I'm doing some research on Bluetooth speakers for a project right now, reading lots of product reviews and basically getting a better idea of what products are out there and which of them are any good. And as I go through these reviews, I'm seeing a lot of comments about Bluetooth reducing sound quality. Most people seem to think that from an audio fidelity standpoint, you're always better off choosing a WiFi-based wireless technology -- such as AirPlay, DLNA, or Sonos -- than you are choosing Bluetooth. But while that's generally correct, it doesn't have to be. There's room in the Bluetooth spec to do better.
Bluetooth was originally designed not for audio entertainment, but to connect phone headsets and speakerphones. It was also designed with very narrow bandwidth. Thus, it applies data compression to an audio signal. That's perfectly OK for phone conversations, but with Bluetooth now used just as often for music reproduction, it's applying this compression on top of data compression that might already be there, as is the case with MP3 or AAC files on a phone, or MP3 or AAC audio streamed through the Internet.
However, a Bluetooth system doesn't have to apply this additional compression. Here's why:
All Bluetooth devices must support SBC, or subband codec. However, Bluetooth devices may also support optional codecs, which are listed in the Bluetooth Advanced Audio Distribution Profile (A2DP) specification. The optional codecs listed are: MPEG 1 & 2 Audio, MPEG 3 & 4 AAC, and ATRAC. Just 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.
Now let's take a look at a couple of lines from the A2DP spec. These are lifted straight from the document, which you can download here as a zipped PDF.
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 is your smartphone, tablet or computer, and the sink is your Bluetooth speaker or headphones or receiver.
What this means is 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 you stored on your phone, tablet or computer, Bluetooth doesn't have to degrade the sound quality if your Bluetooth receiving device also supports that format.
This rule also applies to Internet radio streams that are encoded in MP3 or AAC, which covers the vast majority of audio streams available today. Most Internet radio stations -- in fact, all of them that I've encountered -- use MP3 or AAC. So does Pandora. The big exception among streaming services is Spotify, which uses the Ogg Vorbis codec.
However, 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 Apt-X?
Anyone who's cognizant of what's happening in Bluetooth has heard of the apt-X codec, which is marketed as an upgrade to the mandated SBC codec. Assuming the claims made for apt-X's improved sound quality are true (I have not had a chance to evaluate them), they would yield an improvement over SBC. However, if you're playing MP3 or AAC material, and both your Bluetooth source and sink devices both support that format, the manufacturer would be better off using the native format of the original audio file without applying apt-X or SBC.
Let me end with a little grain of salt: Most Bluetooth audio products are built not by the company whose brand they wear, 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. I learned from my days working at Dolby Laboratories that the more complex a digital product is, and the more engineers who are 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 because almost no Bluetooth receiving devices tell you what the incoming format is.
Still, though, I think it's important that we recognize that while Bluetooth as commonly implemented now does degrade audio quality, it doesn't have to. It's up to the phone manufacturers, primarily, to implement Bluetooth in a way that impacts audio quality the least -- or preferably, not at all. And of course, when you consider that 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.