We're talking a deviation of one second in 1000 years. Basically, the rubidium element disciplines the crystal to its hyperfine oscillation (over 6.8 billion Hz), which produces 100,000 times better accuracy than your Swiss-made quartz-driven watch. When interfaced with the 10M, the OCX switches from its crystal oscillator to the 10M's Rubidium core. The Isochrone 10M Rubidium Atomic Clock, the OCX's sleeker, more expensive 2RU-height companion ($6000 street), is designed to enhance the OCX with atomic clocking technology. (Allen confesses to having mixed a song tracked at 48 kHz at the slower and lower-pitched 44.1 kHz for about twenty minutes before catching on.) It is front-and-center enough to remind you of the sample-rate, which we suspect will prevent the occasional clocking mishap. One design element we particularly liked is the prominent, red LED readout of the sample-rate. This sleek, 1RU-height silver box could easily be the classiest looking piece of gear in your rack. Take a look at its back panel, and you'll find eight BNC word clock outputs two AES/EBU and two S/PDIF outputs and a 256x Superclock (Digidesign) output. The OCX supports sample-rates from 32 to 192 kHz, and-this is a bonus-it is capable of outputting multiple sample-rates simultaneously (though we didn't use this feature). According to Antelope, this translates to higher stability and four to ten times lower jitter in the clock signal compared to the competition. The heart of the Isochrone OCX ($1500 street) is a temperature-controlled oven housing a discrete, transistor crystal oscillator. So as reviewers, we have chosen to sidestep the technological debates as much as possible and move forward with two simple findings: different clocks make the same recordings on the same system sound different and in some cases, that difference is big enough to convince one to buy one clock over another. Regardless of the hard science behind the design and performance of word-clock generators, the aesthetic impact of different clocking devices remains both subjective and context-dependent, as with any piece of gear in our racks.
Why so hotly debated? Because changing the clock source can have a highly significant impact on sound, yet there is no solid consensus as to what clocking scheme is best. The topic of digital clocking is popular right now-on, here in Tape Op (see issue #67's letters section), and generally among recordists. Optionally, a master word-clock signal (generated from an external device or from one of the in-stream devices) can be used to synchronize any devices with a dedicated word-clock input. All standard digital audio transmission formats have an embedded clock signal so that downstream devices remain synchronized to upstream devices. Inconsistent spacing (and/or a misshaped clock signal)-commonly called jitter-results in distortion to the audio signal. Utmost consistency in that spacing is crucial for transparent conversion. It not only regulates data flow so that everything works together synchronously, but more importantly, it controls the spacing between sample words during analog-to-digital and digital-to-analog conversion. In a digital system, the word-clock signal is used to generate a common time reference amongst all system components.
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