Direct Stream Digital
Audio recording process|
|Write mechanism||Super Audio CD|
DSD uses pulse-density modulation encoding—a technology to store audio signals on digital storage media that are used for the SACD. The signal is stored as delta-sigma modulated digital audio; 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/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 Sigma-Delta conversion.
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 originally described in patent 2,927,962, filed by C.C. Cutler in 1954, but was not named as such until a 1962 paper by Inose et al. Previously, decimation had not existed and the intention was to have oversampled data sent as-is. Indeed, the first proposal to decimate oversampled delta-sigma data to convert it into PCM audio did not appear until 1969, in D.J. Goodman's paper "The Application of Delta Modulation of Analog-to-PCM encoding".
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, for further development.
Use of the format
- 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. 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. 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.
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. Examples include the German Stockfisch Records, which releases vinyl editions of albums and DSD-recordings, released as hybrid SACDs.
- 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. 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.
The majority of Telarc International Corporation's releases are on (generally hybrid) SACD, and are DSD recordings. Telarc often works with early audiophile company Soundstream, and has re-released many of its original Soundstream recordings in SACD format. Soundstream, which made the first ever digital recording, 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.
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. 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.
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 sigma-delta 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 and that high frequencies are the first to be affected by hearing loss, though research by Tsutomu Ōhashi et al. has claimed to observe brain changes in subjects exposed to an ultrasound stimulus, which he calls the hypersonic effect.
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 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 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 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 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, 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 raises 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.
The Korg MR-1000 1-bit digital recorder samples at 5.6448 MHz, twice the SACD rate. This is also 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. Additionally a 48 kHz variant at 6.144 MHz has been supported by multiple hardware devices such as the exaSound e20 Mk II DAC.
The Merging Technologies Horus AD/DA Converter offers sample rates up to 11.2 MHz, or four times the SACD rate. This is also 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 48 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.
A further extension to the development of DSD is DSD512, with 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 Player, foobar2000 with SACD plugin and HQPlayer 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.
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. 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.
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 the PCM frames defined in the USB specification; the standard, commonly known as "DSD over PCM", or "DoP", is suitable for other digital links that use PCM. Many manufacturers now offer DACs that support DoP.
While having a different name, DSD-CD is actually the same format as CD-DA. The difference from the standard version of CD is that the sound derives from a DSD master. Other audio CDs, even those derived from DSD masters, are rarely marketed as DSD-CD. A DSD-CD however does not achieve the same sound resolution as SACD because the high-sample rate, low-resolution DSD sound must be converted to 44.1 kHz, 16-bit PCM to comply with the Red Book audio CD standard. DSD-CDs are fully compatible with CD.
DSD vs. PCM
There has been much controversy between proponents of DSD and PCM over which encoding system is superior. In 2001, Stanley Lipshitz and John Vanderkooy from the University of Waterloo stated that one-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. In 2002, Philips published a convention paper arguing to the contrary. Lipshitz and Vanderkooy's paper has been criticized by Jamie Angus. Lipshitz and Vanderkooy later responded.
Fundamental distortion mechanisms are present in the conventional implementation of DSD. These distortion mechanisms can be alleviated to some degree by using digital converters with a multi-bit design. 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. Many modern converters use oversampling and a multi-bit design. It has been suggested that bitstream digital audio techniques are theoretically inferior to multi-bit (PCM) approaches: J. Robert Stuart notes, "1-bit coding would be a totally unsuitable choice for a series of recordings that set out to identify the high-frequency content of musical instruments, despite claims for its apparent wide bandwidth. If it is unsuitable for recording analysis then we should also be wary of using it for the highest quality work."
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 Erich-Thienhaus Institute in Detmold, Germany, seems to contradict this, concluding that in double-blind tests "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." Listeners involved in this test noted their great difficulty in hearing any difference between the two formats.
The future of DSD
DSD has not been broadly successful in the consumer markets, though the SACD format has gained more traction than its direct competitor, DVD-Audio. DSD brings new challenges if immediate manipulation of the recorded data is desired. PCM is far easier to manipulate and is more easily built into existing applications such as the advent of very-high-resolution PCM media and tools, such as DXD. DSD however is used as a master archive format in the studio market and seen as a possible low-noise replacement for analog tapes. As a little quality is lost when converting from DSD to PCM, the debate continues as to whether the ultimate quality digital audio can be found by using DSD players or recording directly into a high quality PCM format in the first place.
In early 2014, AudioFEEL presented a project that proposed to combine the DSD principle with a 'solid state' media type (SD card). The idea is to create a new 'popular' format as able to succeed as historic optical formats: CD, SA-CD, DVD, etc.
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