dbx 128 - Professional vs. dbx II Systems

Consider a 100 dB program which might have loud peaks
at +18 dBm (6.16 volts) and quiet passages as low as - 82
dBm (61.6 microvolts).* If the tape recorder has a maximum
input level of +15 dBm (4.36 volts) before distortion and a
residual noise level of - 40 dBm (7.75 millivolts, a millivolt
is one-thousandth of a volt) (in other words, a 55 dB S/N
ratio whichis typical of many good hi-fi and semi-pro tape
machines), there is no way the original program dynamics
can be captured on tape; peaks will be lost due to saturation,
quiet passages will be lost in the noise, or a combination of
both. dbx encoding (compression) will solve this problem by
bringing the maximum signal level down to +9 dBm (2.18
volts), and by raising the minimum level to - 41 dBm (6.91
millivolts) (compression by a factor of 2:1), so the recorded
program would fall within the usable dynamic range of the
tape. (See Figure 15) That is, the quietest signal remains
above the noise and the loudest signal remains below the
point of tape saturation.
NOTE: Maximum input levels on tape recorders are specified
at the 3% harmonic distortion point; because dbx encoding
lowers maximum recorded levels, it greatly lowers distortion
at the same time it eliminates audible noise.
When the encoded recording is played through the dbx
PLAY circuitry, expansion increases the level of the louder
*The term “dB SPL” is discussed on page 4, and refers to the
acoustic sound pressure level. The term “dBm” refers to the level
of “sound’ ’ while it is in electrical form. dB is a relative term, and
there is no direct equivalence of dB SPL to dBm, although the terms
are proportional, That is, an electrical increase of 1dBm in an
amplifier will create and acoustical increase of 1dB SPL in front of
the speakers. Thus, a live program might increase from 50dB SPL
to 100dB SPL. When translated to an electrical signal by a micro-
phone, the same program might go from —-40dBm to +10d8m...
still a 50dB increase in program level.
passages, and decreases the level of quieter ones. In the pre-
ceding example, the +9 dBm (2.18 volts) recorded level
would be restored to +18 dBm (6.16 volts), the -41 dBm
(69.1 millivolts) level would be restored to -82dBm (61.6
microvolts), and all other levels in between would be pro-
portionately restored (see Figure 15). The tape noise is also
subject to expansion, and it drops from -— 40 dBm (7.75 mil-
livolts) to - 80 dBm (77.5 microvolts) . . . to inaudibility. *
This type of processing for tape recording is far more effec-
tive than filters which operate only during playback of con-
ventional 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 I| processing retains
the full frequency spectrum of the original program.
Many attempts have been made to apply the classical com-
pression/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-MeanSquare) 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 128 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-chan-
*A microvolt is one millionth of a volt.
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