Lindemann & Stello USB-S/PDIF converters
This was due to timing uncertainty in the datastream. When a USB audio device is operated in the ubiquitous isochronous adaptive mode (footnote 3), the host computer controls the flow of data. The sample rate of the data is checked once per millisecond, and the flow is speeded up or slowed down to ensure that the long-term average is close to the appropriate sample rate. However, as a general-purpose computer is not optimized for streaming audio data, there will always be short-term fluctuations in the sample rate that introduce distortion and/or noise to the reconstructed analog signal.
In May 2009 I reviewed Bel Canto's USB Link 24/96 (footnote 4), which offered a solution: convert the jittery USB datastream to S/PDIF so that it could be fed to a high-quality, standalone D/A converter. The input circuit of the DAC would filter the jitter, producing a cleaner, hence better-sounding analog signal. The Bel Canto uses the Texas Instruments TAS1020B USB receiver chip, which includes an embedded microprocessor running code developed by Centrance so that there is no need for the host computer to run a proprietary driver program.
I found that the USB Link 24/96 had quite a high level of jitter on its S/PDIF output, but with a modern DAC like the Benchmark DAC 1 or Bel Canto's own e.One DAC3, both of which have superb jitter rejection, this wasn't an issue. The USB Link 24/96 became a regular member of my system, and Stereophile writers Wes Phillips and Larry Greenhill are also fans. A drop in price from $495 to $249 at the beginning of 2010 made it an even better value, though it no longer comes with a Stereovox S/PDIF cable.
I saw two products at the 2010 Consumer Electronics Show intended to perform the same function as the Bel Canto USB Link 24/96: the Stello U2 from Korea ($349) and the Lindemann USB-DDC 24/96 from Germany ($650). Intrigued, I asked the importers for review samples.
Stello U2 ($349)
Manufactured by April Music of Korea, the Stello U2 is a plain, black-painted, diecast box with a red LED adorning one of its wide sides; on the other are a USB Type B jack, an RCA jack for the S/PDIF output, and a five-pin mini-DIN jack for an I2S data output. Inside, the circuitry and jacks are carried on a single double-sided circuit board. Other than the LED and a pair of electrolytic reservoir capacitors, all components are surface-mount types.
The heart of the device is a 48-pin integrated circuit, the Tenor TE7022L. This chip includes a complete USB 2.0compliant data receiver and both S/PDIF and I2S two-channel inputs and outputs. The datasheet for this chip indicates that it will handle data with sample rates up to 96kHz and bit depths of 16 or 24 bits, and that its integral volume control operates in the digital domain. The Tenor chip is clocked by a temperature-compensated crystal oscillator immediately adjacent to it, and the S/PDIF output appears to use a small transformer to galvanically isolate from each other the grounds of the computer and the audio system.
Power for the Stello's circuit is taken from the USB bus, and no driver program is required; the user plugs a USB cable into the U2's input and an S/PDIF cable into its output to take the audio data to a DAC. The Stello identifies itself to the host computer as "TE7022 w/ SPDIF," and a pair of output volume sliders and a Mute button appear in the Audio Midi Setup program's window. Examining the Stello's USB properties with a Mac utility called USB Prober revealed that it will operate at all legal sample rates between 8 and 96kHz, with the exception of 88.2kHz, with bit depths of 16 or 24. It will also pass encoded data, such as Dolby Digital. USB Prober identified the Stello as the "DigiHug USB Audio" from Tenor Audio.
Lindemann USB-DDC 24/96 ($650)
The German-made Lindemann USB-DDC 24/96, housed in a small enclosure of extruded aluminum, looks more of a finished product than the Stello. Inside, the circuitry and the component complement, as well as the input/output jacksa TosLink optical output jack is provided in addition to the RCA S/PDIF outputare similar to the Stello U2's, with the exception that the 48-in chip has a label reading "DDC 24/96" stuck over it, and no large-value electrolytics are to be seen. The RCA output is isolated with a small transformer, keeping the system and computer grounds separate.
Footnote 1: If they can find it at a used bookstore, younger readers will get a horrific look into the dysfunctional world of computers made before they were born by reading Joe Campbell's The RS-232 Solution (Sybex Computer Books, 1984).
Footnote 2: A basic guide to the various strategies for getting the best sound from a computer can be found here.
Footnote 3: 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. However, only a very few products currently availablefrom Ayre Acoustics, dCS, and Wavelengthfeature this mode.
Footnote 4: Bel Canto Design, 221 N. First Street, Minneapolis, MN 55401. Tel: (612) 317-4550. Fax: (612) 359-9358. Web: www.belcantodesign.com.