Benchmark Media Systems, Inc.
Page 22 of 39
UltraLock™
… What is It?
Accurate 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.
Jitter is present on every digital audio interface. This type of jitter is known as “
interface jitter
” and it is present
even in the most carefully designed audio systems.
Interface jitter
accumulates as digital signals travel down a
cable and from one digital device to the next. If we measure
interface jitter
in a typical system we will find that it
is 10 to 10,000 times higher than the level required for accurate 24-bit conversion. Fortunately, this
interface
jitter
has absolutely no effect on the audio unless it influences the conversion clock in an analog-to-digital
converter (ADC) or in a digital-to-analog converter (DAC).
Many converters use a single-stage Phase Lock Loop (PLL) circuit to derive their conversion clocks from
AES/EBU, Wordclock, or Superclock reference signals. Single-stage PLL circuits provide some jitter attenuation
above 5 kHz but none below 5 kHz. Unfortunately, digital audio signals often have their strongest jitter
components at 2 kHz. Consequently, these converters can achieve their rated performance only when driven
from very low jitter sources and through very short cables. It is highly unlikely that any converter with a single-
stage PLL can achieve better than 16 bits of performance in a typical installation. Specified performance may be
severely degraded in most installations.
Better converters usually use a two-stage PLL circuit to filter out more of the
interface jitter
. In theory, a two-
stage PLL can remove enough of the jitter to achieve accurate 24-bit conversion (and some do). However, not all
two-stage PLL circuits are created equal. Many two-stage PLL’s do not remove enough of the low-frequency
jitter. In addition, two-stage PLL circuits often require several seconds to lock to an incoming signal. Finally, a
two-stage PLL may fail to lock when jitter is too high, or when the reference sample frequency has drifted.
UltraLock™
converters exceed the jitter performance of two-stage PLL converters, and are free from the slow-
lock and no-lock problems that can plague two-stage PLL designs.
UltraLock™
converters are 100% immune to
interface jitter under all operating conditions. No jitter-induced artifacts can be detected using an Audio Precision
System 2 Cascade test set. Measurement limits include detection of artifacts as low as –140 dBFS, application of
jitter amplitudes as high as 12.75 UI, and application of jitter over a frequency range of 2 Hz to 200 kHz. Any
AES/EBU signal that can be decoded by the AES/EBU receiver will be reproduced without the addition of any
measurable jitter artifacts.
The
DAC1
,
DAC-104
and the
ADC-104
employ Benchmark’s new
UltraLock™
technology to eliminate all jitter-
induced performance problems.
UltraLock™
technology isolates the conversion clock from the digital audio
interface clock. Jitter on a DAC digital audio input, or an ADC reference input can never have any measurable
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 the proper phase relationship to the reference clock.
Interface jitter
cannot degrade the
quality of the audio conversion. Specified performance is consistent and repeatable in any installation!
How does conversion clock jitter degrade converter performance?
Problem #1:
Jitter phase modulates the audio signal. This modulation creates sidebands (unwanted tones)
above and below every tone in the audio signal. Worse yet, these sidebands are often widely separated from the
tones in the original signal.
Jitter-induced sidebands are not musical in nature because they are not harmonically related to the original audio.
Furthermore, these sidebands are poorly masked (easy to hear) because they can be widely separated above
and below the frequencies of the original audio tones. In many ways, jitter induced distortion resembles
intermodulation distortion (IMD). Like IMD, jitter induced distortion is much more audible than harmonic distortion,
and more audible than THD measurements would suggest.
