the number of bits (basic unit of information in digital communication) in each sample. This corresponds with the resolution of each sample, as a higher bit depth will create higher resolution audio reproduction. Sample rate determines the number of samples per second that is taken from an audio signal to create a digital signal. Bit rate refers to the number of bits that can be transferred per second. We can use bit depth and sample rate to determine the bit rate by using the equation:

SAMPLE RATE x BIT DEPTH x NUMBER OF CHANNELS (usually 2 for left and right) = BIT RATE.

Using this equation, let’s calculate the bit rate of your standard CD. We can then use this information as a standard of reference when we start looking at different Bluetooth codecs. CDs usually have a sample rate of 44,100 samples per second, and a bit depth of 16-bit. They will also have two channels for your left and right audio. When we multiple these together we get 1,411,200 bits per second, or 1,411.2 kbps.

The last two terms we will want to understand are latency and compression. Latency is the amount of time it takes for a signal to travel through a system. The most common latency when it comes to Bluetooth transmission is audio output latency, which is the time it takes for the sample to play through an audio device after the sample has been created. Compression is a way to make audio data smaller so it can more easily be transferred wirelessly. For example, a Bluetooth codec may reduce the CD-quality 1,411.2 kbps rate down to about 300 kbps. This does result in some loss of audio quality (such as missing higher frequencies that the human ear is less likely to detect), but the stability of the stream from your phone or computer to your headphones is increased due to less data being transmitted.

Now let’s look at five Bluetooth codecs that are commonly used in current devices.

SBC – All Bluetooth devices support the SBC codec, so we can view this codec as the lowest common denominator for Bluetooth connection. Due to the manageable transfer rates of this codec (192 kbps to 320 kbps max), it allows the connection to be stable. Unfortunately, this comes with the caveat of lower audio resolution and quality.

AAC – This is the Bluetooth codec preferred by Apple, and can be inconsistent when used with Android devices. AAC has a max transfer rate of 250 kbps, but can support audio quality up to 24-bit/96 kHz. This low transfer rate keeps stability high, while providing higher audio quality than the SBC codec.

Qualcomm® aptX™ audio – This codec is offered in Qualcomm Bluetooth chips, and is mostly found in Android devices. What separates aptX from the previous two codecs, is that, due to the way aptX encodes data, it provides minimal latency (120 milliseconds, compared to SBC’s 200-300 ms).

Qualcomm® aptX™ HD – This is a higher bandwidth version of the aptX codec, and the higher bit rate allows for higher quality audio to be transmitted. At 576 kbps, right around double the three previous codecs’ bit rates, this may be the first time you can hear an audible difference in quality if given a blind Bluetooth listening test.

LDAC – The last codec we’ll look at is Sony’s LDAC. This codec can transfer data up to 990 kbps, which is significantly higher than the previous codecs, and is the closest to our target CD bit rate. Due to the high bandwidth that this codec takes up, there is the opportunity for latency or loss of connections. Luckily this codec also offers a normal mode (660 kbps) and a connection priority mode (330 kbps), in case you want to slightly sacrifice audio quality for a more stable connection.

In conclusion, we now have a better understanding of the benefits and possible disadvantages of a few of the Bluetooth codecs. Among the key takeaways is the fact that all Bluetooth devices have the SBC codec. This allows there to always be a common denominator that devices can fall back on in order to connect. If you are using an Apple product, then you are usually using the AAC codec when connecting to Bluetooth devices. Qualcomm aptX is a very versatile codec that offers low latency, while alternative versions such as aptX HD can provide higher audio quality. Lastly, if you have two devices that can both support the LDAC codec, you know you are getting the highest audio quality that a Bluetooth transmission can offer.

If you have any additional questions, or want to know what codecs your Audio-Technica Bluetooth wireless product supports, feel free to contact the Audio Solutions Team for more information.