Sidebar 3: Measurements
There are only two meaningful measurements of a disc transport's output quality: how well it copes with disc errors, and the amount of timing uncertainty or jitter there is in the datastream it outputs. To assess the latter, I used my Audio Precision SYS2722 as a digital oscilloscope, overlaying successive snapshots of the dCS Rossini Transport's datastream to show what's called an "eye pattern." With an ideal transmission system, all the pulse transitions in the datastream will overlay one another to produce an image of a wide-open "eye," with apparently just one trace visible (footnote 1).
Fig.1 shows the eye pattern of the dCS Rossini SACD transport's single AES/EBU output, plotted over one "unit cycle" as it fed the Audio Precision a 16-bit AES/EBU datastream comprising the Miller/Dunn J-Test Signal for 60 seconds. The transport was clocked from the Rossini DAC, which is how I performed all my auditioning. The eye is indeed wide open, and the multiple traces overlay exactly.
To make this a worst-case situation, the AES/EBU link was a 45' length of Canare 110-ohm balanced interconnect, which results in a somewhat lengthened risetime compared with the Audio Precision's own AES/EBU output using the same 45' datalink (fig.2). This behavior won't affect sound quality, other than with a D/A processor having a pathologically poor serial data input.
Footnote 1: For a poor example of an eye pattern, see fig.2 here.
Fig.1 dCS Rossini Transport, eye pattern of AES/EBU data output carrying 16-bit, 44.1kHz J-Test data (±3V vertical scale, 175ns horizontal scale).
Fig.2 Audio Precision SYS2722, eye pattern of AES/EBU data output carrying 16-bit, 44.1kHz J-Test data (±3V vertical scale, 175ns horizontal scale).
I tested how well the Rossini Transport coped with disc errors by playing the Pierre Verany Digital Test CD, which has calibrated gaps in its data spiral. The Rossini's error correction was excellent—no glitches in the 500Hz tone were audible until the gaps reached 2mm in length, though the player stumbled at the start of track 31, which has 1mm gaps. (The Compact Disc standard, the so-called "Red Book," requires only that a player cope with gaps of up to 0.2mm.)—John Atkinson
Footnote 1: For a poor example of an eye pattern, see fig.2 here.
