Direct Stream Digital

Direct Stream Digital (DSD) is a trademark used by Sony and Philips for their system of digitally recreating audible signals for the Super Audio CD (SACD).

Direct Stream Digital
Media typeAudio recording process
Disc format
EncodingDigital
Read mechanismDSD
Write mechanismSuper Audio CD
StandardISO/IEC 14496-3
UsageAudio recording
Extended from1999
Extended topresent

DSD uses pulse-density modulation encoding - a technology to store audio signals on digital storage media which are used for the SACD. The signal is stored as delta-sigma modulated digital audio, which is a sequence of single-bit values at a sampling rate of 2.8224 MHz (64 times the CD audio sampling rate of 44.1 kHz, but only at 1⁄32768 of its 16-bit resolution). Noise shaping occurs by use of the 64-times oversampled signal to reduce noise and 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 one-bit delta-sigma conversion.[1]

Development

DSD is a method of storing a delta-sigma signal before applying a decimation process that converts the signal to a PCM signal. Delta-sigma conversion was first described by C.C. Cutler in 1954,[2] but was not named as such until a 1962 paper by Inose et al. Decimation did not initially exist and oversampled data was sent as-is. The proposal to decimate oversampled delta-sigma data before converting it into PCM audio was made by D. J. Goodman in 1969.[3]

DSD technology was later developed and commercialized by Sony and Philips, the designers of the audio CD. However, in 2005, Philips later sold its DSD tool division to Sonic Studio.[4]

See also: Super Audio CD
Major label support

DVD-Audio was endorsed by the Warner Music Group, while the SACD format was endorsed by Sony and Universal Music Group, with an especially high-profile by UMG imprint Virgin Records.[5][6] Despite this, in 2011, The Warner Premium Sound series of albums was released by Warner Music Group, marking the first time the label released titles in a SACD format, with recording in DSD.[7] The series grew to ten rock and pop albums, with Super Audio CD/CD hybrid discs containing both an SACD layer and a standard CD layer.[8]

Sony did not promote SACD actively in North America, with the result that DVD-Audio gained competitive traction in the market. Elsewhere, such as in Europe or Japan, SACD gained more of a foothold.[9] Examples include the German Stockfisch Records, which releases vinyl editions of albums and DSD-recordings, released as hybrid SACDs.[10]

Independent label use

Many music companies that specialize in Super Audio CD products therefore use DSD encoding. A number of independent record labels have also worked directly with Sony to focus on DSD products or the DSD recording process.

DMP Digital Music Products was an early user of the SACD digital audio format.[11] In 1997 their release Alto by Joe Beck & Ali Ryerson was the first commercial recording captured with Sony's Direct Stream Digital recording technology. The label's Just Jobim by Manfredo Fest in 1998 was the first project captured with the new Meitner DSD conversion technology. In 2000, DMP released the world's first multi-channel SACD—Sacred Feast by Gaudeamus.[12]

The majority of Telarc International Corporation's releases were on (generally hybrid) SACD, and are DSD recordings.[13] Telarc often worked with early audiophile company Soundstream, and re-released many of its original Soundstream recordings in SACD format.[14] Soundstream, which made the first digital recording in the United States, recorded in 16 bit PCM at a sample rate of 50 kHz via its own proprietary digital recorder. This 50 kHz PCM format was converted to DSD for release on Telarc SACD.

The record label Mobile Fidelity had engineers who decided to adopt the Super Audio CD over the DVD-Audio disc as a high resolution digital format after listening tests and technical evaluations. On the label's Hybrid SACD releases, the SACD layer is a direct DSD recording of the analog master tape, while the CD layer is a digital down conversion of the DSD, with Super Bit Mapping applied. Post-2001, CD-only are sourced from DSD, but omit the SACD layer.[15]

In 2007, Blue Coast Records was founded in California for the purpose of recording and releasing music recorded with the DSD format, primarily focusing on jazz and acoustic artists.[16]

On August 28, 2013, the Acoustic Sounds label launched SuperHiRez.com, which sells mainstream albums from major record labels that were produced with Direct Stream Digital or PCM audio formats.[17][18] On September 4, 2013, Acoustic Sounds announced an agreement with Sony Music Entertainment to provide the company's new digital download service with albums that have been produced or remastered in Direct Stream Digital format.[19]

The format is used on albums such as Pop, Songs & Death in 2009,[20] and the remastered The Rolling Stones album Their Satanic Majesties Request in 2002.[21][22]

DSD technique
Comparison with PCM

SACD audio is stored in DSD, which differs from the conventional PCM used by the compact disc or conventional computer audio systems.

A DSD recorder uses delta-sigma modulation. DSD is 1-bit with a 2.8224 MHz sampling rate. The output from a DSD recorder is a bitstream. The long-term average of this signal is proportional to the original signal. DSD uses noise shaping techniques to push quantization noise up to inaudible ultrasonic frequencies. In principle, the retention of the bitstream in DSD lets the SACD player use a basic (one-bit) DAC design with a low-order analog filter. The SACD format can deliver a dynamic range of 120 dB from 20 Hz to 20 kHz and an extended frequency response up to 100 kHz—though most current players list an upper limit of 80–90 kHz.

Most professional audiologists accept that the upper limit of human adult hearing is 20 kHz[23] and that high frequencies are the first to be affected by hearing loss.[24]

The process of creating a DSD signal is conceptually similar to taking a one-bit delta-sigma analog-to-digital (A/D) converter and removing the decimator, which converts the 1-bit bitstream into multi-bit 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 one-bit signal cannot be dithered properly: most modern sigma-delta converters are multi-bit.

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 24 bits and a sampling frequency of 88200 Hz.

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 one-bit environment, and because of the prevalence of solely PCM studio equipment such as Pro Tools, the vast majority of SACDs—especially rock and contemporary music, which rely 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 the high sample rate of standard DSD, but uses an 8-bit, rather than single-bit digital word length, yet still relies heavily on the noise shaping principle. DSD-wide is PCM with noise shaping—and is 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 384 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 Digital eXtreme Definition (DXD), a PCM format with 24-bit resolution sampled at 352.8 kHz (or alternatively 384 kHz). DXD was initially developed for the Merging Pyramix workstation and introduced together with their Sphynx 2, AD/DA converter in 2004. This combination meant that it was possible to record and edit directly in DXD,[25] and that the sample only converts to DSD once before publishing to SACD. This offers an advantage to the user as the noise created by converting DSD rises dramatically above 20 kHz, and more noise is added each time a signal is converted back to DSD during editing.

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 typically affects the end-result. 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. The dynamic range of DSD decreases quickly at frequencies over 20 kHz due to the use of strong noise shaping techniques that push the noise out of the audio band, resulting in a rising noise floor just above 20 kHz. The dynamic range of PCM, on the other hand, is the same at all frequencies. However, almost all present-day DAC chips employ some kind of sigma-delta conversion of PCM files that results in the same noise spectrum as DSD signals. All SACD players employ an optional low-pass filter set at 50 kHz for compatibility and safety reasons, suitable for situations where amplifiers or loudspeakers cannot deliver an undistorted output if noise above 50 kHz is present in the signal.

Double-rate DSD (DSD128)

Referred to as DSD128 because the sample rate is 128 times that of CD. Since its establishment content creators have started to make 5.6 MHz DSD128 recordings available, such as the audiophile label Opus3.[26] Additionally a 44.1 kHz variant at 5.6448 MHz has been supported by multiple hardware devices such as the exaSound e20 Mk II DAC.[27] The Korg MR-1000 1-bit digital recorder samples at 5.6448 MHz, twice the SACD rate.

Quad-rate DSD (DSD256)

Referred to as DSD256 because the sample rate is 256 times that of CD. The Pyramix Virtual Studio Digital Audio Workstation allows for recording, editing and mastering all DSD formats, being DSD64 (SACD resolution), DSD128 (Double-DSD) and DSD256 (Quad-DSD). A 44.1 kHz variant of 12.288 MHz has been established. The exaSound e20 DAC was the first commercially available device capable of DSD256 playback at sampling rates of 11.2896/12.288 MHz.[28][29] The Merging Technologies Horus AD/DA Converter offers sample rates up to 11.2 MHz, or four times the SACD rate.

Octuple-rate DSD (DSD512)

Referred to as DSD512 because the sample rate is 512 times that of CD. It has a sample rate of 22.5792 MHz (512 times that of CD), or alternatively 24.576 MHz (512 times 48 kHz). Hardware such as the Amanero Combo384 DSD output adapter, and exaU2I USB to I²S interface, and software such as JRiver Media Center, foobar2000 with SACD plugin (up to DSD256 only), Roon,[30] HQPlayer and Neutron Music Player are all able to handle DSD files of this advanced sampling rate fully natively.

DSD playback options

Sony developed DSD for SACD, and many disk players support SACD. Since the format is digital, there are other ways to play back a DSD stream; the development of these alternatives has enabled companies to offer high-quality music downloads in DSD.[31]

DSD disc format

Some professional audio recorders (from Korg, Tascam, and others) can record in DSD format. Transferring this signal to a recordable DVD with the appropriate tools, such as the AudioGate software bundled with Korg MR-1/2/1000/2000 recorders, renders a DSD Disc. Such discs can be played back in native DSD only on certain Sony VAIO laptops and PlayStation 3 systems.[32] HQPlayer from February 16, 2011, version 2.6.0 beta includes support for direct/native playback from DSD Interchange File Format (DSDIFF) and DSD stream files (DSF) to ASIO devices with DSD support. Moreover, Sony produces two SACD players, the SCD-XA5400ES and the SCD-XE800, that fully support the DSD-disc format. Only the DSF format is supported. DSF is a stereo-only, simplified form of DFF, the format used for SACD mastering and 5.1-channel downloads. However, since most personal computers have only PCM audio hardware, DSD discs must be transcoded to PCM on the fly with the proper software plug-ins with questionable quality benefits compared to native high resolution PCM sources like DVD or Blu-ray Disc Audio.

In June 2012, Pioneer launched a series of SACD players compatible with DSD-disc. The PD-30 and PD-50.

In January 2013, TEAC announced a DSD-disc compatible player, the PD-501HR.[33]

DSD over USB

An alternative to burning DSD files onto disks for eventual playback is to transfer the (non-encrypted) files from a computer to audio hardware over a digital link such as USB.

The USB audio 2.0 specification defined several formats for the more common PCM approach to digital audio, but did not define a format for DSD.

In 2012, representatives from many companies and others developed a standard to represent and detect DSD audio within PCM frames; the standard, commonly known as "DSD over PCM", or "DoP", is suitable for other digital links that use PCM.[34] The 1.1 revision added protocol support for higher DSD sample rates without requiring an increase the underlying PCM sample rate. Many manufacturers now offer DACs that support DoP.

DSD vs. PCM

There has been much controversy between proponents of DSD and PCM over which encoding system is superior. In 2001, Lipshitz and Vanderkooy stated that one-bit converters, as employed by DSD, are unsuitable for high-end applications due to their high distortion.[35] In 2002, Philips published a paper arguing the contrary.[36] Lipshitz and Vanderkooy's paper was further criticized by Angus.[37] Lipshitz and Vanderkooy[38] responded to the criticisms.[39] Stuart also defined sigma delta modulation a "totally unsuitable choice" for high resolution digital audio.[40]

Conventional implementation of DSD has an intrinsic high distortion.[41] Distortion can be alleviated to some degree by using multibit digital to analogue converter. Historically, state-of-the-art ADCs were based around sigma-delta modulation designs. Oversampling converters are frequently used in linear PCM formats, where the ADC output is subject to bandlimiting and dithering.[42] Many modern converters use oversampling and a multi-bit design.

Comparisons of DSD and PCM recordings with the same origin, number of channels and similar bandwidth and noise have yielded contradictory results. A study conducted at the Erich-Thienhaus Institute in Detmold, Germany found that in double-blind tests "hardly any of the subjects could make a reproducible distinction between the two encoding systems."[43] In contrast, a 2014 study conducted at the Tokyo University of the Arts found that listeners could distinguish PCM (192 kHz/24 bits) from either DSD (2.8 MHz) or DSD (5.6 MHz) (but not between the two DSD samplings), preferring the sound of DSD over PCM: "For example, Drums stimulus of DSD (5.6 MHz) has p = 0.001 when compared against PCM (192 kHz/24 bit) in overall preference. This suggests that DSD version was statistically significantly preferred over the PCM version."[44]

DSD met with relatively little success in the consumer market, even though the SACD was actually more successful than its direct competitor, the PCM-based DVD-Audio. Direct manipulation of recorded DSD data is difficult due to limited availability of appropriate software. The advent of new high-resolution PCM standards, such as DXD, further restricted its market niche. DSD, however, is still used as an archival format for studio applications, and it's seen as a possible replacement for analog tapes.

See also

References
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  2. US patent 2967962, Cutler, Cassius C., "Transmission systems employing quantization", issued March 8, 1960
  3. "Data Converter Architectures: Chapter 3" (PDF). Retrieved October 27, 2018.
  4. "Sonic Studio Takes Over Philips ProTech DSD i SACD Products [HFR]". HighFidelityReviews. July 9, 2005. Retrieved October 27, 2018.
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