Bluetooth codecs – The swamp drained

Once upon a time, it was incredibly easy to get music from your smartphone into your ears through earplugs. That went through a port with a diameter of 3.5 mm. You could plug in a plug there and then you had stereo sound. Fortunately, the port is still there on many smartphones, and you can also plug earbuds in via the USB-C port on phones, although that’s not ideal.

Many people find listening to music wired not ideal anyway and wireless earplugs have been difficult to find in recent years, although Bluetooth also has its drawbacks. Almost all smartphone manufacturers have released some sets, and then we are not even talking about the audio brands. Traditionally, however, bluetooth is not intended for audio at all. Bluetooth itself has therefore received many adjustments over the years to make it usable. Bluetooth Low Energy has reduced power consumption, Dual Audio lets you connect to several devices at once, and speed and range have also improved in recent years.

That’s nice, but the codecs used also have an impact on the listening experience and it is usually not clear which codec is being used at any given time. If you haphazardly buy wireless headphones or earbuds, you may not know at all whether the codecs that are best for your devices are supported. For that reason alone, it is useful to know which codecs are on offer and with which they work. Hence this overview article about bluetooth codecs.

Codecs

Many users no doubt know what a codec is, if only from the MP3 era. As a memory refresher, we briefly explain what it is and how it works. A codec is a software or hardware that is capable of encoding and decoding. In this article, we are talking about software that encodes the audio on the sending device side and decodes it on the receiving device.

There are three ways to store audio: uncompressed, with lossless compression, and with lossy compression. For example, you know uncompressed audio from the Wav file format. Saving all your audio in this way is of course possible, but it is often not convenient because it takes up a lot of space. This certainly applies to Bluetooth audio.

You might think that compression is always lossy , so data is lost. That is not the case. Just think of a zip or a rar file. The audio is, as it were, simply unpacked and then played. Examples of this are codecs such as the open source FLAC and Apple’s lossless format ALAC. The data is compressed, but no data is lost. Lossless compressed files are still too large for many applications, but there are more and more streaming services that offer lossless streaming, such as Tidal, Qobuz and recently Apple Music.

In addition to this lossless compression, there is also lossy compression, which you of course know from mp3, but also AAC. Mp3 has a bit rate of up to 320kbit/s and with AAC it is 256kbit/s. That is limited. These two file formats use psychoacoustic models. In fact, it is the essence of these audio formats. These models contain information about which data can theoretically be omitted without being clearly audible. This saves a lot of data and psychoacoustic models are still used. After all, bandwidth and storage are not always abundant and that also applies to bluetooth.

Environment and delay

Bluetooth is subject to disruptive environmental factors. These can be physical obstacles or radio frequencies. For example, a microwave can disrupt your Bluetooth connection. The distance between transmitter and receiver is of course also a factor. Due to all these factors, bluetooth audio must be quite ‘flexible’. While the maximum bandwidth may be sufficient for high bit rates, ideal conditions are rare. Therefore, audio for bluetooth is compressed.

As Bluetooth audio is sent and received, there is a delay, or latency . It is expressed in milliseconds. How much latency there is depends on many factors. Consider the computing power of the devices used, but also the implementation of the Bluetooth standard and the codec used. The latency that stands for the various individual codecs is theoretical and can differ considerably in reality. Nevertheless, we take this factor into account when discussing the various codecs, because it is important for gaming and music production, for example.

When playing video, the problem of lag has been largely solved by A/V sync. The latency of the Bluetooth audio is estimated and the video is delayed by the same number of milliseconds. With the advent of Bluetooth 5, this technology has become commonplace. This works fine on YouTube, for example, and you can see that the video pauses for a moment and then starts playing.

Bluetooth was never intended for audio, but due to recent codecs it now works fine and the quality is acceptable for most. We’re getting somewhat close to wired audio.

A fresh dip in the swamp

Now to the most used Bluetooth codecs of the moment. We start with an overview. It might be good to explain what the different specifications are. The bit rate is simply the amount of data per unit of time, expressed in kilobits per second. The sample rate is the frequency range, expressed in kilohertz. Samples for sound can be compared to pixels in an image. If the sample rate is 44.1 kHz, as with CDs, then the audio contains 44,100 samples per second. The bit depth is the dynamic range. It is the number of bits of information per sample, so actually the resolution per sample. For example, this is 16bit, as with CD audio, or 24bit, which you encounter with DVDs and Blu-rays.

The receiving device, for simplicity we will assume in this article that we are talking about a smartphone that connects to earbuds and communicates with the earbuds on the best codec to use. Sony earplugs will ask for LDAC and Samsung earplugs for the Samsung Scalable Codec. More on that later.

SBC

You can think of SBC as a kind of basic codec. All devices with bluetooth support it in principle. So you can always fall back on it. Usually, however, it is not the ideal codec to use. While the maximum specifications are fine, as you can see in the table, the variable aspect combined with different manufacturers’ implementations means that the quality is often limited compared to the other Bluetooth codecs we discuss here.

AAC

This is the codec Apple prefers. AAC uses psychoacoustic models, but as an Android user it is generally best to avoid it. That’s because different Android manufacturers have very different implementations, and that’s because Android doesn’t have a standardized way of dealing with the codec. It is also a power-hungry codec for use with Android. However, if you have an Apple device, it’s a fine codec and the default bit rate is 245kbit/s.

AptX

AptX is from Qualcomm. This codec is especially important for Android users and almost all Android phones support it. It is not available for iOS users. The reason why it is preferable to SBC for Android users is that the bit rate and sample rate do not vary too much. AptX has a bit rate of 352 or 384 and a sample rate of 44.1 or 48kHz. This makes the quality more consistent in practice.

AptX HD is simply the high-definition variant of AptX. With a bit rate of 576kbit/s, a bit depth of 24bit and a sample rate of 48kHz, this codec is superior to AptX. You can see here which devices support it and thus whether it might make sense to buy earplugs that also support it. The signal-to-noise ratio is also better with AptX HD than with the regular AptX.

There are even more AptX offspring. AptX LL stands for ‘low latency’ and has a maximum delay of only 40ms. It is rare, but may be interesting for gaming and music production. It is being replaced by the fairly new AptX Adaptive. The idea is that it is more scalable without sacrificing quality. At least that’s the goal according to Qualcomm. So the codec has variable bitrates between 279 and 420kbit/s, unlike the AptX codecs above. They choose a fixed bit rate. Whether Adaptive can be as good as AptX HD is open to question, but according to Qualcomm it is. Because the codec handles the data more efficiently, the quality must be higher at the same bit rate than with AptX and AptX HD. Where other AptX codecs and for example LDAC have different bitrate presets,

This is useful, because the disturbances are monitored, so that you can switch back to a slightly lower bit rate if necessary. Failure can then be avoided. In addition, it makes little sense to send audio with lower bit rates that are played by streaming services with higher bit rates. After all, that costs more bandwidth and energy.

LDAC

LDAC is from Sony and is a good high-end codec. Although LDAC, as you can see, potentially has a higher sample rate and bit rate, the steps between the different qualities are much larger. LDAC has a bit rate of 330, 660 or 990kbit/s. AptX Adaptive is therefore more flexible and also has a much lower latency, which is useful for gaming, for example. However, if you stream lossless audio, as is possible with Tidal, among others, you benefit from the higher bit rates and sample rates. Your headphones must of course be able to handle LDAC.

It should be added that the numbers are not really comparable. For example, we already talked about a more efficient way of packing data with AptX Adaptive compared to the other AptX variants, so that the same fidelity can be achieved with a lower bit rate. Still, in theory, this is the best codec out right now for most if you want to listen to high quality audio.

There is a way to ensure that LDAC always operates at the maximum bitrate of 990kbit/s. You then have to repeatedly tap the Build number in the Android settings under ‘info’ or ‘about the phone’ or something similar, depending on the manufacturer. You will then be given developer options at your disposal. Then, while your earplugs are connected, you can choose the audio codec used under networks and, if you choose LDAC, also the desired bitrate. You may experience a less consistent connection, but you know that the sound you hear is of the highest possible quality. As with AptX, LDAC is of little use to you as an iOS user, unless you connect a separate dac to your iOS device.

Samsung Scalable codec

Many Samsung devices support the Samsung Scalable codec. Despite the high variation in bit rate, this codec delivers excellent audio quality and, above all, stability in practice. Its operation is very similar to that of AptX Adaptive. The Bluetooth disturbances are monitored and the bit rate is adjusted accordingly. As with AAC for iOS users, users of Samsung smartphones and Samsung earbuds generally don’t need to think any further; this codec is almost always the right choice.

LHDC (HWA)

Another interesting codec is the Low Latency High-Definition Audio Codec, or LHDC. This one is fairly new and was introduced with Android 10. Not all manufacturers support the codec. Xiaomi, Huawei and OPPO do and that’s quite nice, because if you own a device that can handle it, the maximum bit rate is high. While we can’t find reliable, precise numbers on latency, it would also be lower than many other codecs. For now, there are few headphones that support this codec, but what is not, may yet come.

There are even more Bluetooth codecs than we mention in this article, which are used less often. For example, there is also a low-latency version of LHDC called LLAC. This one is useful, you guessed it, for gamers. However, we have now covered the most important codecs.

Conclusion

So much for this overview of audio codecs. Actually, there is not a lot you need to know to get good sound via Bluetooth, especially as an Apple owner. In fact, most iOS owners are done with AAC. If you buy new earplugs, make sure that AAC is supported, otherwise you will be condemned to SBC.

As an Android user, it is definitely recommended to check which codecs are supported by your sending and receiving device. With the information from this background story you can then determine which codec is the best for you. As mentioned, you can check which codec is being used in Android’s developer options and possibly switch to a different one or, as with LDAC, to a constant bitrate. Especially if you are considering a new pair of headphones, it is of course important to take a good look at which codecs are supported. For example, if you buy a product like the Galaxy Buds Pro and you don’t have a Samsung smartphone, you’re stuck with SBC or AAC, neither of which is an ideal option for Android users.