DACs

Bryston BDA-1 Digital to Analog Converter and Halide Design USB to SPDIF Bridge

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Halide Audio USB to SPDIF Bridge

Very few DACs offer high performance USB inputs. Most USB audio interfaces are synchronous type interfaces, where a digital clock is synthesized on the computer to match the expected clock at the DAC. This technique results in lots of periodic jitter (an unfortunate side effect of digitally synthesized clocks). Another method, called "isosynchronous", uses a synthesized clock on the computer, but adjusts the speed of this clock ~1000 times a second. This should reduce or eliminate low frequency jitter. The Burr Brown PCM2707 chip used in the BDA-1 uses this type of interface. Neither of these methods is really the right way to do things. A better way was developed over the course of several years by Gordon Rankin of Wavelength audio. His method is asynchronous, meaning there is no clock on the computer at all. The USB interface has precision crystal oscillators that clock the USB receiver, which request data from the computer at exactly the right time. He makes his own USB DACs with this technique, and has licensed the technology to several companies, including Ayre for their new USB DAC. Halide Audio is the first company to design a USB to SPDIF converter using the Wavelength interface. This allows you to gain the benefits of the Wavelength asynchronous USB transfer method with any DAC. The Halide Bridge is built right into the USB to digital audio cable. I tested a version with a BNC connector, but a RCA version is available. Behind the connector, a cylinder the same diameter as the connector and about 1.5 inches long contains all the electronics, including a transformer that electrically isolates the computer ground from the audio ground, and a pair of crystal oscillators, one for 44.1 and 88.2 kHz sample rates and another for 48 and 96 kHz sample rates. The bridge works up to 96 kHz and requires no special drivers for either Windows or OSX. The bridge pads all input signals to 24 bit. Halide sells the bridge manufacturer-direct for $450, an especially good deal considering you need to buy no additional cables. I used the bridge with my 2006 vintage Macbook Pro 15", with OSX 10.6 and iTunes 10. Music was stored in Apple Lossless format on a remotely located hard drive accessed over a wireless network (what Apple calls Airdisk). I used my iPhone as a remote with Apple's "Remote" application. My bridge worked perfectly with absolutely no software configuration necessary.