Why UltraLock™ Technology?

Part One of Two

24-bit audio conversion requires a very low-jitter conversion clock. Jitter can very easily turn a 24-bit converter into a 16-bit converter (or worse). There is no point in buying a 24-bit converter if clock jitter has not been adequately addressed.

Unfortunately most converters derive their conversion clocks directly from the poorly filtered output of an AES/EBU receiver IC. These converters can achieve their rated performance only when driven from very low jitter AES/EBU sources through no more than 3 or 4 feet of cable. It is highly unlikely that these converters can achieve much over 16-bits of performance in a typical installation. In short, test bench performance is not repeatable in a typical installation.

The DAC-1, DAC-104 and the ADC-104 employ Benchmark’s new UltraLock™ technology to eliminate all jitter-induced performance problems. UltraLock™ technology totally isolates the conversion clock from the digital audio interface clocks. Jitter on a DAC digital audio input, or an ADC reference input can never have any effect on the conversion clock of an UltraLock™ converter. In an UltraLock™ converter, the conversion clock is never phase-locked to a reference clock. Instead the converter oversampling-ratio is varied with extremely high precision to achieve phase-lock to the reference clock. Jitter cannot effect the audio conversion, and test bench performance is repeatable in any installation!

How does sample-clock jitter degrade converter performance?

Problem #1

Jitter phase modulates the audio signal. The result is that phase modulation sidebands are created above and below every tone in the audio signal. Worse yet, these sidebands are often widely separated from the audio tone they a modulating. Consequently, the distortion caused by jitter is far more audible than THD+N measurements would suggest. Phase modulation sidebands are not musical in nature (they are not harmonically related to the audio), and they are poorly masked (because of the wide separation from the fundamental). In short jitter induced distortion closely resembles intermodulation distortion (IMD) and is much more audible than harmonic distortion (THD). Jitter creates new “audio” that is not harmonically related to the original audio signal. This “audio” is unexpected and unwanted. It will cause a loss of imaging in the stereo signal and when severe, will create what some have called muddiness (a lot of extra low and mid frequency sound that was not in the original). Jitter induced sidebands can be measured very easily using an FFT analyzer.

Just how well does it work?

Figure 1. - FFT Modulation Sideband Analysis of a Tightly Locked A-to-D Converter

Problem #2

Jitter can severely degrade the anti-alias filters in an oversampling converter. This is little known but easily measurable effect. Virtually all audio converters operate at high oversampling ratios. This allows the use of high-performance digital anti-alias filters in place of the relatively poor performance of analog anti-alias filters. By design, digital anti-alias filters have extremely sharp cutoff characteristics, have no negative effect on in-band signals, and often achieve a stop-band attenuation that exceeds 100 dB. But, digital filters are designed using the mathematical assumption that the time interval between samples is a constant. Unfortunately, sample clock jitter in an ADC or DAC varies the time interval between samples. This variation alters the performance of these carefully designed filters. Small amounts of jitter can severely degrade stop-band performance, rendering these filters useless for preventing aliasing. It is also important to note that many of the signals being stopped by these filters are due to digital crosstalk on the circuit board or within the converter ICs. Stop-band attenuation can be measured very easily by sweeping a test tone between 24 kHz and at least 200 kHz while monitoring the output of the converter.

Figure 2. - Digital Filter Performance of the A-to-D Converter Chip at 100 kHz

Figure 3. - Digital Filter Performance of the A-to-D Converter Chip at 26 kHz

We encourage our customers to perform the above tests on UltraLock™ converters. There will be absolutely no change in performance as jitter is added to digital input signals. Try the same test using any converter that is driven directly from an AES/EBU receiver. The results will be very enlightening. Jitter related problems have real (and measurable) effects on ADC and DAC devices. Practitioners of Digital Audio need to understand (and know how to test for) these effects. Tests should be performed with varying levels of jitter and with varying jitter frequencies.

UltraLock™ converters can totally prevent all sample clock jitter problems in your next installation or upgrade. Jitter can only degrade digital audio at an analog-to-digital or digital-to-analog process. Any attempt to cure jitter outside of an ADC or DAC will prove futile. On the other hand, jitter free ADCs and DACs will insure that your entire installation is immune to the ill effects of jitter.

UltraLock™ digital technology compliments Benchmark’s tradition of superb analog design. Together these set our converters apart from the competition.

Continue to Part 2, "Just how well does it work?"