Dbx 3BX Service Manual

Page 2

RMS Detection
The 38X uses a rletectiorr method that acts on the RMS
lRoor-Meart~Stirrarel value at the input signal. HMS ile-
tectrorr is dillerent lrom either peak or average detection.
An HMS detection circuit will not over-react on musical
transrents or noise spikes, yet it responds precisely and
accurately to all changes irt input level, and produces
natural sounding expansion. In tact, the human ear judges
sound levels by their RMS values, which means that the
RMS detection circuit in the 38X electronically parallels
the way the human ear hears music. Until recently, how-
ever, llMS detection was very complex and costly. dlrx
has pioneered the developitrent ol moderately pr iced HMS
detection circuitry, and has led the industry in applying
HMS detection to expantlers, compressor/limiters and tape
noise reduction systems.

Linear dB Expansion

Once tlte signal has been detected," the expander
knows when to increase or decrease its level. The circuit
that actually perlorms this level change is known as a
"voltage controlled amplilier" or VCA. The AVC
(automatic volume control), and "ALC" (automatic levcl
controllon many cassette recorders are examples oi
voltage controlled artrplrliers, as are the level changing
Circuits in any modern expander, compressor or limiter.
The voltage liom the detection circuit increases or decreases
the gain til the VCA which increases or decreases the level
ol the program. While some expander: may rncrease or
decrease the program level by a lixed amount, the 3LlX
increases or decreases the level ol the program on a "linear
decibel" basis. This means that the output dynamic range
and the input dynamic range are linearly related by the
expansion ratio" over the tenure dynamic range (as
described earlier) to: a smooth, natural sound.

Attack and lie t. sc Turn.
The expander must decide how last to react to changes
in program level. The length oi time between the detection
at an input signal level, and its expansion is known as the
ATTACK TIME. Alter the expansion ol an input signal,
the expander allows the input signal to return to its normal
level. The amount ol time to return to normal is known as
RELEASE TIME. These terms also apply lor compressors
and limiters. in general, an expanders detection method,
and its attack and release times are not directly related.
For example, an expander that uses peak detection may
detect an increase Ill program level very rapidly, yet not act
on that increase (by expanding it) until much later. Thus,
peak detection does not necessarily imply last attack time;
the expander just cited would have a slow attack time. Con-
versely, an expander using average detection might have a
very last attack time. While such a unit might not detect an
increase in input level until the actual level had begun to
decrease again, the expander could react quickly, expanding
the program upward at a point where it should he expanded
downward. These are extreme examples, but they illustrate
the distinction between attack and release times and demo
tion method. Evert an expander using an HMS detection
scheme may not have optimum attack and release times lor-
evcry type of musical program. The reason is that the attack
anti release times ol urost expanders (and compressor]
limiters) are fixed (unchangingl. In those units where the
attack and release times are adjustable via front panel
controls, the times are still "lixed" at the control settings--
lixed since they do not change as the program requires.
Dillerent types ol music require dillerent attack and
release times. For example, a smooth classical string quar tet
number might sound best when expanded with a slow
attack and slow release time. where a punchy rock and roll

number might sound best when expanded with a last attack
and release time. Other programs might sound best with a
fast attack and slow release time. The point is that attack
and release times should be allowed to vary according to
the program content for the most natural sound. The 38X

does just that. The SBXs attack and release times auto-

matically and continuously follow the rate of change of
the "envelope of the program. in fact, because they are
not fixed, the 3BX5 attack and release times are specified

as rates which change in response to different program

envelopes, The result is a smooth action that does not alter
the character of the music as dynamics are expanded and

noise is lowered.

Single vs Multi-Banrl Expansion

dbx produces three other models of expanders, the H7,

118 and HS. and a combination expander/dint ll tape

noise reduction system, the model 128. These expanders

are single band units that expand the entire program at all
trequencies at the same time, A single band expander is less
expensive than a mulriband unit, and this method is satis-
factory lor most types of program material. In fact, the 117,
118, HQ and 128 outperlorm every expander marketed
except the new 38X. However, in some cases, when the
program has a very heavy bass line, single band expansion

can cause audible "breathing" as it also raises high ire-
quency noise when a loud level, low frequency note is

detected. Fortunately, the high lrequency content of most
program material masks this high lrequency noise so that
the breathing effect is usually unnoticable. A multi-band
expander has the advantage of being able to control the

expansion of dilierent treouency bands separately,

'The envelope it a graph of rite program level versus time, see the

GIUSIaIy for a rim/rel dehmrron.

eliminating the possibility that a strong note in one band
will cause audible eflects in another frequency band. After
careful consideration, dbx decided that three bands
represented the optimum number of frequency divisions
for an expander.

The 38X

Devices such as "noise gates," "correlators" and
horizontal litters" may be able to subtract noise from
certain types of programs, but they do not significantly
improve the dynamic range of the program in any other ..
way. In addition, such devices may be misled by certain
types of music, causing undesirable changes in the sound,
or they may change the frequency response of the pro-
gram, or even remove parts of the music.

The 38X represents a new state of the art in expansion.
With its three band linear dB expansion, RMS detection,
and program-controlled attack and release rates, it avoids
the midrange coloration of preemphasized units (so called
"weighted" exoanders), the "breathing" at single band
units, and the "noise modulation" ol units that divide the
spectrum into many bands. The 38X does not subtract
any frequency components lrom the music. Instead, it
reduces its gain during quiet passages and increases its gain
during loud passages, achieving 35dB or more of natural
sounding expansion.

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A,

l)
2)
3)
4)
5)
6)
7)
B)

9)

SEQUENCE OF STEPS FOR FINAL TEST

Check Power and Remote LED'S and LED Display
VCA Level and Distortion

Relative Level Set

Relative Level Response

Tone Burst

Noise

Clamp

LED Display

Clipping