Sidebar 3: Measurements
I measured the Hegel Viking using my Audio Precision SYS2722 system. As this player doesn't have digital inputs, I used test signals that I burned on a CD-R. I first used the Pierre Verany Digital Test CD to check the Viking's error correction. It played the tracks with gaps in the data spiral up to 1mm in length without any problems, but there were audible glitches when the gap was longer than 1mm or when there were two 0.5mm gaps in succession. The Compact Disc standard, the so-called "Red Book," requires that a player cope with gaps of only up to 0.2mm, and the Hegel exceeds that standard. But the Hegel's error correction is not as good as the best players or transports that I have measured in recent years.
Fig.1 Hegel Viking, eye pattern of coaxial S/PDIF output carrying 16-bit, 44.1kHz J-Test data (±400mV vertical scale, 175ns horizontal scale).
As the Hegel Viking has a coaxial digital output, to allow it to be used as a CD transport with a separate D/A processor, I examined the quality of that output. Fig.1 was taken from the digital output with a CD playing J-Test data plotted over one "unit cycle." The eye pattern is wide open, with almost no blurring of the leading and trailing edges. The average jitter level, assessed with a 50Hz–100kHz bandwidth, was low, at 389 picoseconds (ps) compared with 340.5ps when I looped the Audio Precision SYS2722's S/PDIF output to its coaxial input. Peculiarly, the Audio Precision's "Active Bits" monitor indicated that, in addition to the expected 16 most-significant bits, the 24th or least-significant bit was active in the Hegel's output datastream.
Fig.2 Hegel Viking, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).
The Viking's single-ended output impedance was a low 22 ohms from 20Hz to 20kHz; the balanced impedance was a still-low 44 ohms, again across the audioband. A 1kHz signal at 0dBFS resulted in an output level of 2.51V, which is 1.9dB higher than the CD Standard's recommended maximum level of 2V. The Hegel's impulse response (fig.2) indicates that the output preserved absolute polarity from both types of analog output and that its reconstruction filter is a minimum-phase type, with all the ringing following the single sample at 0dBFS.
Fig.3 Hegel Viking, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan), with data sampled at 44.1kHz (20dB/vertical div.).
With white noise at –4dBFS (fig.3, red and magenta traces), the Viking's response was flat in the audioband with the level at 20kHz dropping by just 0.23dB compared with that at 20Hz and 1kHz. The player's output rolled off sharply above 20kHz, reaching full stop-band suppression at 24kHz, just above the Nyquist frequency of 22.05kHz (green vertical line). An aliased image at 25kHz of a full-scale tone at 19.1kHz (blue and cyan traces) lies at –101dB (0.0009%), and the distortion harmonics of the 19.1kHz tone all lie below –100dB (0.001%).
Fig.4 Hegel Viking, spectrum of 1kHz sinewave, DC– 1kHz, at 0dBFS (left channel green, right gray) and at –3dBFS (left blue, right red) (linear frequency scale).
Fig.5 Hegel Viking, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with 16-bit data (left channel blue, right red) and with "digital black" (left green, right gray) (20dB/vertical div.).
Channel separation (not shown) was excellent, at 98dB in both directions from 20Hz to 20kHz. With data representing a 1kHz tone at 0dBFS (fig.4, green and gray traces), the random low-frequency noisefloor was low in level, but spuriae of unknown origin were present at 100Hz and its harmonics. These spuriae disappeared when I repeated the spectral analysis with the same 1kHz tone at –3dBFS (blue and red traces), but now the random noise rose by 10dB. The noisefloor in a spectrum taken with a dithered tone at –90dBFS (fig.5, blue and red traces) is due to the dither used to encode the data. Repeating the spectral analysis with "digital black" (green and gray traces) indicates that the Viking's analog noisefloor is almost 20dB lower than that of dithered 16-bit data.
Fig.6 Hegel Viking, waveform of undithered 16-bit, 1kHz sinewave at –90.31dBFS (left channel blue, right red).
With undithered 16-bit data representing a tone at exactly –90.31dBFS, the three DC voltage levels described by the data were well resolved (fig.6) and the waveform was perfectly symmetrical. An inconsequential 25µV positive DC offset is present in the right channel (red trace).
Fig.7 Hegel Viking, spectrum of 50Hz sinewave at 0dBFS, DC–1kHz, into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.8 Hegel Viking, spectrum of 50Hz sinewave at –3dBFS, DC–1kHz, into 100k ohms (left channel blue, right red; linear frequency scale).
As seen in fig.3, the Hegel player featured very low levels of harmonic distortion. With the Viking driving a full-scale 50Hz tone into 100k ohms, the third harmonic was the highest in level (fig.7), but this lay at just –110dB (0.0003%). While higher-frequency harmonics can be seen in this graph, these were all close to –120dB (0.0001%). Commendably, the levels of the distortion harmonics didn't rise when I reduced the load to the current-hungry 600 ohms. With a 50Hz tone at –3dBFS (fig.8), the high-order harmonics disappeared and the third harmonic dropped by 10dB in the left channel (blue trace) and by 17dB in the right channel (red trace).
Fig.9 Hegel Viking, HF intermodulation spectrum (DC–30kHz), 19+20kHz at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.10 Hegel Viking, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit CD data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
Intermodulation distortion with a mix of equal levels of 19 and 20kHz tones was vanishingly low in level (fig.9), though the 100Hz-spaced spuriae seen in fig.4 were present. I tested the Viking's rejection of word-clock jitter with the undithered Miller-Dunn J-Test signal (a high-level tone at one-quarter the sample rate over which is overlaid the least-significant bit toggled on and off at a frequency equivalent to the sample rate divided by 192). The Hegel reproduced the odd-order harmonics of the LSB-level, low-frequency squarewave at the correct levels (fig.10, sloping green line), and no other sidebands were present.
The Hegel Viking offers generally excellent measured performance. The appearance of spurious tones with full-scale data is unusual, but these tones are all very low in level and, as continuous tones at 0dBFS are almost never found in real music, this behavior won't have subjective consequences.—John Atkinson
