Other hardware

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Bluetooth (A2DP)

Advanced Audio Distribution Profile (A2DP)

• forums
• wikipedia
• HABR

Standard baseline codecs:

• Sub Band Codec (SBC) – okay, not great
• LC3 – the new baseline of Bleutooth 5.1. About Opus level, allegedly

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

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 () audio is 229 kbps and 328 kbps for middle and high quality, respectively.

Reference: BLUETOOTH Advanced Audio Distribution Profile 1.0

Bluetooth (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.

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+