Digital Processor Reviews

Glitches: Any recording engineer using A/D converters with better than 16-bit resolution but storage media restricted to 16 bits—CD, CD-R, DAT, etc.—owes it to their sound-quality-conscious customers to use a Meridian 518. However, I did have one operational problem using the 518 in that I could not get my Panasonic 3700 professional DAT recorder to lock to its output, even with the channel bit set to "P" for "Professional." For the Festival project, I used a Meridian CDR as an interim storage medium (footnote 1), as this would lock onto the 518's data output with the channel bit set to "C" for "Consumer." When I had the same problem with the earlier Meridian 618, this was solved by an EPROMectomy. I assume, therefore, that the next 518 software release will fix the bug; I eagerly await the arrival of a new chip before my next recording projects gets underway.

Although the 518 has a female 9-pin RS-232 socket on its rear and the manual tells you how to control the unit with a computer, other than once getting it to completely freeze up, I couldn't get the 518 to respond. This was despite all possible combinations of comms programs, RS-232 cables, and gender changers I tried. Don't blame the 518 in this regard, however, as I am singularly unlucky in matters RS-232.

Magic: During the editing and mastering for Stereophile's Festival CD project, I grew very familiar with the Meridian 518's positive effects when used as a resolution enhancer to reduce digital word lengths. The limited time I had available during the review period meant that I wasn't able to try it as a full-blown digital control center. But one thing I did try at the end of the review period was the inverse: feeding it 16-bit data and instructing it to output 20-bit data to my Levinson No.30.5 (which does have 20-bit resolution) with noise-shaping algorithms "A," "D," or "E" (which one worked best seemed dependent on the source material).

I was expecting nothing to change. After all, both the S/PDIF and AES/EBU datastreams carry 24-bit words at all times. The nominal word length only means that the specified number of MSBs are significant, the rest being set to zero (ie, the digital representation of "digital zero" encoded with 16-bit precision in the datastream is 111111111111111100000000; with 20-bit precision, 111111111111111111110000). In both cases, the actual word length in the signal is 24 bits wide/long. And remember that the Levinson '30.5 uses a FIFO data buffer with a length of 1/8 second to minimize input-data word-clock jitter (footnote 2).

But strap me if there wasn't something quite magical happening. First, the bass became more solid, more deep. The rather lightweight bass synth lines and kickdrums on Steve Winwood's "Valerie" (from The Finer Things boxed set, Island Chronicles 314 516 860-2), for example, became more physically moving, more dancelicious. Orchestral bass drum and organ bass pedals purred more palpably, and the low orchestral strings acquired more bloom. Second, little details in the soundstage, such as the echoes from the walls of the hall, became both more noticeable and paradoxically better integrated with the sound sources. Image depth deepened. But most important, the flow of the music just became more enveloping. It was goosebump city, even with recordings that usually strike me as just being notes by numbers.

I switched to Bypass mode: the sound reverted to being merely very good. The magic had diminished. I switched back to the 16-20 dithered word-length increase. Back came the magic.

I swear I am not making this up! Because I didn't expect anything to happen, I didn't even try it during most of the time I had the 518! So what the heck was going on? Yes, the 518's dual-PLL receiver design means that it acts as a conventional jitter-reduction unit, with the added benefit of stripping out the subcode, including the SCMS bit. (You can see in any of Stereophile's digital processor reviews that the subcode data in the S/PDIF signal produces a ubiquitous 7.35kHz jitter content.) But surely that would also happen with the unit in its bypass mode. I reread the 518's manual. There on p.8 was a warning: "Warning! Systems like this can sound incredibly good!" But this appeared to refer to the 518 used as a digital preamp, where its reduction in jitter is coupled with the complete elimination of the analog preamplifier. I can only assume that using the 518 to increase the word length helps the digital filter in the following processor to do its math more optimally, with reduced round-off error.

Or something.

Was there a downside? But of course. HDCD-encoded recordings would not pass intact through the 518 in its resolution-enhancement mode. But that was trivial given the presence of its Bypass button, which disables any DSP guarantees that all 24 bits presented at the input are passed through to the output.

I generally didn't use the 518's gain-adjust function during CD playback; I routinely monitor recorded level using a Dorrough AES/EBU-input meter, and it's rare that a modern CD doesn't hit -1dBFS or higher some of the time. But there are CDs which are way under-recorded—A Meeting by the River, Kavi Alexander's recording of Ry Cooder and V.M. Bhatt (Water Lily Acoustics WLA-CS-29-CD), for example—and which did benefit from a little noise-shaped volume increase.

Conclusion

When I heard about the Meridian 518, it struck me that it was in some ways an orphan product. While it is an essential tool for quality-oriented engineers, I couldn't see what kind of role it would have in the context of an ordinary high-end audio system.

Now that I've spent time with the 518, I'm going to have to get out my checkbook. Meridian is not getting this review sample back! Yes, it is invaluable used for mastering 16-bit CDs from 20-bit originals. But its effect on CD playback is almost as staggering. And at $1650, it's even a bargain. The Meridian 518 is the Swiss Army Knife of digital audio.

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Footnote 1: Putting Pioneer's $25/disc price for the blank 60-minute "consumer" CD-R media into perspective—see SS's review of the new PDR-99 elsewhere in this issue—the latest price we paid for 74-minute "professional" blanks was just $7.50! No doubt this is a consequence of RIAA pressure not just on Pioneer, but on all companies offering consumer-oriented digital recorders.

Footnote 2: It's hard to believe that any changes upstream of that mighty buffer could affect the sound of the Levinson, yet in previous experiments the differences made by either an Audio Alchemy DTI•Pro or a Sonic Frontiers UltraJitterbug were clearly audible. (Which sounded better, or none at all, was very dependent on the music.)