Sidebar 3: Measurements
I measured the Mytek Liberty with my Audio Precision SYS2722 system (see the January 2008 "As We See It"), using both the Audio Precision's S/PDIF 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. Apple's USB Prober utility identified the DAC SE as "Liberty DAC" from "Mytek Digital," its serial number as "03003-1801-168," and confirmed that its USB port operated in the optimal isochronous asynchronous mode. Apple's AudioMIDI utility revealed that, via USB, the Mytek Liberty accepted 24-bit integer data sampled at all rates up to 384kHz, and that its AES3, TosLink, and coaxial S/PDIF inputs accepted PCM datastreams with sample rates up to 96kHz. With data sampled at 192kHz, however, the outputs muted from all but the USB input.
The Mytek Liberty's maximum output level at 1kHz from the balanced TRS, unbalanced RCA, and headphone outputs was 2.5V, which is 1.9dB higher than the CD standard's 2V. The output impedance was 75 ohms from the line-level outputs at all audio frequencies, but too low to measure accurately from the front-panel headphone jack.
The outputs inverted absolute polarity, and the Liberty's impulse response with 44.1kHz data was a short, minimum-phase inverted pulse (fig.1). With white noise sampled at 44.1kHz (fig.2, magenta and red traces, footnote 1) there was a slow rolloff above the audioband, this typical of the MQA upsampling filter that is applied to PCM data. Consequently, the aliased image of a 19.1kHz tone at –3dBFS (cyan and blue traces) was suppressed by 20dB. I usually use a full-scale 19.1kHz tone for this test, but there was a lot of aliased image energy in the audioband with this signal. However, harmonic distortion is very low in this graph.
Footnote 1: My thanks to Jürgen Reis of MBL for suggesting this test to me.
Fig.1 Mytek Liberty, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).
Fig.2 Mytek Liberty, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at –3dBFS (left blue, right cyan), with data sampled at 44.1kHz (20dB/vertical div.).
Fig.3 shows the Mytek's frequency response at its unbalanced outputs at sample rates of 44.1, 96, 192, and 384kHz. (The headphone-output responses were identical.) The MQA upsampling filter rolls off relatively slowly above each Nyquist frequency (half the sample rate). The Liberty's channel separation (not shown) was excellent at >120dB below 1kHz, and still 110dB L–R and 97dB R–L at 1kHz. The low-frequency noise floor was commendably free from power-supply–related spuriae (fig.4).
Fig.3 Mytek Liberty, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel black, right yellow), 96kHz (left gray, right green), 192kHz (left cyan, right magenta), 384kHz (left blue, right red) (1dB/vertical div.).
Fig.4 Mytek Liberty, spectrum (0Hz–1kHz) of dithered 1kHz tone at 0dBFS (20dB/vertical div.).
Increasing the bit depth from 16 to 24 with a dithered 1kHz tone at –90dBFS lowered the noise floor by 15dB or so (fig.5). This suggests resolution of almost 19 bits. With undithered data representing a tone at exactly –90.31dBFS (fig.6), the three DC voltage levels described by the data were well resolved, with a symmetrical waveform and no DC offset. With undithered 24-bit data, the result was a clean sinewave (fig.7).
Fig.5 Mytek Liberty, 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.6 Mytek Liberty, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit TosLink data (left channel blue, right red).
Fig.7 Mytek Liberty, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit TosLink data (left channel blue, right red).
As revealed in fig.2, harmonic distortion was very low. With a full-scale 50Hz tone into the punishing 600 ohm load, the third harmonic was the highest in level (fig.8), but this was at a negligible –106dB (0.0005%)! The poor ultrasonic rejection visible in fig.2 resulted in a multitude of aliased images in the audioband with a full-scale mix of 19 and 20kHz tones (fig.9). However, reducing the signal level by 3dB produced a significant reduction in the audioband image energy (fig.10). The actual intermodulation distortion is extremely low, the 1kHz difference product, even into 600 ohms, lying at around –120dB (0.0001%). Music rarely has significant energy toward the top of the audioband, so fig.10 more correctly characterizes the Liberty's behavior.
Fig.8 Mytek Liberty, spectrum of 50Hz sinewave, DC–1kHz, at 0dBFS into 600 ohms (left channel blue, right red; linear frequency scale).
Fig.9 Mytek Liberty, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 100k ohms, 44.1kHz data (left channel blue, right red; linear frequency scale).
Fig.10 Mytek Liberty, HF intermodulation spectrum, DC–30kHz, 19+20kHz at –3dBFS into 100k ohms, 44.1kHz data (left channel blue, right red; linear frequency scale).
When I tested the Mytek Liberty for its rejection of word-clock jitter using undithered 16-bit J-Test data, the odd-order harmonics of the low-frequency, LSB-level squarewave were all at the correct levels (fig.11, sloping green line). This excellent behavior was identical when I repeated the test using the USB input—and when I tested with 24-bit J-Test data, the spectrum was completely free from any spurious tones or sidebands (fig.12).
Fig.11 Mytek Liberty, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit S/PDIF data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
Fig.12 Mytek Liberty, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit USB data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
The Mytek Liberty's measured performance indicates excellent digital and analog engineering, especially considering its affordable price.—John Atkinson
Footnote 1: My thanks to Jürgen Reis of MBL for suggesting this test to me.
