It seems DSD is a audio codec. Never heard of it.
DSD is the format used by the "super audio CD", which fortunately never became very popular.
It's based on proper sampling theory and mathematics, but misrepresented to the public as if it were in some way superior to regular PCM. The truth is that all analog-to-digital and digital-to-analog converters in (and around) the audio domain use it as an intermediate format before the final analog stage. This is also known as "oversampling", for a while you'd commonly find things like "128x oversampling" on a CD player, while it's just a modern way of doing digital-to-analog conversion ("modern" means that it's cheaper and usually provides better quality than direct PCM would).
The "misrepresentation" is that it's implied that inaudible frequencies (for humans, above 20kHz, unless you're my age in which case it'll be more like 15kHz) somehow might contribute to the sound. Just like you cannot see ultraviolet light, you cannot "hear" or otherwise observe ultrasone frequencies, even up to the point that the energy of the emitted waves will actually physically harm you.
