Digital Processor Reviews

In hindsight, it was inevitable that two sophisticated digital audio technologies—software-based digital filters and Bitstream D/A converters—were destined to be married in one product. The software-based D/A converters offered by Krell, Wadia, and Theta all used multi-bit ladder DACs, and Bitstream-based units have previously relied on off-the-shelf digital filters.

Having auditioned both types of technologies individually, I've always wondered how they would sound together. This is an important question; fewer and fewer IC manufacturers will make multi-bit DACs as the world—for better or worse—switches to 1-bit type converters.

There are several good reasons why Bitstream converters have never been used in software-based processors. First, the complexity of software-based processors meant long development times—a situation not conducive to adapting products to the quickly changing Bitstream technology. Second, the high cost of developing software-based processors led their designers to use the best DACs available, not the least expensive. Finally, software-based processor designers tend to believe the conversion method was less critical sonically than the filtering method.

All that has changed with the introduction of the new Theta DS Pro Prime digital processor. The Prime incorporates a 4x-oversampling digital filter performed by a Digital Signal Processing (DSP) chip under software control, coupled with a Philips 7350 Bitstream DAC. Theta's reason for using the Bitstream DAC was price: a 7350 is far cheaper to implement than a pair of multi-bit DACs with their attendant Most Significant Bit (MSB) trimmer and need for individual calibration at the factory.

The lower cost of using Bitstream technology is reflected in the Prime's $1250 price—far lower than any other DSP-based processor. Theta's design goal was to make a moderately priced unit that could still use their proprietary filtering technology. They also pulled a few other cost-saving tricks I'll tell you about later.

But first, let's take a closer look at what makes this fascinating amalgam of digital audio technology tick.

Technical description
The Prime looks remarkably like Theta's DS Pro Basic D/A I reviewed in Vol.13 No.8—no frills, but nice appearance and good build quality. The narrow front panel has just one switch that selects between the Prime's coaxial and optical inputs. Two red LEDs indicate if AC power is connected and if the Prime has locked to an incoming digital signal. Unlike the rest of the Theta processor line, no absolute-polarity reversal switching is provided.

The rear panel is similarly minimalist, holding an RCA digital input, Toslink optical input, a pair of RCA analog outputs, and an IEC AC power cord socket. The addition of an optical input is new for Theta; they believe Toslink optical is inferior to coaxial transmission, but included the jack for compatibility with other products. Many Far Eastern CD players (and videodisc players) are being shipped with only Toslink output in an effort to reduce parts cost. The average consumer believes optical is inherently better, but virtually everyone who has listened critically finds Toslink optical inferior to even the worst coaxial digital interconnect.

My first impression on opening the Prime was surprise that I was looking at a $1250 processor. The apparently high parts cost, sophisticated circuit topology, and build quality would have looked at home in a unit costing twice as much.

This is especially true when considering that the Prime, like all Theta processors, is software-based. This means that the digital filtering is performed by a Digital Signal Processing (DSP) chip under the control of instructions (the software) encoded in a programmable Read-Only Memory (ROM) chip. The DSP does the math on the digital datastream under the control of the decoding algorithms (software instructions) contained in the ROM chip. A single DSP chip runs at 4x-oversampling, contrasted with Theta's other products that use two DSPs to operate at 8x-oversampling (software-based filters can run at up to 64x-oversampling). The digital filter outputs 16-bit words 176,400 times per second, four times the CD's sampling frequency of 44.1kHz.

This software-controlled, DSP filtering approach is contrasted with the digital filtering found in most D/A converters—a single inexpensive filter chip connected with a minimum of additional parts. Many very expensive converters use an off-the-shelf digital filter chip. The software-based approach requires an expensive DSP chip (in the Prime, the ubiquitous Motorola 56001), a programmable ROM chip, additional pcb work to accommodate the DSP's 120 closely-spaced pins, and finally, the filter algorithm must be written and tested—a job requiring lots of expensive skilled programing hours (footnote 1).

The Prime's input receiver is the popular Yamaha YM3623B chip. This 16-bit compatible IC receives the incoming data, strips out the subcode, generates a new clock by phase locking to the incoming signal, and formats the data for input to the digital filter.

A Philips SAA7350 Bitstream DAC handles the D/A conversion tasks. As previously mentioned, this is the first time in a commercial product that a Bitstream chip has been mated to a software-based digital filter. The 7350 partially accounts for the Prime's low price; a single Bitstream chip is cheaper to implement than a pair of DACs with their MSB trimmers and associated circuitry.

Each half (left and right channels) of the stereo DAC's differential outputs is summed by a Linear Technology LT1028 op-amp. Any noise common to both polarities of the differential signal will cancel at this summing op-amp. An Analog Devices 707 IC then acts as a DC servo, and an LM6312N op-amp serves as the output driver. There is one set of these output circuits per audio channel.

The power supply uses two transformers, both of which are mounted to the Prime's single printed circuit board. The first transformer powers the ±15V rails for the analog output circuitry and +5V for the analog portion of the Bitstream DAC. The second transformer is used exclusively to supply +5V to the digital circuits—the DSP chip, digital portion of the Bitstream DAC, and associated logic. Power-supply smoothing is provided primarily by four 4700µF and two 2200µF electrolytic capacitors. The +5V digital supply is regulated by a large TO-3 regulator (the same package usually seen in power transistors), and the analog supply rails are regulated by three small-can type regulators attached to heatsinks. This last technique is a Mike Moffat hallmark: he favors metal-can regulators over the more commonly used and less expensive T0-220 plastic regulators.

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Footnote 1: A very rough yet surprisingly accurate estimate of an electronic product's cost is to count the number of IC pins and multiply by some calculated constant.