Slim Devices Transporter network music player Measurements

Sidebar 3: Measurements

All the measurements were performed on the replacement Transporter with its volume control set to its maximum and its internal attenuation set to 0dB. The source signals for almost all the measurements were 16-bit and 24-bit data files accessed from a Mac mini running the necessary SlimServer software via my WiFi network. The 16-bit files were Apple Lossless, the 24-bit files were AIF. Some measurements were repeated using a TosLink optical connection from a PC fitted with an RME soundcard. The Transporter successfully handled files with 44.1kHz, 48kHz, and 96kHz sample rates—but not 88.2kHz, which I felt was a shortfall, as almost all of my own hi-rez files are recorded at that rate. It did play 24-bit AIF files, but while SlimServer would recognize 24-bit WAV files stored as 32-bit data words, the Transporter would not play them.

The maximum output levels at 1kHz were to specification at 3.11V RMS (balanced) and 2.07V RMS (unbalanced), and both outputs preserved absolute polarity; ie, were non-inverting. The source impedance from the unbalanced RCA jacks was a low 100 ohms across the audioband, and exactly twice that from the balanced XLRs. The cute VU-style meters registered a sinewave at –8dBFS as "0dB" and had a very short attack time.

The Transporter's frequency response at –12dBFS is shown in fig.1, taken with both 44.1kHz and 96kHz data from the balanced outputs (the unbalanced output response was identical). The high sample-rate response rolls off rapidly above 40kHz, but is flat almost to that frequency rather than continuing the slight rolloff in the top audio octave with CD data. Channel separation was superb, with any crosstalk buried in the noise floor: below 3kHz for the unbalanced jacks, and below 10kHz for the balanced jacks (fig.2).

Fig.1 Slim Devices Transporter, balanced frequency response at –12dBFS into 100k ohms at 44.1kHz and 96kHz sample rates (right channel dashed, 1dB/vertical div.).

Fig.2 Slim Devices Transporter, channel separation (from top to bottom): R–L, L–R, unbalanced; R–L, L–R, balanced (10dB/vertical div.).

That the Transporter offers superb resolution can be seen in fig.3, which shows the 1/3-octave spectra of the player's balanced output while it decoded dithered WiFi data representing a 16-bit/1kHz sinewave at –90dBFS, 24-bit data representing the same signal, and 24-bit data representing a 1kHz tone at –120dBFS. With 16-bit data (top pair of traces), the spectrum's noise floor is free from power-supply and harmonic spuriae, and the trace shows only the recorded dither noise. Increasing the word length to 24 bits drops the level of the noise by an astonishing 21dB, suggesting that the Transporter has almost 20-bit resolution! The –120dBFS tone is clearly resolved (bottom traces), and the trace is free from any distortion harmonics. Though a power-supply component at 120Hz appears in the 24-bit traces at –136dB, it won't bother anyone. Extending the measurement bandwidth to 200kHz and driving the Transporter with 16-bit data representing a –1LSB DC offset (not shown) reveals a modest amount of ultrasonic noise from the DAC's noiseshaping.

Fig.3 Slim Devices Transporter, 1/3-octave spectrum with noise and spuriae of (from top to bottom at 3kHz): dithered 1kHz tone at –90dBFS, 16-bit data; dithered 1kHz tone at –90dBFS, 24-bit data; dithered 1kHz tone at –120dBFS, 24-bit data. (Right channel dashed.)

The plot of the Transporter's linearity error (fig.4) is dominated by the test signal's dither. With the Transporter fed undithered data representing a 1kHz sinewave at exactly –90.31dBFS, the signal's three DC voltage levels are clearly defined and free from any DC offset (fig.5). Increasing the bit depth to 24 gave a good facsimile of a sinewave, even at this very low level (fig.6).

Fig.4 Slim Devices Transporter, left-channel departure from linearity, 16-bit data (2dB/vertical div.).

Fig.5 Slim Devices Transporter, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data.

Fig.6 Slim Devices Transporter, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data.

Not only did the Transporter offer superb measured performance in the digital domain, it also did so in the analog domain. Fig.7, for example, shows an FFT-derived spectrum of the player's unbalanced output reproducing a full-scale 24-bit sinewave. The THD (true sum of the harmonics) was just 0.0005% (left) and 0.0008% (right), with the third harmonic the highest in level in the left channel, at just –108.4dB. The right channel had equal amounts of second and third harmonic, still at a vanishingly low level of –105.6dB. These spectra were taken into 8k ohms, about the lowest impedance the Transporter will see in real life. Dropping the load impedance to a punishing 600 ohms increased the third harmonic, but at –94dB (0.002%), this is still negligible .

Fig.7 Slim Devices Transporter, unbalanced spectrum of 1kHz sinewave at 0dBFS into 8k ohms (linear frequency scale).

Repeating the test with 24-bit data representing a 1kHz tone at –90dBFS gave the spectrum shown in fig.8: all the distortion harmonics are buried in the noise at –128dB, which is very low. (This is an FFT-derived spectrum plotted on a linear frequency scale, which is why the noise floor is flat compared with the middle traces in fig.3, which were taken with a 1/3-octave bandpass filter swept logarithmically with frequency.) Intermodulation distortion was also vanishingly low (fig.9).

Fig.8 Slim Devices Transporter, unbalanced spectrum of 1kHz sinewave at –90dBFS into 8k ohms (linear frequency scale).

Fig.9 Slim Devices Transporter, unbalanced HF intermodulation spectrum, 19+20kHz at 0dBFS peak into 8k ohms (linear frequency scale).

I tested the Transporter's rejection of word-clock jitter using the Miller Jitter Analyzer, which examines a narrowband, FFT-derived spectrum of the analog output of the device under test (DUT) for pairs of sidebands around a high-level tone at one quarter the sample rate, while the LSB is toggled on and off at 1/128 the sample rate. (Both signals are exact integer fractions of the sample rate, meaning that any spuriae that appear in the spectrum are due to the behavior of the DUT, not to quantizing distortion.) Fed 24-bit data via the WiFi network, the Transporter developed just 235 picoseconds peak–peak of jitter with no data-related components (not shown). Decreasing the word length to 16 bits gave the spectrum shown in fig.10. Here the jitter level has increased slightly, to 268ps p–p; though there are data-related sidebands (red numeric markers) at the test signal's residual level. The primary jitter components lie at ±15.6Hz (purple "1") and ±1435Hz (purple "10"), but this is still excellent performance.

Fig.10 Slim Devices Transporter, high-resolution jitter spectrum of analog output signal (11.025kHz at –6dBFS sampled at 44.1kHz with LSB toggled at 229Hz), 16-bit WiFi data. Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

Repeating the measurement with 16-bit data fed to the Transporter via an optical TosLink S/PDIF connection increased the jitter to 313ps p–p (not shown), mainly due to rises in the level of the sidebands at the primary data-related frequency of ±230Hz and the levels of the ±1435Hz sidebands.

Despite it receiving data via a WiFi link, and despite its relatively affordable price, Slim Devices' Transporter offers state-of-the-art D/A converter performance. To judge from the Transporter's measurements, Silicon Valley is fast becoming a center of high-end audio design.—John Atkinson

Slim Devices
455 National Avenue
Mountain View, CA 94043
(650) 210-9400
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