Spirit Digital 328, Руководство пользователя, Страница: 4 / 20

affecting it slightly. An analogy might be building a
model out of clay or out of Lego™ bricks. You can
make copies of both, but even with the best cast-
ing methods, you won’t be able to make a perfect
copy of the clay model. No matter how many
times you make copies, or copies of copies of the
Lego ™ model, each can be exactly like the origi-
nal.
One of the first areas of the audio to benefit
from the introduction of digital technology was
recording and playback equipment, and digital
recorders have now almost entirely replaced ana-
logue recorders in every application. Apart from
the fact that a well-designed recorder with good
converters offers very high audio quality, they offer
two distinct advantages. Firstly, the quality of digi-
tal recordings remains unchanged even after
repeated playback. Analogue tape recordings, on
the other hand, shows a progressive drop in quali-
ty after repeated passes. Secondly, digital recorders
can use random-access rather than linear record-
ing media, which allows them to play back record-
ed material in many ways other than simply ‘as
recorded.’ You can repeat sections (for choruses,
for example), perform cut-and -paste editing
operations, and so on.
328 FOCUS: CONVERTERS
Converting between analogue and digital is one of
the most critical stages of an audio signal path.
Once a signal is in digital form, it’s fairly robust,
but converting to and from analogue offers a
chance for audio quality to degrade significantly.
In order to capture input signals as faithfully as
possible, analogue-to-digital converters should
offer high bit resolution – 16 or 18-bit converters
are looking rather outgunned in these days of 20
and even 24-bit converters. The Spirit Digital 328
has 24-bit 128-oversampling A-to-D and D-to-A
converters, with considerably greater resolution
than established media such as CD and DAT (16-
bit), and allowing use with 24-bit systems such as
newer PC-based recording platforms.
At the output stage, the 328’s conversion is also
24-bit with 128-times oversampling.
SECTION
1: The Basics Of Digital Mixing
4
B. Bits and Sample Rates
The two most important factors determining the
quality of a digital audio system are sample rates
and bit resolution. A properly designed digital
recording system can reproduce audio frequen-
cies up to half of its sampling rate. So, in order to
cover the full range of human hearing, a sample
rate of 40kHz is required – hence the two most
common sample rates of 44.1kHz (the CD stan-
dard) and 48kHz.
Bit resolution, on the other hand, determines
the signal-to-noise ratio of the system - that is,
how much difference there is between its quietest
and loudest output. You may have heard that digi-
tal systems can achieve a performance of 6dB of
signal-to-noise per bit, so a 16-bit system (CD or
DAT, for example) manages 96dB. A 20-bit system
can in theory manage 120dB, though in higher
resolution systems it becomes harder and harder
to achieve theoretical performance.
328 FOCUS: USING DITHER
When you take a 24-bit audio signal down to 16-
bit resolution, you inevitably lose quality. Whereas
the 24-bit signal could have a signal-to-noise ratio
of 144dB, the 16-bit version of it has a signal-to-
noise ratio of 96dB. The Spirit Digital 328’s stereo
digital outputs, both AES/EBU amd SPDIF, use a
technique called dithering to preserve as much
quality as possible when going down to lower-res-
olution media, such as DAT, which have poorer
signal-to-noise performance than the mixer’s
internal signal path. Dithering involves adding a
small amount of noise to the 24-bit signal at just
about the theoretical noise floor of a 16-bit signal.
Odd though it may sound, this trick means that
you can hear low level sounds (reverb tails, for
example) well down below the noise floor, and the
16-bit signal actually achieves a dynamic range of
around 115dB. This improvement is audible no
matter what DAT player the recording is played on
- it does not require any special decoding.
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