Asus is best known for building motherboards and video cards, but the company also has a solid reputation for designing and manufacturing high-end PC sound cards and other audio kit. Asus now takes audio processing outside the PC and laptop, with the spectacular Asus Xonar Essence One. See all audio accessories reviews.
The Xonar is a sample-rate converter, a USB DAC (digital-to-analogue converter), a stand-alone DAC (with both coaxial and optical digital inputs), a sample-rate converter, a pre-amp (with both XLR and RCA stereo outputs), and a sweet headphone amplifier.
As a USB audio device, the Xonar keeps the PC's stereo audio output in the digital domain until it reaches the Xonar. It can accept signals with either 16- or 24-bit resolution, with sampling rates of between 44.1- and 192kHz. When the bit stream reaches the Xonar, an onboard DSP (digital signal processor) upsamples the signal to 32-bit resolution, and it symmetrically upconverts the sampling rate by a factor of up to eight.
A 16-bit, 44.1kHz signal (the resolution and sampling rate of an audio CD) is converted to 32-bit resolution at a sampling rate of 352.8kHz (44.1 x 8), as would input signals with sampling rates of 88.2- or 176.4kHz). A 16-bit, 48kHz signal, meanwhile, gets converted to 32-bit resolution and a sampling rate of 384kHz (48 x 8), as would signals with sampling rates of 96- or 192kHz.
Upsampling is known to improve the quality of the signal that's presented to the DAC (digital-to-analogue converter), much in the same way that upsampling a digital photo can enable you to enlarge the image without seeing jagged edges. Theoretically, the higher the resolution and sampling rate, the higher the audio quality. Icing the cake, Asus claims the Xonar delivers a signal-to-noise ratio of 120dB.
Asus provides optional ASIO (Audio Stream Input/Output) drivers for the Xonar, so that ASIO-compatible playback software (I happened to use Fubar with the ASIO add-on) will bypass the normal Windows audio path and connect directly to the hardware.
There's a twofold advantage to using an ASIO driver: It reduces latency (which isn't all that important for this application, but is critical for professional audio-recording applications), and it bypasses the Windows audio API so that the bit stream goes straight to the Xonar without undergoing any processing or being changed in any way. When the ASIO driver is used, a “bit perfect” LED glows on the Xonar's faceplate, indicating that the device is receiving the exact bitstream from the recording.
[Note that the Asus Xonar Essence One will not operate correctly with Mac OS X's Core Audio, and Asus offers no software drivers to allow this device to be used with the Mac platform – Reviews Editor]
I connected the Xonar to my Windows PC's USB port and performed my listening tests using a pair of Ultrasone HFI-2400 headphones. These cans aren't particularly difficult for a headphone amp to drive, being rated at 70 ohms of impedance, but Asus claims the Xonar's amp can handle models presenting up to 600 ohms of impedance. I used several recordings that I'm intimately familiar with after repeated listening sessions, including Accidental Powercut Session 1, from Bowers & Wilkin's Society of Sound subscription music service. This album of live binaural recordings was made specifically for headphone listening, using microphones inside a dummy head on the stage. The tracks were encoded at the studio as losslessly compressed FLAC files at a sampling rate of 44.1kHz with 16-bit resolution.
Listening to the instrumental track “Ant House,” from Speakers Corner Quartet, I was impressed with the Xonar's ability to render the band's unique combination of cello, flute, contrabass, and drums with incredible detail without adding coloration. If you find this transparency too sterile for your taste, you can replace the Xonar's operational amplifiers (op/amps) with something warmer, since the ICs come installed in user-serviceable sockets instead of being soldered to the circuit board, as is the more common manufacturing technique.
I also used several traditional studio recordings during my evaluation, including Peter Gabriel's Half Blood. Another Society of Sound release, this album features a collection of Gabriel's biggest hits arranged for and re-recorded with a full orchestra. These tracks were encoded in FLAC at a 48kHz sampling rate with 24-bit resolution. The Xonar rendered the chanting chorus in “The Rhythm of the Heat” so cleanly that I could discern each individual singer's voice straight through the driving percussion.