Computer Audio Reviews

When I reviewed this $200 soundcard for Windows PCs in the January 2010 issue, I concluded that the Xonar Essence STX was by far the least expensive way of turning a PC into a genuine high-resolution audio source that I had encountered. Neither the Essence's resolution nor its low level of noise was compromised by having to operate in the electrically unfriendly environment of a computer chassis. However, I ran into a problem playing files with sample rates of 96 or 192kHz. The card was installed in a quiet Shuttle PC (AMD Athlon 3.1GHz dual-core processor, 2GB RAM) running Windows Vista Home Premium with SP2. Using Foobar 2000 (v.0.9.6.9) or Adobe Audition 3.0 and setting the appropriate sample rate with the Xonar Audio Center control program didn't produce sound from the STX card that was noticeably better than standard CD-sourced files. It appeared that 96 and 192kHz files were being downsampled by the STX to 48kHz, with spectral components higher than 48kHz aliased into the audioband; eg, a 40kHz tone was reproduced as 8kHz.

I managed to solve the problem by downloading, installing, and then selecting either ASIO4.DLL or WASAPI as the default sound device in Foobar's Playback Preferences dialog. I let ASUS's technical support know about the problem, but they had not gotten back to me by press time for the January issue. In the new year, however, I did hear back from them: A beta version of a new driver was available that fixed this problem. I installed the new driver in the Shuttle PC, which by then had been upgraded to Windows 7. The original driver was v.6.12.8.1762; the new beta driver was v.7.12.8.17731, dated 2/3/2010. The original version of the Xonar Audio Center software was v.0.2.3.20; the new one was v.0.2.3.27, though the appearance of the user interface was identical (fig.1).

Fig.1 Xonar Audio Center user interface.

With the new driver, bit-perfect playback by the STX card is guaranteed at all sample rates supported by the DMA buffer—44.1, 48, 88.2, 96, 176.4, and 192kHz—and, unlike with the earlier driver, ASIO-compatible applications take exclusive control of the audio device. However, the card's volume control is disabled when it is used in this mode, which makes it inconvenient for headphone listening.

With the Xonar Audio Center open and playing files with Foobar 2000, selecting "Speakers (ASUS Xonar Essence STX Audio Device)" and 24-bit resolution in the Foobar Output Preferences dialog now did give true 96kHz playback, a 40kHz tone being decoded as 40kHz. However, material recorded at 88.2kHz was still sample-rate–converted to whatever rate had been chosen with Audio Center, the 88.2kHz sample rate not being available.

Changing the preferred output device to "Xonar Essence STX bit-perfect ASIO driver" and choosing "Digital Output (ASUS Xonar Essence STX Audio Device)" in Windows 7's Control Panel/Hardware and Sound/Audio Devices dialog did give the correct sample rate for the file being played at the digital output. I tested this configuration with data having sample rates of 44.1, 48, 88.2, 96, 176.4, and 192kHz, and bit depths of 16 and 24; in every case, the data output by the Essence STX were identical to those in the original file. I tested this by recording the output to a new file, inverting its polarity, then playing the old and new files simultaneously in Adobe Audition 3.0. They nulled completely, confirming that the data were the same.

Fig.2 Xonar Essence STX, eye pattern of S/PDIF data output carrying 16-bit J-Test signal (±500mV vertical scale, 175ns horizontal scale).

Tested with the digital input of the Audio Precision SYS2722 as the PC played the 16-bit Miller-Dunn J-Test data, the Essence STX offered low jitter, even with a TosLink connection, at 690 picoseconds peak with a correspondingly clean eye pattern (fig.2). However, I did find one anomaly with the Essence STX's digital output. I was playing files on one PC, feeding the soundcard's TosLink optical output to an RME soundcard fitted to another PC so that I could monitor the state of the bits in the digital datastream using RME's DIGICheck program. To my surprise, when I paused playback, instead of all bits changing to zero (which is what usually happens with digital sources), the DIGICheck meter indicated with red flags that some bits were still active (fig.3). Recording the STX's digital output with Adobe Audition showed that the data remained latched at the value of the last word of the audio data before playback was paused, resulting in a digital DC offset (fig.4).

Fig.3 Xonar Essence STX, bit status of digital output with playback paused.

Fig.4 Xonar Essence STX, waveform of digital output before and after playback paused.

I was at first alarmed by this behavior—if you happen to pause playback just when a waveform peaked at its maximum possible level, the resultant DC offset will be equivalent to a full-scale signal. But this offset is in the digital domain; it would present a problem only if it translated to an equivalent DC offset in the analog domain, where it might be amplified by the power amplifier, with possible damage to the loudspeakers' woofers if the entire playback chain was DC-coupled. I tried the Xonar soundcard's digital output with every D/A processor I had in the house, ranging from the budget-priced Benchmark DAC1 to the super-expensive dCS Puccini. In every case, when I paused playback on the PC, the resultant digital offset did not give rise to an analog offset. Even when I deliberately arranged for the latched data in the Xonar's digital output to be equivalent to a full-scale signal, I couldn't measure any related DC offset in any of the processor's analog outputs. So while this behavior is curious, it isn't pathological.

Headphone listening
I didn't have the space in the original review to discuss the Essence STX card's sound quality with headphones, though I did say that the fact that the headphone output's maximum level could be adjusted to match the headphone impedance was a blessing. The output impedance of the ¼" headphone jack was 10.7 ohms at all frequencies and settings, which is usefully low. However, there was a relatively high DC offset on this output, at 19mV left and 15mV right, this unrelated to the anomaly in digital output noted above.

Through my favorite headphones, Sennheiser's HD-650, I was consistently surprised by the quality of the Xonar soundcard's output. Yes, in comparison with the CEntrance DACport USB headphone amplifier ($395), which I reviewed in June (p.99), the highs weren't quite as silky smooth; and my reference Benchmark DAC1 ($995), fed with the Essence STX's digital output, offered tighter, deeper lows and a generally greater feel of dynamic swings. For example, Mark Flynn's snare drum on Attention Screen's Live at Merkin Hall (CD, Stereophile STPH018-2) exploded from the Sennheisers as it should when the headphones were driven by the Benchmark; driven by the Essence STX at the same level, the drum sounded as loud, but without quite the same jump factor. The low frequencies did have a greater feeling of power and drive than they did with the DACport, however, and backgrounds were silent, with no noise interference from the PC. Overall, the Essence STX's headphone output was better than you would expect from so affordable a product.

As we prepare this issue of Stereophile for publication, I'm working on the first mixes, using Adobe Audition 3.0 in multichannel mode, of the concert Attention Screen played last April at the Yamaha recital hall in midtown Manhattan (see "Update" in this issue). Whether listening to the mixes through the Sennheiser headphones driven directly by the Xonar soundcard or via the card's optical output fed to my big rig, I get no sense of missing anything that I should be aware of.

Summing Up
With its driver update, the Xonar Essence STX and its PCI-bus equivalent, the Xonar Essence ST, can be recommended to those on restricted budgets who wish to incorporate a PC into their high-end rigs.—John Atkinson