POW-r ：dither机器，3500美元一台，Pro Audio Review Magazine说：如果你想要世界上最好的 16bit 声音，就一定要拥有它。
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Review: POW-r by Bob Katz for Pro Audio Review Magazine
Gambit Audio (Weiss) Advanced Noiseshaping Redither with POW-r Algorithm
They laughed when I sat down at the piano, but they cried when I began to play. I was laughing at first when I received the new Weiss redithering box for review. For about 8 years, I've been doing shootouts of redithering algorithms, and every system I tried was unable to recover ambience as well as my venerable Meridian model 518. But when I heard the Weiss, I began to cry, because I couldn't believe my ears---this box brings 16-bit audio closer to the 24-bit source than I had dreamed possible. This includes previous tests of UV-22, Waves IDR, Sony Super Bit Mapping, Prism, Sonic Solutions Turbo Bit Mapping, and other lesser-known varieties of dither. Until I heard the Weiss, I thought that every dither was a compromise; some sacrificed ultimate depth and space for a pleasant tonality, or vice-versa. The POW-R dither seems to have broken a new barrier, removing the compromise of tonality versus depth.
But I digress. Let's go back to the beginning. What is POW-R dither? POW-R is a clever acronym, short for Psychoacoustically Optimized Wordlength Reduction. POW-R dither is owned and licensed by the POW-R Consortium LLC, a group formed by the owners of four otherwise-independent audio companies. In 1997, John La Grou, president of Millennia Media, Inc., Placerville, California, brought together an international complement of talent: David McGrath of Lake DSP, Australia, Daniel Weiss from Switzerland, and Glenn Zelniker of Florida's Z-Systems---with the goal of producing the most sonically-transparent dithering algorithm possible. The process they came up with is not just dither with noise-shaping, but a unique patent-pending approach shrouded in considerable secrecy. According to the consortium, "POW-r is similar to other wordlength reduction schemes in its use of a random noise signal as part of the signal/quantization error decorrelation process. Where POW-r differs, though, is in how this noise signal is generated and what its spectral characteristics and statistical properties are." Not much to go on, but perhaps we'll learn more when their patent is granted.
POW-R dither comes in three "flavors", intended to fit different kinds of music. More about that in the listening tests, below. The POW-R consortium will license the algorithm non-competitively to its own members, and to additional licensees, which include notable DAW, processor, and converter manufacturers. Sonic Solutions intends to include pow-r as a plug-in on its HDSP processor, Sadie has expressed interest, and Z-Systems currently has an equalizer and a planned compressor-limiter with the algorithm.
The Weiss box is the first implementation of the algorithm in a standalone hardware product. It accepts four input formats, SPDIF (RCA), AES-EBU (XLR), SDIF-2 (BNC), and SDIF-1 (9-pin). Any selected input simultaneously feeds all outputs, so the box also functions as a distribution amplifier and format converter, since the bypass mode is bit-transparent. The algorithm can be switched between 16, 18, and 20 bit output, with an option to blank the output to digital black (mute) if the source has been black for a certain number of samples. A bypass switch that also selects the type of algorithm completes the simple complement of controls. I only tested the box with the SPDIF interface, as my digital system has been converted to 75 ohm throughout, mostly on BNC's.
Dithering and noise-shaping. Noise shaping is a method of improving the signal-to-noise-ratio of dither, resulting in an SNR more like the longer wordlength source. But it can't be perfectly done; if we could fit 24 pounds into a 16 pound bag then we wouldn't need the bigger bag---that is, if you weigh things on a logarithmic scale, but let's not quibble! Noise shaping curves are tailored to the limits of human hearing (the well-known Fletcher-Munson curves), and each designer has his ideas of how to do it best. The object is to keep the total power of the curve at 16 bits, but to lower the noise in the areas where the human ear is most sensitive. Figure 1, a Spectrafoo FFT, illustrates the dilemma. Here are three 9th-order curves: Meridian's shape D (blue curve), POW-R dither type 3 (yellow), and Waves Ultra dither type 1 (green). This is the new, improved Waves dither from their L2 Ultramaximizer limiter. Notice how each manufacturer follows the same general shape, but each one chooses fine variations on that shape. Noise masks signal in the same frequency band. Thus, in the frequency ranges where the dither noise is reduced, more of the source music and ambience is revealed (unmasked).
Is This Thing On? That's the theory, but now it's time to hook up the box and listen. Based on my experience with many dithering boxes, it looked like the Weiss ought to perform better in some areas, and worse in others. But I was wrong. My first reaction when I hooked up the POW-R box was "is this thing on?" I thought I was listening to the source. I rechecked my connections, inspected my bitscope, and switched around until I was sure there was no wrong connection. This was amazing! Every other dithering system shrinks the soundstage slightly and changes the tonality of the sound. But apparently not the pow-r dither.
Next I began to familiarize myself with the box's operation. There are three types of pow-r dither. Previous experience with dither has shown me that rock and roll music frequently benefits from a gentle curve, or UV-22; I think these curves cover up some of the "evils" in the source and tend to be less "edgy" sounding on brashy material. Classical music tends to benefit from the 9th order curves, like Meridian shape D, which has greater depth and width, yet has a bit of a "sheen" to it. Because each dither affects the tonality of the sound, I must integrate it into the mastering process. Ideally, a dither should be tonally neutral, so that the 16-bit CD master sounds like the 24-bit. In the FFT of Fig. 2, note that pow-r type 1 (blue) is a "near-Nyquist dither" second order curve, which most resembles UV-22 in its shape. Type 2 (green) is a slightly steeper fifth-order curve, which resembles Meridian type E. And type 3 (red) is the severe ninth-order curve.
Serious Listening. As a serious dither connoisseur, I expected to hear meaningful differences between the pow-r curves. But again, I was wrong. Using a delicate 24-bit classical piano solo recording, I could not reliably distinguish the three 16-bit curves from each other or from the 24-bit source! This has never happened before. I usually notice some degradation and a meaningful difference between any box's curve shapes. Next, I tried an excellent recording of a "country-rock" group, which has a lot more high-frequency information than the classical piano. This recording came in on 1/2" 30 IPS analog tape and I had previously mastered it using various digital processors and Meridian D for the dither. Starting with pow-r type 3 (the sharpest curve), I was amazed at the clarity, depth, and lack of congestion in the music. With this music, I was finally able to hear the smallest difference between the 24-bit source and the 16-bit dithered version; it's hard to describe, the 16-bit being a little bit more "closed in", but a respectable facsimile of the original in all respects. I then tried pow-r type 1, and heard a bit more degradation compared to type 3, but nothing to write home about. I certainly couldn't use any of the dither types as a "tone control"; it's much too subtle for that. Basically, after a few tests, I decided that the way to use pow-r dither for me is to stick with the best-sounding curve (type 3). If you need to cover up some evils in your source, this is not the box to use. It neither disguises nor enhances any part of the spectrum. We can also conclude that the effect is music-dependent; complex stereophonic music appears to expose dither problems more than simple.
Next, as a control, I compared the pow-r against my reference dither, Meridian's shape D. For this music I had originally chosen shape D over other dithers because it revealed more of the source and added a little "bite" which was desirable with this music. So I was concerned that pow-r's tonal neutrality might produce a more recessed sound. But I found that pow-r reveals so much more of the source, that I no longer needed the Meridian's artificial edge. Instead, the transparency of the original source carried the day, translated through the pow-r algorithm. Listening comparisons showed that the Meridian is more congested, with a slightly squashed lateral and depth image compared to the 24-bit source. In other words, I was using the Meridian's slight bite to make up for its lack of resolution. Yet the Meridian's resolution is slightly better than many other dithers. It's all a matter of degree, but it didn't take long for me to conclude that the pow-r dither is better than anything else out there. It's negligibly different from the source, an incredible achievement.
Let's put this in perspective. There are many people who are insensitive to the virtues of the different dithering systems. What order of magnitude of differences are we talking about? If you are inclined to music that has depth, space and purity of tone, you will recognize the qualities provided by a superior dither, or for that matter, 20-24 bit recording. A high resolution monitor system is useful to distinguish the sound of subtly-different dithers from each other and from the source. Similarly, it's hard to hear the difference between MP3 files and the 16-bit source played on a boombox. But seekers of good-quality sound should rejoice that the bar has been raised for 16-bit. I'd like to see a shootout between pow-r dither and HDCD. Because pow-r is completely compatible with standard 16-bit, and does not require a special decoder. Can we really fit 24 pounds in a 16-pound bag? I don't know, but this process comes real close. I have no reservations about this box except for the price, which does cover the cost of the multiple professional interfaces. I guess it's time to remortgage the house. It would be nice if Weiss produce a lower-cost version with just AES or SPDIF.
Bob Katz, president and mastering engineer of Digital Domain in Orlando, Fla, is a contributor to Pro Audio Review. Katz has made available useful information about digital audio and mastering at the Digital Domain Web site:
www.digido.com.
At a Glance
Applications: Dither processor for mastering studios.
Key Features: Digital inputs and outputs. Accepts 44.1 kHz and 48 kHz sampled audio signals on any standard digital interface.
Price: $3500.00.
Contact: G-Prime at (212) 765-3415 or circle Reader Service XXX
Product Points
Weiss Gambit Advanced Noiseshaping Redither with pow-r algorithm. 1-U rack mount. Reduces 24-bit and 20-bit sources to 16-bit virtually transparently.
Plus
Listening tests show this to be the best-sounding dither---virtually indistinguishable from the 24-bit source
For the first time, word-length reduction delivered with little or no sonic "congestion"
Distribution amplifier and format converter, including SDIF-2
Minus: Expensive
The Score: If you want to make the best-sounding 16-bit product in the world, you have to own this box.