Stereo Amplifier

Review: Pioneer PD-S801 CD-player

When any hi-fi manufacturer claims to have fathered a revolution in audio technology, others in the industry dispute paternity. Look back far enough and hard enough into audio history and precedents can usually be found to today's 'breakthroughs'. As co-creator of the moving-coil loudspeaker Chester Rice once observed, expressing the frustration of every creative thinker: "The ancients have stolen our inventions". Still, whether you conceived it or not, there is nothing wrong in recognizing a good idea and seizing upon it.

This is what Pioneer appear to have done in the case of Legato Link, the digital filter fitted to their latest generation of middle-market and up-market CD players. They cannot legitimately claim to have originated the underlying concept; nor, in a fashion not atypical of the Japanese majors, can they claim always to have offered clear-eyed descriptions of precisely what Legato Link is and what it achieves. But so what? When all pedantry is said and done, Legato Link confers a distinct and alluring sound quality on the new Pioneer range. In the case of the cheapest PD-S801, it arguably makes for the best-sounding £300 CD player currently available.

Legato Link also serves to illuminate something that the starring role accorded digital-to-analogue converters has often obscured: that the digital filter makes as important a contribution to the sound quality of a modern CD player as do its D/A converter chips.

What is Legato Link?

It might be helpful, before proceeding to describe what makes Legato Link distinctive, to repeat quickly what function digital filtering performs in a CD player.

Digital filtering in the context of CD players is synonymous with oversampling, the function of which is to increase the apparent sampling rate of the digital signal and thereby force the ultrasonic distortions produced by analogue-to-digital conversion to still higher frequencies. This relieves the requirement for steep post-D/A converter analogue filtering, with corresponding benefits to phase distortion, temperature stability, product consistency and cost. (Oversampling also provides the opportunity to reduce the digital word length required for each sample, which relieves demands on DIA converter accuracy-but let's not complicate the story unnecessarily.)

Oversampling works-simply enough in principle-by interpolating between the genuine samples read off the disc a number of pseudo-samples, the calculation of which is under the control of a mathematical function which the digital filter applies to the incoming data. This algorithm, as it is known, determines the contour of the reconstructed analogue waveform between the genuine sampling points and as a consequence influences not only out-of-band signals but also those at the top of the audible range.

Conventional digital filter algorithms are contrived to imitate the 'brick wall' post-D/A converter analogue filtering which used to be applied. Frequency response errors within the audible band are minimized, while just above it the response is rapidly curtailed in order to suppress ultrasonic spuriae. Performing this in the digital rather than the analogue domain allows a linear phase characteristic to be easily achieved and al so overcomes the cost and consistency problems of complex analogue filters. But that other bugbear of steep, high-order filtering-transient oscillation or 'ringing'-remains.

Not everyone is of the prevalent view, shared by the vast majority of digital engineers, that this ringing is inconsequential. American specialist manufacturers Wadia it was who first contended that it should be suppressed, even at so me inevitable expense to in-band frequency response. With Legato Link, Pioneer has taken up a similar theme.

Indeed, Pioneer's development of Legato Link echoes the approach adopted by Wadia. Digital signal processing (DSP) chips were used to implement a variety of filter algorithms and the results assessed by listening test. The upshot in both instances was a preference for gentler-than-normal filtering, with decreased ringing, despite the fact that this introduces a significant in-band high frequency roll-off (-2.8dB at 20kHz in the case of Legato Link) and worsens ultrasonic rejection. The latter consequence Pioneer claim as a benefit rather than a handicap (despite the fact that the ultrasonic components are distortion products) on the basis that it retains a more natural spectral content, rather than one which is suddenly truncated above 20kHz.

Wadia retained the DSP filter realization for their products, an d in the course of development have offered a succession of different filter algorithms (Frenchcurve, Digimaster and now Bio-Digital). Pioneer have chosen instead to design and manufacture a dedicated digital filter chip, a move which precludes updating of the filter algorithm but has the enormous benefit of substantially reducing costs. Pioneer says in addition that the dedicated chip sounds better than the DSP realization - an intriguing claim but one which, of course, is subject to Mandy Rice-Davies' famous observation.