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DAC2 Series Instruction Manual Rev C Page 30

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 maximum

allowable level for accurate 24-bit conversion.

Fortunately, interface jitter has absolutely no

effect on the audio unless

it influences the

conversion clock in an analog-to-digital

converter (A/D) or in a digital-to-analog

converter (D/A).

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 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 usually 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.

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 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. 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 –160 dBFS, 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 will be reproduced without

the addition of any measurable jitter artifacts.

The DAC2 employs Benchmark’s

UltraLock2™ technology to eliminate 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.

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