Balancing Noise And Distortion
As always, the design is a trade-off between noise and
distortion. As level increases, the noise level drops, but the
distortion rises as you approach 0dBFS. The challenge is to
get the right balance. Low impedance sources, proper current
management, and factors as small as how the tracks on the
circuit board are routed are important in minimising distortion,
while high amplitude helps to minimise noise.
It’s always a good idea to look at the output with an FFT (Fast
Fourier Transform) analyser, to see if there are any periodic
functions, which might be caused, for example, by a track on the
board acting as an antenna. Each component also generates its
own noise, and a haphazard arrangement of components on the
board can result in random components defining the noise floor. A
great deal can also be learned by listening to the noise: how ‘white’
or broadband is it? The broader its bandwidth, the better.
From Analogue To Digital
On the digital side, there is the question of clocking. Clocking
would be easy if the clock rate didn’t have to change — but of
course, it does, as devices need to lock to each other. And the
minute a phase-locked loop is involved, the situation gets more
complex and simplicity goes out of the window. But readily
available modern clocking solutions, such as the tried-and-
tested Jet PLL™ system for clock recovery and reclocking, allow
hardware manufacturers to focus on the analogue side of the
equation. Similarly, jitter-elimination is more straightforward
today, because the majority of jitter created by modern systems
is random, whereas in the past it tended to be cyclic, for
example.
In Summary
As we’ve found, several different stages are involved in the
design and development of digital conversion systems.
Today, many of the old issues of digital audio have been
surmounted, and yesterday’s solutions to the grand challenges
of the conversion process are today largely the stuff of myth
and legend, and not of reality, and are of limited relevance.
Today the task is to push back the boundaries of dynamic
range, striving towards the 144 dB theoretical maximum for
a 24-bit system. This requires the regular advances in the
silicon of conversion chips to be matched by ever-improving
componentry, board layout and analogue circuit design.
© Focusrite Audio Engineering Limited
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