What You Might Not Know About Bluetooth, Sound Quality, and aptX

Reasons Why Bluetooth Can Reduce Audio Quality

The Bluetooth logo and symbol

Bluetooth has quickly become the most common way to enjoy wireless audio through speakers and headphones. However, one concern that some have is with regards to Bluetooth and the overall reduction of sound quality. There are those who feel that – from an audio fidelity standpoint – you're always better off choosing one of the WiFi-based wireless technologies, such as AirPlay, DLNA, Play-Fi, or Sonos.

While that belief is generally correct, there's more to using Bluetooth than you might know.

Bluetooth was originally created not for audio entertainment, but to connect phone headsets and speakerphones. It was also designed with a very narrow bandwidth, which forces it to apply data compression to an audio signal. While this may be perfectly fine for phone conversations, it's not ideal for music reproduction. Not only that, but the Bluetooth could be applying this compression on top of data compression that might already exist, such as from digital audio files or sources streamed through the Internet. But one key thing to remember is that a Bluetooth system doesn't have to apply this additional compression. Here's why:

All Bluetooth devices must support SBC (stands for Low Complexity Subband Coding). However, Bluetooth devices may also support optional codecs, which can be found in the Bluetooth Advanced Audio Distribution Profile (A2DP) specification.

The optional codecs listed are: MPEG 1 & 2 Audio (MP2 and MP3), MPEG 3 & 4 (AAC), ATRAC, and aptX. To clarify a couple of these: The familiar MP3 format is actually MPEG-1 Layer 3, so MP3 is covered under the spec as an optional codec. ATRAC is a codec that was used primarily in Sony products, most notably in the MiniDisc digital recording format.

Let's take a look at a couple of lines from the A2DP spec sheet, which can be found as a PDF document on Bluetooth.org.

4.2.2 Optional Codecs

The device may also support Optional codecs to maximize its usability. When both SRC and SNK support the same Optional codec, this codec may be used instead of Mandatory codec.

In this document, SRC refers to the source device, and SNK refers to the sink (or destination) device. So the source would be your smartphone, tablet, or computer, and the sink would be your Bluetooth speaker, headphones, or receiver.

This means that Bluetooth does not necessarily have to add additional data compression to material that is already compressed. If both the source and sink devices support the codec used to encode the original audio signal, the audio may be transmitted and received without alteration. Thus, if you're listening to MP3 or AAC files that you have stored on your smartphone, tablet, or computer, Bluetooth doesn't have to degrade the sound quality if both devices supports that format.

This rule also applies to internet radio and streaming music services that are encoded in MP3 or AAC, which covers much of what is available today. However, some music services have been exploring other formats, such as how Spotify uses the Ogg Vorbis codec.

As overall internet bandwidth increases over time, we could be seeing more and better options in the near future.

But according to Bluetooth SIG, the organization that licenses Bluetooth, compression remains the norm for now. That's mainly because the phone must be able to transmit not only music but also rings and other call-related notifications. Still, there's no reason that a manufacturer couldn't switch from SBC to MP3 or AAC compression if the Bluetooth receiving device supports it. Thus the notifications would have the compression applied, but native MP3 or AAC files would pass unaltered.

What About aptX?

The quality of stereo audio through Bluetooth has improved over time. Anyone who's cognizant of what's happening in Bluetooth has heard of the aptX codec, which is marketed as an upgrade to the mandated SBC codec. The claim to fame for aptX is its ability to deliver "CD-like" audio quality over Bluetooth wireless. Just remember that both the Bluetooth source and sink devices must support the aptX codec in order to benefit. But if you're playing MP3 or AAC material, the manufacturer might be better off using the native format of the original audio file without additional re-encoding through aptX or SBC.

Most Bluetooth audio products are built not by the company whose employees wear their brand, but by an ODM (original design manufacturer) you've never heard of. And the Bluetooth receiver used in an audio product probably wasn't made by the ODM, but by yet another manufacturer. Those who have been in the industry learn that the more complex a digital product is, and if there are more engineers working on it, the more likely it is that no one knows everything about what's really going on inside the device. One format could easily be transcoded into another, and you'd never know it because almost no Bluetooth receiving device will tell you what the incoming format is.

CSR, the company that owns the aptX codec, claims that the aptX-enabled audio signal is delivered transparently over the Bluetooth link. Although aptX is a type of compression, it's supposed to work in a way that doesn't greatly impact audio fidelity (versus other compression methods). The aptX codec uses a special bit rate reduction technique that replicates the entire frequency of the audio while allowing the data to fit through the Bluetooth "pipe" wirelessly. The data rate is equivalent to that of a music CD (16-bit/44 kHz), hence why the company equates aptX with "CD-like" sound. 

But it's important to recognize that every step in the audio chain affects the output of sound. The aptX codec can't compensate for lower-quality headphones/speakers, lower-resolution audio files/sources, or the varying capabilities of digital-to-analog converters (DACs) found in devices. The listening environment has to be considered as well. Whatever fidelity gains made through Bluetooth with aptX can be obscured by noise, such as running appliances/HVAC, vehicle traffic, or nearby conversations. With that in mind, it may be worth choosing Bluetooth speakers based on features and headphones based on comfort rather than codec compatibility.

It's important to recognize that while Bluetooth (as commonly implemented) does degrade audio quality (to varying degrees), it doesn't have to. It's primarily up to the device manufacturers to use Bluetooth in a way that impacts audio quality the least – or preferably, not at all. Then you have to consider that the subtle differences among audio codecs can be hard to hear, even on a really good system. In most situations, Bluetooth will not have a significant impact on the sound quality of an audio device. But if you ever have reservations and want to eliminate all doubt, you can always enjoy music by connecting sources using an audio cable.