When it comes to getting audio from a PC via its USB port, the buzzword du jour is asynchronous. This cryptic term refers to which device has control over the timing of the audio data being streamed from the computer: the computer itself, or the device receiving the data. It might seem logical to have the computer control the timing, but this is not so. When digital audio data are converted to analog by a D/A converter, control over exactly when each dataword is converted is critical for the best quality of sound. Any uncertainty in that timing manifests itself as analog distortion, aka jitter.

It is therefore good engineering practice to place the timing circuit, for obvious reasons referred to as a “clock,” as close as possible to the D/A circuit. But a PC clock several feet away, at the other end of a USB cable, is clearly not “as close as possible.” Worse, a computer must attend to other tasks, so its clock is inherently less stable than the crystal oscillator found in a DAC. The optimal solution is to allow the DAC to control the flow of USB data from the computer; this is what is meant by asynchronous.

Not all USB receiver chips can be operated in asynchronous mode, an exception being the Texas Instruments TAS1020B. This USB 1.1 device includes an onboard microcontroller that can be programmed to run the interface in asynchronous mode. Gordon Rankin of Wavelength wrote proprietary code for the TAS1020B’s microcontroller; as well as using it in his own USB products, he licenses this Streamlength code to hardware manufacturers. One such is Halide Design, who used Streamlength in their S/PDIF Bridge converter ($395), which I reviewed in December 2010. The Bridge comprises a USB cable with a tubular housing at one end, terminated in either an RCA or a BNC jack. The Bridge operates in asynchronous mode to convert USB-streamed audio data to the S/PDIF serial format required by traditional D/A processors. Because there is no jitter-inducing S/PDIF cable—the Bridge connects directly to the processor’s input jack—the Bridge became my preferred means of connecting a computer to DACs that lacked an asynchronous USB port.

It seemed an obvious development for Halide to marry the Bridge’s asynchronous USB receiver to a DAC chip, to create a complete USB/analog converter (footnote 1). This they have done with the subject of this review, the DAC HD. Initially priced at $550, the DAC HD will be available for $495 by the time you read this review.

Asynchronous Conversion
Like the Bridge, the DAC HD includes captive cables, in this case a black version of Wireworld’s Starlight USB cable, which features silver-clad conductors and a unique geometry said to allow use in lengths up to 7m. (A 2m cable is standard; 7m and other lengths are available as options when ordering.) Analog output is via two 6″ lengths of silver-conductored cable terminated in Eichmann silver Bullet RCA plugs. (These RCAs are favorites of mine because they make the ground before the signal connection—a commendable feature.)

The circuitry, all surface-mount, is contained in a small (1.875 cubic inches), black-anodized, machined-aluminum enclosure, and is carried on a small double-sided printed circuit board. Power is taken from the 5V USB bus and first filtered (two-pole passive), then regulated; there are eight regulators and separate power rails for the digital circuitry, the master clock, and the output stage. The TAS1020B is mounted behind the USB port and feeds a Wolfson WM8716 DAC chip—a two-channel, multi-bit, sigma-delta, 24-bit type. Although the WM8716 can operate at sample rates up to 192kHz, the TAS1020B handles sample rates only up to 96kHz, including the important 88.2kHz rate. The Wolfson DAC includes fast- and slow-rolloff digital reconstruction filters, but also can be used with an external filter. The output stage uses high-performance op-amps.

No driver programs are required with either Macs or PCs. The user simply plugs the DAC HD into one of the computer’s USB ports and selects it as the preferred audio output device.

As I was about to start this review, I retired my 2006-vintage G4 Mac mini and replaced it with a 2.7GHz, i7-powered Mac mini with 8GB of RAM. Both the Mac mini and the Pure Music program recognized the Halide when I plugged it in, and I set it as the default playback device. Pure Music was set to upsample CD-sourced files to 88.2kHz with its Maximum Fidelity sample-rate converter.

Halide Design states that the DAC HD is cryogenically treated to give a “smoother, more resolved sound.” Whether or not that was the reason, my first impression was indeed of a very smooth sound. There wasn’t quite the wealth of recorded fine detail that I’m used to hearing with the megabucks D/A converters that usually grace my system, nor was there quite as expansive a soundstage. In my recording of Carol Wincenc performing Mozart’s Flute Quartet in D, K.285, at the Santa Fe Chamber Music Festival (ALAC file sourced from CD: Serenade, Stereophile STPH009-2), while the flute was unambiguously placed in front of the string trio, the acoustic of St. Francis Auditorium sounded a little smaller and drier than it should. But that was a minor criticism, considering that when I rose from the listening chair to find my notebook, I realized I had listened to all three movements of the quartet. There was something very seductive about the sound of the DAC HD.

Footnote 1: Halide Design’s earlier, less expensive Devilsound DAC uses the Burr-Brown PCM2706 USB receiver, which operates in the more conventional adaptive isochronous mode, in which the computer controls the timing of the data flow.

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