dbx 128 - Bandpass Filtering and Noise Reduction

Bandpass Filtering
To further improve the performance of the dbx system,
bandpass filters are placed in the signal path, restricting the
response to the audible frequency spectrum. This does not
degrade the audio signal’s frequency response in any way.
Bandpass filters are also placed in the RMS level detection
path (Fig. 18, 4 & 4A) so that subsonic and supersonic
signals (such as air conditioning noise or tape bias noise) are
less likely to create encode/decode mistracking.
Pre-Emphasis & De-Emphasis
Tape modulation noise is a phenomenon that occurs with
all tape recordings. It consists of noise sidebands which
appear on either side of the signal which is being recorded,
and it is caused by inherent characteristics of the tape.
Modulation noise levels are significantly higher than the
residual background noise of the tape, although the modula-
tion noise falls off as the frequency moves away from the
recorded signal. The signal masks modulation noise com-
ponents that lie nearby in frequency, but it does not mask
noise which is several octaves above. For this reason,
modulation noise is most often a problem when a strong,
low frequency signal is recorded (what might be heard, for
example is a low organ or bass guitar note that is accom-
panied by a rushing, hissing sound . . . as the note dies, so
does the noise). dbx applies pre-emphasis and de-emphasis
to reduce modulation noise by up to 12dB (Fig. 18, 2&2A).
Levels
When using linear compression and expansion there is no
threshold at which the compression and expansion takes
place. Therefore no pilot tones or routine calibration are re-
quired. The 128 is provided with a REC LEVEL MATCH
control, and a PLAY LEVEL MATCH contro}. These con-
trols adjust the dbx record and play gain to correspond to
the nominal levels of your equipment. These level adjust- 31
ments let you maintain the same levels in record, play and
bypass modes for monitoring convenience. Level matching
is not essential for proper encode/decode tracking. (Refer to
pages 2, 3, 4 &15 for level adjustment information.)
Fig. 18 — Block Diagram of dbx II ‘‘Noise Reduction Circuitry”
(Diagram on next page)
A. The Signal Bandpass Filter
With a 3cB roll off at 20Hz and at 27kHz, this filter prevents
non-recordable signals from entering the dbx record processor.
It does not affect the audible spectrum, but it does prevent encode/
decode mistracking by preventing subsonic and supersonic signals
from entering the compander.
B. Record Signal Pre-Emphasis
A high frequency boost that matches reciprocal high frequency
reduction upon decoding (curve F), thereby reducing modulation
and asperity noise.
C. Level Detector Bandpass Filter
The same filter is used for encoding and decoding, and rolls off
3dB at 27H2z and 10kHz. The filter affects only the RMS level
detection, preventing expansion and compression circuits from
reacting to subsonic or supersonic signals or from being misled, by
poor frequency response in the tape recorder at the extremes of
the audio spectrum. This avoids encode/decode mistracking without
affecting overall signal frequency response.
D. RMS Level Detector Pre-Emphasis
The same RMS Level Detector pre-emphasis curve is used for
encoding and decoding. It complements the signal pre-emphasis and
de-emphasis curves, avoiding excessive high frequency levels which
might otherwise cause tape saturation or self-erasure.
E. Overall Record Processing (Encode) Response
This is the single sine wave response of the encoding circuitry.
Given a nominal level input at the dbx 128 input (a sine wave
signal swept across the audible spectrum), this is what the dbx
record output will do. Note that the overall encode and decode
response (curve H}) is essentially flat.
F. Play Signal De-Emphasis
The complementary curve for the record signal pre-emphasis
(curve B), containing a high frequency roll off to reduce modulation
and asperity noise components by some 12dB.