Theta DS Pro Generation V digital processor

Some high-end audio companies develop reputations for having a particular "sound." This reputation develops when every product the company makes has a similar sonic flavor. These products appeal to certain customers who like the company's sound, and who therefore tend to stay with that company's products year after year. Unfortunately, such an approach can limit a manufacturer's appeal to a broader audience.

This situation has been especially true for Theta Digital, a pioneer in advancing digital playback, and one of the first US companies to market an outboard digital processor. Theta's processors have always sounded a little forward and incisive to me, rather than subtle and refined. That's why the sound of Theta's top-of-the-line DS Pro Generation V will surprise Theta fans and detractors alike. Though the Gen.V represents a departure from the "Theta" sound, it manages to retain the qualities that have set Theta processors apart from the competition (footnote 1).

Technical description
Theta, in my view, has always concentrated spent more of their build budget on what's heard than on what's seen. For example, the best Thetas use Teflon printed circuit boards and Vishay resistors—both extremely expensive items, but unseen by the customer.

The antithesis is the product with a thick gold faceplate and machined chassis filled with mediocre electronics—all show and no go. Although the cosmetics and finish of Theta products have always been acceptable, they're more business-like in appearance than other high-end processors.

The Generation V carries on that tradition, but is the best-looking Theta yet. A sculpted plate fits over the front panel, giving the unit a more refined look. In addition, all front-panel edges are nicely rounded and finished.

Four toggle switches select among inputs, choose between source and tape, turn the power on, and invert absolute polarity. Two blue LEDs indicate when the unit is powered and locked to a source. Note that the front-panel power switch turns off the computer and the power-indicating LED, but leaves the audio stages powered for the best sound.

Three digital inputs are included as standard: AES/EBU, coaxial on an RCA jack, and BNC digital connections. The review sample was fitted with Theta's LaserLinque single-mode optical input, an $800 option (footnote 2). ST-type optical is also available for $300. For those who want to use the Gen.V with a laserdisc, TosLink output is available for an additional $100. A digital tape loop (on RCA jacks) provides for connecting a digital recorder to the Gen.V. Analog output is via a pair of XLR connectors (balanced), or RCA jacks (single-ended). The Gen.V is available single-ended ($3795) or balanced ($5595).

The Gen.V is an evolution of Theta's highly regarded DS Pro Generation III processor (footnote 3). The V uses the same DSP engine for the digital filtering, and even the same filtering algorithm. The big differences in the Gen.V are the all-discrete analog output stage (no op-amps, as in the Generation III) and the updated power supply.

The Gen.V's build is almost unique. The unit is divided into three distinct sections, each shielded from the other: the power supply is housed in a subchassis below the digital section and the analog output stage, and the digital compartment occupies about two-thirds of the upper area. These subchassis act as Faraday cages to isolate each subsystem from the others' electromagnetic radiation.

The power supply has four separate power transformers supplying 13 regulators. Eight of the power-supply rails are regulated in the power supply, then re-regulated on the analog board next to the circuits they supply. All the regulators in the Gen.V are three-pin ICs; filter capacitors are the excellent Nichicon Muse types. The power supply of the Gen.V was significantly updated from that in the Generation III, to accommodate the new discrete output stage.

The digital board features a Crystal CS8412 input receiver. The input stage's Phase Locked Loop (PLL) has been tightened for lower jitter, and is now supplied from a dedicated power-supply regulation stage.

As with previous iterations of Theta's Generation series, the heart of the V is the custom digital filter. Three Motorola DSP56001 digital signal processors, providing a total computing power of 129 MIPS (Million Instructions per Second), perform the 8x-oversampling digital filtering, and a pair of Erasable Programmable Read-Only Memory (EPROM) chips contain the software code that contains the filtering instructions. In essence, the DSPs are the muscle and the EPROMs are the brain. The DSPs, the amount of computing horsepower, and the filter algorithm are unchanged from the Generation III. De-emphasis is performed in the digital domain by the DSP chips.

The EPROMs are socketed for easy replacement should there be a software change. The three DSPs, their EPROMs, and support electronics replace a single, drop-in, off-the-shelf filter used by most manufacturers. Note that the Gen.V's filter will pass 20-bit data should you have a 20-bit source—or Audio Alchemy's DTI Pro, which outputs 20-bit words. The NPC filter used in most processors truncates the input word length to 18 bits.

The digital board has two gate arrays—one to control the input selection logic, the other to convert the incoming serial datastream to parallel form for input to the DSP chips. These two gate arrays replace dozens of support chips.

The analog board is an impressive piece of work. The balanced version I had for review has four Burr-Brown PCM 63 DACs for true differential operation. In a fully balanced processor such as the Gen.V, the digital datastream is split into left +, left –, right +, and right –, with each signal converted to analog by its own DAC, then processed by four separate analog stages. Conversely, the shortcut method uses just two DACs, then puts an analog phase splitter on the output to derive the left and right + and – phases of the balanced signal. Doing it right, as in the Gen.V, doubles the circuitry—and the power-supply requirements. Balanced operation requires four DACs, four current-to-voltage converters, and four output buffers. It's like having two DACs in one chassis.

A fully balanced processor such as the Gen.V benefits from not having another analog stage (the phase splitter) in the signal path, and from rejection of any noise or artifacts common to both DACs. In my experience with digital processors, I've found that true balanced circuits have a decided sonic advantage over single-ended designs, and especially over "balanced" outputs derived from an analog phase-splitter.

This doubling of the circuitry is what makes the balanced Gen.V cost $1800 more than the single-ended version. You can convert your single-ended Gen.V to fully balanced for a little more than the cost of buying it balanced in the first place. (Theta hadn't set a price on the upgrade by press time.)

Previous balanced Theta products have had single-ended outputs that took the positive phase of the balanced signal and presented it to the RCA output jack, ignoring the negative phase. This technique has none of the advantages of balanced operation, even though the processor may have used four DACs. The alternative approach, introduced by Theta in the Gen.V, is to combine the two phases of the balanced signal differentially. The differential amplifier's output drives the single-ended RCA jack. This technique brings the benefits of balanced operation to single-ended users.

Footnote 1: The Gen.V is the last Theta product that designer Mike Moffat worked on. Mike left Theta to start a company that makes surround-sound processors.

Footnote 2: LaserLinque is Theta's trade name for an optical communication system used in the aerospace industry. Generically called "single-mode" optical, LaserLinque uses an optical cable with a much smaller inside diameter than usual. The narrower optical conductor minimizes dispersion of the light as it travels down the fiber, which would otherwise introduce jitter in the S/PDIF clock. In addition, the wavelength is shorter than that used in the AT&T ST-type optical interface. AT&T ST-type optical and TosLink are both "multi-mode" optical interfaces. Single-mode has been likened to rolling a basketball through a tunnel just bigger than the basketball; standard optical is like throwing a tennis ball down the same tunnel.

Footnote 3: There was no Generation IV—the numbering system jumped from III to V. (The number four in one of the High End's main markets, Asia, is equivalent to the West's thirteen.)

Theta Digital/ATI
1749 Chapin Road
Montebello, CA 90640
(323) 278-0001