Radio Shack Optimus CD-3400 portable CD player More measurements

Sidebar 3: More measurements from November 1994 (Vol.17 No.11)

I measured the effects of three jitter-reduction devices on a digital processor with the Meitner LIM Detector (see Vol.16 No.1, p.114), which looks at the jitter on the word clock driving the D/A converter chip—the point where jitter degrades sound quality. In previous measurements, I've found that the greater the S/PDIF jitter, the greater the word-clock jitter. Although using the LIM Detector to measure the effects of S/PDIF jitter-reduction devices appears to be a step removed from measuring the actual device, it may actually be more subjectively relevant because it's DAC word-clock jitter that ultimately degrades sound quality.

I chose an original PS Audio UltraLink processor for the measurements—it uses the older Yamaha YM3623 input receiver, a chip with poor jitter-rejection performance compared to the Crystal CS8412 now used in nearly all processors (footnote 1). The following measurements were made on the 352.8kHz (8x) clock on the appropriate pin of the NPC 5803 digital filter chip. The test data, taken from the CBS Test Disc, represented a full-scale (0dBFS), 1kHz sinewave.

To establish a baseline, I measured the UltraLink's jitter when driven by a PS Audio Lambda transport—a very low-jitter model. The RMS jitter level, measured over a 400Hz-22kHz bandwidth, was 338 picoseconds. Fig.9 is the same measurement, but with the UltraLink driven by a RadioShack Optimus CD-3400 portable CD player used as a transport. The spectrum is slightly "dirtier" with the RadioShack—the presence of many periodic jitter components is indicated by the spikes in the plot—and the RMS jitter level was nearly doubled to 610ps (who says transports don't make a difference?).

Fig.9 PS Audio UltraLink I, word-clock jitter spectrum measured at digital filter with the Meitner LIM Analayzer, when processing a 1kHz sinewave at 0dBFS sourced from RadioShack CD-3400 transport (linear frequency scale, 10dB/vertical div., 0dB=1ns).

I then attempted the same measurement with an Audio Alchemy DTI•Pro inserted between the Optimus CD-3400 and the Ultralink, but the DTI•Pro wouldn't double-lock to the RadioShack. The DTI•Pro's first phase-locked loop engaged, but the secondary lock wouldn't; the CD-3400's output frequency was either too far out of spec, or its output was too jittered. (When developing the No.30.5 processor, Madrigal found that the Optimus CD-3400 had the least accurate output frequency of any transport they'd tested.)—Robert Harley

Footnote 1: UltraAnalog measures the jitter of all incoming CS8412s, and has discovered that their intrinsic jitter varies from a low of 140ps to a high of more than 400ps. Crystal specifies the CS8412 as typically 200ps, but doesn't guarantee the device's jitter performance. If you've ever heard sample-to-sample variability in a model of digital processor, the CS8412's varying jitter performance may be the culprit.—RH
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