Digital Processor Reviews

I measured the behavior of the Empirical Audio Off-Ramp 4 using the Audio Precision SYS2722 system (see www.ap.com and "As We See It" in the January 2008 issue). I installed the correct version of the driver program for the Snow Leopard operating system, and played test tones at various sample rates and bit depths from BIAS Peak 7 on an Intel MacBook running OS X 10.6.8. The Macintosh USB Prober utility reported both the manufacturer and product strings as "Empirical Audio Async 192." The interface was identified as "vendor-specific," with no information offered about the number of operating mode, channels, bit depth, or compatible sample rates, presumably because these are all aspects of the driver program.

The Off-Ramp 4 operated at sample rates of 44.1, 48, 88.2, 96, 176.4, and 192kHz, provided the sample rate had been set with Audio MIDI Setup on the Mac and its output correctly followed the bit depth of the file being played, 16 or 24. Most important, the output bits were the same as those in the file.

Fig.1 shows the eye pattern of the AES/EBU output, the Off-Ramp 4 driving the digital input of the SYS2722 with the 1.5m length of Kimber Orchid cable I used for my auditioning. The eye is wide open, with no blurring at the ends that would indicate the presence of significant amounts of jitter. The Audio Precision calculated the level of jitter to be just 343 picoseconds peak, over a 50Hz–100kHz bandwidth. For reference, the BNC version of the Halide Bridge's S/PDIF output produced 440ps peak measured over the same bandwidth (footnote 1); the Musical Fidelity V-Link's output measured 395ps peak. This measurement was with the AudioQuest Coffee USB cable and the Off-Ramp powered from the Monolith's battery; the jitter number didn't change when I switched to the wall-wart supply or to a generic USB cable.

Fig.2 shows spectral analyses up to 20kHz of the timing uncertainty in the Off-Ramp 4's AES/EBU output (red trace) and the BNC Halide Bridge's S/PDIF output (blue), both performed in the digital domain while the datalinks were carrying 16-bit J-Test data. While the absolute levels of the spectral components are all low, at 20ps or below, the spectra are very different. Though it has a relatively strong component just below 19kHz, the Halide's spectrum is very clean. If you look closely at the blue trace, you can see very low-level components at 10,796 and 11,254Hz, these equivalent to the frequency of the Fs/4 J-Test tone ± the frequency of the Fs/192 LSB-level squarewave. These are both accentuated with the Off-Ramp 4, with higher-order components visible (red trace). When the AES/EBU datastream is fed to a D/A converter, these higher-frequency timing variations will be reduced in level by the low-pass filter action of the DAC's Phased-Locked Loop receiver circuit. I have shown them here to indicate that the Off-Ramp's jitter signature is very different from the Halide's as well as from the Musical Fidelity V-Link's, which was noisier across the spectrum (not shown). Interestingly, the Off-Ramp's spectrum was slightly cleaner at higher frequencies with the generic USB cable than with the AudioQuest Coffee.

As in previous reviews of USB converters, I examined how the Off-Ramp 4 affected the analog output of the D/A converters with which it was used (footnote 2), in this case the Logitech Transporter and dCS Debussy. Starting with the Transporter, fig.3 shows the spectrum of its output fed a 24-bit version of the J-Test signal from the Off-Ramp 4 via the 1.5m length of Kimber Orchid AES/EBU cable. The floor is relatively noisy, and while the central spike that represents the 11.025kHz tone is sharply defined, sidebands are visible at ±229Hz and its harmonics. These shouldn't be there; obviously, there is something in the Transporter's receiver circuitry that is sensitive to datastream jitter. This graph was taken with the Off-Ramp powered from its battery supply; changing to AC power didn't affect the spectrum, and neither did changing from AudioQuest Coffee to a generic USB cable, or the AES/EBU connection to S/PDIF.

For comparison, fig.4 shows the spectrum of the Transporter's analog output while it was fed the 24-bit J-Test data via the Halide Bridge and the BNC connection. The ±229Hz sidebands are 6dB higher in level and the noise floor is now dominated by closely spaced sideband clusters. So while the Halide Bridge's datastream output has fewer timing variations, the Transporter appears to have more of a problem rejecting those timing variations than it does with the Off-Ramp 4.

Does this explain why I found the Empirical Audio converter to sound better than the Halide with the Transporter? Perhaps. I did find the differences between the converters to be much harder to hear with the dCS Debussy. Fig.5 shows the Debussy's output fed USB data representing 16- and 24-bit J-Test signals via the Halide Bridge. All that can be seen with 16-bit data (cyan and magenta traces) are the odd-order harmonics of the low-level squarewave, which lie at the residual level. Changing to 24-bit data eliminated these completely, though a couple of low-level discrete tones remain between 9.5 and 10.5kHz. Repeating the spectral analysis with the Debussy fed by the Off-Ramp 4 gave an almost identical result (fig.6), though the spectral spreading at the base of the central spike that represents the Fs/4 tone is reduced in both amplitude and width.

These measurements indicate that the Empirical Audio Off-Ramp 4 does a superb job of transforming USB audio data into an AES/EBU or S/PDIF stream. However, whether or not it offers an audible improvement with a specific D/A processor will depend on the processor's inherent rejection of datastream jitter.—John Atkinson

Footnote 2: A primer in why and how I measure jitter can be found here.