A Bit About Bluetooth
Bluetooth was not originally created 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 design 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 Low Complexity Subband Coding. However, Bluetooth devices may also support optional codecs, which can be found in the Bluetooth Advanced Audio Distribution Profile specification. The optional codecs listed are: MPEG 1 & 2 Audio, MPEG 3 & 4, ATRAC, and aptX. ATRAC is a codec that was used primarily in Sony products, most notably in the MiniDisc digital recording format.
Optional Codecs
The official Bluetooth standard, at section 4.2.2, states: “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. By design, Bluetooth does not necessarily 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 experiment with other formats, such as how Spotify uses the Ogg Vorbis codec. 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 MP3 or AAC files would pass unaltered.
What About aptX?
The quality of stereo audio through Bluetooth has improved over time. The current aptX codec, which is marketed as an upgrade to the mandated SBC codec, delivers “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 natural 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 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. 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 affect 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.
Factors Beyond Codecs
The aptX codec can’t compensate for lower-quality headphones and speakers, lower-resolution audio files and sources, or the varying capabilities of digital-to-analog converters 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, the HVAC system, 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. While Bluetooth as commonly implemented does degrade the 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 diminishes 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.
