Sustainability of Digital Formats
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One-bit Delta Sigma Audio Encoding (DSD)

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Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name One-bit Delta Sigma (or Sigma Delta) Audio Modulation; Direct Stream Digital (DSD; trade name)
Description

Encoding for audio using pulse-density modulation; the signal is stored as delta-sigma (some say "sigma delta") modulated digital audio, a sequence of single bit values at a high frequencies. Although delta-sigma encoding is applicable to a variety of digital signals, this discussion is focused on sound, where the encoding is sometimes described as "oversampling." In published sound recordings, one-bit delta sigma modulation is disseminated on the tangible media format called Super Audio CDs (SACDs), developed by SONY in association with Philips and documented in the 1999 Scarlet Book. In this context, the prevalent term is Direct Stream Digital. The associated DSDIFF_1_5 file format is used by music producers, e.g., to store DSD content or to deliver it "to the label" for reproduction on SACDs.

For audio, a delta-sigma converter digitizes the signal 1-bit deep with a very high sampling rate: 64 times the CD Audio sampling rate of 44.1 kHz, for a rate of 2.8224 mHz (1 bit times 64 times 44.1 kHz). Advocates for this encoding state that the oversampling and subsequent digital processing reduces aliasing and other distortion or noise, potentially to a greater degree than customary PCM encoding. Meanwhile, delta-sigma elements are a normal part of the PCM encoding process. There are also hybrid recording systems that use what is called DSD-wide, with 8-bit samples.

Production phase Typically used as a final-state format, although there is some use in initial- and middle-state settings.
Relationship to other formats
    Used by DSDIFF_1_5, DSDIFF, Direct Stream Digital Interchange File Format, Version 1.5
    Used by SACD, Super Audio CDs, a tangible format not documented at this Web site
    Other DST, Direct Stream Transfer format for lossless encoded DSD audio, not documented at this Web site

Local use Explanation of format description terms

LC experience or existing holdings No experience with DSDIFF files, although the collections of the Motion Picture, Broadcasting, and Recorded Sound Division include SACDs.
LC preference None. For preservation reformatting, the Library prefers Broadcast WAVE (WAVE_BWF_1 and WAVE_BWF_2).

Sustainability factors Explanation of format description terms

Disclosure Conceptual entity widely discussed in the literature
    Documentation See Useful references for conceptual discussion. The SACD specification is called the Scarlet Book (1999); it is not clear to this writer how a copy may be obtained nor whether it includes implementation guidelines.
Adoption

Adoption of file version unknown. Tangible media SACD disks are on the market from SONY, in competition with the LPCM-based high resolution (96 kHz, 24-bit) DVD-Audio disks; neither disk type appears to have a significant hold on the music marketplace. Interest from SONY and others in the industry in promoting this format turns up from time to time; in September 2013, for example, National Public Radio's Morning Edition program presented a feature on DSD tied to SONY's announcement of "a new line of high-end audio components."

This encoding is mentioned in Audio Engineering Society specifications like the High-Resolution Multi-channel Audio Interconnection (HRMAI; AES-R6-2005).

    Licensing and patents None for the conceptual entity; licensing applies to SACD.
Transparency Although signal processing elements make DSD audio challenging to record, the one-bit delta-sigma encoding is a simple, uncompressed representation and thus relatively transparent.
Self-documentation Not applicable. Metadata is embedded in the DSDIFF_1_5 file format or on SACD disks.
External dependencies None
Technical protection considerations None intrinsic to the encoding; SACD includes technical protection features.

Quality and functionality factors Explanation of format description terms

Sound
Normal rendering Good support.
Fidelity (high audio resolution) Delta-sigma encoding supports high audio resolution. DSD on SACDs have a sampling rate of 2.8 MHz which enables a frequency response of 100 kHz and a dynamic range of 120 dB.
Multiple channels Not intrinsic to the bitstream. DSDIFF_1_5 files can be structured for 2 channels or for "5- or 6-channels" (presumably for 5.1 mixes). Almost all commercially released SACDs have included both stereo and multi-channel mixes, generally in surround mode. Non-surround multi-channel mixes on SACD have included Living Stereo reissues that use the three front channels to reproduce the original three-track (3.0) stereo recordings, and there is at least one reissue of a quadraphonic title as a 4.0 mix.
Support for user-defined sounds, samples, and patches Not applicable
Functionality beyond normal rendering Not applicable

File type signifiers Explanation of format description terms

Tag Value Note
Filename extension Not applicable.   
Internet Media Type Not applicable.   
Magic numbers Not applicable
 

Notes Explanation of format description terms

General

Notes from selected Web sites:

  • Uwe Beis writes about delta sigma encoding on SACDs: "The whole system from the analogue input to the analogue output now requires only one modulator and one low pass filter instead of two, each with conventional digital recording. Please remember: The bitstream of modern, high quality ADCs and DACs use an oversampling rate of 64, e.g. the amount of data is 64 bits for each audio sample - compared to 16 or up to 20 or 24 of conventional bits required for a comparable quality. Technically, SACDs are DVDs and based on a sampling frequency of 2.8224 MHz (= 2.8224 MBits net per channel). Not that much more than on audio CDs (0.7056 MBits net per channel), and no problem at all for a DVD with its high capacity."
  • Digital Dharma of Audio A/D Converters (1997) offers this explanation (excerpt): "Essentially a delta-sigma converter digitizes the audio signal with a very low resolution (1-bit) A/D converter at a very high sampling rate. It is the oversampling rate and subsequent digital processing that separates this from plain delta modulation (no sigma). [Regarding] quantizing noise it is possible to calculate the theoretical sine wave signal-to-noise (S/N) ratio (actually the signal-to-error ratio, but for our purposes it's close enough to combine) of an A/D converter system knowing only n, the number of bits. Doing a bit (sorry) of math shows that the value of the added quantizing noise relative to a maximum (full-scale) input equals 6.02n + 1.76 dB for a sine wave. For example, a perfect 16-bit system will have a S/N ratio of 98.1 dB, while a 1-bit delta-modulator A/D converter, on the other hand, will have only 7.78 dB! . . . . One attribute shines true above all others for delta-sigma converters and makes them a superior audio converter: simplicity. The simplicity of 1-bit technology makes the conversion process very fast, and very fast conversions allows use of extreme oversampling. And extreme oversampling pushing the quantizing noise and aliasing artifacts way out to megawiggle-land, where it is easily dealt with by digital filters (typically 64-times oversampling is used, resulting in a sampling frequency on the order of 3 MHz). To get a better understanding of how oversampling reduces audible quantization noise, we need to think in terms of noise power. . . . With oversampling the quantization noise power is spread over a band that is as many times larger as is the rate of oversampling. For example, for 64-times oversampling, the noise power is spread over a band that is 64 times larger, reducing its power density in the audio band by 1/64th."
  • Christopher Hicks states that "by using some digital signal processing, the stream of 16-bit words at 44.1kHz can be transformed to a stream of shorter words at a higher rate. The two data streams represent the same signal in the audio band, but the new data stream has a lot of extra noise in it resulting from the wordlength reduction. This extra noise is made to appear mostly above 20kHz through the use of noise-shaping, and the oversampling ensures that the first image spectrum occurs at a much higher frequency than in the multi-bit case. This new data stream is now converted to an analogue voltage by a DAC of short word length; subsequently, most of the noise above 20kHz can be filtered out by a simple analogue filter without affecting the audio signal." (This statement was formerly posted at a site called RecAudioPro, www.vex.net/~pcook/RecAudioPro/digital.html, not accessible in September 2006.)
  • In September 2005, the Wikipedia entry for Direct Stream Digital reported "There is much debate over whether DSD or PCM is ultimately the better digital encoding format, as can be seen in the format war between SACD and DVD-Audio. The DSD technology is pioneered by Ed Meitner, an Austrian sound engineer and owner of EMM Labs. It is now developed by Sony and Philips, the initial starters of the audio CD."
  • Good introductory information at Direct Stream Digital (DSD) Encoding includes this fact: "Using single stage FIR [finite impulse response] digital filtering and noise shaping, 1-bit DSD can be down-converted into standard 24, 20 or even 16-bit PCM audio for CD distribution while still retaining the maximum possible audio quality. The system's 2.8224 MHz sampling rate is specifically designed for high precision down-conversion to all current PCM sampling rates using simple integer multiplies and divides."
History Philips' DSD tool division was transferred to Sonic Studio, LLC in 2005 for ongoing design and development. The Sonic Studio Web site had disappeared by 2013; Comments welcome

Format specifications Explanation of format description terms


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Last Updated: 02/14/2014