Direct Stream Digital

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Direct-Stream Digital (DSD) is the trademark name used by Sony and Philips for their system of recreating audible frequencies which uses pulse-density modulation encoding, a technology to store audio signals on digital storage media which is used for the Super Audio CD (SACD).

The signal is stored as delta-sigma modulated digital audio, a sequence of single bit values at a frequency sampling rate of 64 times the CD Audio sampling rates of 44.1 kHz, for a rate of 2.8224 MHz (1 bit times 64 times 44.1 kHz). Noise shaping occurs by use of the 64× oversampled signal to reduce noise/distortion caused by the inaccuracy of quantization of the audio signal to a single bit. Therefore it is a topic of discussion whether it is possible to eliminate distortion in 1-bit Sigma-Delta conversion (see Audio Engineering Society Convention Paper 5395 in external links section below). Due to the way 1-bit Sigma-Delta converters work, DSD encoded audio for lower frequencies has better resolution than standard CDs, while for higher frequencies the resolution, and therefore the fidelity, drops down below that of standard CDs.

There has been much controversy between proponents of DSD and PCM over which encoding system is superior. Professors Stanley Lipshitz and John Vanderkooy from the University of Waterloo, in Audio Engineering Society Convention Paper 5395 (2001), stated that 1-bit converters (as employed by DSD) are unsuitable for high-end applications due to their high distortion. Even 8-bit, four-times-oversampled PCM with noise shaping, proper dithering and half data rate of DSD has better noise floor and frequency response. However, in 2002, Philips has published a convention paper arguing against this Convention Paper 5616. Lipshitz and Vanderkooy's paper has been criticized in detail by Professor James Angus at an Audio Engineering Society presentation Convention Paper 5619. Lipshitz and Vanderkooy responded in Convention Paper 5620.

Practical DSD converter implementations were pioneered by Ed Meitner, an Austrian sound engineer and owner of EMM Labs. Global DSD technology was developed by Sony and Philips, the designers of the audio CD. Philips' DSD tool division was transferred to Sonic Studio, LLC in 2005 for on-going design and development.

DSD technology may also have potential for video applications. A similar structure based on pulse-width modulation, which is decoded in the same way as DSD, has been used in Laserdisc video.

[edit] External links

• Audio Engineering Society Convention Paper 5395: Why 1-Bit Sigma-Delta Conversion is Unsuitable for High-Quality Applications
• Audio Engineering Society Convention Paper 5616: Enhanced Sigma Delta Structures for Super Audio CD Applications
• Audio Engineering Society Convention Paper 5619: The Effect of Idle Tone Structure on Effective Dither in Delta-Sigma Modulation Systems
• Audio Engineering Society Convention Paper 5620: Toward a Better Understanding of 1-Bit Sigma-Delta Modulators - Part 3
• Poking a Round Hole in a Square Wave (DSD vs PCM comparison)