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发表于 2011-4-2 11:04
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《stereo phile》前主编Robert Harley 对THETA DSPRO BASIC的评价
Theta Digital DSPro Basic D/A processor
By Robert Harley • Posted: Nov 5, 2009 • Published: Aug 5, 1990
As an equipment reviewer, I find it helpful to talk to audiophiles and music lovers about their systems and upgrade plans. Fortunately, Stereophile's computer supplier and troubleshooter, Michael Mandel, also happens to be an avid audiophile. I say "fortunately" because I rarely get a chance to talk to people who put down their hard-earned money for hi-fi components. Instead, I usually converse with equipment designers, technicians, and marketing types, hardly people who reflect the buying public. It is thus a valuable education to get feedback from real-world consumers to find out what kind of products they want, their priorities, and how much they're willing to spend for certain levels of performance. They have a view distinctly different from that of the often jaded reviewer who is used to enjoying the best (albeit temporarily) without agonizing over its cost.
It is significant, therefore, that whenever Michael sees me, the first words out of his mouth, even before "Hi Bob," are "What's good in reasonably priced digital processors?" Although he would like to own a Stax, Krell, Wadia, or the Theta DSPre, he cannot afford such an expenditure, especially with the knowledge that remarkable price/performance breakthroughs are occurring very rapidly. He is, however, willing to fork over a couple of kilobucks, provided the processor provides truly musical performance and isn't likely to be significantly surpassed at the price point anytime soon. I see his quest for affordable digital as representative of many audiophiles who own CD players, even good ones, who are ready to take the next step to an outboard processor. Like Michael's attitude, there has been a "wait and see" approach to investing in good digital playback. Inherent in his question is a growing impatience, with the implied secondary question of "How much longer do I have to wait?"
Since Michael comes from the computer industry, he is well aware of the huge technological leaps, coupled with equally large and often simultaneous price reductions, in any competitive electronic field. Buy too early in the cycle and risk paying too much for soon-to-be-obsolete performance. Buy too late and miss owning the product during the wait for better performance and lower prices that are not immediately forthcoming.
What does all this have to do with the new $2000 Theta DSPro Basic?
Everything.
Technical description
As its name implies, the DSPro Basic is a simpler, more cost-effective version of the highly regarded DSPre digital processor/preamplifier from Theta Digital. When it was introduced, the DSPro set the standard for digital playback, making it into Class A of Stereophile's recommended components. The DSPro was, to my knowledge, the first software-based digital processor, employing Motorola 56001 DSP (Digital Signal Processing) chips.
The DSPro Basic is rather simple, with the front panel home to just two toggle switches and two LEDs. One switch selects between CD (44.1kHz) and DAT (48kHz) inputs, while the other inverts absolute polarity. The left LED indicates the unit is locked onto the incoming data, while the right indicates power is applied. The rear panel contains the analog outputs, two digital inputs (CD and DAT), and a digital tape-out loop for driving a DAT machine input, all on gold-plated RCA jacks. An IEC AC power-cord jack finishes off the rear panel. No power on/off switch is provided, suggesting that the DSPro Basic should be left on continuously. Construction is a bent-steel chassis with a ¼"-thick front panel. The top panel is screwed into pem-nuts in the chassis with hex-head bolts, an improvement over sheet-metal screws into chassis holes. Overall, I found the DSPro Basic's appearance attractive and its fit and finish quite good.
All the DSPro Basic's electronics are carried on a single printed circuit board, separated into digital and analog sections. The unit has two separate power supplies, one for the digital section and one for analog circuitry. The digital supply consists of a transformer, rectifier, two 4700µF filter caps, and a large heat-sunk regulator in a TO-3 package. The analog supply is more elaborate, with a similar transformer, two rectifiers, and four 2200µF filter caps. Additional analog power-supply components are located at the other end of the PCB, next to the DACs and analog output stage. Six voltage regulators are used in the analog supply, but not the typical TO-220 type (footnote 1) found in most digital processors. Instead, they are small round metal cans which, according to DSPro Basic designer Mike Moffat, offer superior sound over the less expensive and easier-to-implement TO-220s.
The digital section features the ubiquitous Yamaha YM3623B 16-bit S/PDIF decoder, along with a few chips whose markings were painted over. The Yamaha decoder extracts the clock information from the signal with an internal phase-locked loop (PLL), strips out the subcode, demultiplexes left and right audio, and sends the data to the next processing stage.
The wire carrying the digital data stream from the RCA input jack to the digital board was carefully chosen for its sonic virtues. This low-propagation delay wire was suggested by Dave Magnan, maker of the highly regarded Magnan Type V interconnects. According to Mike Moffat's listening tests, even this short (3") piece of wire carrying ones and zeros affects the processor's sound. Other design aspects of the DSPro Basic include careful attention to timing relationships and clock signals to avoid jitter, an input circuit to keep RF out, and minimizing ground-plane noise.
The DSPro Basic's heart is the computing section consisting of two Motorola 56001 DSP chips and two ROMs that contain the decoding algorithm for the 56001s. The DSPs perform 8x-oversampling which, according to Mike Moffat, is optimum for this amount of computing power and DACs. He contends that DACs start to sound worse as they are driven at faster and faster speeds by high oversampling rates. An additional benefit is that some computing power is reserved for other functions that are done in software, rather than hardware. For example, the Basic performs de-emphasis in the digital domain with the DSP chips instead of additional analog circuitry switched in by a relay or FET, as is common.
It should be noted that the Wadia X-32 (reviewed elsewhere in this issue) also uses two DSP chips, these the slightly more powerful AT&Ts. Although it is touted as 32x oversampling, the X-32 performs 8x oversampling in the digital domain and achieves a final 32x rate by using a staggered DAC array. A full discussion of this technique is provided in the X-32 review. I bring this up to dispel any confusion over why the DSPro Basic is 8x-oversampling and the Wadia is "32x," when both use about the same amount of computing power.
The DSP chips drive two Analog Devices AD1860N-K 18-bit DACs, one for each channel. The "K" designation refers to the premium-grade version of the AD1860, which is also used in the sonically well-respected Kinergetics KCD-40 player. The DAC is said to be glitch-free, thus requiring no deglitch circuit. A trim pot next to each DAC provides MSB adjustment at the factory after a two-day burn-in. The current-to-voltage converters (I/V) are OP42s, a high-quality op-amp IC from Precision Monolithics. Mike Moffat feels that a premium IC like the OP42 outperforms discrete circuits for I/V conversion due to its faster speed, a result of monolithic construction. A second op-amp IC, an OP27, drives the output buffer. Both these ICs have superior performance characteristics over most other monolithic devices. The output section consists of an op-amp per channel, in metal-can packages rather than the typical 8-pin DIP (Dual In-line Package). Six voltage regulators near the DACs, again in metal cans, support the DACs, I/V converters, and output buffers.
Overall, the DSPro Basic's design is a departure from most other digital processors: It uses Analog Devices DACs instead of the ubiquitous Burr-Brown, metal-can regulators and output buffers instead of TO-220s and 8-pin DIPs, digital-domain de-emphasis, and very few signal-path capacitors. The Basic was definitely not designed with a "follow the herd" mentality.
Problems
The first sample I auditioned worked fine with the 44.1kHz signal from the Esoteric P2 transport. However, the DSPro Basic had problems with the 48kHz output of the JVC DAT machine. Although music could be heard, it was superimposed on a metallic sound that increased in level with the music. As soon as I heard this, I remembered where I had heard this sound before: track 9 of the Sony Classical sampler CD. The noise is probably due to a problem synchronizing to the incoming data. Evidently, the same phenomenon that caused the DSPro Basic to make this sound during playback is the same as what occurred in the recording/mastering chain on that track on the Sony CD. In addition, after this happened, the Basic put out a loud thump when the input selector switch was toggled. Before the noise problem, no thump had been evident. Temporarily disconnecting AC power so the computer could reset corrected the thump problem, but the noise remained during 48kHz decoding.
I was dismayed to discover that the second sample also exhibited the same noise when decoding a 48kHz signal from the DAT machine. The second sample was sent because it incorporated new updates, not because of the decoding problem. I'm sure the DAT machine's data output is good, as it was successfully decoded by the Proceed PDP, Wadia 2000, Wadia X-32, and the Stax DAC-X1t. This is not the first time a digital decoder under review has had problems with a 48kHz signal. In my review of the British Fidelity Digilog converter (Vol.12 No.10), there was a ticking sound from the loudspeakers when the converter tried to playback a 48kHz data stream. I tweaked a trim pot inside the unit that adjusted the edge detection threshold and cured the problem.
Mike Moffat informed me that the problem is most likely that decoding 48kHz requires more software instructions for the DSP chips. Since the DSPro Basic uses the same ROM as the DSPre, which apparently has no problem with DAT machines, he was surprised by this development. Interestingly, I'm the first to report this problem (footnote 2).
These experiences point to a big potential problem when people start buying DAT machines (assuming they do) and try decoding the signal with their outboard converters. Since they have never tried the 48kHz input, they may not discover that it doesn't work until years later, after the warranty has expired. I therefore suggest that any purchaser of a digital decoder ask their dealer to try their unit in the store with a DAT machine. Make sure that the tape is not the prerecorded variety (recorded at 44.1kHz), but one made on a DAT machine at 48kHz.
Listening
Moving on to the listening impressions, I was not prepared for the level of performance offered by the DSPro Basic, especially at the "budget" $2000 price point. This is one terrific processor. The DSPro Basic's overall presentation was clearly more in the league of the megabuck units, rather than sounding like merely an improved version of an inexpensive decoder.
Specifically, the DSPro Basic had a textural liquidity and soundstage transparency rivaling any processor I've heard except the $12,000 Stax DAC-X1t. The sense of soundstage depth, coupled with the ability to resolve instruments in the front-to-rear perspective, was refreshing to hear in digital playback, much less from an affordable decoder. I had the distinct impression of instruments being behind one another, with hall reflections behind the instruments. There was a real sense of the three-dimensionality so often missing from digital. Listen to the percussion instruments on "Treme Terra" from Three-Way Mirror (Reference Recordings RR24CD). They are behind the other instruments, giving the impression of a soundstage miles deep. Not only was the soundstage deep, but wide, open, and airy as well. This remarkable recording, with its natural ambience and sense of space, reveals just how wide and deep a processor can throw a soundstage. Through the DSPro Basic and with my eyes closed, it was easy to be transported to the California Civic Auditorium where it was recorded.
The DSPro Basic's soundstage depth seemed to combine synergistically with its remarkable transparency. There was no sense of veiling, congestion, or fog. Instead, the Basic provided a transparent window on the music that was luscious and inviting. Music had an openness and clarity seldom heard from digital playback at any price. This transparency, coupled with the ability to reveal spatial nuance, allowed me to see into the music as though it were clear crystal. Instrumental outlines were resolved in three dimensions, with air and space around them. Images floated between, above, and behind the loudspeakers. Female vocal (Diane Reeves on David Benoit's This Side Up, En Pointe ENP 0001) was pure, clear, and seemed to exist in my listening room. In addition, the DSPro Basic had the ability to convey instruments' individualities during complex passages without homogenizing their sounds. The only other processor I've heard that surpassed the DSPro Basic in this regard was the $12,000 Stax DAC-X1t.
Equally impressive was the DSPro Basic's ability to reveal musical detail. Percussion instruments had sharp, clean, transient attacks without being up-front or aggressive. Instead, they had that quick leading edge, but in their correct spatial locations behind the musicians. Detail tended to be immediate rather than laid-back. The word "vivid" is an appropriate description of the DSPro Basic's presentation. The acoustic guitar on my duet recording (track 12 of the Stereophile Test CD) had a clean, sharp attack, at the same time surrounded by a dynamic bloom. Playing back the CD Dick Hyman Plays Fats Waller (Reference Recordings RR33CD) confirmed this impression of the DSPro Basic's quick transient ability. The sound of the hammers hitting the strings was apparent, but not overly so. It was very similar to the actual live sound of the Böosendorfer I heard during the recording session.
The tonal balance was smooth, with a surprisingly clean treble. No trace of digititis here. The upper octaves were devoid of the hash and grundge that so often mars CD playback. Although I would not characterize the HF presentation as sweet and silky in absolute terms like the Stax X1t, it was nevertheless relatively free from fatiguing grain and harshness.
Dynamic impact and ability to reveal subtle dynamic contrast were superb. The DSPro Basic's ability to deliver punch and visceral impact added a rhythmic urgency to music. Low frequencies were round and liquid, yet detailed. The string attack of acoustic bass was readily audible (Eddie Gomez on the first Steps Ahead album, Elektra/Musician 9 60168-2), its pitch and body intact. Bass drum was clean and taut, without tubbiness or bloat. This added a rhythmic intensity to music that involved the listener in the performance. Overall, the DSPro Basic provided a thoroughly involving and inviting musical experience.
To find fault with the DSPro Basic, I must turn to the extraordinary Stax DAC-X1t. The Basic is that good. The DSPro Basic lacked the analog-like sense of ease that made the X1t so pleasurable. In addition, the treble presentation from the tubed X1t was smoother and warmer. During the auditioning, I thought that the DSPro Basic's rendering might be detailed to the point of being overly analytical. Whether this is an artifact of the processor or a revelation of what is actually on the disc, I don't know. This has been a source of recent debate. It has been argued that some of the good-sounding processors tamper with the presentation to make it more "musical" at the expense of fidelity to the source. This is a difficult but vitally important question to resolve.
There is a way, however, to know exactly what a digital processor is doing to the sound, though I was unable to implement it in time for this review. As soon as I've finished this review, I'm meeting Bob Katz in California, bringing with me the original ½" analog master tapes of Stereophile's Poem LP for conversion to digital. The tapes will be played back on Kavi Alexander's machine, on which they were recorded, and converted to digital with several leading-edge A/D converters. After editing (assembling the tracks in the correct order with the appropriate amount of space between them), CDs will be manufactured from the edited digital master.
Concurrently, Stereophile has purchased an Ampex ATR-100, a full-on professional analog tape machine for playing back our ½" original master tapes. We will thus have the uncorrupted and ultimate reference (the analog master tape) and a CD made from that tape, decoded by the processor under scrutiny (footnote 3). This is by far the most revealing test of a digital processor's editorial influence on the sound: it will stand naked with its flaws and euphonic colorations exposed.
Conclusion
To say I liked the Theta DSPro Basic is an understatement. It provided a level of musicality I would never have expected at this price. In fact, the DSPro Basic is the best digital processor I've heard at any price, except for the $12,000 tubed Stax DAC-X1t. It had all the attributes of the best digital playback: a deep and transparent soundstage, smooth tonal balance, spatial detail galore, and textural liquidity. It was the antithesis of the flat, harsh, sterile sound that characterizes many digital processors, especially the budget models. I can't wait to hear Theta's DSPro Generation II, their top model.
It's significant that I'm forced to compare the DSPro Basic to the Stax DAC-X1t to reveal the former's shortcomings. No, the Basic didn't have the Stax's sense of ease, smoothness, freedom from fatigue, and ability to resolve finely woven detail. But for $2000, it came far closer than one might imagine. My only criticism is that the DSPro Basic's presentation tended toward the overly detailed and analytical, which did create fatigue after extended listening sessions. Whether this is an accurate portrayal of what's on the disc or an artifact of the processor is an open question.
I'm concerned, however, about what appears to be the DSPro Basic's fundamental inability to decode a 48kHz signal from a DAT machine. This may become a significant issue as DAT machines hit the stores soon. (See my CES report next month.) I confirmed, however, that the problem can be corrected in software with new ROM chips: Theta has a good track record of updating customers' units.
The DSPro Basic clearly breaks new ground in affordable digital playback. It represents a quantum leap in what we can expect from a $2000 processor. All contenders for the title of best reasonably-priced converter must regard the Theta DSPro Basic as the benchmark against which all others are judged.
Get out your checkbook, Michael. The Theta DSPro Basic is exactly what you've been waiting for. |
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