Instruction manual

DAC1 HDR Instruction Manual Rev I Page 24
assumption that the time interval between
samples is a constant. Unfortunately, sample
clock jitter in an A/D or D/A varies the
effective time interval between samples. This
variation alters the performance of these
carefully designed filters. Small amounts of
jitter can severely degrade stop-band
performance, and can render these filters
useless for preventing aliasing.
The obvious function of a digital anti-alias
filter is the removal of audio tones that are
too high in frequency to be represented at the
selected sample rate. The not-so-obvious
function is the removal of high-frequency
signals that originate inside the converter
box, or even originate inside the converter IC.
These high-frequency signals are a result of
crosstalk between digital and analog signals,
and may have high amplitudes in a poorly
designed system. Under ideal (low jitter)
conditions, a digital anti-alias filter may
remove most of this unwanted noise before it
can alias down into lower (audio) frequencies.
These crosstalk problems may not become
obvious until jitter is present.
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.
Put UltraLock™ converters to the
test:
We encourage our customers to perform the
above tests on UltraLock™ converters (or let
your ears be the judge). There will be
absolutely no change in performance as jitter
is added to any digital input on an
UltraLock™ converter. Try the same tests
on any converter using conventional single or
two-stage PLL circuits. Tests should be
performed with varying levels of jitter and
with varying jitter frequencies. The results will
be very enlightening. Jitter related problems
have audible (and measurable) effects on A/D
and D/A devices. Practitioners of Digital Audio
need to understand these effects.
Is it possible to eliminate all of
the effects of jitter in an entire
digital audio system?
Interface jitter will accumulate throughout
even the most carefully designed digital audio
system. Fortunately, interface jitter can
only degrade digital audio if it affects the
sampling circuit in an analog-to-digital or
digital-to-analog converter. Any attempt to
cure jitter outside of an A/D or D/A will prove
expensive and, at best, will only partially
reduce jitter-induced artifacts. Dedicated
clock signals (word clock, and super clock,
etc.) are often distributed to A/D converters
and D/A converters in an attempt to reduce
jitter. Again, these are only partial solutions
because jitter even accumulates in these
clock distribution systems. Furthermore, a
poor quality master clock generator can
degrade the performance of the entire system
(if converter performance is dependent upon
reference clock quality). Jitter free A/D and
D/A converters are the only true insurance
against the ill effects of jitter. UltraLock™
converters are jitter-immune under all
operating conditions (they will never add
audible jitter induced artifacts to an audio
signal).
What UltraLock™ converters
cannot do:
UltraLock™ converters cannot undo damage
that has already been done. If an A/D with a
jitter problem was used to create a digital
audio signal, then there is nothing that can be
done to remove the damage. Jitter-induced
sidebands are extremely complex and cannot
be removed with any existing audio device.
Therefore, it is very important to attack jitter
at both ends of the audio chain. The DAC1
HDR is a great start, as it will allow accurate
assessment of various A/D converters. It is
impossible to audibly evaluate A/D
performance without a good D/A. The
consistent performance delivered by the
DAC1 HDR eliminates one major variable:
jitter.