Other hardware
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Bluetooth
Bluetooth has two unusual requirements: codec complexity and latency. Complexity decides battery life, but with the codec doing less work, it's not usual to see bitrate/quality trades way worse than conventional codecs.
A2DP
Advanced Audio Distribution Profile (A2DP) is the traditional music profile.
Standard baseline codecs:
- Sub Band Codec (SBC) – okay, not great
Standard optional codecs:
- MPEG-1,2 Audio: = MP3, almost never used
- AAC: relatively good decode support, transparency possible at bluetooth bitrates, but latency can be an issue
- ATRAC: never used
Vendor extension codecs:
- AptX, LDAC, LLDC: in general lower complexity but higher bitrate (still within BT limits) than AAC, transparency possible; AptX and LDAC has wide support
- FastStream: Qualcomm's bidirectional SBC variant, providing a mono back channel
- Opus 0.5: PipeWire's little experiment with nearly unlimited channels in either direction. Zero hardware support.
- Opus Android: The Android 13 approach to adding Opus support. Stereo only, could be bidirectional? (check a2dp_vendor_opus_constants.h!) Supported by Pixel headphones.
Bitrate is limited by bluetooth link. For all relevant codecs, there is a maximum limit of 2 channels: this is a headphone protocol, not a speaker protocol. Microphone-back channel is not available except in FastStream and AptX LL duplex.
Sub Band Codec
- Sampling Frequency - 16000, 32000, 44100, 48000 (some are optional, see references)
- Channel Mode - mono, dual channel, stereo, joint stereo
The bitrate used for CD-rate (16-bit, 44.1 KHz) audio is 229 kbps and 328 kbps for middle and high quality, respectively.
Reference: BLUETOOTH Advanced Audio Distribution Profile 1.0
HSP/HFP
This is the mode with mono input and output, both at voice sample rates. Only very crude encodings (CVSD, PCM, optionally SBC 16KHz as the higher-quality thing) are supported, so do not expect audio quality.
LE Audio
This is the new, rewritten general-purpose bluetooth audio stack. This mode allows bidirectional audio of various channel configurations. Battery use is allegedly lower than A2DP.
LC3
- See also: Wikipedia:LC3 (codec)
LC3 is the baseline LE Audio codec. It uses some of the same ideas as Opus CELT, but comes with some new patented techniques and runs at extremely low complexity.
- It's allegedly "better than Opus" with respect to error tolerance in speech environments. Except the test is done with speech samples in the non-speech, non-FEC mode of Opus (CELT), at minimum complexity (effort), and an old version at that.
- No joint stereo is available. The rationale given is that this allows True Wireless Stereo (TWS) to work with the audio source transmitting direct to each headphone, without needing an intermediate decode.
- LC3plus, LC3's "high-res" cousin available over A2DP, is worse than Apple and Android (FDK) AAC at 144 kbps. The Japan Audio Society nevertheless thinks that's enough for a "Hi-Res AUDIO WIRELESS" sticker.
In any case, it's much better than SBC and retains the low complexity character. It is not the best use of the bandwidth available, but a lot of things can be excused with "low complexity". Google's open source liblc3 implementation is written to the word of the BT spec.
Headphones
Some Styles (smallest to largest)
- In the ear - outputs directly into the ear canal
- Earbuds - sits in the outer ear
- Supra-aural - sits on the ear
- circumaural - completely cover the ear
Terminology used for comparing headphones
- Noise Cancelling
- active
- passive
- Frequency response - The range of frequencies the headphones can reproduce
- Impedance - Doesn't mean much by itself, but in general, impedence should be matched across a system.
- SPL@1kHz, 1V rms - Sound Pressure Level. How efficient the unit converts electrical energy to sound energy.
- Neodymium - The magnet of choice for high end headphones.
Popular Headphones
- Sub $30
- Sub $100
- $100 - $300
- $300+