dCS Purcell D/D converter
The Purcell's latest software upgrade has added downsampling capabilities of 96kHz to 48kHz and 96kHz to 44.1kHz, which allow 24/96 DVD recordings to be copied to CD-R and MD for use in car playback systems and other portable disc players, "where permitted by the copyright owner." The upgrade package also includes downsampling from 48kHz to 44.1kHz for making CD-R copies. And hang in there, vinylphiles—the Purcell Plus is coming to town. It'll feature an analog input to digitize and upsample your precious records! Paging Michael Fremer!
So what, exactly, is the difference between upsampling à la dCS and the industry-standard 8x-oversampling converters that take "Red Book" 16-bit/44.1kHz up to a 352.8kHz datastream before hitting the DACs? That's...hard to answer. As John Atkinson mused in his December "As We See It," it may all be down to a question of the difference between digital filters!
If you're interested in the subject, visit the dCS website and click on "Technical Papers." There you'll find, in PDF format, "Timing Errors and Jitter," "A Suggested Explanation for (Some of) the Audible Differences Between High Sample Rate and Conventional Sample Rate Audio Material," "Resolution, Bits, SNR, and Linearity," and, finally, the long-awaited "Effects in High Sample Rate Audio Material."
You might also want to visit dCS importer Audiophile Systems' website, and download the Purcell's user manual. It contains interesting technical Information about word-length reduction and dither, a process that, distilled to its basics, adds a signal to transform the sonically ugly quantization products due to truncation from a long word length to a shorter one into benign random noise.
For its part, dCS states that "upsampling reveals information that is present in the master source, but which is not audible when the CD is played back normally. Upsampling cannot increase the amount of information in a signal, and the exact mechanism behind the perceived sonic improvements is currently not clear. We are continuing our research into this subject."
So while the true differences between upsampling and oversampling remain murky, my pleasant mission is to report on the sound of the Purcell and compare it to the latest version of the pro-audio version, the 972—and that I shall do forthwith!
The Purcell is attractively finished in brown and taupe. There are a grand total of three buttons on the front panel: two on the left, one on the right, with the display window between. It was very easy to use—and to place, for that matter, designed as it is to dock below a dCS Elgar or Delius DAC.
Far left, the Input button makes the same precise relay sounds when pressed as the other two control buttons, and cycles through AES, S/PDIF (RCA), and ST AT&T optical (if installed). The Input button is also used to page backward through the Function Menu if you're not using the remote.
The Output button cycles through the output sample rates: 32, 44.1, 48, 88.2, 96, 176.4, and 192kHz. (The top two are available only when running dual AES connections.) dCS warns that, with most DACs, the system should be muted before changing sample rates, but Elgar and Delius owners can set their inputs to Non Audio Mute and relax. The Output button is used to page forward through the Function Menu when the remote is not used.
Cascading down the display window's left side are bright yellow input indicators, while the main display shows input and output sample rates. Down the right side of the display are indicators for Output word length and Noise Shaping Off. If variable noiseshaping is used, telltales for first-, third-, and ninth-order shaping filters light up when selected.
The last button to the right on the simple and architecturally elegant front panel is the Function button, which allows the user to access additional features. Future enhancements will be released on CD; you'll be able to upload them into the software simply by playing the disc!
The rear panel seems densely packed with connectors, but don't let that put you off—it's dead simple to hook up. You've got several choices for input: S/PDIF on RCA (the one I used) and BNC (there's no BNC S/PDIF output on the Accuphase DP-100 SACD/CD transport), plus AES/EBU on an XLR connector, all capable of passing up to 96kHz sampling rates. Next in line is a serial-type communications port "for future enhancements." You're warned to keep outta there to avoid provoking digital mayhem within. An ST optical port is next, if you've elected that option. (ST is dead in the dust these days. Here it reliably handles signals only up to 48kHz; higher speeds are possible but depend on the quality of the receiver.)