A Transport of Delight: CD Transport Jitter Page 9
Fig.36 Mark Levinson No.31 (solid) and C.E.C. TL 1 (dotted), jitter in S/PDIF data signal, 1-50kHz (vertical scale, 5-80ps, 100µV = 1ps).
Fig.37 C.E.C. TL 1 (solid) and Mark Levinson No.31 (dotted), jitter in S/PDIF data signal, 20Hz-1kHz (vertical scale, 1-20ps, 100µV = 1ps).
There is now no question that jitter in CD transports and digital interfaces affects digital audio sound quality. Not only do different transports and interfaces sound different, they produce varying amounts of jitter and have their own "jitter signatures," seen in the jitter's spectral distribution.
Moreover, we can see that transport jitter goes right through the digital processor's input receiver (even the Crystal CS8412) and affects the amount of jitter at the DAC's word clock—the point where jitter makes an audible difference. If the word-clock timing is different, the sound will be different.
The revelation that digital interconnects and their direction can introduce large differences in measured jitter was quite a shock. The differences heard between digital interconnects—and in their directionality—have now been substantiated by measurement.
Although the CD-transport measurements presented here are fascinating, it is impossible to draw conclusions about how a transport will sound solely by looking at its jitter measurements. Based on the measurements and listening impressions of the Audio Alchemy DTI, we can confidently conclude that the jitter differences the DTI imposes on both high- and low-jitter sources are easily audible, and that lower jitter always correlates to better sound. But when examining the jitter performance of other transports, a direct correlation is less clear.
As described earlier, there are many variables that influence how much jitter, and jitter of what spectral distribution, appears in the recovered clock. This could be a significant factor, suggested by the example of the PDT 1 transport. The PDT 1 had slightly lower RMS jitter and an almost identical spectrum compared with the Mark Levinson No.31, despite the No.31's vastly better sonic performance (footnote 8).
This paradox illustrates the problem of interpreting an entirely new set of test data. We don't know what's significant in the measurements and what isn't. Remember, these are the first transport-jitter measurements ever presented by any publication: It will take some time and a lot more experience to determine which jitter characteristics are of actual sonic importance. Do the slight differences between the C.E.C. TL 1's and the No.31's jitters (figs.36 and 37) make an audible difference? Are we using an appropriate amplitude scale in examining jitter differences between products? Does jitter in a certain frequency band produce a much greater audible change than a similar amount of jitter in a different frequency band? If so, what are the subjective effects?
These are all unanswered questions. With traditional measurements we have a well-established framework for drawing conclusions about the audibility of measured performance. For example, we know that a frequency-response rise of 0.1dB over an octave of bandwidth is just audible. But what is the threshold of audibility for transport jitter? Our examination of transport jitter may be analogous to looking for a 0.1dB amplitude difference using a vertical scale of 20dB per division—it's there in the data, but we can't see it. Similarly, tiny differences between traces in our graphs may produce large subjective differences. We just don't know.
The measurements presented here are far from the last word in quantifying a transport's technical or musical performance. Instead, this article should be considered a primitive first step toward understanding jitter and its effects on sound quality. We need a concerted effort by critical listeners and audio engineers to understand transport jitter—and to correlate measured data with its subjective effects on the musical presentation. Only then will jitter be eliminated as a source of variability in the quality of digitally reproduced music.
Footnote 8: The question of whether the transport's jitter spectrum appears on the DAC's word clock is an important one. I intend to combine the Meitner LIM Detector and the UltraAnalog transport-jitter analyzer in a future investigation to answer this question.—Robert Harley