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参考：
BENCHMARK DAC1 的用户手册对DAC jitter的介绍
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 singlestage
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 twostage
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 slowlock
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 jitterinduced
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.
Benchmark Media Systems, Inc.
Page 23 of 39
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.
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 clock jitter in an ADC or DAC 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 ADC and DAC 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-toanalog
converter. Any attempt to cure jitter outside of an ADC or DAC 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 ADC’s and DAC’s 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). |
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发表于 2010-12-14 22:17
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