Benchmark DAC2 D Instruction Manual page 37

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 UltraLock™ technology
eliminates jitter-induced performance
problems. UltraLock™ 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 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. The clock isolation of the
UltraLock™ 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.
Manual for DAC2 D with 2.X Firmware - Rev. H
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 Graph 15 - JITTER TOLERANCE
FFT). 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. The graph plots
the output spectrum of the DAC2 when
exposed to 31 different jitter frequencies
ranging from 100 Hz to 100 kHz. All 31
output spectra are identical and are free from
any signs of jitter-induced distortion.
Problem #2: Jitter can severely degrade the
anti-alias filters in an oversampling converter.
This is a little known but easily measurable
effect. Most audio converters operate at high
oversampling ratios. This allows the use of
high-performance digital anti-alias filters in
place of the relatively poor performing analog
anti-alias filters. In theory, digital anti-alias
filters can have extremely sharp cutoff
characteristics, and very few negative effects
on the in-band audio signal. Digital anti-alias
filters are usually designed to achieve at least
100 dB of stop-band attenuation. But, digital
filters are designed using the mathematical
assumption that the time interval between
samples is a constant. Unfortunately, sample
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