Usual caveat: Measurements are merely slices of reality. They can never encompass the full reality of listening experience. Yet when properly interpreted, they do reveal underlying physical mechanisms that correlate, to a limited degree, with listening experience. With the recognition of their limitations and capabilities, they can be a useful guide in evaluating the performance of audio equipment.
Please note that the jitter from the SPDIF signal is only a part of the picture. What ultimately matters to the sonic quality is the sampling jitter, measured at the analog output of the DAC.
Period Jitter Histogram
This shows the histogram of period jitter, one of the several ways of characterizing jitter. The measured period jitter is about 17 ps rms, which is near the limit of our measurement system. After the jitter of the measurement system (13 ps rms) is subtracted, the remainder, the U2's contribution, is estimated to be about 11 ps rms.
Period Jitter vs. Coax Cable Length
The jitter remains essentially constant over a wide range of coax cable lengths, even with inexpensive cables used for these measurements (RG-59 cables by TPI, about $20 for 25 ft). If you need to extend the distance beyond the 15 ft limit of the USB cable, please do not hesitate to use a long RG-59 coax cable at the SPDIF output. The U2's performance is robust even with a significant mismatch in the termination resistance.
SPDIF Output Waveform
The SPDIF output of the Anedio U2 shows a fast, well-behaved response, free from excessive overshoot and ringing. The rise time is about 6 ns, and the output swing is 0.58 Vp-p into 75 ohms. Such a clean response is attributed to the high-quality transformer, specifically designed for low-jitter transmission of pulses.
What happens when a poor-quality transformer is used?
The quality of the transformer used for galvanic isolation is critical. The waveform on the right shows the degradation in waveform when a poor-quality transformer is substituted. Especially disconcerting is the kink in the middle of the rising edge, which can drastically increase the jitter.
Notes on Measurement Condition:
- The period jitter measurements were performed with the Agilent 53230A Frequency Counter, which is capable of 20 ps time-interval resolution, one of the best in its category. Its intrinsic jitter is approximately 13 ps rms, depending on the setting under which it is measured.
- Download the U2 USB-to-SPDIF Converter Manual