Channel Islands Audio Transient Mk.II & VDC•5 Mk.II USB D/A processor & power supply 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 Channel Islands Audio Transient Mk.II's measured behavior. I used my 2012 Apple MacBook Pro running on battery power to examine the processor's performance via its USB port. For all tests, the Transient was powered from its VDC•5 Mk.II supply.

Apple's USB prober utility identified the Transient as the "CIAudio USB Audio 2.0" and confirmed that it operated in the optimal isochronous asynchronous mode with 24-bit resolution. The Transient worked with data sampled at 44.1kHz up to 192kHz, and made all stops in between. The maximum output level with the DAC's volume control set to its maximum (top blue LED lit) was 2.07V and the output preserved absolute polarity (ie, was non-inverting). The output impedance was a moderately low 551 ohms at low and middle frequencies, dropping slightly to 536 ohms at the top of the audioband.

Fig.1 shows the Transient's ultrasonic behavior with two different signals: white noise at –4dBFS, and a 19.1kHz tone at 0dBFS, both sampled at 44.1kHz. (This test was suggested to me by Jürgen Reis of MBL.) The cyan and blue traces show the spectrum of the DAC's analog output with the 19.1kHz tone: the aliasing product at 25kHz (44,100–19,100) can be seen to be suppressed by 60dB, and while all the distortion harmonics lie at –92dB and lower (<0.003%), there is a tone present at 69kHz. However, this does lie at –70dB, and so will be inconsequential.

314CIATfig01.jpg

Fig.1 CIAudio Transient Mk.II, 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.).

The magenta and red traces in fig.1 show the spectrum with white noise. The rolloff above 21kHz is a little slower than is usually found, and there is a striking series of low-level peaks in the reconstruction filter's ultrasonic stopband. This behavior is very similar to "Filter 1" in the data sheet for the Wolfson WM8742, which other reviews have suggested is the DAC chip used in the Transient. However, it also resembles the behavior in the data sheet for the filter in Wolfson's WM8523 DAC/line-driver chip. When I inspected the circuit, this chip appeared to be connected directly to the Transient's analog output jacks—I couldn't see a WM8742, but some of the circuit board was inaccessible—and, like the WM8742, the WM8523 offers a digital volume control. On the other hand, the Transient's impulse response (fig.2) reveals the reconstruction filter to be an asymmetrical, non-halfband type, which is one of the options offered by the WM8742 but not, according to its data sheet, by the WM8523. Perhaps CIAudio's Dusty Vawter can clear up this matter in his "Manufacturer's Comment."

314CIATfig02.jpg

Fig.2 CIAudio Transient Mk.II, impulse response (4ms time window).

Fig.3 shows the frequency response of the Transient plotted with sample rates of 44.1, 96, and 192kHz. The rolloff is very similar at the two higher rates, the only difference being the sharp drop-off above 47kHz with 96kHz-sampled data (cyan and magenta traces). The DAC behaves a little differently with 44.1kHz data (green and gray), there being a small ripple in the top audio octave. Channel separation (not shown) was good, at 74dB in both directions from 20Hz to 20kHz.

314CIATfig03.jpg

Fig.3 CIAudio Transient Mk.II, 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) (1dB/vertical div.).

With both 16- and 24-bit dithered data representing a 1kHz tone at –90dBFS (fig.4), the second harmonic at 2kHz was visible 20dB below the level of the primary component. Even-order distortion correlates with waveform asymmetry; if you look at figs. 5 and 6, which show the waveforms of undithered data at –90.31dBFS, you can see some asymmetry: in each graph, the top half of the waveform is smaller than the bottom half. There is also more analog noise in these graphs than I usually find with high-performance D/A processors; looking again at fig.4, you can see that the increase in bit depth has dropped the noise floor by only 8dB or so. This implies that the Transient offers resolution of just over 17 bits, even with 24-bit data.

314CIATfig04.jpg

Fig.4 CIAudio Transient Mk.II, 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.).

314CIATfig05.jpg

Fig.5 CIAudio Transient Mk.II, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).

314CIATfig06.jpg

Fig.6 CIAudio Transient Mk.II, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red).

At high levels, the Transient Mk.II offers low levels of harmonic distortion, with the subjectively benign second harmonic the highest in level, at –90dB (fig.7). The third and fifth harmonics lie around 6dB lower in level than the second, but, commendably, none of the distortion harmonics increased in level when I reduced the load impedance to a punishing 600 ohms. Intermodulation distortion was also very low (fig.8), though the "leaky" reconstruction filter allows the ultrasonic images of the primary tones to be seen in this graph. Again, this spectrum didn't change into a 600 ohm load.

314CIATfig07.jpg

Fig.7 CIAudio Transient Mk.II, spectrum of 50Hz sinewave, DC–1kHz, at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

314CIATfig08.jpg

Fig.8 CIAudio Transient Mk.II, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

When I examined the Transient's rejection of jitter, I got anomalous results. Though the Miller-Dunn J-Test signal, which comprises a high-level tone at exactly ¼ the sample rate and an LSB-level squarewave at 1?192 the sample rate, is not formally diagnostic for the streaming of audio data via USB, I have nevertheless found it a useful test signal, as it can reveal problems other than word-clock jitter. Fig.9 shows a narrowband spectrum of the Transient's output while it decoded 16-bit J-Test data sampled at 44.1kHz. The central spike that represents the 11.025kHz tone is well defined, with no "skirts" that might result from random low-frequency variation in the clock frequency. And while a pair of low-frequency sidebands can be seen, these are at –125dB and presumably innocuous. However, what is missing in this graph are almost all of the odd-order harmonics of the low-frequency squarewave, which should slowly decrease from –124dB at the left of the graph to –129dB at the right.

314CIATfig09.jpg

Fig.9 CIAudio Transient Mk.II, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit data via USB from MacBook Pro (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

These harmonics are apparent in a digital-domain spectral analysis of the same J-Test data output from the Transient's S/PDIF port (fig.10), so these low-level data are being lost somewhere in the Transient's D/A circuitry. (The DAC's volume control was set to its maximum for these tests.) And by comparing figs. 9 and 10, you can see that, via the analog outputs, the pair of sidebands at ±229.6875Hz is being accentuated by almost 10dB.

314CIATfig10.jpg

Fig.10 CIAudio Transient Mk.II, high-resolution jitter spectrum of digital S/PDIF output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit data via USB from MacBook Pro (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

The Channel Islands Audio Transient Mk.II's measured performance was a little disappointing, though there is no reason why it shouldn't sound acceptably good with CD-resolution recordings. As Jon Iverson noted, it "holds its own comfortably" in its price class.—John Atkinson

COMPANY INFO
Channel Islands Audio
567 W. Channel Islands Boulevard, PMB 300
Port Hueneme, CA 93041
(805) 984-8282
ARTICLE CONTENTS
Share | |

X
Enter your Stereophile.com username.
Enter the password that accompanies your username.
Loading