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although the terms are proportional. That is, an electrical
increase of 1dB in an amplifier will create an acoustical
increase of 1dB SPL in front of the speakers. Thus, a live
program which increases from 50dB SPL to 100dB SPL,
when translated to an electrical signal by a microphone,
might go from —40dBm to +10dBm... still a 50dB
increase in program level.
When the encoded recording is played through the dbx
PLAY circuitry, expansion increases the level of the louder
passages and decreases the level of quieter ones. In the
preceding example, the +9dBm recorded level would be
restored to +18dBm, the —41dBm level would be restored
to —82dBm, and all other levels in between would be pro-
portionately restored (See Figure 19). The tape noise is also
subject to expansion, and it drops from —40dBm to
—80dBm ... to inaudibility. This type of processing for
tape recording is far more effective than filters which
operate only during playback of conventional recordings
(like the rumble and scratch filters on your receiver or
amplifier). Moreover, such playback-only filters may cut
off the higher frequency program harmonics in order to
reduce hiss, whereas dbx II processing retains the full
frequency spectrum of the original program.
Many attempts have been made to apply the classical
compression/expansion concept to tape noise reduction,
but most of these have been less than successful for
technical reasons. The dbx approach, however, is unique.
A voltage controlled amplifier (VCA) serves as the gain
control element, and a true rms (root-mean-square) level
sensing detector insures perfect encode/decode tracking for
accurate transient response . . . regardless of phase changes
that may be induced by the tape machine. Moreover, there
is no audible breathing, pumping, or other coloration of
the sound. Critical level matching is not needed because
the dbx system encodes and decodes accurately despite
differences in record and playback levels.
The Two dbx Systems: dbx (professional) and dbx II
The original dbx noise reduction system was developed
for use in professional recording studios. With the
increasing popularity of dbx noise reduction in models
like the 4-channel 187 and the 16-channel 216, many home
recording enthusiasts sought better quality tapes. In
response to the demands of consumers and smail studios,
dbx introduced a variety of semi-professional units (the
150 series). These units utilize the same signal processing as
the professional recording studio models, so tapes made
with the professional series may be decoded with the semi-
professional series, and vice-versa.
Broadcasters realized the potential for improved signal
quality if they could use dbx processing. However, broad-
cast cartridges and telephone transmission lines do not
offer the excellent frequency response available in pro-
fessional
recording
studios
and
in
better
hi-fi
tape
machines;
the
low
and
high
end
of
the
frequency
spectrum
often
fall
off
considerably.
With
the
dbx
professional
system,
poor
high
and
low
frequency
response
in
a
tape
recorder
can
cause
mistracking
of
the
rms
detection
circuitry.
Also,
the
rumble
and
low
frequency
errors
of
warped
discs,
or
the
limited
low
and
high
frequency
response
of
telephone
and
microwave
transmission
lines
would
create
mistracking
of
the
rms
detection
circuitry.
Therefore,
the
dbx
Il
system
was
developed,
represented
by
dbx
Models
122,
124,
128
and
the
142.
The
basic
principle
of
operation
of
dbx
pro-
fessional and dbx II are identical, and the amount of noise
reduction is the same, yet the two systems are not com-
patible. A tape encoded with either system cannot be
decoded by the other. Similarly, a dbx encoded disc (dbx
It processed) cannot be decoded with the original dbx