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Professor Malcolm Omar Hawksford is Director of the Centre for Audio Research and Engineering, Department of Electronic Systems Engineering, at University of Essex. Chris Dunn is a Research Officer at King's College, London. This work was funded by the UK's Science and Engineering Research Council, and was originally presented as a paper, "Is the AES/EB U S/PDIF digital audio flawed?" (Preprint 3360), at the 93rd Audio Engineering Society Convention, October 1992, in San Francisco. It is reproduced here with the kind permission of the AES. High-quality digital audio systems require that all digital interfaces in the signal path exhibit signal transparency. The widely adopted AES/EBU and S/PDIF interfaces have been criticized for a lack of signal transparency; here we address possible problems with such interfaces and present methods for improving the interface standard. In a correctly functioning (uniformly quantized and sampled) digital audio system, the only observable signal impairments should be attributable to band-limitation and an additive noise residue. Thus, although digital audio's subjective sound quality has been criticized since the launch of the Compact Disc medium 13 years ago, the theoretical performance obtainable from the 16-bit linear PCM format sampled at 44.1kHz is superb to any analog sources available to the consumer . When correctly dithered using triangular PDF dither, a 16-bit digital audio signal possesses a dynamic range of 93.3dB, with zero distortion and zero noise modulation. The 16-bit format holds the possibility of even higher subjective dynamic range with minimally audible noise-shaping employed during CD mastering. Lipshitz et al1 show that an increase in subjective dynamic range of up to 18dB is readily achievable when making the final truncation to 16 bits. Since any practical digital audio system will err from this ideal performance,attempts are made to minimize measurable errors in digital components. For digital/analog converters (DACs), circuit-architecture advances including 1 S.P.Lipshitz, J. Vanderkooy,and R.A.Wannamaker, 'Minimally Audible Noise Shaping,' JAES, November 1991, Vol39, pp.836-852. |
oversampling, noise-shaping, and 1-bit conversion result in greatly improved low-level resolution - the compact disc's theoretical performance can now be realized at a relatively low cost, at least upon replay. In the quest for resolution, many "outboard" DAC units have appeared on the consumer market, with their sensitive D/A conversion process removed from the harsh electromagnetic environment inside the typical CD transport. Digital data is transmitted from the transport to the DAC along a coaxial or optical link (fig.1) in a serial format known as the Sony/Philips Digital Interface Format (S/PDIF).]" The S/PDIF standard is very similar to the AES/EBU format commonly used to interconnect professional digital components, and differs only in details, including transmission
amplitude and subcode format. For much of this article, both interface standards will simply be referred to as the digital audio interface. Some users have reported subjective differences between various implementations of the interface. Peter Van Willenswaard 2 was among the first to note a change in out-board DAC sound quality when switching between different CD transport units; he linked this to measurable differences in interface signal risetime. Audio reviewers' claims 2 Peter van Willenswaard, Stereophile November 1988, Vol.11 No.11, pp.51-53. |
