Digital Processor Reviews

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.

Alternatives
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.

Summing up
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.

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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.