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Direct-Stream Digital (DSD) is the trademark name used by Sony and Philips for their system of recreating audible signals which uses pulse-density modulation encoding, a technology to store audio signal 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 the External Links section below).

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 Waterloomarker, 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 published a convention paper arguing against this in Convention Paper 5616. Lipshitz and Vanderkooy's paper has been criticized in detail by Professor James Angus at an Audio Engineering Society presentation in Convention Paper 5619. Lipshitz and Vanderkooy responded in Convention Paper 5620.

Practical DSD converter implementations were pioneered by Ed Meitner, an Austrianmarker 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.

History

DSD is a method of storing a Delta-Sigma signal before applying a "decimation" process that converts the signal to a PCM signal. When Delta-Sigma conversion was originally described in patent 2,927,962 filed by C. C. Cutler in 1954 (But not named as such until a 1962 paper by H. Inose, Y. Yasuda, and J. Murakami), decimation did not exist and the intention was to have oversampled data sent as-is. Indeed, the first proposal to decimate oversampled delta-sigma data to convert it in to PCM audio didn't appear until 1969, in D. J. Goodman's paper "The Application of Delta Modulation of Analog-to-PCM encoding".

DSD

SACD audio is stored in a format called Direct Stream Digital (DSD), which differs from the conventional PCM used by the compact disc or conventional computer audio systems.

DSD is 1-bit, has a sampling rate of 2.8224 MHz, and makes use of noise shaping quantization techniques in order to push 1-bit quantization noise up to inaudible ultrasonic frequencies. This gives the format a greater dynamic range and wider frequency response than the CD. The SACD format is capable of delivering a dynamic range of 120 dB from 20 Hz to 20 kHz and an extended frequency response up to 100 kHz, although most currently available players list an upper limit of 80–90 kHz.

The process of creating a DSD signal is conceptually similar to taking a 1-bit delta-sigma analog-to-digital (A/D) converter and removing the decimator, which converts the 1-bit bitstream into multibit PCM. Instead, the 1-bit signal is recorded directly and in theory only requires a lowpass filter to reconstruct the original analog waveform. In reality it is a little more complex, and the analogy is incomplete in that 1-bit sigma-delta converters are these days rather unusual, one reason being that a 1-bit signal cannot be dithered properly: most modern sigma-delta converters are multibit.

Because of the nature of sigma-delta converters, one cannot make a direct comparison between DSD and PCM. An approximation is possible, though, and would place DSD in some aspects comparable to a PCM format that has a bit depth of 20 bits and a sampling frequency of 96 kHz . PCM sampled at 24 bits provides a (theoretical) additional 24 dB of dynamic range.

Because it has been extremely difficult to carry out DSP operations (for example performing EQ, balance, panning and other changes in the digital domain) in a 1-bit environment, and because of the prevalence of studio equipment such as Pro Tools, which is solely PCM-based, the vast majority of SACDs — especially rock and contemporary music which relies on multitrack techniques — are in fact mixed in PCM (or mixed analog and recorded on PCM recorders) and then converted to DSD for SACD mastering.

To address some of these issues, a new studio format has been developed, usually referred to as "DSD-wide", which retains standard DSD's high sample rate but uses an 8-bit, rather than single-bit digital word length, but still relies heavily on the noise shaping principle. It becomes almost the same as PCM (it's sometimes disparagingly referred to as "PCM-narrow") but has the added benefit of making DSP operations in the studio a great deal more practical. The main difference is that "DSD-wide" still retains 2.8224 MHz (64Fs) sampling frequency while the highest frequency in which PCM is being edited is 352.8 kHz (8Fs). The "DSD-wide" signal is down-converted to regular DSD for SACD mastering. As a result of this technique and other developments there are now a few digital audio workstations (DAWs) that operate, or can operate, in the DSD domain, notably Pyramix and some SADiE systems.

Another format for DSD editing is DXD (Digital eXtreme Definition), a "PCM-like" signal with 24-bit resolution sampled at 352.8 kHz.

Note that high-resolution PCM (DVD-Audio, HD DVD and Blu-ray Disc) and DSD (SACD) may still technically differ at high frequencies. A reconstruction filter is typically used in PCM decoding systems, much the same way that bandwidth-limiting filters are normally used in PCM encoding systems. Any error or unwanted artifact introduced by such filters will typically affect the end-result. If the filter designer choose a wide, flat passband, then the length of the impulse response will inevitably increase (Time-domain analysis of filters). A claimed advantage of DSD is that product designers commonly choose to have no filtering, or modest filtering. Instead DSD leads to constant high levels of noise at these frequencies. DSD's dynamic range decreases quickly at frequencies over 20 kHz due to the use of strong noise shaping techniques which push the noise out of the audio band resulting in a rising noise floor just above 20 kHz. PCM's dynamic range, on the other hand, is the same at all frequencies. (Some high-end SACD players employ an optional low-pass filter set at 30 kHz for compatibility and safety reasons, suitable for situations where amplifiers or loudspeakers can't deliver an undistorted output if noise above 30 kHz is present in the signal.)

DSD Disc Format

Some professional audio recorders can record in DSD format. Transferring this signal to a recordable DVD disc with the appropriate tools, such as the Korg MR-1/1000/2000-bundled 'AudioGate'-software, will render a DSD Disc. Such discs can be played back/transcoded to PCM on the fly on most PCs (this requires a DSD plug-in for Windows Media Player) and in native DSD on Sony VAIO laptops and PlayStation 3. Moreover, Sony has released a new SACD-player (autumn 2008), the SCD-XA5400ES, which fully supports the DSD-disc format. Only so-called DSF format is supported. DSF is a stereo-only, simplified form of DFF, the format used for SACD mastering and 5.1-channel downloads.

DSD-CD

DSD-CD is a variant of the Compact Disc Digital Audio format. The difference with the regular version of CD is that the sound is derived from a DSD master. A DSD-CD however does not achieve the same sound resolution as SACD because the high-resolution DSD sound has to be down-converted to 44.1 kHz, 16-bit PCM in order to be compliant with the 'Red Book' audio CD standard. DSD-CDs are fully compatible with CD.

DSD-CD has been marketed as an audiophile medium, primarily in Hong Kong with music by Cantopop artists such as Sally Yeh.

Double-rate DSD

The Korg MR-1000 1-bit digital recorder samples at 5.8MHz, twice the SACD rate. It's also referred to as DSD128 because of the sample rate 128x that of CD.

Sound quality of DSD vs. (24-bit/176.4 kHz) PCM

When comparing a DSD and PCM recording of the same origin, the same number of channels and similar bandwidth/SNR, some still think that there are differences. A study conducted at the Erik-Thienhaus Institute in Detmold, Germany, seems to contradict this, concluding that "hardly any of the subjects could make a reproducible distinction between the two encoding systems. Hence it may be concluded that no significant differences are audible".

References

  1. Data Converter Architectures Chapter 3 page 3.136 references these papers. Retrieved June 10, 2009
  2. Extremetech.com, Leslie Shapiro, July 2, 2001. Surround Sound: The High-End: SACD and DVD-Audio. "Bottom line, SACD recordings can achieve a high-frequency response of 50kHz and a dynamic audio range of 120dB" Retrieved on June 10, 2009.
  3. DVD-Audio versus SACD: Perceptual Discrimination of Digital Audio Coding Formats; Blech, Dominic; Yang, Min-Chi. Erich-Thienhaus-Institute (Tonmeisterinstitut), University of Music Detmold, Germany 2004


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