A Transport of Delight: CD Transport Jitter Page 4
What caused this reduction in measured jitter?
Changing the direction of the digital interconnect between the transport and the jitter analyzer.
This phenomenon was easily repeatable: put the cable in one direction and read the RMS jitter voltage, then reverse the cable direction and watch the RMS jitter voltage drop. Although I'd heard differences in digital-cable directionality, I was surprised the difference in jitter was so easily measurable—and that the jitter difference was nearly double.
To confirm this phenomenon, I repeated the test five times each on three different digital interconnects. One was a generic audio cable, the other two were Mod Squad Wonder Link and Aural Symphonics Digital Standard, both highly regarded cables specifically designed for digital transmission. The generic cable wasn't directional: it produced the same high jitter in either direction. But both the Wonder Link and the Aural Symphonics had lower jitter levels overall, but different jitter levels depending on their direction. Moreover, the generic cable had higher jitter than either of the two premium cables—even in the latters' "high-jitter" direction.
Fig.8 shows the jitter difference between cable direction in Wonder Link using the Panasonic '3700 as the source (the difference was about the same in the Aural Symphonics). Note that, at these high levels, small differences in the trace are significant. Between "10m" and "0.1" on the vertical scale, each horizontal division is 100ps. The overall RMS jitter was 4050ps with the Wonder Link connected in one direction, and 2700ps with the cable reversed.
Fig.8 Panasonic SV-3700 DAT recorder, jitter in S/PDIF data signal, 20Hz-50kHz, with Mod Squad data cable one way around (solid) vs the other (dotted). RMS jitter measured 4050ps vs 2700ps (vertical scale, 1ps-2ns, 100µV = 1ps).
I also plotted the SV-3700's jitter spectrum through the generic audio cable, Wonder Link, and Aural Symphonics Digital Standard (fig.9). The Wonder Link and Aural Symphonics were both in their "low-jitter" directions for this plot. The top trace (highest jitter) is the generic cable, the next-lower trace is Wonder Link, and the lowest is Digital Standard. You can easily see that the premium digital interconnects had significantly lower jitter than the generic cable. As we saw earlier, in figs.2 and 3, this jitter in the S/PDIF signal directly affects a digital processor's word-clock jitter, which in turn degrades sound quality.
Fig.9 Panasonic SV-3700 DAT recorder, jitter in S/PDIF data signal, 20Hz-50kHz, using a generic audio cable (solid), Wonder Link (dashed), and Aural Symphonics Digital Standard (dotted) (vertical scale, 1ps-2ns, 100µV = 1ps).
These measurements confirm the reports of critical listeners—see elsewhere in this issue—that digital interconnects sound different when connected in different directions.
I performed the same tests using the low-jitter PS Audio Lambda transport as source. The results were very different. With a good source, cable direction didn't make a difference in the measurable jitter (fig.10). This suggests that the SV-3700—or any poor-quality transmitter—reacts with the cable's impedance to create jitter-inducing reflections in the interface. The directionality was probably caused by differences in the way the two RCA plugs were soldered to the cable; any bumps or discontinuities in the solder or RCA plug will cause a change in the characteristic impedance, which will cause higher-amplitude reflections in one direction than in the other. These reflections set up dynamically changing standing waves in the interface, introducing jitter in the embedded clock. These problems were reduced by the Lambda's higher-quality output circuit.
Fig.10 Panasonic SV-3700 DAT recorder, jitter in S/PDIF data signal, 20Hz-50kHz, with Mod Squad data cable one way around (solid) vs the other (dotted) (vertical scale, 1ps-2ns, 100µV = 1ps).
In short, the worse the transport, the more cables—and their direction—affect sound quality. Incidentally, a $2.99 Radio Shack 75 ohm coaxial video cable had lower jitter than the generic audio cable, but higher jitter than either the Wonder Link or the Aural Symphonics (footnote 7).
While we're on the subject of the digital interface, I should point out that the engineering for transmitting wide-bandwidth signals was worked out nearly 50 years ago in the video world. In video transmission, the source has a carefully controlled output impedance, the cable and connectors have a precisely specified characteristic impedance and are well-shielded, and the load impedance is specified within narrow tolerances. If these practices aren't followed, reflections are created in the transmission line that play havoc with video signals. This issue is so crucial that a whole field called Time Delay Reflectometry (TDR) exists to analyze reflections in transmission lines.
The audio community should adopt the standard engineering practices of video engineering for digital interfaces. This means designing transports with a carefully controlled 75 ohm output impedance, precisely specified characteristic impedance of the cable (75 ohms with a narrow tolerance), and junking RCA connectors in favor of true 75 ohm BNC connectors. By applying standard video engineering techniques—in use for decades—the high-end product designer can greatly improve the performance of the transport/processor interface. We've seen what happens with a poorly implemented interface with the SV-3700 and different cables: higher jitter in the recovered clock and degraded sound quality. The engineering needed to optimize the digital interface is readily available. Let's use it.
Audio Alchemy DTI
The next job was to measure the effect of the $349 Audio Alchemy DTI "jitter-reduction" box on the measured jitter—using the same digital interconnects in the same direction for each test. In my review of the DTI last May, I concluded that it improved some transport/processor combinations and made others sound worse. I wrote: "The Audio Alchemy DTI's ability to improve the sound of a digital front end varied greatly with the transport and digital processor used....Although the DTI can improve the sounds of some digital systems, it is no substitute for a topnotch transport....Careful evaluation in one's own system is mandatory before purchasing the DTI." (emphasis in original). JA's independent auditioning also revealed that the DTI degraded the sound of some transport/processor combinations.
Footnote 7: Starting this project was like opening a can of worms. Although this article is confined to transport-jitter measurements, I'll be reporting in an upcoming issue on other peripheral factors. Phenomena worth exploring include a survey of the jitter performances of digital interconnects, the effects of CD tweaks and the ASM spatial filter (fitted standard on the Lambda) on transport jitter, the effects of vibration on transport jitter, and anything else of interest.—Robert Harley