direct stream digital (CD) (DSD)
Direct Stream Digital (DSD) was developed by Philips and Sony and is used to store high-resolution audio signals. The DSD recording method uses pulse density modulation( PDM), it supports 6 audio channels and is used in the Super Audio CD( SACD). It features improved sound quality over pulse code modulation( PCM) based on the high sampling frequency of 2.8224 MHz.
The sampling rate of 2.8 MHz is obtained in a sigma-delta converter from the sampling rate of 44.1 kHz and a 64-fold oversampling ratio( OSR) for a 1-bit signal. This high resolution allows for an almost pristine reproduction of the original sound, since the bit stream is stored directly; hence the name Direct Stream Digital. To reduce noise, the noise components resulting from the 1-bit conversion are shifted to frequency ranges outside the audible range by noise shaping.
In the simplest case, DSD bitstreams are reproduced via lowpass filters, with a frequency range of up to 100 kHz and a dynamic range of 120 dB. The quantization noise generated in the sigma-delta converter is shifted into an inaudible frequency range by noise shaping.