Bluetooth Audio Codecs Explained

The main reason for discussing Bluetooth audio codecs is the fact that Bluetooth is the future of wireless headphones and earbuds. We’ve had some doubts for a while but the increasing number of new smartphones without aux ports has reassured even the most suspicious ones. Apple was the first company to remove the headphone jack from their iPhone 7 and some other manufacturers did the same.

Does the lack of headphone jacks imply that we don’t need them anymore? Well, we’ll see about that later but the fact is that the modern world is all about convenience. Considering the fact that we all live faster and that we are constantly “on the go”, the necessity to make everything light, portable and simpler is perfectly understandable.

Subsequently, Bluetooth technology is rapidly developing and improving, and it is probably going to replace the old way of listening to music. The quality of Bluetooth can still be argued but it has become obvious that we can’t fight the future. Like it or not, you should get ready for the new Bluetooth era.



Moreover, manufacturers of Bluetooth-enabled phones and headphones don’t always publish the complete specifications. Even if they do, they certainly don’t explain the meaning of each term and most of us can’t really understand their meaning. This is the main reason why so many people make their decisions based on the product’s design and price.

In order to help you get a better understanding of Bluetooth audio codecs, we have prepared this short article and tried to explain everything in simple and understandable words. 

Key Terminology

Before we proceed to the codec description, we will explain some of the keywords for understanding the whole story about Bluetooth audio codecs.

Bluetooth audio codec

Codecs define the way Bluetooth signal is transmitted from a source (e.g. smartphone) to a recipient device (e.g. headphones, speakers). It encodes and decodes the digital audio signal into a specific format. The main purpose of any codec is to transmit quality signal at a certain bit rate. This requires high compression of your audio files because the Bluetooth connection is still not capable enough. Unfortunately, the compression results in lower audio quality. This is the reason why low-res audio files can be stored in high amounts on your phone but high-res files require the storage space not smaller than a CD.

Sample rate

The sample rate is one of the most common words when it comes to audio files and Bluetooth audio codecs. It is expressed in hertz (Hz) and it represents the number of data points per second of an audio file. High sample rate implies higher file resolution and size. In this case, high-res files are the files exported at the sample rate equivalent to 96 kHz or higher.

Bit depth

Bit depth represents the number of bits contained in one audio sample. It determines the resolution of an audio file and it’s expressed in bits. If we take a CD-quality file as an example, we can say that it supports 16-bit and DVD-quality files, while Blu-ray discs usually support 24-bit audio files. The situation with bit depth value is the same as the situation with sample rate – the higher it is, the larger the audio file.

Bit rate

Bit rate represents the number of bits processed per each second of one audio file. It is expressed in kilobits/megabits per second (kbps/Mbps) and you can calculate it by multiplying sample rate, bit depth, and the number of channels.

Data transfer

In order to understand Bluetooth audio codecs, it is important to know a few things about Bluetooth data transfer. It has two main forms – ACL (Asynchronous Connection-Less) and SCO (Synchronous Connection Oriented). ACL implies asynchronous transfer executed without connection establishment while SCO represents synchronous transfer through connection establishment.



In general, data transfer is completed in accordance with the time-division scheme and through a process of changing frequency channels for each data packet that is transmitted. By the time-division scheme, we mean time division in slots/intervals, each lasting 625 microseconds. A single data packet can be placed in 1 to 5 slots and the number of occupied slots depends on the data packet size and the mode of transmission.

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Bluetooth EDR values of 2 or 3 Mbps that you can often find in the specs list represent the maximum data transfer rates for the total amount of data. In reality, data transfer rates significantly differ from the advertised values. When more than one device is connected to the source device, the data transfer rate is significantly lower due to the time-division principle. For example, if your phone is connected only to your headphones, the data transfer rate will be higher while it lowers down as you add more devices (smartwatch or a speaker) due to the fact that now all the three devices share the same data slots.

This is why you shouldn’t completely believe the advertised EDR values. Whenever you see the advertised transfer rates for a certain codec, you should be aware that these values are not constant. They vary with the number of connected devices and they also depend on the ambient conditions (indoor or outdoor, with or without obstacles, with or without interference). So, maintaining constant transfer rates is practically impossible.

Psychoacoustics

Psychoacoustics actually explores the way we perceive sound and it represents a model applied to all the digital media. It determines the data that can be compressed or eliminated without any consequences in the sound quality department. 

Compression

Compression can be defined as an algorithm used to remove certain frequencies from the audio spectrum in order to reduce the actual file size. The process of compression has been improved over the years and it now removes mostly those frequencies that are already inaudible to most humans. After all the improvements, we have two main types of compression now – lossy and lossless as well as two matching groups of audio formats.

The first one is more common and it creates 256 and 320-bit files in several formats: OGG, MP3, M4A, etc. 256-kbps M4A files are mostly used by Apple Music and iTunes.

On the other hand, lossless formats have bit rates higher than 500 kbps. They come in several formats: FLAC, WAV or ALAC. Lossless formats preserve all (or most of) the detail and frequencies from the original studio recordings. Subsequently, they deliver a much better sound quality

Different audio codecs have different compression rates. Some of these codecs are lossy, while others are lossless. Unfortunately, all the available Bluetooth codecs are lossy. However, there’s a very good chance that Bluetooth will soon be just as good as wired connection. 

The List of Bluetooth Audio Codecs

Whenever music is played via Bluetooth, it’s transmitted via one high-quality transmission profile called A2DP. This profile was established and certified in 2003, and it has suffered only minor changes ever since. On the opposite side, the number of codecs has increased since the introduction of the first codec. As a result, we have 14 different codecs today.

The most common Bluetooth audio codecs and their bit rates

The most common Bluetooth audio codecs and their bit rates

SBC

It’s safe to say that SBC is the basic codec that all or almost all devices support. It’s a low-complexity sub-band codec that supports audio files with minimum audio quality at medium bit rate. This was actually the first audio codec that became certified. Its maximum transfer rate is 320 kbps while the max bit rate is 345 kbps. It’s a lossy codec and it’s the least capable of all the available codecs. 

However, SBC is everywhere and this happens for a reason. Many devices support it as their only codec and the rest of the devices has it as a backup codec. Some call it the worst method of compression and we can’t say they are completely wrong. SBC has problems with frequency cut-outs as it focuses mostly on the lows and mids. On the bright side, this codec is quite flexible and it has many settings that allow the users to configure the codec in the most suitable way and control the whole data transfer process. Bitpool parameters can be adjusted and, subsequently, you can set the max bitrate. This helps when there is a chance of connection disruption because the codec adjusts these values until the connection becomes stable again.

AptX Codec Family

There are several aptX codecs and all of them are developed by Qualcomm. As most of the Android devices have Qualcomm processors, they usually support some aptX codecs. On the other hand, Apple devices don’t support them (except for the Mac devices). The performance of these codecs depends on the type of Android device and its overall quality but the best results are achieved when the aptX source device is paired with a pair of headphones or speakers that have aptX codecs. The three most popular and common aptX codecs are aptX, aptX HD, and aptX LL.

All these codecs are lossy. aptX codec supports files with a 352 kbps bit rate while the aptX HD is more capable with 576 kbps bit rate. Many people claim that this codec is significantly better than the regular aptX and that it offers better, “near-lossless” audio quality. However, both of them allow you to configure the number of channels or even to adjust the preferred mode. 

aptX divides audio files into 4 frequency bands with the same continuous number of bits. If the sample rate is 44 kHz, the first frequency band spans from 0 to 5.5 kHz and it gets 8 bits. The second spans from 5.5 to 11 kHz with 4 bits, the third has 2 bits and it spans from 11 to 16.5 kHz, while the fourth also has 2 bits and it spans from 16.5 to 22 kHz.

While SBC cuts off certain frequencies, aptX doesn’t but it reduces the dynamic range by adding quantization noise to those frequencies.

aptX HD is far less common than aptX and it represents the improved version in terms of bits per frequency band. If we take the same example (44 kHz sample rate, division into 4 bands), we will see that the first band gets 10, the second 6, the third 4, and the fourth 4 bits.

Besides these two, we also have aptX-LL, which is short for aptX Low Latency codec. It uses the same technology as regular aptX. It’s also lossy but it lowers the latency and its main purpose is to make watching TV and video content while using Bluetooth headphones/speakers possible. The latency you get with this codec is approximately 32ms, which is barely noticeable to the human eye. In comparison to this, other Bluetooth codecs have 80-140ms latency.

AAC

AAC stands for advanced audio coding and it’s, along with SBC, the most common Bluetooth audio codec. It’s more complex than the previous ones and it’s mostly used for streaming YouTube music and playing games using Sony PlayStation. It’s the second most popular codec after MP3 and it supports 256-320 kbps files. It works better with Apple than Android devices due to the fact that its streaming quality gets rather inconsistent with Android devices. Only the Android devices with the latest operating systems work nicely with this codec.

LDAC

This is the codec created by Sony, which implies that is almost exclusively found on Sony’s devices and it’s compatible with Android system versions 8.0 and later. It has a variable bit rate and it can transfer three times more data than the SBC. It also offers the highest audio quality among all the existing Bluetooth audio codecs. At least in theory. In reality, it has three modes: 990, 660, and 330kbps. In most cases, the data will be transferred at 330kbps. LDAC supports files with up 96 kHz sampling rate and 24-bit depth.

The first two modes (990kbps and 660kbps) lose fidelity above 20Hz, while the lowest one (330kbps) is worse than aptX and SBC. The 990-mode is the most desirable but the problem with smartphones is that they are preset to LDAC 330 mode, which means you have to set it up manually to make it work in more demanding mode. LDAC is closer to lossless codec than aptX HD.

This codec splits files in 12 to 16 bands. 12 bands are used for 44.1. or 48 kHz while 16 are used for 88.2 or 96 kHz.

Other codecs

Now that we have described the most common codecs, we will only list the rest of the 14 codecs that are available on the Bluetooth device market. There are 7 codecs left: MPEG-1/2 Layer 1/2/3, MPEG-2/4 AAC, ATRAC, FastStream, HWA LHDC, and the three Samsung codecs – Samsung HD, Samsung Scalable, and Samsung UHQ-BT.

All of these codecs are rarely used and available only on a limited number of devices. ATRAC has never been used as a Bluetooth codec. FastStream was created as a result of adding some support to the two-way SBC files and making a one-way A2DP standard transmission. It supports 44.1 to 48 kHz files and transmits them at 212 kbps. HWA LHDC is quite new and currently used by a few devices, while the three Samsung codecs also have extremely limited support.

Knowing the basics of Bluetooth codecs technology ensures you better Bluetooth audio experience. It enables you to match your devices properly and get the most out of them. It teaches you that you can’t use the same equipment for every activity and that you have to set your priorities and choose the equipment accordingly.



If you’ve been wondering if it’s really possible for Bluetooth to replace the good old wired connection, you should know that the transition has already begun but it’s not going to be quick.

At the moment, replacing the cables with Bluetooth comes at a cost and the cost is lower audio quality. A certain part of the original recording is lost during the Bluetooth transmission, even if the most capable Bluetooth codecs (like LDAC and aptX HD) are used. Another big issue with all the Bluetooth audio codecs is the latency (audio lag). However, we all appreciate the convenience of a wire-free environment and that’s what enables further development of Bluetooth. New Bluetooth audio codecs are being introduced every year and each brings some improvement in terms of transfer rates and latency reduction.  

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