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-to-
analog 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).
