November 10, 2017 by Mixcder
Bluetooth is a short-range, low-power wireless standard for transmission of digital data that has been around since 1998. It has many uses, but of course the one we are most interested in here at The Headphone List is audio streaming.
Bluetooth has evolved a number of specifications and profiles over the years, which can be somewhat confusing and are rarely explained all in one place. The purpose of this article is to provide a brief introduction to Bluetooth technology pertinent to audio streaming.
Bluetooth devices come in all manners of shape, size, and function, but as far as audio is concerned there are two basic types – transmitters and receivers. A Bluetooth audio transmitter is any audio source with Bluetooth functionality, for example an iPhone, computer, or dongle that receives audio input from another device. A Bluetooth audio receiver can be a wireless headphone or speaker, or a dedicated audio receiver.
The transmitter sends out a digital signal encoded in a way the receiver can recognize. The receiver then decodes this signal and converts it to analog for audio playback. To establish a link between transmitter and receiver and determine which Bluetooth profiles can be used (see Bluetooth profiles section below), an initial setup step called “pairing” is necessary.
Bluetooth profiles are sets of specifications for how certain functionality must be implemented by Bluetooth devices. There are dozens of profiles for everything from file transfer to wireless printing, but the ones we are interested in are those applicable to headphones and headsets.
HSP (Headset Profile) – allows basic headset functionality with microphone input, 64kbps mono audio, and limited remote control
HFP (Handsfree Profile) – a more advanced version of HSP also designed for mono headsets; among other things adds support for additional remote functions such as redialing and voice dialing. The latest version (1.6) adds optional support for better-quality audio via a mono configuration of the SBC codec used by A2DP
A2DP (Advanced Audio Distribution Profile) – designed for multimedia and used for stereo audio transmission over Bluetooth. This is the profile we are interested in as headphone users
AVRCP (Audio/Video Remote Control Profile) – provides remote control of media playback (e.g. play/pause and skip/previous track functions); used in conjunction with A2DP. The latest AVRCP versions (1.4 and 1.5) allow direct control of device volume as well. With previous versions, volume controls affected headset volume independently of the connected device.
All stereo headphones support A2DP and AVRCP, and – if a microphone is present – at least HSP for headset functionality. Most stereo Bluetooth headsets implement all four of the above profiles.
So far so good, but what determines how audio is sent to a wireless headphone? That would be the codec. After the transmitter selects the appropriate profile, it chooses a codec to digitally compress the audio for sending to the headphone. The headphone then decodes the file for playback. Although it is theoretically possible to send mp3s or any other digital format straight to the headphones over Bluetooth, this is not what happens. Instead, the A2DP profile specifies its own set of audio codecs.
SBC (Subband Coding) – this is the mandatory default codec for A2DP and is supported by all Bluetooth stereo audio devices. It was designed to provide reasonably good audio quality over the limited bandwidth of Bluetooth without heavy processing power requirements for encoding/decoding. SBC is capable of bitrates up to about 328 kbps at a 44.1kHz sampling rate (source). This is not far off from the maximum for mp3 encoding, though mp3 is superior to SBC in terms of audio quality given similar bitrates (source).
aptX – aptX is a proprietary audio codec developed for demanding audio applications. Its use in Bluetooth devices is entirely optional. In fact, aptX is currently supported by only one manufacturer of Bluetooth chipsets – CSR – who acquired parent company APT back in 2010 (source).
aptX is designed to encode a CD-quality (16-bit / 44.1kHz) audio stream without loss of sound quality through a combination of slightly higher data transfer rate compared to SBC as well as more efficient audio encoding. The caveat is that both the headphone and the source must support aptX – if either one lacks aptX support, the default SBC codec will be used instead.
Mixcder HD401 adopts aptX tech, while Mixcder MS301 and HD601 utilize aptX LL (low latency) tech which reduces delay and improves end-to-end speed of the audio transmission, resulting in a high quality, synchronised user experience.
AAC (Advanced Audio Coding) – unlike SBC and aptX, AAC is a codec popular outside of wireless applications, including Apple’s iTunes platform and YouTube. It was designed to achieve better sound quality than mp3 at similar bit rates. Unlike aptX, it is supported by iOS devices. Apple implements AAC over Bluetooth at about 250 kbps (source – pdf), which should compete in fidelity with the best-quality mp3s or any other lossy codec. However, at this time AAC support in wireless headphones is not very common.
At least in theory, Bluetooth transmitters automatically default to the “best” quality codec available. The default Bluetooth stereo audio codec, SBC, can sound quite good with the right implementation, especially for on-the-go use. However, it can also produce some horribly compressed-sounding audio at its worst. aptX takes out the guesswork, maintaining consistently high bitrates, but both the transmitter and receiver have to support it. AAC is supported by Apple devices, but not many other transmitters, and has been picked up by few headphones. For those concerned with audio quality, at least for the time being, aptX seems to be the solution to beat.
There are a few more Bluetooth functions that are supported by some current headphones. These are independent of the profiles and protocols listed above and don’t affect audio quality, but are worth mentioning for the sake of completeness.
NFC (Near field communication) – NFC is a wireless standard for close-proximity data exchange. While it can technically be considered a Bluetooth competitor, NFC has a very different set of strengths and weaknesses. It has slower transfer speeds than Bluetooth and a much shorter range (a few inches at most), but also uses far less power and does not require “pairing”.
There are many different uses for NFC ranging from mobile payments to data transfer. When it comes to headphones, however, NFC is used as a “handshaking” shortcut to establish connection. Bluetooth devices that support NFC can be paired and connected by simply bringing their NFC chips together. It allows users to pair, connect, and disconnect NFC-enabled headsets without ever opening the phone’s Bluetooth menu. Doesn’t seem like a big deal at first, but it is surprisingly convenient in everyday use. The caveat, of course, is that that both devices have to support NFC.
If you are interested in this NFC, Mixcder HD601 is worth trying since it not only has NFC but also aptX low latency.
Multipoint – Multipoint (as opposed to Singlepoint) refers to the ability of a Bluetooth accessory – most likely a headset – to maintain two Bluetooth connections simultaneously. For instance, a Multipoint headset can have a tablet and phone connected to it at the same time. If a call comes in on the phone while music is streaming from the tablet, music playback on the tablet will be paused and the user will be able to take the phone call on the headset. When the call is terminated, the headset will switch back to the tablet and resume music playback automatically.
Like NFC, multipoint is not at all vital and has no bearing on audio fidelity, but it’s a pretty nice feature to have that I can see become more widely desired as the popularity of wireless headsets grows.
Original article: Wireless fidelity: making sense of Bluetooth headphone technology
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