dbx 122 - Page 35

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developed and patented analog techniques to achieve
excellent rms detection at a moderate cost.
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 leve! detection
path (Figure 22 — 1, 4 & 4A) so that subsonic and super-
sonic signals (such as air conditioning noise or tape bias
noise) are less likely to create encode/decode mistracking.
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Fig. 22 — Block Diagram of dbx I! Noise Reduction Circuitry
The functions are explained in the text and on the following
pages.
A. The Signal Bandpass Filter
With a 3cB roll off at 27Hz 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 avoid 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 27Hz 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 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 modula-
tion and asperity noise components by some 12dB.
G. Overall! Play Processing (Decode) Response
This is the single (swept) sine wave response of the decoding
circuitry. Given a nominal level input at the dbx input (a sine wave