A Transport of Delight: CD Transport Jitter High Jitter vs. Low Jitter
What is the sonic effect of jitter in the S/PDIF datastream? To answer that, I listened to the same music through the same system, but switched between the PS Audio Lambda and the SV-3700 as the S/PDIF data source.
The difference was not subtle. On the Harmonia Mundi recording of Handel's Water Music (HMU 907010), the violins sounded almost unrecognizable through the high-jitter SV-3700. Instead, they sounded like a single synthesizer, devoid of all nuance, detail, and the sonic cues that tell you a bow is moving across strings. In place of these cues was a steely hardness and screechiness that literally made me cringe during loud passages. It sounded as though the violins were being played with hacksaw blades.
In addition, low-level detail was lost; the harpsichord seemed to disappear, and all the instruments blended into a giant roar. The presentation was drier, and reverberation decay became truncated. Overall, the music was sterile, mechanical, and uninvolving.
Moving to a very different selection, Michael Ruff's Speaking in Melodies (Sheffield CD-35), I heard a similar steeliness to the treble. Sibilance was emphasized, sounding like spikes of barbed wire sticking out on vocal "s" and "ch" sounds. The treble was edgy and aggressive. The vocal was more forward, drier, and didn't sound integrated with the band.
One change between the Lambda and the SV-3700 not heard on the Handel but clearly revealed on the Ruff disc was how jitter affects bass. The SV-3700 had more bass, but it was fatter, slower, and devoid of pitch. Through the Lambda, I could clearly hear all the notes the bass player was playing at the beginning of track 11. Through the SV-3700, the bass in this same passage was a featureless morass. Moreover, the SV-3700 destroyed this music's upbeat rhythmic intensity. The band seemed less enthusiastic through the SV-3700.
When we hear the effects of jitter, we're hearing voltage errors at the DAC's analog output caused by word-clock timing variations. This magnitude of the voltage error is a function of the signal amplitude and the jitter. Although low-level signals introduce more jitter in the interface, we're less likely to hear jitter on low-level signals. The voltage error may be on the order of a few microvolts with a low-level signal, but several millivolts with a high-level signal. Consequently, the audibility of jitter will vary greatly with the music's spectral content, level, and dynamics. According to HDCD® developer Pacific Microsonics—who has done extensive research into jitter audibility—the jitter's frequency and spectral distribution largely determine a transport's sonic signature. Jitter at one frequency may produce a certain sonic characteristic, while jitter at another frequency will create a very different subjective effect.
Clearly, jitter in the data stream driving a digital processor is audible—and a significant contributor to "digital sound." Because jitter adds artifacts we associate with digital audio in general, reducing jitter is an important step toward truly musical digital reproduction.—Robert Harley