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 signals, which uses pulse-density modulation encoding, a technology to store audio signals on digital storage media that are 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 sampling rate of 2.8224 MHz (64 times the CD Audio sampling rate of 44.1 kHz, but only at 1⁄16th its 16-bit resolution). 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.
Practical DSD converter implementations were pioneered by Andreas Koch and Ed Meitner, the original founders of EMM Labs. Andreas Koch later left EMM Labs and along with Jonathan Tinn, founded Playback Designs who have pioneered the transfer of DSD files over USB connections. DSD technology was later developed and commercialised by Sony and Philips, the designers of the audio CD. However, Philips later sold its DSD tool division to Sonic Studio, LLC in 2005 for further development.
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 into PCM audio did not appear until 1969, in D. J. Goodman's paper "The Application of Delta Modulation of Analog-to-PCM encoding".
A Direct Stream Digital (DSD) recorder uses sigma-delta modulation. DSD is 1-bit, has a sampling rate of 2.8224 MHz. The output from a DSD recorder alternates between levels representing 'on' and 'off' states, and is a binary signal (called a bitstream). The long-term average of this signal is proportional to the original signal. DSD makes use of noise shaping techniques in order to push quantization noise up to inaudible ultrasonic frequencies. In principle, the retention of the bitstream in DSD allows the SACD player to use a basic (one bit) DAC design which incorporates a low-order analog filter. 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 and 20 kHz is the upper limit of human hearing.
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 format 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. 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. 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 5.6 MHz, twice the SACD rate. This is also referred to as DSD128 because the sample rate is 128 times that of CD.
The Merging Technologies Horus AD/DA Converter offers sample rates up to 11.2 MHz, four time 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).
Sony developed DSD for Super Audio CD 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, will render a DSD Disc. Such discs can be played back in native DSD only on certain Sony VAIO laptops and PlayStation 3. HQPlayer from Feb 16 2011 ver 2.6.0 beta includes support for direct/native playback from DSDIFF and DSF files 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 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. However, since PCs (and Macs) have only PCM-audio hardware, DSD discs must be transcoded to PCM on the fly with the proper software plug-ins (for Windows Media Player or foobar2000), with questionable quality benefits compared to native high resolution PCM sources like DVD- or Blu-ray Disc Audio.
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 (Pulse-code modulation) approach to digital audio, but did not define a format for DSD. Playback Designs' players and converters and pro-audio company Mytek Digital's 192 Stereo DAC both sport DSD over USB capability. In 2012 representatives from these companies and others developed a standard to represent and detect DSD audio within the PCM frames defined in the USB specification; the standard is suitable for other digital links that use PCM.  However this format may be of little practical value because as John Atkinson of Sterophile found with the pioneering Playback Designs DoP DAC /SACD player "The relatively high level of background noise limits the MPS-5's resolution with SACD and external 24-bit data to not much better than 16-bit CD."
Note about DSD-CD
Despite its confusing name, DSD-CD is actually the same format as CD-DA. The difference from the standard version of CD is that the sound is assured to be derived 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 has to be 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 vs. PCM
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 published a convention paper arguing against this in Convention Paper 5616[dead link]. Lipshitz and Vanderkooy's paper has been criticized in detail by Professor Jamie Angus at an Audio Engineering Society presentation in Convention Paper 5619[dead link]. Lipshitz and Vanderkooy responded in Convention Paper 5620.
There are fundamental distortion mechanisms present in the conventional implementation of DSD. These distortion mechanisms can be alleviated to some degree by using digital converters with a multibit 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 (Hawksford 1995). Many modern converters use oversampling and a multibit design. It has been suggested that bitstream digital audio techniques are theoretically inferior to multibit (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 "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."
In the popular Hi-Fi press, it had been suggested that linear PCM "creates [a] stress reaction in people", and that DSD "is the only digital recording system that does not [...] have these effects" (Hawksford 2001). This claim appears to originate from a 1980 article by Dr John Diamond entitled Human Stress Provoked by Digitalized Recordings. The core of the claim that PCM (the only digital recording technique available at the time) recordings created a stress reaction rested on "tests" carried out using the pseudoscientific technique of Applied Kinesiology, for example by Dr Diamond at an AES 66th Convention (1980) presentation with the same title. Diamond had previously used a similar technique to demonstrate that rock music was harmful due to the presence of the "stopped anapestic beat". Dr Diamond's claims regarding digital audio were taken up by Mark Levinson, who asserted that while PCM recordings resulted in a stress reaction, DSD recordings did not.
A double-blind subjective test between high resolution linear PCM (DVD-Audio) and DSD did not reveal a statistically significant difference. 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 studio recording or the consumer markets, even though the SACD format has gained more traction than its direct competitor, DVD-Audio. The advent of very-high-resolution PCM media and tools e.g. DXD has led to a decrease in the uptake of DSD in the studio market. Also, consumer Blu-ray Discs are considered to provide similar or superior audio quality to DSD-based SACDs.
- Stanley P. Lipshitz; John Vanderkooy (2001-05-12), Why 1-Bit Sigma-Delta Conversion is Unsuitable for High-Quality Applications, Audio Engineering Society, retrieved 2011-07-06
- Data Converter Architectures Chapter 3 page 3.136 references these papers. Retrieved June 10, 2009
- 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.
- David W. Robinson (2012-04). "The Higher End: From an Editor's Notebook - Downloadable DSD". Positive Feedback Online. Retrieved 2012-05-14.
- "PS3 SACD FAQ". PS3SACD.com. Retrieved August 8, 2010.
- Andreas Koch et al (2012-03-30). "Method for transferring DSD Audio over PCM Frames Version 1.1". DSD-Guide.com. Archived from the original on 2012-05-14. Retrieved 2012-05-14.
- Atkinson, John. "Playback Designs MPS-5 SACD/CD player Measurements". Stereophile.
- "What is DSD-CD?". PS3SACD.com. Retrieved September 1, 2009.
- Hawksford, M. (2001). "SDM versus LPCM: The Debate Continues", 110th AES Convention, paper 5397.
- Coding for High-Resolution Audio Systems, J Robert Stuart, J. Audio Eng. Soc., Vol. 52, No. 3, March 2004 pp139-142
- 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 http://www.hfm-detmold.de/eti/projekte/diplomarbeiten/dsdvspcm/aes_paper_6086.pdf
- http://www.diamondcenter.net/digitalstress.html[dead link]
- Are the Kids All Right?: The Rock Generation and Its Hidden Death Wish, John Grant Fuller, ISBN0812909704, pp130-135
- Audio Engineering Society Convention Paper 5396: Why Direct Stream Digital is the best choice as a digital audio format
- 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[dead link]
- 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 at the Wayback Machine (archived October 11, 2007) - DSD vs PCM comparison
- DSF and DSD Disc Format specifications by Sony
- 'How to create a DSD Disc' guide including DSD plug-in for Windows Media Player
- Multi-channel DSD over USB DSD recording playback over USB software and hardware development kit
- DSD downloads