SACD and the DSD principle

SACD is currently available in different carrier formats. Where CD uses a single layer for music/data storage, SACD uses either a single high-density (HD) layer disc (4,7GB) or a dual HD layer disc (9GB). A third group of SACD is defined by the hybrid disc. This is the most important one for the near future. It incorporates a single HD layer but also a conventional CD layer. This combined disc makes it possible to play normal CD on a conventional CD player.

Direct Stream Digital (DSD) encoding

Standard CD recording uses multibit PCM technology. In most cases this is 16 bit wide with a sampling frequency of 44,1 kHz. Improved technologies often use higher bit rates(18-20-24 bit) and/or higher sampling rates. However, the improvements are getting smaller and smaller. Every PCM system requires steep filtering at the input to absolutely block signals above half of the sampling frequency (conventionally this is 22,05 kHz). It also requires the addition of re-quantization noise for its decimation (down-sampling) digital filters and interpolation (up-sampling) digital filters. These problems limit the actual fidelity of the reproduced audio.

In conventional PCM systems the analog signal is first converted to digital by 64x oversampling delta-sigma modulation. The result is a 1-bit digital signal. Where conventional systems immediately decimate the 1-bit signal into multi bit PCM code, DSD records the 1-bit pulses directly. Using a negative feedback, if the input waveform, accumulated over one sampling period, rises above the value accumulated in the negative feedback loop during previous samples, the converter outputs a digital "1." If the waveform falls relative to the accumulated value, a digital "0" is output. As a result, full positive waveforms will be all 1s. Full negative waveforms will be all 0s. Alternating 1s and 0s will represent the zero point. The resulting pulse trains "looks" remarkably like the analog audio waveform that it represents, this in contrast to PCM. Furthermore digital to analog conversion can be as simple as running the pulse train through an analog low pass filter!

DSD is designed to capture the complete information of today's best analog systems. The best 30ips half-inch analog recorders can capture frequencies past 50 kHz. DSD can represent this with a frequency response from DC to 100 kHz. To cover the dynamic range of a good analog mixing console, the residual noise power was held at -120 dB through the audio band. This combination of frequency response and dynamic range is unmatched by any other recording system, digital or analog.

pcmanddsd.gif (6790 bytes)

converterchain.gif (2860 bytes)

DSD in recording

DSD samples music at 64 times the rate of CD (64x 44,1 kHz). This yields a sampling rate of 2,822MHz. To compare, CD uses 16 bit/sample, so the bit rate is 16 bit x 44,1 kHz= 705,6 kbit/s. DSD uses 1 bit/sample, so the bit rate is 2,822 Mbit/s. This is only 4x the data of CD and that makes storage on current technologies relative easy.

DSD downconversion

dsdbitstream.gif (11591 bytes)Using single stage FIR 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.

dsddownconvert.gif (6070 bytes)

Theory based on DSD (Direct Stream Digital) and SACD (Super Audio Compact Disc) technology developed by Royal Philips Electronics and Sony Corporation copyright 2000 and 2001.

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