Digital Processor Reviews

Much has been written about the Theta DS Pro Generation V, which I still consider among the best D/A processors available. I level-matched the Muse Model Two and my Gen.V to within 0.04dB (with the help of the two remote-controlled Rowland preamps) and drove them both with, in turn, the Theta Data II, the excellent Theta Data Basic (which I like even better than the recently discontinued Data II), and the Micromega T-Drive transport.

Qualitatively, the two processors were surprisingly close. They sounded remarkably alike when properly level-matched—as should be expected from reference-quality processors that don't editorialize much and use the same DAC chips. That many processors using the same DACs don't sound similar is testament to varying levels of designer skill, or indicates a deliberate choice to tailor the sound toward a desired tonal balance.

Perhaps because of its DSP filter, the Gen.V had the very faintest edge in the dimensional aura of harmonics emanating from an instrument. For instance, the spray of cymbal aftertones on Tiger Okoshi's Two Sides to Every Story (JVC 2039-2) seemed to project in all directions through both processors, but with the very slightest possible nod in expansiveness going to the Gen.V. Keep in mind that the Muse was excellent in this area as well, and making this distinction in dimensionality without careful matching and repeated comparisons would be impossible. That's how close they were to one another in this area.

On the other hand, the Model Two had a slight yet notable advantage over the Gen.V in transient purity, focus, and rendition of very fine inner detail. Also, the Muse seemed to possess a bit more grip and definition in the bass—a traditional Theta strong point. Again, these differences were subtle, yet certainly perceptible.

Overall, the Theta seemed to sound ever so slightly smoother, with a softer rounding of transient edges, while the Muse was spot on in its ability to accurately capture transient attack and subsequent decay, resulting in impressive rhythmic drive.

Both processors were hard to fault in soundstage presentation. Stage width and depth sounded limited only by the recording. More importantly, the distinction of individual instruments in orchestral settings under intense crescendos was well-delineated and beautifully rendered. In this area the Muse was spectacular, due largely to its outstanding transient performance and focus.

Not surprisingly, the separation of individual voices in a large choral setting was magnificent. Reference Recordings' wonderful Postcards CD (RR-61CD), performed by the 200-member Turtle Creek Chorale, is a knockout both musically and sonically, and has become one of my favorite reviewing tools. It contains the richest variety of music, covering many styles and a broad range of dynamics. On "Sakura," the purity of the fundamental notes from a solo flute through the Muse Model Two was awesome. Neither did the Muse harden up or become flustered with the rich tapestry of exotic percussion combined with intense massed vocals.

I should note that my DS Pro Generation V was made in March of 1994, a year ago as I write this, and that Theta constantly refines their products. I'm told current versions of the Gen.V sound even better than mine. In the near future, both Theta and Muse will likely also offer an HDCD option in conjunction with their advanced filtering schemes.—Shannon Dickson

Shannon Dickson wrote about the HDCD upgrade in Janaury 1996 (Vol.19 No.1):

This is yet another follow-up describing an update to Theta Digital's flagship processor, now known as the DS Pro Generation V-A. Theta seems to offer a significant revision about every year and a half, consistent with the pace of refinement in digital processor design. In the process, they've kept their promise to DS Pro owners of every vintage to provide a steady upgrade path, maintaining cutting-edge performance. If you bought a Gen.I in 1989, you could have taken that same unit through a dramatic metamorphosis over the years, even if you skipped certain stages, arriving now at what's certainly one of the best digital processors available.

As a longtime owner of Theta's flagship series of processors—most recently the original version of the Generation V—it was only natural that I compared its performance to that of the wonderful Muse Model Two during my review of the same in the July '95 Stereophile (Vol.18 No.7). For those who missed that article, I found the first-generation Muse surprisingly competitive with the twice-as-expensive Gen.V, and actually preferred it in the areas of focus and transient purity; however, the Theta maintained a slight edge in "dimensionality" and expansiveness of the soundscape. I also acknowledged that the Gen.V used for that comparison was an early model, prompting an offer from Theta's Neil Sinclair to bring my unit up to snuff with what he promised were significant sound-quality enhancements. In addition, I'd have the opportunity to compare two otherwise-identical Gen.V-As using either Theta's software-based filter—now updated with a new dither scheme—or the HDCD PMD-100 digital filter.

Gen.V-A Upgrade Particulars
Much has been written about the Gen.V in this and other audio journals, so I'll just stick to the hardware improvements that constitute the $495 "A" update.

Four principal changes define the differences between the original Gen.V and this latest incarnation. In no particular order, they are: an improved PLL (phase-locked loop) circuit; a revised version of Theta's famous software digital-filter algorithm; a new analog board with matched transistors; and an improved analog reconstruction filter. The PMD-100 HDCD filter chip can be added to the Gen.V-A, the Gen.V, or the Gen.III as a $459 option; it's used solely for decoding HDCD CDs—Theta's own filter takes over for all non–HDCD-encoded discs.

A phase-locked loop circuit is almost always physically contained within the digital input receiver found in almost every DAC (the Meitner BiDat is an exception). The role of the PLL, in this application, is to extract and "lock" onto the master clock from the signal in order to reduce the rate at which the clock frequency can change—the very definition of jitter reduction. Absolute timing accuracy of the DAC as a whole is related to the precision of the PLL's output signal. Some designers attempt to lower jitter in the master clock even further by employing a secondary PLL wired between the latch-enable signal at the output of their processor's input receiver or digital filter and the corresponding input on the actual DAC chips. While this technique can work well, it also can be tricky, and makes the quality and frequency of the received signal more critical. Theta chose a different route.

All PLLs contain three essential elements: a voltage-controlled oscillator (VCO), a phase comparator, and a low-pass filter. One leg of the phase comparator senses the VCO's output frequency; the other is attached to the latch-enable signal from the input receiver or oversampling digital filter. A proportional voltage, relative to any difference in phase between the two signals, appears at the output of the phase comparator. This voltage is then used to control the VCO's frequency, completing the loop. However, if you connected the VCO directly to the incoming signal, its output would almost exactly match the input signal's frequency and be vulnerable to corruption by any instantaneous changes in the same—ie, jitter. Ideally, the circuit should follow the average, not instantaneous, frequency of the signal. Inserting a low-pass filter between the phase comparator and the VCO's input will change the frequency at a rate determined by the filter's characteristics. This is where Theta made enhancements in the Gen.V-A; through changes to the low-pass filter, the PLL's output frequency more closely matches the average frequency of the input, in effect increasing the jitter attenuation of the circuit.

The optional HDCD filter board is mounted directly above the existing input receiver. Also, a new EPROM chip containing the revised software filter algorithm is added between two of the three Motorola 56001 DSP computers on the main digital board. This is the first significant change to Theta's proprietary oversampling algorithm in five years. According to Jim White, Theta's man of many hats, the decision to make this change stemmed from a suggestion by their original algorithm designer that adding a particular dither shape to the program would improve the low-level linearity of the processor's DAC. After two years of extensive comparisons between various dither options, Theta settled on a specific solution for their algorithm. The sonic result is said to be better low-level resolution and a cleaner, more natural presentation of decay.

Shifting to the new analog board, Theta began "multitasking" their receptionist as a transistor-matching demon. She sifts through and measures literally thousands of transistors to find six matched octal sets (a total of 48 critical transistors) for the new analog board in each Gen.V-A. The payoff is reported as a sweeter, more detailed performance with less listener fatigue—not that this was much of a problem in the original Gen.V. By the way, both halves of the balanced analog circuit—totaling four Burr-Brown PCM63P-K DACs and six new AD-707 DC servos—are located on one board, instead of the two used in earlier balanced versions of the processor.

The analog reconstruction filter that follows the I/V converter has a major impact on sound quality. This section not only smooths the transition from the "staircase" voltage found at the output of the I/V stage to a clean analog signal; it must also minimize the amount of high-frequency interference that rides along with the signal from the processor to the preamp, and thus the rest of the system, all the while minimizing group delay (the delay of certain frequencies in relation to other portions of the audible band).

Theta uses a third-order Bessel filter for this purpose, which ensures superb group delay characteristics. However, they now execute this filter without using inductors. According to Neil Sinclair, maintaining the correct filter slope without inductors while also replacing the original capacitors used in this circuit with high-quality Wima caps produces tighter image focus and less sibilance. All in all, these changes represent a fairly major update, one that could just as easily merit a new model number as its mere suffix.

Sound Effects
So how does the Gen.V-A rate compared to its predecessor? To answer this question fairly, it's important to emphasize just how far along the knee of the curve we've come in the development of our present—though limited—digital standard. As recently as two years ago, significant advances in performance accompanied nearly every product released by the key digital design houses. Today, the upgrades offered by those companies more often provide subtle yet important refinements rather than breakthroughs in sound quality. On the other hand, many of these changes that provide subtle enhancements to products already possessing such high resolution reflect deeper insight into the inner workings of D/A conversion, and often result in a wonderful cumulative effect.

The Gen.V-A is a case in point. I can verify that the sonic claims Theta has made for the A-series upgrade, listed above, are indeed true. The first thing I noticed, vis-è-vis the original Gen.V, was a real increase in a "see-through"–like openness. Theta's famous, already immense soundscape is even better defined, with various instrumental images spotlit more clearly both in relation to one another and with respect to the recording venue as a whole.

In the Muse review I mentioned that the original Gen.V slightly softened or rounded the leading edges of transients, giving the processor a very subtle but consistent sonic signature. The Gen.V-A simply has less of that sonic signature now, providing a beautifully balanced spectrum and a more transparent window into every recording. It just seems that there's less processor between you and the music. The new version possesses the smooth character of the original, but with noticeably better focus and snap, conveying an improved sense of pace. Indeed, the Theta now focuses on a par with the original Muse Model Two—and that's saying a lot! This enhanced focus and pace seems to stem from a tighter, better-defined bass and lower midrange than was found in the earlier version.

One mark of a genuinely excellent product is the Gen.V-A's ability to present music with a greater sense of ease and effortlessness, yet at the same time with enhanced clarity, transient definition, and dynamic contrast. This attribute makes long-term listening far more relaxing and inviting, as opposed to the overly laid-back perspective found in components that seem relaxed at the expense of slurred transients and a veiled, rounded-off sound.

Theta sent me their updated software algorithm with the new dither scheme when it became available in late September '95—after the other physical changes of the upgrade had been made. I was able to confirm that their choice of dither does improve the perception of ambience and low-level decay.

Buy it:
By any measure, the Gen.V-A is a superb processor, and although the relative changes from its predecessor are small in scale, their sonic impact makes this upgrade a highly recommended step forward for owners of earlier models. In the context of digital playback ca late 1995, the Gen.V-A's overall performance is as good as I've heard.—Shannon Dickson