6.2 Ax Sidechain Path
After leaving the OPERATING LEVEL gain normalizing circuit (Section
6.1.3), a split of the main audio signal feeds a state variable filter that pro
vides control of TUNE, PEAKING, and NULL FILL. The filter output signal
splits into three paths to feed a Downward Expansion circuit for NR
THRESHOLD Control, a Harmonic Generator with control of HARMONICS
and TIMBRE, and a VCAtt (voltage controlled attenuator) where all the
Excitement components are summed together. The output from the VCAtt is
routed through the MIX Control and is then sent back as the Ax signal to the
Electronic Switching Stage (Section 6.1.4)
6.2.1 State Variable Filter for TUNE, PEAKING, and NULL
FILL
U402A-C is a state variable filter configured as a high-pass filter at the output
of U402. The damping or "Q" of the filter is adjusted with the PEAKING
Control, VR103. VR 402A and B provides TUNE Control for selecting the
starting frequency of the high pass filter. VR404 is the NULL FILL Control,
with a taper correction resistor R430 in parallel, which introduces a band
pass signal from U402B to the input of U402D. The high-pass and band-pass
signals are together in inverting amplifier U402D which provides proper
signal polarity to the remaining circuits. The output signal from U402D
splits to the VCAtt, Harmonics Generator, and the Noise Reduction circuits.
6.2.2 Harmonics Generator
One signal from the state variable filter reaches VR405, a variable HAR
MONICS Control. The potentionometer wiper feeds a chosen amount of
signal to an Aphex MAX 1502 integrated circuit for transient processing of
fast attack signals. Transient processing helps produce more harmonics in
the attack, rather than the sustain, portion of the incoming signal.
The output signal at pin 10 of the MAX 1502 is routed to a waveform genera
tor circuit, composed of U406C and U406D. U406C is a half-wave rectifier
and U406D is a summing amplifier. The TIMBRE Control provides an
adjustable mix of the rectified signal and the compressed signal to produce
the desired balance of odd and even harmonics. The output of the summing
amplifier U406D is tied to VC of the VCAtt circuit (see Section 6.2.5).
6.2.3 Noise Reduction (NR) MODE A
NR MODE A is based on a downward expansion circuit that can provide up
to 30 dB of noise reduction through the use of a voltage controlled attenuator
(VCAtt). A signal from the output of U402D feeds a cascaded logarithmic
amplifier made up of U406A and 406B. The combination of these two op
amps provides a greater open loop gain than a single integrated circuit.
Back-to-back precision diodes contained within the VCAtt (pins 10 and 12)
Circuit Description
6-5
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