ASUS Xonar Essence One Muses Edition D/A processor–headphone amplifier Measurements

Sidebar 3: Measurements

I used Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (see www.ap.com and the January 2008 "As We See It") to examine the ASUS Xonar Essence One Muses Edition's measured behavior. I used my 2012-vintage Apple MacBook Pro running on battery power to examine the processor's performance via its USB port. Apple's USB Prober utility confirmed that the USB port operated in isochronous asynchronous mode and reported the processor's product string as "ASUS Xonar Essence One\000" from "ASUSTek Computer Inc." Peculiarly, the Mac's AudioMIDI utility identified the Xonar as "Speaker," but showed that its USB input operated at all sample rates from 44.1 to 192kHz with a 24-bit word length. Unusually, the TosLink input locked to a datastream sampled at 192kHz. (TosLink's limited bandwidth usually precludes operation at sample rates greater than 96kHz.)

Unless otherwise stated, the measured data refer to all three outputs. The maximum output level at 1kHz was 4.03V from the balanced XLR jacks, 2.02V from the single-ended RCA jacks, and 6.98V from the headphone jack. The output impedance was a low 100 ohms at all audio frequencies from the single-ended outputs; twice that, as expected, from the balanced jacks; but 11.1 ohms from the headphone output. All the outputs preserved absolute polarity (ie, were non-inverting). Channel separation from all outputs was excellent, at >120dB in both directions below 2kHz, and still 108dB at 20kHz (not shown).

With oversampling switched off, the Xonar's impulse response with 44.1kHz data (fig.1) revealed the reconstruction filter to be a conventional linear-phase, half-band type, with time-symmetrical ringing before and after the impulse. Wideband analysis of 44.1kHz-sampled white noise at –4dBFS (fig.2, magenta and red traces) indicated a very steep rolloff above the audioband with the ultrasonic sampling image of a full-scale 19.1kHz tone at 25kHz suppressed by 100dB (cyan, blue). Harmonic distortion of the tone was low, with the third harmonic at 57kHz the highest in level, at –83dB (0.007%).

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Fig.1 ASUS Xonar Essence One Muses Edition, impulse response (4ms time window).

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Fig.2 ASUS Xonar Essence One Muses Edition, no oversampling, wideband spectrum of white noise at –4dBFS (left channel blue, right magenta) and 19.1kHz tone at 0dBFS (left cyan, right red), with data sampled at 44.1kHz (20dB/vertical div.).

Repeating this analysis with oversampling on gave a very different picture (fig.3). The white noise now starts to roll off above 14kHz, the 19.1kHz tone is suppressed by 46dB, and, most significant, the noise floor above the audioband rises, while that within the audioband drops slightly. The "oversampling" actually appears to be inband noiseshaping, which increases audioband resolution at the expense of resolution at ultrasonic frequencies. The residual effect of this noiseshaping can be seen by comparing fig.4, which shows the waveform of an undithered 16-bit/1kHz tone at exactly –90.31dBFS without oversampling, with fig.5, which shows what happens when oversampling is switched on. There is no increase in resolution with oversampling, the waveform remaining superbly symmetrical in both graphs, with the three DC voltage levels described by the data clearly evident. But with oversampling, the waveform is now overlaid with some noise of very high frequency. With 24-bit data, the waveform is a well-formed sinewave (fig.6) that, with oversampling, is again overlaid by high-frequency noise (fig.7).

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Fig.3 ASUS Xonar Essence One Muses Edition with oversampling, wideband spectrum of white noise at –4dBFS (left channel blue, right magenta) and 19.1kHz tone at 0dBFS (left cyan, right red), with data sampled at 44.1kHz (20dB/vertical div.).

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Fig.4 ASUS Xonar Essence One Muses Edition, no oversampling, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).

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Fig.5 ASUS Xonar Essence One Muses Edition with oversampling, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).

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Fig.6 ASUS Xonar Essence One Muses Edition no oversampling, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red).

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Fig.7 ASUS Xonar Essence One Muses Edition with oversampling, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red).

Fig.8 shows the Essence One's frequency response without oversampling with data sampled at 44.1kHz (green and gray traces), 96kHz (cyan, magenta), and 192kHz (blue, red). While the 96kHz sampling gives the correct bandwidth increase compared with 44.1kHz, the 192kHz traces exactly overlay the 96kHz traces, there not being the expected octave increase in bandwidth. With oversampling switched on (fig.9), the 44.1kHz traces now roll off sharply above 10kHz, and while the 192kHz traces now do extend higher in frequency than the 96kHz traces, this is because the latter's bandwidth has been reduced.

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Fig.8 ASUS Xonar Essence One Muses Edition, no oversampling, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel green, right gray), 96kHz (left cyan, right magenta), 192kHz (left blue, right red) (0.5dB/vertical div.).

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Fig.9 ASUS Xonar Essence One Muses Edition with oversampling, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel green, right gray), 96kHz (left cyan, right magenta), 192kHz (left blue, right red) (0.5dB/vertical div.).

The Xonar Essence One intrinsically offers high resolution, as shown in fig.10, which plots the spectra of a dithered 1kHz tone with 16-bit data (cyan and magenta traces) and 24-bit data (blue, red). With 16-bit data, the noise floor is actually the dither noise used to encode the signal. The increase in bit depth drops the noise floor by 20dB, suggesting that the Essence One has around 19 bits of effective resolution, which is excellent at its price. And while a power-supply–related spurious tone can be seen at 120Hz, this lies more than 140dB down from full level (0.00001%). I can safely predict that this will be completely inaudible to all listeners under all circumstances! Fig.10 was taken with S/PDIF data via TosLink without oversampling; repeating the analysis with USB data gave the same result, indicating that the Xonar's USB port does operate correctly with 24-bit data. Switching in oversampling (fig.11) didn't increase the level of the 24-bit noise floor below 10kHz, but did introduce some very low-level harmonic distortion.

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Fig.10 ASUS Xonar Essence One Muses Edition, no oversampling, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with: 16-bit data (left channel cyan, right magenta), 24-bit data (left blue, right red) (20dB/vertical div.).

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Fig.11 ASUS Xonar Essence One Muses Edition, with oversampling, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with 24-bit data (left channel blue, right red) (20dB/vertical div.).

At high levels, the Essence One featured very low distortion from its unbalanced and balanced outputs, even into a very low 600 ohm load (fig.12). The third harmonic at –96dB (0.0015%) was the highest in both channels with a full-scale 50Hz tone, but the second harmonic was almost as high in the right channel only (red trace). The third harmonic didn't rise with the headphone output driving 300 ohms at maximum level, but now the second harmonic was the highest in level, at a still-low –86dB (0.005%, fig.13). Intermodulation distortion without oversampling was very low (fig.14), though switching in oversampling with these 44.1kHz-sampled data (fig.15) rolled off the primary tones, as anticipated from fig.7. Comparing figs. 10 and 11, you can see that the inband noiseshaping with oversampling engaged drops the noise floor below 10kHz by 10dB.

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Fig.12 ASUS Xonar Essence One Muses Edition, balanced outputs, spectrum of 50Hz sinewave, DC–1kHz, at 0dBFS into 600 ohms (left channel blue, right red; linear frequency scale).

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Fig.13 ASUS Xonar Essence One Muses Edition, headphone outputs, spectrum of 50Hz sinewave, DC–1kHz, at 0dBFS into 300 ohms (left channel blue, right red; linear frequency scale).

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Fig.14 ASUS Xonar Essence One Muses Edition, no oversampling, balanced outputs, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

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Fig.15 ASUS Xonar Essence One Muses Edition with oversampling, balanced outputs, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

Fed, via TosLink, 16-bit S/PDIF data representing the Miller-Dunn J-Test signal, the Xonar DAC wasn't very effective at rejecting word-clock jitter. Fig.16 shows that there is some emphasis of the data-related sidebands at ±229Hz, as well as a pair of lower-level sidebands at ±120Hz. (Given the very low level of 120Hz in the analog out signal, these sidebands may well arise from some power-supply–related interference on the DAC chip's voltage-reference pin.) With USB data, the spectral spike that represents the high-level tone at one-quarter the sample rate acquires wide "skirts." Though these skirts, which suggest the presence of random low-frequency jitter, were reduced a little with oversampling (fig.17), the noise floor with USB data was not as clean as it had been with TosLink data.— John Atkinson

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Fig.16 ASUS Xonar Essence One Muses Edition, no oversampling, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit data via TosLink from AP SYS2722 (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

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Fig.17 ASUS Xonar Essence One Muses Edition with oversampling, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit data via USB from MacBook Pro (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

COMPANY INFO
ASUSTeK Computer Inc.
US distributor: ASUS Computer International
44370 Nobel Drive
Fremont, CA 94538
(812) 282-2787
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COMMENTS
rockoqatsi's picture

And in any case, I think it's great that you're reviewing these entries from ASUS (this, and the Xonar ST), though I'm surprised to find them releasing such an expensive and esoteric item—for a motherboard manufacturer at least. How good have dacs in the $700-$900 range gotten since the Lynx cards were a hot commodity? Any idea? (I'd try selling my Lynx for the Benchmark DAC2, but nobody wants a legacy PCI card these days.)

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