Sidebar 3: Measurements
I performed a full set of measurements on the Ideon eos using my Audio Precision SYS2722 system, then checked some of the test results with the magazine's higher-resolution APx555 analyzer. The coaxial S/PDIF inputs accepted data sampled at rates up to 192kHz, while Apple's AudioMIDI utility revealed that the USB port accepted 16-, 24-, and 32-bit integer data sampled at all rates from 44.1kHz to 768kHz. The USB Prober app identified the Processor as "Ideon USB Audio" from "IDEON AUDIO" and indicated that the USB port operated in the optimal isochronous asynchronous mode.
The Ideon eos preserved absolute polarity from the balanced and unbalanced outputs. The processor's output levels with a full-scale 1kHz signal and the Output Level set to High via the switch on the rear panel were 9.24V balanced and 4.62V unbalanced. The maximum output levels with the switch set to Low were 6dB lower, at 4.63V balanced and 3.315V unbalanced. Except where noted, I performed all the measurements with the Output Level set to High. The output impedances were commendably low, at 20 ohms from the balanced outputs and 10 ohms from the single-ended outputs, both values consistent from 20Hz to 20kHz.
Footnote 1: See, for example, fig.4 here.
Fig.1 Ideon eos, impulse response (one sample at 0dBFS, 44.1kHz data, 4ms time window).
Fig.2 Ideon eos, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan) into 100k ohms with data sampled at 44.1kHz (20dB/vertical div.).
Fig.1 shows the Ideon eos's impulse response with data sampled at 44.1kHz. It is the familiar hybrid type seen in our reviews of digital processors that use the ESS Sabre chip set (footnote 1). The magenta and red traces in fig.2 show the Ideon's wideband spectrum with 44.1kHz white noise data at –4dBFS. The response rolls off sharply above the audioband, with full stopband attenuation reached at exactly half the sample rate. The reconstruction filter is therefore an apodizing type, and the image at 25kHz of a full-scale 19.1kHz tone (cyan, blue traces) is completely suppressed.
Fig.3 Ideon eos, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel green, right gray), 96kHz (left cyan, right magenta), and 192kHz (left blue, right red) (1dB/vertical div.).
Fig.4 Ideon eos, spectrum of 24-bit 1kHz tone at 0dBFS, DC–1kHz with Output Level set to High (left channel blue, right red) and to Low (left green, right gray) (20dB/vertical div.).
The Ideon eos's frequency response with 44.1kHz, 96kHz, and 192kHz data (fig.3) is flat in the audioband, with a small amount of passband ripple in the top octave. It follows the same basic shape at all three sample rates, with a very sharp rolloff just below half of each rate. With 44.1kHz data (green and gray traces), the response starts to roll off above 17kHz and is down by 16dB at 20kHz. The frequency response was identical from both output types and was not affected by the Output Level switch. Channel separation was superb, at >115dB in both directions below 1kHz and still 106dB at the top of the audioband. The low-frequency noisefloor was very low in level and free from power supply–related spuriae (fig.4). The blue and red traces in this graph were taken with the Output Level set to High. The noisefloor remained at the same level with the Output Level set to Low (green, gray traces), but, as the signal is now 6dB lower (as can be seen at the top right of the graph), the noisefloor was effectively 6dB higher.
Fig.5 Ideon eos, left channel, 1kHz output level vs 24-bit data level in dBFS (blue, 20dB/vertical div.); linearity error (red, 1dB/small vertical div.).
Fig.6 Ideon eos, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with 16-bit data (left channel green, right gray) and 24-bit data (left blue, right red) with Output Level set to High (20dB/vertical div.).
Fig.7 Ideon eos, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).
The red trace in fig.5 plots the error in the analog output level as a 24-bit, 1kHz digital tone stepped down from 0dBFS to –140dBFS. The amplitude error is negligible down to –110dBFS but rapidly increases below that level, due to the presence of random noise. The Output Level was set to High for this graph, as it was for fig.6, which shows spectra with 16- and 24-bit dithered coaxial data representing a 1kHz tone at –90dBFS. (The spectra were identical with 16- and 24-bit USB data.) The increase in bit depth lowered the noisefloor in fig.6 by 12dB, which suggests a measured resolution of 18 bits. Setting the Output Level to Low reduced the measured resolution by 6dB or 1 bit. With undithered data representing a tone at exactly –90.31dBFS, which consists of data at –1LSB, digital zero, and +1LSB, the waveform was symmetrical, but the three DC voltage levels described by the data were obscured by noise (fig.7).
Fig.8 Ideon eos, spectrum of 24-bit 50Hz sinewave, DC–1kHz, at 0dBFS into 200k ohms (left channel blue, right red, linear frequency scale).
Fig.9 Ideon eos, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 600 ohms, 24-bit, 96kHz data (left channel blue, right red; linear frequency scale).
The Ideon eos's distortion signature primarily comprised the second and third harmonics (fig.8). These were very low in level, lying below –110dB (0.0003%) with a 50Hz signal at 0dBFS. Intermodulation distortion with 24-bit data representing an equal mix of 19 and 20kHz tones, each at –6dBFS, was also extremely low, even into 600 ohms (fig.9). (Note that I used data sampled at 96kHz for this measurement due to the early rolloff above 16kHz with 44.1kHz data seen in fig.3.)
Fig.10 Ideon eos, 16-bit coaxial data, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
The Ideon eos was immune to jitter with both its coaxial and USB inputs. Fig.10 shows the spectrum of its output when it was fed 16-bit coaxial J-Test data. The odd-order harmonics of the undithered low-frequency, LSB-level squarewave all lie at the correct levels, though the noisefloor between those harmonics is higher in level than I find with other high-performance D/A processors.
Other than the different reconstruction filter, the Ideon eos's measured performance is similar to that of the Ideon Ayazi mk2 processor that Alex Halberstadt reviewed in August 2022, though with better rejection of jitter and an even lower level of intermodulation distortion.—John Atkinson
Footnote 1: See, for example, fig.4 here.
