Instruction Manual for DAC2 HGC and DAC2 L with
2.X Firmware
Page 34
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 often 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 PLLs 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. Two-
stage PLL circuits may fail to lock when jitter
is too high, or when the reference sample
frequency has drifted.
UltraLock™ converters exceed the jitter
attenuation performance of two-stage PLL
converters while achieving near instantaneous
lock time. They are free from the slow-lock
and no-lock problems that can plague two-
stage PLL designs. UltraLock™ converters
have extremely high immunity to interface
jitter under all operating conditions.
The UltraLock™ system is so effective that
no jitter-induced artifacts could be detected
using an Audio Precision System 2 Cascade
test set while the inputs to the DAC2 were
exposed to high levels of interface jitter. The
measurement limits included the ability to
detect artifacts as low as –144 dBFS, but
none could be detected, even while applying
jitter amplitudes as high as 12.75 UI, over a
frequency range of 2 Hz to 200 kHz. Any
AES/EBU signal that can be decoded by the
AES/EBU receiver in the DAC2 will be
reproduced without the addition of any
measurable jitter artifacts.
Benchmark’s UltraLock2™ technology
eliminates jitter-induced performance
problems. UltraLock2™ technology isolates
the conversion clock from the digital audio
interface clock. Jitter on a D/A digital audio
input, or an A/D reference input can never
have any measurable effect on the conversion
clock of an UltraLock2™ converter. In an
UltraLock2™ 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. The clock isolation of the
UltraLock2™ system ensures that interface
jitter can never degrade the quality of the
audio conversion. Specified performance is
consistent and repeatable in any installation
with cables of any quality level!
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.
Jitter creates ‘new audio’ that is not
harmonically related to the original audio
signal. This ‘new audio’ is unexpected and
unwanted. It can cause a loss of imaging, and
can add a low and mid frequency ‘muddiness’
that was not in the original audio.
Jitter-induced sidebands can be measured
using an FFT analyzer while the converter
plays a pure high-amplitude tone. We
typically use a full-scale 10 kHz test tone to
test for the presence of jitter-induced side
bands (see Plot 15). This FFT shows that the
DAC2 is free from any jitter-induced
sidebands to a measurement limit of about
-144 dB relative to the level of the test tone.