If you've read Bob Katz's informative articles about jitter and his recommendations on clocking (www.digido.com), you'll understand that in certain situations, it's better to sync your A/D converter using an internal clock instead of an external word clock (WC) for a couple reasons. First, cables can distort the signal and therefore add jitter. Second, an external WC signal must be "followed" by a phase-locked loop (PLL) circuit in any downstream device; if that PLL is poorly designed or overtaxed, the PLL will exhibit more jitter than an internal crystal-based clock will. But Bob also explains clearly (and cites ample evidence) that monitoring a signal thru daisy-chained digital devices (with each downstream device sync'ed to the embedded clock in the digital stream that originates at the A/D converter) can result in significant amounts of accumulated jitter at the final stage of conversion back to analog. In a situation like this, an external WC can make a huge difference in the sound that you ultimately hear, especially if the D/A converter has a well-designed PLL circuit or if there's a jitter-reduction system ahead of the DAC circuit. Not everyone agrees, and I think ultimately, as with any gear decision, you'll have to listen for yourself to see if your setup sounds better (or at least different) with an external WC. There's also the very strong argument for convenience and consistency. If you sync all your digital devices off a single master clock (assuming all your devices have some way of receiving an external clock-see my comments on the Drawmer clock below), you will never need to change the sync settings of your devices, nor will you make the mistake of having any devices out of sync.
We've been avid users of Apogee's products at The Lodge for many years. At our New York music-production facility, the main room has a Pro Tools HD rig with Apogee Rosetta 800 converters (equipped with X-Series-HD cards for direct connection to the HD cards). Because we also have many other digital devices in this studio, it was a no-brainer to go with a Big Ben master word clock to keep everything in sync. The Rosetta 800's sound fantastic (and personally, I wish I had gone with a pair of Rosettas instead of the Digidesign 192 I/O that I have in my Boston studio), and they incorporate a technology that Apogee calls Intelliclock for jitter reduction. With the Big Ben driving the whole studio, we never have clocking problems, and our digital audio remains pristine. The only gripe I have is that I'm not a fan of Apogee's user-interface standards. Going back to the first Apogee device I ever used (AD-8000), I've often had to pull out the manual (or do a web search) to figure out how to change even simple settings. The lack of dedicated buttons and detailed front-panel labeling forced me to write explanatory notes onto strips of artist tape that I keep affixed to the Apogee devices at The Lodge!
Contributing writer Mike Jasper, who owns and operates Deceptive Sound in Austin, TX, returns to the Gear Reviews section of Tape Op after a long hiatus to write about his recent experiences with the Big Ben in his studio. Immediately following his review is my take on the Drawmer clocking system that I installed in my Boston studio. But first, here's what Mike had to say about the Apogee Big Ben. -AH
It had been a few years since I bought my state-of-the- art Apogee PSX-100SE (Special Edition) AD/DA converters, so I figured it was time to look for an upgrade. When I checked out Apogee's website, I took a close look at the Rosetta 200, but was annoyed to discover it doesn't include Apogee's top- of-the-line digital clock, the new "C777." Fortunately, the C777 is sold separately in a product called Big Ben. Since I record at 24-bit, 44.1 kHz, then mix down to 16/44.1, the need for a Rosetta 200 with its ability to record and playback in 192 kHz is far from urgent, so I decided to give Big Ben a try and use it with my old PSX-100SE-which I'm convinced is now a vintage digital device.
Conventional wisdom has it that the addition of an external clock will create more digital jitter from the cable connection to the converters than the clock itself can possibly prevent. But others say that a well-constructed external clock takes that added jitter into account and counteracts the ill effects. To use an eyesight analogy (because we are talking about clarity, after all), sometimes putting on eyeglasses can improve your vision, even though it's not as ideal as your OEM eyeballs working at 20/20. The Big Ben works like eyeglasses for your digital devices.
For the A/D test, I used StormDrum to create a groove on what would turn out to be a cover of the first verse of "My Girl" by the Temptations. I played bass, electric guitar, and electric piano through the DI of a Great River MP-2NV, then recorded acoustic guitar and vocals with a Neumann U 87 to the same preamp. The song was recorded twice, once with the PSX-100SE alone, and again using the converters clocked to Big Ben. I heard in the recording pretty much what I heard in my playback tests-better clarity in the highs and low mids when clocked to Big Ben. But this time, I wasn't entirely sure I liked the result, until I mixed down to 16-bit and burned a CD. When I heard the two versions over my stereo and then in my Walkman, the Big Ben version was clearly superior. While both recordings sounded very good, the Big Ben recording sounded the way it was supposed to- especially the tone of the electric guitar.
Me? I say to hell with conventional wisdom, just plug it in, and let me hear for myself. What I heard was more clarity overall, especially in the highs and low mids, and when played back at 24-bits, the difference was striking. Still, I would have liked to have had the time and resources to try a test with a Rosetta 200 clocked to a Big Ben. Would the combination of the two be ideal for my two-channel setup? Beats me. All I can really tell you is that the Big Ben improved my sound. That said, given the wide variety of digital setups available today and the number of manufacturers making A/D and D/A converters, my best advice for this particular piece is to listen to the old saw nobody likes to hear-try before you buy. ($1495; www.apogeedigital.com)
-Mike Jasper, www.deceptivesound.com
Drawmer sells a two-piece clocking system comprised of the DMS-1 M-Clock generator and the DMS-2 D-Clock distributor. The M-Clock provides extremely stable clock signals with rates up to 192 kHz (as well as Digidesign Superclock), assignable across eight WC outputs on BNC connectors and four AES/EBU (XLR), four S/PDIF (phono), and four TOSLINK outputs. Its counterpart, the D-Clock, takes either a WC input or an incoming AES/EBU signal from which it strips the embedded clock and redistributes WC to sixteen rear-panel and four front-panel BNC outputs (with zero latency). It also has a front-panel readout that displays the incoming clock's frequency and drift (with 2 ppm resolution). You can use the two units together, or you can use them separately.
In my Boston studio, I have a Sony DMX-R100 digital desk with a MADI interface that allows me to hook up a bajillion channels of digital gear to it. I love how the console sounds, especially because its analog front-end starts to distort before the level to the A/D converters reaches full-scale; therefore, it sounds like an analog desk! Additionally, the converters themselves sound great when running on the console's internal crystal clock-I think better than the converters on my Digidesign 192 I/O. I had all my other devices clocked to the DMX-R100, believing that this would give me the most stable A/D conversion. Last month, I installed the Drawmer M-Clock and D-Clock in my studio, replacing the Lucid GENx6-96 that I was never fond of.
I used to think my ears needed to "settle" into my room whenever I'd first sit down at my console because things wouldn't sound as good as they would a half hour or so later, but according to the error display on the D-Clock, it's the DMX-R100's internal clock that's doing the settling as it's warming up (from around 10 ppm down to 0-2 ppm, the D-Clock's limit of accuracy). I confirmed this using my Tektronix digital oscilloscope. I let both the D-Clock and scope warm up for an hour, then powered up the DMX-R100 and measured its WC output at intervals as it warmed up. I also compared the WC outputs of both the M-Clock and the DMX-R100 on the scope. Not only does the M-Clock come online at the correct clock-rate immediately (it has a temperature-compensated oscillator), but holy schmoly, the square wave coming out of the Drawmer clock is rock solid (both in freq and shape), while the signal from my console has lower voltage, a curved trailing edge, and a leading edge that's not as surgical. (I swapped cables and made sure I had proper termination into the scope to make certain that my measurements were valid. In the process, I also discovered that Monster Cable digital patchcords suck compared to Mogami (Redco-assembled) and Apogee cords.) Well, that was a revelation. Although the A/D conversion on my DMX-R100 sounds fine running on its internal oscillator-but only after the console is warmed- up-if I sync downstream devices from my console, the downstream conversions suffer from the console's crappy outgoing WC signal. Sure enough, the sound of my 192 I/O converters improved dramatically when I installed the M- Clock as my master clock! To the point where they went from god-awful to sounding pretty dang fine (but still not as smooth as my console's converters). This makes me think that my console's WC output's driver circuitry is either poorly designed or defective. The sound of the DMX-R100's converters when properly warmed-up also changed with the switchover to the M-Clock. It's as if my ADAM S3-A monitors jumped a couple feet closer to me (i.e., the soundstage moved toward me, especially in the midrange)-something I never experienced clocking off my now retired GENx6-96.
I should also mention the two reasons why I chose the M-Clock over the Big Ben. First of all, the M-Clock has dedicated controls and indicators that are well-labeled-no manual needed. It's dead simple to use, as a master clock should be. Second, it has four built-in sample-rate converters (referenced internally to the clock), with one SRC input available on the front panel and outputs for all four on both the rear and front. That means I can connect my
CD player to my Z-Sys digital patchbay through one of the M-Clock's SRC's and have the player's output reclocked to sync with the rest of my studio. Same for plugging a portable MD recorder or digital keyboard into the SRC input on the front. In other words, devices that aren't capable of being externally clocked can still be sync'ed to my studio via the M-Clock. Cool! (M-Clock $1375 MSRP, D-Clock $1000; www.drawmer.com) -AH
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