Focusrite Saffire PRO 40, Руководство пользователя

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important. If you’re dealing with a professional manufacturer,
then the answer to the question “what converter chip do you
use in this product?” is likely to be, “the best available part for
the job”. In a similar sense, you really don’t need to ask BMW
whether the engine they have built their new model around is
up to the job: you can happily assume that it is.
The boundaries of converter chip design are being pushed back
all the time, and the main impact of these developments is
improvement in the dynamic range offered by the part. Current
converter chips offer around 121—123dB in this area; however
the theoretical maximum for a 24-bit system is 144dB. There is
quite a way to go before we approach that limit, and each time
the performance moves forward, it’s the result of analogue as
well as digital developments.
Analogue Is Critical
At the same time, just having a good quality conversion part
on the circuit board does not make a high-quality converter:
there is still a very distinct art and science of conversion design,
which manifests itself at several levels, not least board layout
and circuit design. Some early manufacturers of digital studio
equipment believed that with the advent of “pure, perfect”
digital sound, analogue design would no longer be as important,
but today we know that in fact, if anything, the opposite is the
case. Above all, developing a clean, well-designed analogue
section is vital.
just having a good quality conversion
part on the circuit board does not make
a high-quality converter: there is still a
distinct art of conversion design
As part of this process, the analogue section needs its own
power supply regulation too, as power supply lines — and the
supplies themselves — can introduce noise. Indeed, power
supply noise is one of the biggest factors affecting the noise
floor. Traditionally it was felt that power supplies had to be
linear, as early switch-mode designs often raised the noise
floor by inserting (and sometimes even radiating) digital noise
into the system. Today, however, switch-mode PSU design
has advanced to the point where they actually reduce noise: a
modern switcher simply takes the noise out of the audible range.
The analogue section also needs to be physically separated
from the digital section, and requires judicious use of ground
planes and multilayer boards. In an A-D design, low-noise
amplifiers are needed to feed the input to the conversion chip
itself, which also requires a very low input impedance: low
impedances mean higher current, which in turn requires larger
circuit board track areas, and more careful design.
PRINCIPLE INTO PRACTICE
When you look at a 192kHz A-D conversion system like the
digital board in Focusrite’s ISA One Digital, you’re looking at
what is effectively a fourth-generation design, where all the
factors we discuss here have been tweaked, developed, revised
and improved to take advantage of advances in converter silicon
design. Gone are the days when ‘cost-effective’ meant ‘limited
quality’ analogue I/O: the ISA One Digital at least equals the
performance of available standard interfaces, and its on-
board conversion makes the unit self-contained and means
that it does not occupy any valuable line inputs on an audio
interface. Instead it can go straight into your system via AES/
EBU or S/PDIF, thus bypassing the entire standard interface
path. Further research and development went into the design
of the optional 192kHz A-D card for the ISA828, which combines
eight ISA series transformer-based mic preamps in a single 2U
unit. This used a new converter part and a specialised very low
noise, high current amplifier for the front end of the converter.
Once again, the key to a good design was component choice,
including the selection of low-noise components with high
dynamic range and low distortion; and careful board layout. By
taking a more sophisticated, more cost-effective approach to
this multichannel product, it was possible to reduce the chip
count on the digital side of the board from around 20 to just
four chips. The result is a conversion system delivering a level
of performance that sets the standard for the marketplace as a
whole.
THE MEANING OF R&D
Of course, it’s always possible to do better, even with existing
components. Running conversion chips in parallel can improve
performance, for example, but at a significant additional cost.
The challenge is to push back the boundaries of performance
without unwarranted cost to the purchaser: there’s no point
in having a world-beating system that is so much more
expensive than its competitors that nobody buys it. Indeed, we
could characterise the “Research” part of R&D as the path
to producing innovation as cost-effectively as possible. The
result goes to the “Development” phase, where the aim is to
produce actual products to put into people’s hands. The fact
that there are different acceptable price points for different
market segments also means that there will be performance
differences too across those segments. To an extent, you get
what you pay for.
The epitome of modern converter design: The A-D/D-A option
card for the ISA428 MkII and ISA828 provides Focusrite’s
multi-channel preamps with 192kHz/24-bit digital outputs
with a dynamic range of 122dB.