"How sour sweet music is
When time is broke and no proportion kept!"
With those lines from Richard II, Shakespeare unwittingly described a phenomenon in digital audio called "word clock jitter" and its detrimental effect on digitally reproduced music. "Clock jitter" refers to timing errors in analog/digital and digital/analog converters—errors that significantly degrade the musical quality of digital audio.
Clock jitter is a serious and underestimated source of sonic degradation in digital audio. Only recently has jitter begun to get the attention it deserves, both by high-end…
There's more. Clock jitter can raise the noise floor of a digital converter, reducing resolution, and can introduce spurious artifacts. If the jitter has a random distribution (called "white jitter" because of its similarity to white noise), the noise floor will rise. If, however, the word clock is jittered at a specific frequency (ie, periodic jitter), artifacts will appear in the analog output as sidebands on either side of the audio signal frequency being converted to analog. It is these periodic artifacts that are the most sonically detrimental; they bear no harmonic relationship to the…
Results
The test procedure was as follows: an FFT-derived spectral analysis was performed on the processor's clock jitter (the LIMD output) when the processor was driven by the digital code representing a 1kHz sinewave at six input levels ranging from 0dBFS to -90dBFS. The FFT plots show energy vs frequency. The spectral analysis was repeated with the processor under test driven by digital silence (all data words are zero), a 1kHz squarewave, and a 10kHz sinewave at 0dBFS (full scale). The LIMD output was then connected to a true-RMS voltmeter and the jitter voltage measured with all…
Despite my negative feelings about its sound quality, the Vimak DS-2000 (reviewed last month) had astonishingly low levels of jitter: just 34.8ps compared with their specified 50ps. However, although the jitter might be thought to be random in nature, there are a number of discrete low-amplitude spectral lines visible below 6kHz (fig.16). These measurements were made right at the DAC's word-clock pin. Interestingly, the word clock was even cleaner when measured where it enters the analog board, indicating that these discrete components are introduced after the input receiver. Compare fig.16…
The last processor measured was the Bitwise Musik System Zero (reviewed in September 1992). At full scale the Zero had a smooth spectrum, with very few spikes and an overall jitter level of between 2.0 and 2.8ns. It maintained this performance until -50dBFS (worst case), where the number and amplitude of the periodic jitter components increased dramatically. The Zero's jitter spectrum is shown when driven by a full-scale signal (fig.27) and at -50dB (fig.28).
Fig.27 Jitter Spectrum, Bitwise System Zero processing 1kHz sinewave at 0dBFS (linear frequency scale, 0dB = 226.8ns).…
Sidebar 1: The Timing Problem
For the mathematically inclined, the signal/noise ratio (SNR) in a digital system is affected by time-base jitter in the following manner:
SNR = 20 log10(2 T PI F Square Root(2 F0/FS))
where: T = RMS jitter
PI = 3.1416
F = signal frequency
F0 = bandwidth of noise measurement (20kHz)
FS = sampling frequency (352,800Hz for 8x oversampling)—Robert Harley
Sidebar 2: Politics
When John Atkinson and I first learned that the Meitner/Museatex LIM Detector was available for purchase, we were tremendously excited by the possibility of using it to measure jitter in digital audio processors and report the results in Stereophile. We were, however, aware of the political implications of buying a test-measurement system from a manufacturer who makes digital processors and competes with other manufacturers of digital processors. It is possible for a manufacturer to devise a test of questionable worth on which his product performs well but makes other…
The Jitter Game Revisited, April 1993 (Vol.16 No.4):
This "Follow-Up" also provides an opportunity to clarify a perplexing issue raised in the January jitter article. From those measurements, it would appear that processors with single-bit converters have much lower jitter than the more common 8x-oversampling units that use multi-bit DACs. Indeed, 1-bit converters typically have only a few picoseconds of jitter, rather than several hundreds of picoseconds or even several nanoseconds for multi-bit converters. Is the jitter performance of 1-bit machines intrinsically lower, thus "better…
The Jitter Game: an Update, October 1993 (Vol.16 No.10):
Since last January's "The Jitter Game," Stereophile has been able to measure and report on clock jitter in digital processors and CD players under review. In the intervening nine months, the technique has been improved with a second-generation Meitner LIM Detector (the jitter-measurement instrument), more sensitive measurement methods, and improved interpretation of the results.
First, Ed Meitner redesigned the LIM Detector. He added a "phase-detected" output that integrates the jitter, producing a flatter output with…
It was John Atkinson, that legendary ornithologist, who first pointed it out: "Have you noticed how frequently you see women using the iPod?" I hadn't. I'd been so darn happy striding about the streets of New York listening to Tom Russell and Carla Bley that I hadn't been paying attention. Gimlet-eyed, I now began examining my fellow pedestrians for the telltale flash of the distinctive white-and-chrome player and the giveaway white headphone cable that announced the iPod's earbuds.
What an astoundingly acute observer of the human condition Stereophile's editor proved to be! Of…
