dbx 128 - HOW dbx II TAPE NOISE REDUCTION WORKS; dbx System Overview

26
HOW
dbx
I!
TAPE
NOISE
REDUCTION
WORKS
A
Short
Explanation
To reduce tape noise, the dbx 128 utilizes a sophisticated
version of the classical compressor/expander (compander)
concept. The RECORD processor compresses the input to
the tape recorder by a 2:1 ratio, linear over a 100 dB range.
Upon playback, the PLAY processor provides 1:2 expansion
TAPE RECORDER
{COMPRESSOR}
{dbx RECORD MODE}
FROM PREAMP
“TAPE OUT"
\
LEVEL IN oBm
>
-40 —
-60 —
380°
Fig. 15 — How the dbx System Works
A. Typical program of wide dynamic range (about 100dB).
B.
2:1
compressed
program
(encoded)
reduces
dynamic
range
by
half,
so
that
signal
can
be
placed
below
the
tape
saturation
tevel
and
above
the
tape
noise
level.
C.
1:2
expanded
program
(decoded)
restores
the
original
dynamic
py)
of
the
recorder’s
output.
The
expansion
is
a
mirror
image
of
the
compression
so
the
recorded
signal
cannot
be
distinguished
from
the
original
audio
source,
full
dynamics
have
been
pre-
served,
and
virtually
no
audible
tape
noise
has
been
added.
TAPE PLAYBACK
(EXPANDER)
(dbx PLAY MODE)
TO PREAMP
“TAPE IN",
OR "MONITOR" IN
—+27
— +20
TAPE NOISE
(NOISE FLOOR OF dbx UNIT) — -88
— -100
range
(100dB).
Tape
noise
is
always
well
below
the
quietest
part
of
the
program.
D.
Dynamic
range
of
the
dbx
compressor
and
expander
is
about
100dB.