Pioneer PD-91 CD-player
With its PD-91 Compact Disc player, Pioneer claims use of the "world's first true 18-bit D/A [digital-to-analog] converter." Even though the PD-91 has been scooped by another true 18-bit machine using the same converter chips, Pioneer's 18-bit entry does offer a measurable advance in the approach to the ultimate CD player: The PD-91's level of performance is unique among CD players.
An 18-bit D/A converter (DAC) won't resolve finer waveform detail than is present in the 16-bit data on a CD, but it can do the decoding more precisely than a 16-bit DAC. Specifically, the same degree of error relative to their respective capabilities constitutes a much smaller absolute error for the 18-bit decoder.
In a way, that's not unlike the principle of oversampling, which the PD-91 also employs. In the PD-91, an eight-times resampling of the basic 44.1-kHz CD sampling rate results in a 352.8-kHz data-reconversion rate; it also pushes switching noise and various conversion artifacts much farther beyond the audio band than is the case with lower resampling rates. Pioneer's resampling technique lowers the performance requirements of the analog filter needed to block these effects and, in the PD-91, permits the filter to be a relatively simple third-order Butterworth design for low ripple and phase shift in the final outputs.
There are a host of other technical refinements as well, some shared with other products in Pioneer's Elite Series and some also available in high-end equipment from other brands. For example, exceptional care has been taken to avoid mechanical resonances, including the use of Pioneer's massive honey-comb-chassis construction. And in addition to vibration-damping feet under the main chassis, a fifth foot at the back directly supports a large power transformer that is further isolated from the chassis by a compliant coupling.
Four separate power supplies feed the electronics via 11 (!) voltage regulators. In contrast, a single digital reference frequency prevents interference that can be generated by numerous timing circuits found in other players. A further hedge against extraneous noise is the front panel's fluorescent display, which can be switched off once programming chores are finished. However, I never encountered any audio noise attributable to the elegant, uncluttered display (and very little noise attributable to any cause).
In addition to gold-plated analog-output pinjacks, the back panel contains two sets of direct digital outputs-electrical (pinjack) and optical. A front-panel switch permits analog-only, digital-only, or both outputs, so you can defeat whatever is not needed or use both simultaneously.
Most of the CD-player operating modes familiar to you are available on the PD-91. The front panel has controls for random track playback, 24-selection programming, index-number cueing, various repeat modes, autospace (which adds three seconds to the interselection blanks), automatic programming for dubbing onto a cassette (whose length you enter into the player, which then programs as many tracks as will fit on a cassette side), and "time fade edit."
This last control employs a function rarely found on CD players-the ability to program by time cues (minutes and seconds). In this case, it's coupled to an automatic fader, so you can (for example) turn the PD-91 into the ultimate in sleep-timers-when time runs out, it won't jar you back to consciousness with a sudden silence. The feature makes a little more sense, however, when you look at the supplied wireless remote, which gives you the ability to manually fade in (from pause) or fade out. This is an elegant touch that's welcome when you're out of reach of the volume control.
Also on the remote are controls for programming as many as eight "music windows"-segments whose beginning and end points can be marked manually (typical phrase-repeat functions allow the definition of only one window). Windowing proved quite fascinating, though its ultimate utility in normal listening is questionable. In programming the function, you can use the track-seek, index-seek, and manual-scan controls to find the passages you want. Each press on MUSIC window marks a beginning or an end, even though the music continues to play during the process. When you then stop the player and press play once again, your sequence of passages is recalled, neatly faded in at the beginning and faded out at the end of each window. Even if you skip most of the disc in between marked passages, cueing is effortlessly fast-Pioneer rates maximum access time at 1/2 second. Also noteworthy is the seamless joining of contiguous tracks in the regular programmed mode, though many other current models match the PD-91 in this ability.
Most of the front-panel controls are repeated on the remote. The only omissions I regret are the level control for the headphone jack and the display switch. But given the modest size of the display elements and the length of most headset cords, neither function is of much use at any distance from the player.
The sound of the PD-91 is, as one would expect, exemplary. Sonically, some CD players have pleased me more and some less. In the exalted realm inhabited by such top-end players as the PD-91, the differences among them seem essentially undemonstrable-at least in comparison with the obvious differences between CDs and LPs. However, Pioneer seems to have left no chip unturned in its quest for perfection.
That progress has occurred is beyond doubt. All of Diversified Science Laboratories' data can be summed up in one statement: No other CD player has measured this well overall, and none has provided so little opportunity for specific complaint. In fact, the worst that can be said of the PD-91's lab tests is that its frequency response with pre-emphasized recordings is only a little better than average among the premium models we've tested (response without pre-emphasis, though, is outstanding). And even this statement quibbles over fractions of a decibel except at the extreme top of the band, where Pioneer's generous over-sampling allows filtering that leaves no telltale response droop whatsoever.
Pioneer must be given high marks for pursuing perfection and for having produced a CD player that luxuriously satisfies both sonically and functionally. While the audible gains of 18-bit DACs may not be obvious, the PD-91 is clearly a superior CD player.
