Audio Alchemy DTI•Pro 32 Jitter Filter Page 2

Setting up the DTI•Pro 32 and choosing its output word length requires the owner's manual; there's no direct readout of the 32's operating status. You must instead interpret a code provided by the three power-indicating LEDs. For example, 20-bit output is indicated by illumination of only the bottom two LEDs. Without the owner's manual (or a great memory), you won't be able to correctly set up the Pro 32 for your system.

To change the output word length, press and hold the "Phase" button until the three power-indicating LEDs switch from power indication to operational status indication. Still holding the Phase button, press the input button to scroll through the output word length options. Note that you must go through this procedure for playing back HDCD-encoded CDs (if you have an HDCD-based processor), then reset the Pro 32 to its previous settings for conventional CDs. The process is a little clumsy, but workable. Fortunately, the new Pro 32 remembers the input and output settings in nonvolatile memory when powered down, meaning you don't have to reset the unit after it's been turned off. This important feature was lacking in the DTI•Pro. The Pro 32's output mode selection and display are also improved over those of its predecessor.

The Pro 32 uses the same jitter-reduction technology as that of the original DTI•Pro. A Crystal CS8412-based input receiver locks to the incoming data stream with a Phase-Locked Loop. A second, low-jitter PLL locks to the first PLL's output, then generates a low-jitter master clock for output to your outboard digital processor. The first PLL recovers the data stream, the second attenuates jitter. A Voltage-Controlled Crystal Oscillator (VCXO) provides the timing reference. The quality of the VCXO's output largely determines the quality of the output signal. The Pro 32's jitter attenuation cutoff frequency (the frequency above which the circuit attenuates jitter) is reportedly a low 5Hz. For comparison, a Crystal CS8412 without a dual-stage PLL has a JACF of 25kHz. The Pro 32's second PLL has a slightly narrower bandwidth than that of the Pro, a characteristic that reportedly improves the product's jitter performance.

The recovered signal is then processed with Audio Alchemy's Resolution Enhancement algorithm. A Texas Instruments TMS320C31 Digital Signal Processing (DSP) chip running at 40MHz executes the processing. The Texas Instruments chip is a 32-bit floating-point device, compared with the Star SPROC DSP which is a 24-bit fixed-point processor. Although the Resolution Enhancement algorithm remains unchanged from the original DTI•Pro, the new Pro 32's greater computing horsepower and longer interpolation filters means the algorithm can be executed with greater mathematical precision. A Programmable Read-Only Memory (PROM) chip contains the Resolution Enhancement software that tells the DSP chip what to do. This software is easily upgradeable in the field simply by replacing the socketed PROM.

The DTI•Pro 32's build quality is classic Audio Alchemy: no cosmetic frills or lavish construction, just solid engineering in a spartan package.

Listening
I've had the DTI•Pro 32 in my system for a few months, removing it temporarily while auditioning the Avalon/Spectral/MIT system reviewed in the January Stereophile. I did, however, listen to the DTI•Pro 32 in that system before dismantling it for return to the manufacturers.

I'm glad I had a chance to hear the DTI•Pro 32 with the Avalon/Spectral/MIT package before sending the system back. This system's soundstaging, image specificity, and amazing ability to resolve spatial cues provided great insight into the DTI•Pro 32's effect on spatial presentation. If you think that a Mark Levinson No.31 transport driving the Spectral SDR-2000 Pro processor couldn't benefit from an Audio Alchemy processing device inserted between them, you'd be wrong—as I was. The Spectral's sound was taken up another full notch in quality by adding the Pro 32. The system had a greater sense of air, space, and soundstage size with the Pro 32. The presentation opened up, with more resolution of the recorded acoustic surrounding instrumental images.

This character was illustrated on the acoustic bass opening of the tune "Leather Cats" from the superb new CD Beyond Words by Oregon (Chesky JD130). Glen Moore plays a restored 1715 Klotz bass, recorded with a stereo microphone arrangement in St. Peter's Episcopal Church in New York City. Putting the Pro 32 in the system expanded the size of the recorded acoustic and better resolved reflections from the church's walls. With the DTI•Pro 32 in the chain, I could hear the reflections from the rear wall, which deepened the already spacious soundstage. And this was with what I consider to be the finest digital front-end extant: the mighty Mark Levinson No.31 transport and Spectral SDR-2000 Pro digital processor.

In the Sonic Frontiers/ARC/Genesis system, the Pro 32's effect on the spatial presentation was less pronounced, though still significant. The Pro 32 widened and deepened the soundstage, revealing more space around the presentation. Concurrently, image focus tightened up, and the sense of air around images was better resolved. The result was a greater impression of individual instruments hanging in three-dimensional space. The Pro 32 also made the background sound "blacker," with greater intertransient silence and heightened dynamic contrast.

The Pro 32's improvement varied with the program material and the converter with which it was used. When connected to the DDE v3 via the I2S interface, the Pro 32's effect was more significant. All the benefits I heard with other converters increased in magnitude with the DDE v3, making this already high-value processor sound like a much more expensive converter. The jitter reduction abilities of the I2S interface combined synergistically with the Resolution Enhancement processor for a greater improvement than either benefit alone.

When the Pro 32's resolution enhancement was bypassed (meaning the Pro 32 was acting only as a jitter filter), I thought that the Pro 32 slightly degraded the sound of the SDR-2000. The presentation was overall cleaner and more highly resolving without the DTI•Pro 32. I heard a slight loss of low-level detail with the Pro 32, and a trace of hardening in the mids and treble. Engaging the resolution enhancement turned the tables, however, making the Pro 32 an asset to the system's sound. This experience also suggests that the resolution enhancement processing does provide benefits. I preferred the Pro 32 set to Resolution Enhancement, but no dither when using it with the Spectral. With all other converters, the Pro 32's dither was a decided advantage.

No matter what the converter, the Pro 32 improved the bass presentation. The sound took on more weight and solidity, and seemed to have more power and articulation in the lower bass and mid-bass. With acoustic and electric bass, I could hear more dynamics in the strings' attack and decay, more body and weight, and greater pitch articulation. There was no question about the Pro 32's overall improvement in bass performance.

Compared to the original DTI•Pro, the Pro 32 sounded only slightly better. The difference with and without the DTI•Pro was much greater than the marginal improvement the Pro 32 offered over the Pro. I thought, however, that the Pro 32 produced smoother midrange textures and a more liquid treble. The Pro 32's bass was also somewhat tighter and deeper, but not significantly different from the Pro.

Conclusion
It's hard to imagine that a world-class digital front-end such as the Mark Levinson No.31 transport and Spectral SDR-2000 Pro digital processor could be improved by a little Audio Alchemy box, but it was. The DTI•Pro 32 tightened and deepened the bass, made the presentation bigger and more expansive, and better revealed space and bloom around instrumental outlines. The DTI•Pro 32 made the presentation sound more like real instruments existing in a real space.

I didn't really appreciate what the DTI•Pro 32 did for my system until I took it out for comparison listening. The urgency to put it back in the system perhaps says more about the Pro 32's benefits than any specific description of what it did to the sound. I simply enjoyed music that much more with the Pro 32.

Moreover, the DTI•Pro 32 worked its wonders over a wide range of processors and transports. Every combination of transport and processor benefited from the Pro 32, and the improvements were the same regardless of the digital components. The real synergy, however, happened with the DTI•Pro 32 driving the DDE v3 processor through the I2S interface, making the sound of the v3 competitive with much more expensive processors.

In this age of products that offer marginal musical benefits, the DTI•Pro 32 stands out as a significant improvement. You may not have heard what your system can do until you've experienced Audio Alchemy's DTI•Pro 32.

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