Other hardware

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Bluetooth

A2DP

Advanced Audio Distribution Profile (A2DP) is the traditional

• forums
• wikipedia
• HABR

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-quality (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) are supported, so do not expect audio quality.

LE Audio

This is the new, rewritten general-purpose bluetooth audio stack. LC3 is the baseline codec.

• 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), and an old version at that.
• LC3plus, the "high-res" branch, is worse than Apple and Android (FDK) AAC at 144 kbps. So LC3 is probably not as good as Opus after all.

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+