Mackie 8-BUS Series, Руководство пользователя

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D-1
O w n e r ’ s M a n u a l
Apogee UV22
Appendix D
Apogee UV22
Dither And UV22 :
The Apogee Difference
In the analog world, as a signal dies away, it
does so smoothly (if nothing is wrong with the
system). As the level drops, the signal gets
progressively quieter. At some point it reaches
the same level as the noise, and as the signal
level continues to drop, you can still hear it,
even though it lies below the “noise floor.” This
constitutes an important aspect of the way that
analog signals behave—you can hear coherent
audio information even when it has a significantly
lower level than random noise.
In the raw digital environment, things are dif-
ferent. As the level of a signal drops, fewer and
fewer binary digits represent the audio informa-
tion. Ultimately, you simply run out of bits, and
when this happens, the signal just stops, and in
a 16 bit system, this happens at an audible level.
This behavior is one of the factors that gave
early digital recordings a bad name, and led
some pundits to claim that digital audio sounded
fundamentally inferior to analog.
Adding noise to the signal provided one
solution to the problem. At low levels, this
effectively turns the last few bits on and off at
random, smoothing out the sound and ensuring
that everything does not simply disappear as the
level falls. We call this noise “dither noise” or
simply “dither.” The word literally means to
tremble or quiver—a reference presumably to the
least significant bits turning on and off at random.
The disadvantage of this process: you
introduce noise into the system and therefore
effectively degrade performance. More than
that, the noise actually sounds quite objection-
able. Truly random (white) noise contains all
frequencies and it sounds particularly obnoxious.
As a result, several manufacturers and research-
ers have attempted to improve the situation by
developing methods of hiding or “shaping” the
noise created by the dithering process.
This may appear even more important when
you record a signal, say, in 24-bit form and want
to reduce it to 16 bit for compact disc. If we
could preserve the detail of a 24-bit recording by
making the noise floor more transparent—more
like analog—then we would achieve an audible
improvement in the quality of the final CD, and
we would enjoy audible benefits by recording be-
yond the 16 bit level, even for a conventional
16-bit compact disc.
Most of these “noise shaping” techniques
rely on the fact that the ear seems more sensi-
tive to midrange frequencies (around 4kHz)
than to either low or high frequencies. In trans-
ferring a 20-bit recording to the 16-bit world of
compact disc, for example, we remove the last
four bits of the 20-bit signal and feed them
back into the input signal through a filter that
both adds dither and changes the spectral
shape. Originally, the filter shape proposed by
researchers (primarily at the Audio Research
Group at Waterloo University, Ontario) and
based on psychoacoustic principles added more
noise in the upper frequencies while lowering
the noise floor at around 4kHz—the frequency
at which measurements indicate the ear’s
maximum sensitivity.
In fact, they could have achieved even better
results by adding the noise back in at low fre-
quencies, to which the ear is even less sensitive
(hence “loudness” controls on your receiver or
car stereo) but this requires significantly more
processing.
More recently, a number of manufacturers
have claimed that their own proprietary filter
shapes sound audibly superior to the theoretical
designs. Unfortunately, these noise shaping
techniques can cause problems. First, although
they lower the noise floor at the most audible
frequency, they unavoidably increase the overall
noise. They also, according to independent mea-
surements, add audible artifacts to the sound.
The fact remains that dithering is an impor-
tant tool for digital audio. But how can we do it
so that the results sound good? Apogee has the
answer. Apogee takes a completely different
approach with the UV22
® process, incorporated
in Apogee’s converters and now, for the first
time, in your Mackie Digital 8•Bus console.
UV22 does not constitute a “new flavor” of
dither noise. Instead, UV22 essentially modu-
lates the data from the least significant bits of a
signal on to the 16-bit signal according to a
special algorithm, which adds an inaudible high-
frequency “bias” to the digital bit stream,
placing a “clump” of energy at around 22kHz.
This results in an essentially flat noise floor, at
the theoretical 16-bit level—4 to 5 dB below
that of conventional “flat dither.” In addition, the
noise floor does not have the distinctive and an-
noying “hissiness” of conventional dither. The
UV22 noise floor sounds audibly quieter and
less objectionable than other techniques. In ad-
dition, you cannot hear any audible artifacts.
Yet, as with analog, you can hear coherent audio
signals several dB below the noise “floor”—thus
retaining much of the detail and audio quality
inherent in the original signal.
SUPER CD ENCODING