How The Dbx Works - Tascam 414MKII PortaStudio Owner's Manual

Care and Maintenance
Cleaning the Pinch Roller
Clean the pinch roller at least once each day the deck
is used. Use a good rubber cleaner.
1. Clean the pinch roller with a cotton swab
moistened with rubber cleaner, until there is no
visible residue on the pinch roller.
2. Using a clean cotton swab, wipe off all excess
rubber cleaner from the pinch roller. Make certain
that there is no foreign matter remaining on the
pinch roller.
Cleaning the Capstan Shaft
After cleaning the pinch roller, clean the capstan
shaft with a cotton swab moistened with head
cleaning fluid.
Erase head Capstan
Record/Play head
Degaussing the Tape Path
Hold the degausser about 1 m (3 feet) away from the
recorder. Turn it on, slowly move into the tape path.
Move the degausser slowly back and forth, touching
lightly all metal parts in the tape path. Slowly move it
away again to at least 1 m (3 feet) from the recorder
before turning it off.
CAUTION
If the surface of the unit gets dirty, wipe the
surface with a soft cloth or use a diluted
neutral cleaning fluid. Clean off thoroughly.
Do not use thinner, benzine, or alcohol, as
they may damage the surface of the unit.
36
Pinch roller

How the dbx Works

The dbx system is a wide-band compression-
expansion system which provides a net noise
reduction (broadband, not just hiss) of a little more
than 30 dB. In addition, the compression during
recording permits a net gain in tape headroom of
about 10 dB.
A compression factor of 2:1 is used before recording;
then, 1:2 expansion on reproduce. These
compression and expansion factors are linear in
decibels and allow the system to produce tape
recordings with over a 90 dB dynamic range – an
important feature, especially when you're making
live recordings. The dbx system employs RMS level
sensors to eliminate compressor-expander tracking
errors due to phase shifts in the tape recorder, and
provides excellent transient tracking capabilities.
To achieve a large reduction in audible tape hiss,
without danger of overload or high-frequency self-
erasure on the tape, frequency pre-emphasis and de-
emphasis are added to the signal and RMS level
sensors.
Á
Encoder Á
Input
Dynamic range
Encode
of
Record
(Compress)
input signal 80dB Á 40dB
+20dB
+10dB
0dB
Dynamic range of tape
–30dB
–60dB
dbx encoding/decoding level diagram
SUBSONICS AND INTERFERENCE
The dbx system incorporates an effective bandpass
filter. This filter suppresses undesirable subsonic
frequencies to keep them from introducing errors
into the encode or decode process. However, if
rumble from trains or trucks is picked up by your
microphone and fed to the dbx system, modulation
of the program material during low level passages
may occur. This low-frequency component will not
itself be passed through the recorder and so, will not
be present at reproduce for proper decoding. If this
low-level decoding error is encountered, and
subsonics are suspected, we suggest the addition of a
suitable high-pass filter in the microphone line.
Á
Tape deck Decoder Output
Decode
Playback
(Expand)
Saturation
Á 40dB Á 80dB
level
+25dB
+20dB
+15dB
(65dB)
–50dB
–60dB
–80dB Noise level