Sidebar 3: Measurements
I measured the AVM Ovation MP 8.2 with my Audio Precision SYS2722 system (see the January 2008 "As We See It"), using both the Audio Precision's optical and electrical digital outputs and USB data sourced from my MacBook Pro running on battery power with Pure Music 3.0 playing WAV and AIFF test-tone files. I measured the behavior with both the MP 8.2's Steep and Smooth reconstruction filters in "native" mode, the output level set to "fixed," and the USB port set to Type 2. Apple's USB Prober utility identified the AVM as "AVM Class 2 Audio" from "XMOS," and the serial number as "0 (none)." The USB port operated in the optimal isochronous asynchronous mode. Apple's AudioMIDI utility revealed that, via USB, the AVM accepted 16- or 24-bit integer data with sample rates from 44.1 to 384kHz and all stops in between. The optical inputs locked to datastreams with sample rates up to 96kHz, the AES/EBU and S/PDIF inputs to streams of up to 192kHz.
The maximum output level at 1kHz was 2.46V from both the balanced and single-ended outputs, which is 1.8dB higher than the Compact Disc standard's 2V. Both sets of outputs preserved absolute polarity (ie, were non-inverting), the XLR jacks being wired with pin 2 hot. The single-ended output impedance was a low 47 ohms at all audio frequencies, this doubling as expected from the balanced outputs. The CD transport demonstrated superb error correction, there being no glitches or dropouts in the player's output until the single gaps in the data spiral on the Pierre Verany Digital Test CD reached 2.5mm in length. (The CD standard, the so-called "Red Book," requires only that a player cope with gaps of up to 0.2mm.)
The Steep filter's impulse response (fig.1) indicates that it is a conventional finite impulse response (FIR) type, with the symmetrical ringing either side of the single sample at 0dBFS mapping the filter coefficients. The Smooth filter (fig.2) is also an FIR type, but is much shorter. As its name suggests, the Steep filter rolls off rapidly above half the sampling frequency when the MP 8.2 decodes white noise sampled at 44.1kHz,1 though it doesn't reach the full passband suppression until 60kHz and above (fig.3, magenta and red traces). Note that while the aliased image of a full-scale 19.1kHz tone at 25kHz (44,100–19,100Hz) is absent with this filter, the distortion harmonics of that tone are evident, the second and third lying at –68dB (cyan and blue traces). The Smooth filter behaves very differently (fig.4), with very little suppression of the image at 25kHz, some images appearing below 20kHz, and a slow rolloff above the audioband with white noise. This rolloff starts in the top audio octave, reaching –1.5dB at 20kHz (fig.5, gray and green traces). At higher sample rates, the rolloff continues smoothly until half of each sample rate, at which point the response drops rapidly. The blue and red traces in this graph were taken with data sampled at 192kHz. With 384kHz data (not shown), the response below 100kHz mirrors that with 192kHz data.
As suggested by figs. 3 and 4, visible distortion harmonics were present in the Ovation MP 8.2's output. With a full-scale 50Hz tone, the second and third harmonics lay close to –80dB (0.01%, fig.10), but commendably, this was not affected by reducing the load impedance to 600 ohms (fig.11). There was also more intermodulation distortion than I usually find using a full-scale mix of 19 and 20kHz tones. The spectrum with the Steep filter is shown in fig.12; the level of the intermodulation products was not affected by reducing the signal level, but neither did the levels rise with a 600 ohm load. The leaky nature of the Smooth filter is demonstrated by fig.13: while the intermodulation products are no higher in level than in fig.12, there are now a lot of aliasing images of the twin tones.
Footnote 1: My thanks to Jürgen Reis of MBL for suggesting this test to me.
Fig.1 AVM Ovation MP 8.2, Steep filter, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).
Fig.2 AVM Ovation MP 8.2, Smooth filter, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).
Fig.3 AVM Ovation MP 8.2, Steep filter, 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.).
Fig.4 AVM Ovation MP 8.2, Smooth filter, 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.).
Fig.5 AVM Ovation MP 8.2, Smooth filter, 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.).
Channel separation was >100dB in both directions below 1kHz, but decreased steadily above that frequency, reaching 75dB at 20kHz (fig.6). This was most likely due to capacitive coupling between the channels, perhaps in a shared tube. The MP 8.2's noise floor was free from power-supply–related artifacts, but there was a little more noise present than in the quietest digital products. When I increased the bit depth from 16 to 24 with a dithered 1kHz tone at –90dBFS (fig.7), the noise floor dropped by around 13dB, meaning that the AVM offers a little more than 18 bits' worth of resolution. Nevertheless, with undithered data representing a tone at exactly –90.31dBFS (fig.8), the three DC voltage levels described by the data were well resolved. With undithered 24-bit data the result was a slightly noisy but otherwise well-formed sinewave (fig.9).
Fig.6 AVM Ovation MP 8.2, channel separation.
Fig.7 AVM Ovation MP 8.2, 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.).
Fig.8 AVM Ovation MP 8.2, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit TosLink data (left channel blue, right red).
Fig.9 AVM Ovation MP 8.2, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit TosLink data (left channel blue, right red).
Fig.10 AVM Ovation MP 8.2, Steep filter, spectrum of 50Hz sinewave, DC–1kHz, at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.11 AVM Ovation MP 8.2, Steep filter, spectrum of 50Hz sinewave, DC–1kHz, at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.12 AVM Ovation MP 8.2, Steep filter, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 100k ohms, 44.1kHz data (left channel blue, right red; linear frequency scale).
Fig.13 AVM Ovation MP 8.2, Smooth filter, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 600 ohms, 44.1kHz data (left channel blue, right red; linear frequency scale).
Finally, the MP 8.2 offered excellent rejection of word-clock jitter. All the odd-order harmonics of the J-Test signal's low-frequency, LSB-level squarewave are at the correct levels, as shown by the sloping green line in fig.14, and no other sideband pairs are visible. This graph was taken with AES/EBU data; there was very slightly more noise with both CD and USB data, but the results were still excellent in absolute terms, as is shown by repeating the test with 24-bit data (fig.15).
Fig.14 AVM Ovation MP 8.2, Steep filter, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit AES/EBU data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
Fig.15 AVM Ovation MP 8.2, Steep filter, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit AES/EBU data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
AVM's Ovation MP 8.2 offers generally excellent measured performance. I suspect that the distortion and channel-separation results are due to the use of tubes in the analog circuitry.—John Atkinson
Footnote 1: My thanks to Jürgen Reis of MBL for suggesting this test to me.
