SBC (Bluetooth codec)
The mandatory audio codec in Bluetooth A2DP — simple enough for a cheap earbud chip, and therefore the one every device is guaranteed to speak.
Low Complexity Subband Coding came out of Philips and splits audio into four or eight frequency bands, quantising each separately. No psychoacoustic model, no heavy transform; the point was decoding on next to no power. Quality is governed by a parameter called bitpool. The spec recommends 53 for high quality, around 330 kbit/s for 44.1 kHz stereo, and at that setting SBC sounds a good deal better than its reputation.
The reputation comes from manufacturers turning the bitpool down to save airtime, and from SBC always being the fallback when nothing better can be negotiated. aptX and LDAC spent years being marketed against it specifically. Its successor in LE Audio is LC3, which the Bluetooth SIG claims sounds better at roughly half the bitrate.