Meridian 518 Digital Audio Processor Page 3
Table 1 Meridian 518, Noise-Shaping Algorithms
|Noise Advantage (44.1kHz)|
|(White-spectrum TPDF dither)|
|(Additive, high-pass TPDF dither)|
|(Flat dither, 2nd-order MAF*)|
|(Flat dither, 9th-order MAPÝ)|
|(Flat dither, 9th-order MAF)|
|(High-pass dither, 9th-order MAF)|
|Shape E||Not specified|
|(High-pass dither, 2nd-order MAF)|
* MAF = Minimum Audible Field, the threshold curve for human listeners with stereo loudspeakers.
Ý MAP = Minimum Audible Pressure, the threshold curve for human listeners using stereo headphones.
All my auditioning of the 518 was done in my usual reference system: Mark Levinson No.31 CD transport feeding a No.30.5 D/A processor via a 1.5m length of Illuminati's excellent new Orchid AES/EBU datalink ($750 [choke, cough, splutter!]). With the Meridian in the chain, a 1.5m length of Madrigal AES/EBU cable took the output of the transport to the 518. The D/A processor was connected to a Levinson No.38S preamplifier via 0.5m lengths of AudioTruth Diamond x2, with then 5m lengths of AudioTruth Lapis or XLO Signature 3.1 feeding a Levinson No.333 power amplifier (review underway) or a Cary CAD-300SEI single-ended amplifier. Loudspeakers were B&W Silver Signatures (mainly), the four pairs of speakers I am currently reviewing—JMlab Micron Carats, Totem MANI-2s, Joseph RM7sis, and PSB Stratus Mini Mk.IIs—or the Epos ES25s that JE reviews in the next issue. The B&Ws were used with their dedicated silver speaker cables, the other speakers with an 8' bi-wired set of Cardas Cross.
Masterful: To investigate the subjective effect of the 518's resolution enhancement while reducing data word length, I used the 20-bit master data for both Stereophile's new Festival recording and for the forthcoming Robert Silverman Liszt Piano Sonata recording. These were played back from a Sonic Solutions hard-disk editing system mounted in a Macintosh Quadra 650, with 6 Gigabytes of hard-drive space available.
I have no doubt that, along with other devices/algorithms to achieve the same end—such as Apogee's UV22 processor and Sony's Super Bit-Mapping process, neither of which I have yet gotten any hands-on experience with—Meridian's 518 (and its predecessor, the 618, which I used for our Concert project) achieves a revolution in CD sound quality.
After spending many hours listening to the 20-bit Sonic Solutions data fed straight to the '30.5 via a Sonic Frontiers UltraJitterbug, simply truncating the word length by setting the Sonic Solutions' output to 16-bit and switching off its dither gave a horrendously audible degradation. Midrange textures sounded clangorous rather than sweet, treble acquired an extra helping of fine glassy grain, and the recorded soundstage shrank. And most tellingly, applause no longer sounded like the sound of many hands clapping, but was reduced to a bland impulse-noise-like, er, noise.
Enter the 518 with its input word length set to "20" and its output length to "16." With the dither shape set to "Flat," some of the sound's dimensionality returned and digital applause started to sound more like applause. "High-Pass" gave more improvement, as did "Shape A." And with Shapes B through E, I could not hear much of a difference between the original and the processed data, nor could I hear any consistent differences between the different dither algorithms.
Mission accomplished; the magic became real. I therefore used the Meridian to convert the edited 20-bit data for Festival to 16-bit.
Meridian recommends using the 518's adjustable digital gain to ensure that the signal occupies the full dynamic range of the output medium. As the Festival data already peaked at 1 LSB below 0dBFS, I didn't add gain. But as I had recorded Aaron Copland's Appalachian Spring 6dB hotter than the percussion-heavy Milhaud and Kohjiba works in the same program, I used the 518 to reduce its level by 6dB when I transferred the 20-bit data to 16-bit CD-R. In this way, not only would I achieve the correct level match among the three works, but I would do so with the minimum resolution penalty. As the 518 indicates when the signal reaches 0dBFS, you don't have to worry about clipping.
Meridian also recommends using the 518 to add pre-emphasis, Bob Stuart correctly pointing out that wrapping the digital data in a pre-emphasis/de-emphasis loop will result in a 1-bit (6dB) improvement in S/N, a 2-bit improvement in HF resolution, and a more than 10-fold reduction in D/A converter noise. I therefore tried mastering my data with pre-emphasis (adjusting the overall gain to avoid clipping). To my surprise, not only could I not hear any difference between emphasis and no emphasis played back through the Levinson, occasionally I thought it sounded worse. There did seem to be an improvement with my budget reference, the Assemblage DAC-1, however.
I suspect that the reason for this disparity between theory and practice is due to how the D/A processor implements the de-emphasis. The whole idea is to place the de-emphasis as far back in the chain as possible. In this way, all the high-frequency errors and artifacts produced by a processor or CD player's digital filter and DAC will be rolled off, reducing their audibility. The logically optimal de-emphasis implementation will therefore be in the analog domain, right before the processor's output amplifier. Indeed, the Assemblage's de-emphasis is achieved with a FET-switched network in the feedback loop around the I/V converter op-amp. But the Levinson's de-emphasis, as in almost all processors, is achieved in the digital domain, within the digital filter chip. While this is not subject to the vagaries of analog component tolerance, giving a perfect de-emphasis curve, it sort of throws away the reason for using it in the first place (as Bob Stuart points out in the 518's manual).