By John Atkinson
The speed with which audiophiles have adopted a computer of some sort as their primary source of recorded music might be thought breathtaking. But with the ubiquitous Apple iPod painlessly persuading people to get used to the idea of storing their music libraries on computer hard drives, the next logical step was to access those libraries in listening rooms as well as on the move. A few months back, I wrote a basic guide to the various strategies for getting the best sound from a computer: "Music Served: Extracting Music from your PC." Since then, Minnesota manufacturer Bel Canto Design has released a product that aims to simplify matters even further.
The USB Link 24/96 is a small box, about the size and weight of a pack of cigarettes, with a USB Type B jack at one end and a 75 ohm BNC jack at the other. The user hooks the Bel Canto's USB input up to a USB port on his PC or Mac computer, which supplies power to the Link, thus illuminating a red LED next to the Link's BNC jack. He then uses a 9" length (footnote 1) of Stereovox XV2 S/PDIF datalink (supplied) to feed his audiophile DAC. (This cable is fitted with BNC connectors at both ends; an RCA adapter is provided.) No driver programs are required—the Link uses the native drivers provided with the Mac OSX and Windows operating systems. The computer automatically recognizes the Link as "Bel Canto 2496 USB," and once the Link has been selected as the default audio output device, programs such as iTunes will direct their output to it, and thence to the owner's high-end system.
The Bel Canto USB Link 24/96 disables the computer's volume control, ensuring that the maximum sound quality is obtained from music files. The sample rates supported run up to 96kHz, with a depth of 24 bits.
Under the hood
The USB Link 24/96 is housed in a small aluminum extrusion with black plastic endcaps. Undoing the four Phillips-head screws at each end allows the multilayer circuit board to be slid out. All the components used are surface-mount types. The USB data are fed to a Texas Instruments TAS1020 chip, which converts the audio data to i2C format. The TAS1020 is clocked by an adjacent 6MHz crystal oscillator; despite its thumbnail size, this complex TI chip includes an embedded microprocessor that runs, I believe, code developed by Centrance, obviating the need for the host computer to run a proprietary driver program with the USB Link. The i2C audio data are then fed to a Crystal CS8406 chip, which converts them to the S/PDIF serial format and drives a small pulse transformer adjacent to the output BNC jack, in order to galvanically isolate the computer and audio system and thus avoid injecting high-frequency noise via the linked grounds.
While the Bel Canto USB Link 24/96 is a simple device, reviewing it wasn't so simple. Not only are there two different computing platforms to be considered, PC and Apple Macintosh, there are also the various flavors of their operating systems, and the multitude of possible music-playback programs. I settled on using the Bel Canto with two computers: a dual-core Pentium PC running Windows XP with Service Pack 3, and a G4 Mac mini (footnote 2) running OS10.4.11. I played music files using iTunes 8.0 and the open-source Audacity DAW freeware, both of which are available for both computer flavors. On the Mac, I also used my regular music-editing program, BIAS Peak Pro 6.0.5; on the PC, I also used Winamp and Foobar2000, both of which I prefer to iTunes on that platform because they allow the audio data path to be optimized, and Windows Media Player.
With the Mac, the Bel Canto needs to be selected with the Audio Midi Set-Up utility and have its sample rate set to match that of the music to be played. (Go to Applications/Utilities/Audio Midi Set-Up; in the right-hand portion of the panel, select the Bel Canto as the output device; then click on Properties at the left.) If you don't do this, the Link has no way of knowing what the file's sample rate is—unlike a specific audio serial format, such as AES/EBU or S/PDIF, USB doesn't include a data field to specify sample rate, but defaults to the sample rate of whatever was the last file played, using the host computer's sample-rate converter to transcode the audio data. Some programs, such as Peak, use the Mac's CoreAudio interface to switch the USB datastream to whatever is required, but with iTunes on the Mac, you need to manually change the sample rate with Audio Midi Set-Up whenever you select a file with a different sample rate. Otherwise, you'll get no audible benefit from playing a hi-rez file.
Windows XP is more friendly in this respect; the sample rate of the USB Link 24/96 automatically follows that of the audio file selected with Windows Media Player, Foobar2000, and Winamp (though not with iTunes, at least that I could see). None of my PCs runs Windows Vista, but during the review period, Erick Lichte, musical director of Cantus, visited so that we could do some further work mixing the group's next CD. Erick's VAIO laptop runs Vista, and as he had some hi-rez files of pianist Robert Silverman playing Brahms he wanted me to hear, he plugged the USB Link 24/96 into his laptop's USB port, loaded the files into the Soundforge program, and pressed Play.
Nothing. No matter what Erick did, we couldn't get the files, which were recorded at the sample rate of 88.2kHz, to play through the Bel Canto, even though CD files did. It turns out that Vista's audio codec has a bug that doesn't allow playback at 88.2kHz.
As a format converter, Bel Canto's USB Link 24/96 shouldn't have a sound of its own, of course; it should be a neutral intermediary. I mainly used Bel Canto's own e.One DAC3 for my auditioning, as that would seem a natural match. As you can read in my November 2007 review, the DAC3 also has a USB data input. Why, then, should a DAC3 owner consider a USB Link 24/96? Because the DAC3 is limited to 16-bit files sampled at 44.1kHz. The Link allows a DAC3 to handle computer-sourced hi-rez files, and I found it did so with ease, other than occasionally emitting three or four clicks when I switched sample rates on the Mac.
I couldn't hear much difference in sound quality between feeding the DAC3 USB data sourced from iTunes on the Mac as it played an AIF or WAV file, and feeding it AES/EBU data from the original CD as played by my Ayre C-5xe universal player. Perhaps the Ayre produced a slightly more solid-feeling bass, with slightly better-defined, better-extended low frequencies, but it was not a night-or-day difference. Changing from the DAC3 to my early sample of the Benchmark DAC1, even that difference between the two sources disappeared, though I slightly preferred the sound of the two Bel Canto devices overall to that of the USB Link 24/96 or Ayre driving the Benchmark, which had a slightly less silky high end. My own 24-bit/88.2kHz files, such as the hi-rez masters for Cantus's While You Are Alive (CD, Cantus CTS-1208) and Attention Screen's Live at Merkin Hall (CD, Stereophile STPH018-2), sounded convincingly better than the "Red Book" CD versions, with my Mac mini feeding the combination of Bel Canto's USB Link and DAC3.
Footnote 1: The conventional wisdom with datalinks is that they should be either very short, around 9", or at least 2m in length, to minimize jitter due to impedance-mismatch-induced reflections.
Footnote 2: A major advantage of the Mac mini is that although this has a fan, this rarely comes on, unless the computer is doing some major number crunching. And even when it does come on, it is still quiet enough not to be heard. It is not a big deal, therefore, to place a Mac mini adjacent to your D/A processor in the equipment rack, controlling iTunes with the Remote app running on an iPod Touch or iPhone. By contrast, the typical PC is too noisy for use in the same room as a high-end system, unless you can put it in a closet.
USB-sourced audio data do not necessarily have low jitter, though Bel Canto does claim that the USB Link 24/96 offers low-jitter clock recovery from the USB data (footnote 3). This may be a moot point with the Benchmark and Bel Canto D/A processors, of course, which are modern designs featuring superb rejection of timing errors in the datastreams they're fed. But what about older products? I retrieved from storage a sample of the Assemblage DAC-1 that I'd built from a Parts Connection kit in early 1995, when we published a review by Wes Phillips (Vol.18 No.4). Costing $449 back then, the Assemblage uses the respectable Burr-Brown PCM1702 DAC chip, but a data-receiver circuit, based on a Crystal CS8412 chip, that was never as good at eliminating datastream jitter as I had expected.
Driving the Assemblage with the Ayre, converting the latter's AES/EBU output to S/PDIF with a dCS 972 format converter, the sound was okay if a little grainy, with a less well-defined soundstage than the Bel Canto or Benchmark DACs. Not bad for a 15-year-old design, I thought, though the Assemblage will run at only the 44.1 and 48kHz sample rates. However, switching to the USB Link 24/96–derived datastream resulted in a reduction in sound quality. The soundstage flattened a bit, the midrange became slightly coarser, adding a bit of clanginess to piano sound, and the lows lost definition, double bass sounding slightly more wooly.
I returned to the Bel Canto DAC3, with levels matched to within 0.1dB at 1kHz. The soundstage came back into focus and away went the coarseness. And back into the closet went the Assemblage DAC-1.
Other products provide the same USB-to-S/PDIF conversion as the Bel Canto USB Link 24/96, and one of them is the M-Audio Transit USB, which costs just $99.95; another, recommended by a poster on Stereophile's online forum, is the E-Mu 0404 USB ($199.95).
The M-Audio Transit USB is housed in a plastic box a little smaller than the Bel Canto Link; it has a USB input at one end and, at the other, analog and optical digital inputs and outputs. I bought a sample a couple of years ago and have found it a reliable means of getting audio data out of PCs and Macs. Inside, the Transit USB uses the same TAS1020 chip driven by a 6MHz crystal oscillator as the Bel Canto, but feeds its output to an AKM4585 chip, which provides A/D and D/A conversion as well as S/PDIF input and output. Unlike the Bel Canto, the M-Audio needs to have a driver program installed; like the Bel Canto, its sample rate needs to be set manually using this program. Nor is the Transit compatible with Windows Vista, I am told.
The E-Mu 0404 USB is a complete two-channel recording device with microphone and line inputs, 24/96 A/D conversion, a headphone amplifier, a MIDI interface, and optical digital I/O. It, too, uses a driver program with both Macs and PCs, but while it will convert USB data to S/PDIF optical at sample rates of up to 96kHz with PCs, it is limited to 48kHz with Macs.
Both the M-Audio and E-Mu devices provide the same basic conversion as the Bel Canto, with the added complication of the user having to install a driver program, but at significantly lower cost. When I played music CDs through them and the Benchmark DAC1, I could hear no appreciable differences among the three USB-S/PDIF converters. With the Assemblage DAC-1, the Bel Canto Link gave a sound that was cleaner than the E-Mu's but, to my surprise, was not appreciably different from the cheap M-Audio's, even with high-sample-rate files.
Provided it is used with a D/A processor that offers effective jitter rejection, the USB Link 24/96 does what Bel Canto promises it will do, and can be recommended. However, I can't pretend that the $495 USB Link doesn't come under strong competition from M-Audio's $100 Transit USB. Both handle sample rates up to 96kHz, and for a Mac user like me, the potential advantage of the Bel Canto of not having to manually set playback sample rate with Windows is moot. But with its aluminum enclosure, the made-in-America Bel Canto does feel like a high-end product; with the Chinese-made M-Audio, plastic is as plastic does.
Do you need such a product? The beauty of the Internet, in combination with something like Bel Canto's USB Link 24/96 to feed the music from your computer to your high-end audio system, is that eventually everything will be available at a click or two of a mouse button. Back in spring 1970, I was driving home from a gig, listening to a BBC broadcast of Fleetwood Mac performing live on, if I remember correctly, John Peel's radio program. The music, "The Green Manalishi (with the Two Prong Crown)," was like nothing I had heard before—lead guitarist Peter Green was evolving his band from British Blues Revival to a unique form of progressive rock. I pulled over and listened to the rest of the broadcast. I have tried in vain to find a reissue of that broadcast, either on LP or CD. Oh well.
As I finish writing this review, I am using the combination of the Bel Canto USB Link 24/96 and e.One DAC3 to drive my system with a live concert streamed from one of my favorite music sites, Wolfgang's Vault, which started off making available the late Bill Graham's live recording archive but has since expanded to include live rock recordings from many sources. It wasn't that BBC concert, but it was an April 1970 Fleetwood Mac performance, this one from London's Roundhouse, the last London gig Peter Green did with the band before his breakdown. Mono the sound might have been, with analog tape noise. And, of course, streamed audio suffers from all the ills of lossy data compression. But, oh wow! Thanks, Peter Green. Thank you, the administrators of Wolfgang's Vault. And thank you, Bel Canto, for providing the USB Link 24/96 and e.One DAC3, which so effectively communicated the music.
Footnote 3: As with almost all the products available on the market, the Bel Canto Link allows the host computer to control the flow of data. A PC is not optimized for uninterrupted streaming, and has operating-system housekeeping chores to attend to—while the sample rate of the output data, averaged over a longish period, will indeed be the specified 44.1kHz or 48kHz, there will be short-term fluctuations or jitter. It is possible to operate the USB interface in what is called "asynchronous mode," which allows the DAC to control the flow of data from the PC, which in turn very much reduces the amount of jitter, but there are very few products currently available, from Wavelength, dCS, and Ayre, that feature this mode.
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