A Transport of Delight: CD Transport Jitter Page 5
Fig.11 is the SV-3700's jitter spectrum with the DTI in the S/PDIF signal path. The DTI reduced the jitter by a factor of 10, from 4250ps to 450ps (with a 1kHz, full-scale signal), easily seen in the comparison with fig.4 (the SV-3700's jitter without the DTI). The jitter reduction was less apparent, but still significant, with music as the test signal. Compare fig.12 (with the DTI) with fig.5 (without the DTI). The reduction in broad-band RMS jitter was from 3830ps without the DTI to 289ps with the DTI. Overall, we can confidently say the DTI produces a significant reduction in jitter—when fed with a high-jitter source.
Fig.11 Panasonic SV-3700 DAT recorder with Audio Alchemy DTI, jitter in S/PDIF data signal, 20Hz-50kHz, when transmitting digital silence (bottom solid trace), a 1kHz sinewave at -90dB (middle, dashed trace), and a 1kHz sinewave at 0dBFS (top, light dotted trace) (vertical scale, 1ps-2ns, 100µV = 1ps). Compare fig.4.
Fig.12 Panasonic SV-3700 DAT recorder with Audio Alchemy DTI, jitter in S/PDIF data signal, 20Hz-50kHz, when transmitting music #1 (Firebird, solid) and music #2 (Steve Morse, dashed) (vertical scale, 1ps-2ns, 100µV = 1ps). Compare fig.5.
But look what happens when we put the DTI in the digital signal path from the PS Audio Lambda, a low-jitter source. Fig.13 is the Lambda's jitter spectrum with the DTI. Compare it to fig.6 (Lambda without the DTI) shown earlier. Now the jitter level increased, from 32ps to 133ps with the DTI (with a full-scale 1kHz test signal). Moreover, the DTI imposed a very similar jitter signature—seen as the curve's characteristic shape—on both the SV-3700 and the Lambda. The effect will be to make the high-jitter source sound better, the low-jitter source sound worse. Further, the DTI's common jitter signature will reduce audible differences between transports and interfaces, making them all sound similar—and relatively mediocre. With music as the test signal, the Lambda/DTI combination produced the plot of fig.14. The RMS jitter level with music was 37ps without the DTI and 129ps with, both measured using musical signal #2. (Note that the 129ps figure is lower than the CS8412's 200ps of intrinsic jitter owing to the 30kHz measurement bandwidth.)
Fig.13 PS Audio Lambda with Audio Alchemy DTI, jitter in S/PDIF data signal, 20Hz-50kHz, when transmitting digital silence (bottom solid trace), a 1kHz sinewave at -90dB (middle, dashed trace), and a 1kHz sinewave at 0dBFS (top, light dotted trace) (vertical scale, 1ps-2ns, 100µV = 1ps). Compare fig.6.
Fig.14 PS Audio Lambda with Audio Alchemy DTI, jitter in S/PDIF data signal, 20Hz-50kHz, when transmitting music #1 (Firebird, solid) and music #2 (Steve Morse, dashed) (vertical scale, 1ps-2ns, 100µV = 1ps). Compare fig.7.
In addition to revealing that the DTI quantitatively improves some transports and degrades others, as was suggested by the listening, these measurements suggest something else: That the DTI imposes its intrinsic jitter on its digital output signal, and still passes some of the jitter in the incoming data stream. Note the very much higher jitter at the DTI's output when fed by the high-jitter Panasonic SV-3700 compared to the low-jitter Lambda.
Further, these test results demonstrate that the differences seen in the jitter spectra and levels with and without the DTI are easily audible. As described in my review, the DTI's sonic effects—good or bad—aren't subtle. I'm at a loss to explain why some reviewers have gone on the record to state that the DTI improves the sound of every transport and processor combination—even that of the Mark Levinson No.30 and No.31!
I repeated these measurements using a prototype jitter-reduction box that uses the new Analog Devices AD1890 Asynchronous Sampling Rate Converter chip described in last May's "Industry Update" (Vol.16 No.5, p.41). The prototype box, designed by Bob Katz (Digital Domain, Chesky), greatly reduced the jitter of high-jitter sources (from 4250ps to 45ps—an order of magnitude greater reduction than the DTI), and very slightly degraded the low-jitter Lambda (from 32ps to 42ps). All jitter-reduction devices are, however, limited by the fact that their reduced-jitter outputs must still then pass through another jitter-inducing interface between their output and the digital processor.
On to the transports
For the RMS jitter figures for each transport with each test signal, see Table 1.
Table 1: Transport Jitter (figures in picoseconds)
|Proceed PDT 3||39||127||70||43||33||62|
|Mark Levinson No.31||34||98||66||38||29||53|
|Proceed PDT 1||37||81||62||40||32||50|
|C.E.C. TL 1||30||90||61||33||24||48|
|PS Audio Lambda||32||51||66||36||29||43|
|PS Lambda W/DTI||133||158||129||120||139||136|