DSD (Direct Stream Digital)
Sony and Philips's audio format that stores a delta-sigma converter's 1-bit stream as is, rather than turning it into PCM.
The base rate is 2.8224 MHz, 64 times the CD's 44.1 kHz, with a single bit per sample. The argument went like this: delta-sigma converters produce such a stream anyway, so why filter and decimate it to PCM only to convert it back again on playback? The format launched on Super Audio CD in 1999, specified in the Scarlet Book, one of the Rainbow Books.
The catch is that you can't mix 1-bit audio: even a volume change needs more bits. Studios ended up converting to high-resolution PCM for editing and back again afterwards, which rather undermined the whole point. SACD never broke through, but DSD survives as a niche download format for enthusiasts.