PDM (pulse-density modulation)
A 1-bit format in which signal level is encoded by how densely the ones arrive, and the standard output of digital MEMS microphones.
A PCM sample states exactly what level the signal is at. A PDM stream only says up or down, a few million times a second, and the information lives in the proportion of ones: almost all ones is full positive scale, half and half is zero. It's exactly what a delta-sigma modulator produces before decimation, and DSD is essentially PDM stored on a disc.
The format's big market is the microphones in phones and laptops. A MEMS mic with PDM output needs just two wires, clock and data, and two mics can share the same data line by answering on opposite clock edges. Conversion to PCM, filtering and decimation included, is left to the processor, which has silicon to spare anyway.