Aphex AURAL EXCITER III User manual

AURAL EXCITER® TYPE III
MODEL 250
OPERATING GUIDE
&
SERVICE MANUAL
APHEX SYSTEMS LTD.

WARNING
TO REDUCE THE RISKS OF FIRE OR ELECTRIC
SHOCK, DO NOT EXPOSE THIS APPLIANCE TO
RAIN OR MOISTURE

TABLE OF CONTENTS
1.0 INTRODUCTION 1-1
2.0 OPERATIONAL CHARACTERISTICS 2-1
2.1 New Features 2-1
2.2 Typical Applications 2-3
2.3 Specifications 2-4
2.4 Warranty and Service Information 2-6
3.0 INSTALLATION 3-1
3.1 Unpacking 3-1
3.2 Setting Input Sensitivity 3-1
3.3 Audio I/O (Input/Output) Connectors 3-1
3.4 I/O Connection Considerations 3-2
3.5 AC Line Connection 3-2
3.6 Remote Control Connection 3-4
3.7 Installing the Type III in an Equipment
Rack 3-4
3.8 Applying Power and Initial Checkout 3-5
4.0 FRONT PANEL CONTROLS 4-1
4.1 NR MODE Switch 4-2
4.2 NR THRESHOLD Switch 4-4
4.3 TUNE Control 4-5
4.4 PEAKING Control 4-4
4.5 NULL FILL Control 4-5
4.6 HARMONICS Control 4-6
4.7 TIMBRE Control 4-8
4.8 MIX Control 4-8
4.9 Ax Switch 4-8
4.10 SOLO Switch 4-9
4.11 SPR Switch 4-9
4.12 RELAY Switch 4-10
4.13 POWER Switch 4-10
5.0 APPLICATIONS 5-1
5.1 Initial Settings 5-1
5.2 Exciting a Recorded Tom or Kick Drum 5-3
5.3 Recording a Snare Drum with
Excitement 5-4
9310HTW
Table of Contents 1

TABLE OF CONTENTS
5.0 APPLICATIONS (continued)
5.4 Exciting FM Audio for Television
and Radio 5-5
5.5 Exciting AM Broadcast Audio 5-6
5.6 User Templates 5-7
6.0 CIRCUIT DESCRIPTION 6-1
6.1 Main Audio Path 6-2
6.2 Ax Sidechain Path 6-5
6.3 Power Supply 6-7
7.0 TEST AND NULLING PROCEDURES 7-1
7.1 Main Audio Board 7-1
7.2 Control Boards 7-2
8.0 PARTS LIST 8-1
8.1 Main Audio Board 8-1
8.2 Control Board 8-5
8.3 Power Supply 8-8
9.0 SCHEMATICS
2Table of Contents

1.0 INTRODUCTION
A little history...
The new Aural Exciter III
It all started back in 1956 when Curt Knoppel, an inventor and entrepreneur
who was tinkering with a Heathkit amplifier, accidentally stumbled across
the principles behind a new audio processor that could effectively restore the
essential high-frequency musical harmonics that are lost in the recording and
playback process. Nineteen years later, after moving to the United States
from Sweden, Knoppel met Marvin Caesar, then an accountant and land
developer. An audiophile himself, Caesar marvelled at Knoppel's invention,
and the two became partners in 1975, forming Aphex Systems in Framing
ham, Massachusetts.
After moving to Hollywood, California in 1976, the company began to
establish itself by leasing its provocatively-named "Aphex Aural Exciter" to
record companies for $30 per minute of finished product. The original
Model 402 was tube-powered and was subsequently updated to a solid state
version, the Model 602. This two-channel unit provided controls for input
sensitivity, output level, Aphex mix, de-esser, and signal phase reversal.
The unique combination of circuitry made this psychoacoustic processor an
essential ingredient in producing the "California" sound on numerous
projects, including albums by Heetwood Mac, Linda Ronstadt, James Taylor,
and Jackson Brown. As the hit list grew, so did criticism from detractors,
and, more importantly, competition from manufacturers with knock-off
products.
In 1982, in an effort to stay innovative and also keep ahead of the field,
Aphex introduced the second generation Model 201, the Aphex Aural Exciter
II. This version provided additional features for tuning the high pass filter,
as well as controlling the range and type of harmonics generated within the
patented Aphex process. Also, the VU meters were replaced with electronic
bar graph indicators having selectable ballistics and reference level settings.
This new design was a great advancement over the aging Model 602, yet the
biggest change was the dropping the policy of lease only in favor of outright
purchase. Popularity soared as numerous engineers and producers found
out that they could now buy the famous Aphex Aural Exciter. The increased
sales volume propelled the Model 201 into every conceivable sound applica
tion. It seemed as though every sound specialist was using the Aural
Exciter II in recording studios, concert venues, film dubbing theaters,
broadcast, and video post-production houses throughout the world.
The beginning of 1989 marks the introduction of the latest generation Aphex
Aural Exciter III. The slimmer and more powerful Model 250 is the latest
successor to the premium product in the field of Aural Excitement. As you
read the following sections you will find information on Operational Char
acteristics, Installation, Front Panel Controls, Applications, Circuit Theory,
and Maintenance Procedures. Technicians and other interested users will
find complete schematics located at the end of the manual.
Introduction 1-1

2.0 OPERATIONAL CHARACTERISTICS
Special features...
Main and Sidechain
audio paths
A and B noise
reduction
The Aphex Aural Exciter Type III, Model 250, is the latest generation, fully -
professional Aural Exciter, loaded with powerful new processing features
and improved circuitry. The dual-channel Type III adds thrilling new
dimensions to Aural Excitement. Noise and distortion are virtually elimi
nated in the Type III circuitry, with new special features that make enhance
ment possible and actually erase noise from noisy sources. The Model 250 is
the third successor to the original Aphex Aural Exciter that started an audio
revolution back in 1975.
2.1 N ew Features in the M odel 250
The Aphex Type III Aural Exciter utilizes a patented audio process that will
recreate and restore missing harmonics. These harmonics are musically and
dynamically related to the original sound. When added, they restore natural
brightness, clarity and presence, and can actually extend audio bandwidth.
These harmonics are so low in level, however, they add little power to the
signal. Unlike an equalizer or other "brightness enhancers," which can only
boost high frequencies, the Type III Aural Exciter extends the high frequen
cies. It is a single-ended process that can be applied at any point in an audio
chain, and needs no decoding.
2.1.1 Two Audio Paths
The Type III Aural Exciter process consists of two audio paths, the main path
and the process side-chain path. The main path transparently conveys the
audio signal directly from the input stage to the output stage, maintaining
unity gain with wide dynamic range. The side-chain path contains all of the
Aural Exciter processing circuits and receives audio from the input stage. A
mixing circuit in the main path allows the sidechain output signal to be
mixed with the main signal. You adjust the amount of MIX to set the
strength of the effect.
2.1.2 Two Noise Reduction Modes
Additional new design features include two modes of noise reduction. Mode
A and Mode B, for sound enhancement without adding to the noise floor of
reasonably noise free sources. It can also enhance the brightness, detail and
clarity of seriously noisy audio sources while, at the same time, erasing much
of the original noise.
Mode A operates as a linear sidechain expander with variable threshold. The
expansion ratio of 2.5-to-l permits the sidechain MIX to follow the signal
level below threshold. When the higher frequencies of the input signal
drop below threshold, the MIX will drop at a proportional 2.5 to-1 ratio. In
this way, the original signal-to-noise is not affected even with a great deal of
enhancement.
Operational Characteristics 2-1

Mode B is a revolutionary new noise reduction technique which operates to
actually "erase" source audio noise while the Type III enhances the signal.
This feature allows you to restore brightness and intelligibility to noisy
recorded tracks or other noisy sources and improve the signal-to-noise ratio!
Both modes are fast and easy to use, and effective in any application from
live sound to broadcasting and recording.
2.13 Spectral Phase Refractor (SPR)
Correct bass delay The SPR, or Spectral Phase Refractor, function of the Type III is a totally
new concept in psychoacoustic enhancement which can produce some
amazing results. Through the many steps of recording, duplicating, distrib
uting and reproducing sound, the phase of the low frequency audio spec
trum becomes delayed compared to mid and high frequencies. This is a
natural and unavoidable effect which becomes worse with each generation.
When the bass frequencies become delayed in time compared to other
sounds, the bass loses fullness and definition and seems to become less
powerful, even though there may be no actual loss of bass frequency re
sponse. The high end also loses definition, seeming to get duller.
Amplitude equalization at this point will not fully restore the clarity and bass
power. In fact, it may worsen the condition causing clipping or overload
distortion. SPR corrects the bass delay anomaly to restore clarity and
openness and significantly increases the apparent bass energy level without
adding any amplitude equalization or "bass boost. " The SPR function
works harmoniously with the new Aural Exciter circuitry to give the Type III
Aural Exciter amazing new capabilities.
2.1.4 Adjustable Harmonics Level and Timbre Mixing
The DRIVE Control of previous Aural Exciters has been eliminated. In its
place are two new controls that provide adjustable mixing of the exact
amount of desired harmonics level and timbre. These features make the
Type III more flexible and even easier to use.
2.13 Servo Balanced Inputs and Outputs
Short-circuit proof Although more costly than conventional designs, servo-balancing offers
many advantages. Servo-balanced input circuits absorb high common-mode
voltages found in long cable runs without sacrificing headroom. Servo-
balanced outputs are not only short-circuit proof, but can be used single-
ended at any time without the usual 6 dB loss of conventional circuits.
To use input or output in a single-ended (unbalanced) mode, you need only
to ground the unused pin. In addition, both inputs and outputs are fully RF
protected. The unity gain I/O structure is normalized for both+ 4 dBu
(professional) and -10 dBv (IHF) operating levels by switch selection from
the rear panel.
2-2 Operational Characteristics

2.1.6 Null Fill
Phase Nulling... NULL FILL is a new and useful tuning adjustment introduced for the first
time with the Aural Exdter Type III. The addition of NULL FILL to the
PEAKING and TUNE controls gives the Type III more power and flexibility
to enhance all types of audio sources. To understand how it works, it is nec
essary to understand a physical phenomenon called "Phase Nulling" which
occurs with all Aural Exciters when the MIX control is adjusted to obtain
high frequency enhancement.
There is a time delay associated with the sidechain signal which is an impor
tant part of the operating theory of the Aural Exciter. This time delay causes
transient waveforms to be slightly "stretched" as the sidechain signal is
added to the main audio path. The "stretched" transients are then perceived
by the ear as more pronounced or "louder". The side effect of the time delay
is a "dip" or "null" in the output equalization curve. The null can be a
desirable characteristic because it compensates for the slight additional
power added to the signal by the high frequency shelving boost. The null
de-emphasizes the frequency range around the TUNE control setting, thus
giving even greater emphasis to the higher frequencies.
There are times, however, when the Phase Null is unwanted. The NULL
FILL control allows the user to "fill-in" the phase null to any desired amount,
thus further improving presence.
2.2 Typical Applications
Operational Characteristics 2-3
The Aural Exciter may be used in many ways for audio enhancement.
Depending upon the requirements, either pre- or post-processing may be
selected. Either source or system optimization, or a combination of both, is
possible.
2.2.2 Sound Reinforcement
Increase intelligibility For example, a P.A. system may be greatly enhanced by using the Aural
Exciter to increase the intelligibility of the loudspeakers, thus improving
penetration of the sound around comers and in areas usually difficult to fill.
In another case, the source may sound dull and hard to understand. The
Aural Exciter will compensate for this deficiency by adding brightness and
clarity to the sound more effectively than use of equalization alone.

Pre-process recordings
Extend bandwidth
2.2.2 Tape Duplication
The Aural Exciter may be used to "pre-process" recordings to anticipate the
audio degradation in the medium or during subsequent reproduction. Much
of the detail added by the Aural Exciter will survive filtering and distortion
of the reproduction equipment and provide a better quality audio playback.
Audio and Video Cassette duplication are examples of this application.
Broadcasting is another good example.
2.23 Sound Recording
In the recording studio, post production suite or similar environment, post
processing of previously recorded sound tracks can restore lost vibrance and
realism, even to the extent of saving dialogue or sound effects which were
thought to be unusable. Instruments and vocals can be made to stand out in
the mix without substantially increasing the mix levels or using equalization.
2.2.4 Electronic Music
Many electronic instruments are limited by their sampling rate (bandwidth)
and word length (resolution); they can sound lifeless. The Aural Exciter
actually extends bandwidth and adds details making synthesizers, samplers
and drum machines come alive.
2.23 Broadcasting
Broadcast audio is constrained in dynamics and frequency response to fit the
parameters of AM and FM transmission. AM and FM radio, as well as tele
vision audio, all have unique, critical bandwidth and signal level transmis
sion limits. The Type III is especially useful in creating the perception of
higher frequencies and greater dynamics without pre-emphasis distortion or
overmodulation. As a result, the final audio product has more presence and
clarity, with no apparent increase in noise.
2.3 Specifications
2-4 Operational Characteristics
VCA: A phex1001
Frequency Response: +0,-ldB from lH z to 190 kHz
THD: .01% Max@ +27dBu
IMD: .01 % Max @ + 27 dBu
Max Input/Output Level: +27 dBu
Nominal Gain: 0 dBu

InputInpedance: 19.5 k Ohms/600 Ohms (switch selectable
from rear panel)
Load Impedance: 600 Ohms or greater
Output Impedance: 65 Ohms
Unweighted Output Noise (20
Hz-20 kHz BANDWIDTH):
-86 dBu
Unweighted SNR(for +4dBu
operating level):
-90 dB
Dynamic Range: 114dB
Crosstalk: Better than 85 dB up to 20 kHz
Output Circuits: Servo Balanced Transformerless
Input Circuits: Servo Balanced Transformerless
Input CMRR: Better than 50 dB 20 Hz to 10 kHz
Input RF Rejection: Better than 40 dB at 800 kHz, better than 60
dB above 2 MHz
Power Requirements: 100/120/220/240VAC (+/- 10%), 50/60 Hz
16 W.
Size: 1 3/4" (44.5 mm) h x 19" (482.6 mm) wx9"
(228.6 mm) d
Weight: 8 lbs (s.63 kg)
Accessories: Model 51 Plexiglass Security Cover
Aphex Aural Exdter covered by U.S. Patent Number 4,150,253. Additional
Patents Pending. Foreign Patents granted and pending.
Aphex Systems is constantly striving to maintain the highest professional
standards. As a result of these efforts, modifications may be made from
time to time to existing products without prior notice. Specifications and
appearance may differ from those listed or shown. Aphex and Aural Exciter
are registered trademarks of Aphex Systems, Ltd.
Operational Characteristics 2-5

2.4 Warranty and Service Information
Aphex Systems, Ltd. warrants parts and labor for the Aural Exciter Type ID
for a period of one year from the date of purchase.
If it becomes necessary to return a unit for repair, repack it in the original
carton and packing material, if possible. For warranty repair, enclose a
copy of proof of purchase and send package to:
APHEX SYSTEMS, LTD.
11068 RANDALL ST.
SUN VALLEY, CA 91352-2621
PH: [phone] FAX: [phone]
2-6 Operational Characteristics

3.0 INSTALLATION
To install the Aphex Aural Exciter Type III for use with your system, per
form the instructions described in the following sections.
CAUTION... H Id Thoroughly read Section 3.5 before you apply power to the unit. An
incorrect line voltage setting can damage the unit.
3.1 Unpacking
Your Aural Exciter Type III was carefully packed at the factory, and the
container was designed to protect the unit from rough handling. Neverthe
less, we recommend careful examination of the shipping carton and its
contents for any sign of physical damage which could have occurred in
transit. If damage is evident, do not destroy the container or packing mate
rial. Immediately notify the carrier of a possible claim for damage. Shipping
claims must be made by the consignee.
3.2 Setting Input Sensitivity
Before using the Aural Exciter Type III, you need to set the unit for the
correct operating level (OVU). Choose either -10 or +4 dBu by pressing the
OPERATING LEVEL button IN or OUT for each channel located on the back
panel, as shown in Figure 3-1.
The IN position engages a -10 dB Operating Level, while the OUT position
is suitable for Operating Levels from 0 to +10 dB.
3.3 Audio I/O (Input/Output) Connectors
The audio inputs and outputs are made with standard XLR type 3-pin
connectors on the rear panel. Equal polarity is maintained from input to
output, so that you may elect to use either pin 2 or pin 3 as high to match
your balanced system. Also be sure to use proper shielding techniques (i.e.
lifting the shield wire to the output connection and connecting the shield
wire to the input connection).
Installation 3-1

3.4 I/O Connection Considerations
Prior to installing the Aural Exciter Type III into your system, take note of
the following considerations for 1/O impedance matching and system
wiring.
3.4.1 Impedances
The input is of high (19.5 kOhms) impedance and may be easily driven by
any output source. If your source needs to see a 600 Ohm load, press the 600
OHM INPUT TERMINATION button located next to each audio input
(Fig-3-1.). Pressing the switch IN connects an internal 602 Ohm (1%) resistor
across pins 2 and 3 of the input XLR connector.
The output is of an active, transformerless type, with an output impedance of
65 Ohms between pins 2 and 3. When driving certain transformer coupled
loads, the unit may need 270 Ohm 5% resistors installed in a series bridge
with pins 2 and 3 to create a 600 Ohm source impedance, as shown Fig. 3-2.
3.4.2 Balanced vs. Unbalanced Use
Although of balanced configuration, the inputs may be driven single ended
(unbalanced), on either pin 2 or 3, by tying the unused pin to pin 1 ground.
3.5 AC Line Connection
A.C. line power is supplied to the unit via an integral receptacle/fuse holder
on the rear panel which meets all of the international safety certification
requirements and also doubles as a radio frequency line filter.
Verify that the unit is configured to match your A.C. line voltage by inspect
ing the voltage programming tag located with the fuse holder, as shown in
Fig. 3-3. If you need to change the voltage setting, follow the procedure
outlined in the following two sub-sections.
3-2 Installation

3.5.1 AC Voltage Selection
As shipped, the Aural Exciter Type III is set for 120 VAC operation, but you
can change your unit to match any standard AC line voltage simply by
reprogramming the voltage programming card in the fuse holder in the
following way:
1. Remove the power cord from the chassis receptacle.
2. Slide the clear plastic cover to the left to uncover the fuse
compartment.
3. Remove the fuse by prying out the "fuse pull" lever.
4. Pull out the small printed circuit programming tag.
The tag has four voltages printed on it which are labelled 100/220 on one
side and 120/240 on the other side, as shown in Fig. 3-3. Orient the tag so
the required voltage is readable on the top left side of the tag, and reinsert
the tag in the fuse holder. You should now be able to read your correct line
voltage through the window of the fuse holder.
3.5 .2 Fuse Selection
After reprogramming the AC line voltage, be sure one of the following rated
fuses is inserted into the fuse holder before you apply power to the unit:
0.25A Slo-Blo for 100-120 ACV operation
0.125A Slo-Blo for 220-240 ACV operation
Installation 3-3

3.6 Remote Control Connector
The Aural Exciter Type Etl can be set up for remote bypass control use via the
REMOTE CONTROL connector, as shown in Fig. 3-4.
This four-pin, modular telephone-type jack (Fig. 3-1) provides the follow
ing control signals:
Pin 1 - Logic Ground
Pin 2 - Channel 2 Relay Control
Pin 3 - Channel 3 Relay Control
Pin 4 - +15 VDC (for remote indicator lamp use)
CAUTION...^/ Shorting pin 4 to ground will temporarily disable the Type III and eventually
cause damage.
You may want to construct the suggested circuit (Fig. 3-4) for remote on-line
control of the Aural Exciter Type III in an automated broadcast system,
production studio, or audio duplication chain.
3.7 Installing the Type III in an Equipment Rack
The Aural Exciter Type III occupies one standard rack unit of space
(1 3/4") at a width of 19" and a depth of 9". Allow at least an additional 3'
of depth for connectors and at least 1/2" of air space around the unit for
cooling.
3-4 Installation
Mount the unit in the desired space by using the cushioned rack screws
provided with the package.

3.8 Applying Power and Initial Checkout
After you have completed the previous instructions, apply power to the unit
by depressing the POWER switch (located on the front panel). If none of
the indicator LED's light, check fuse and power cord installation before
proceeding with the remaining checks.
Position all rotary knobs at 12 o'clock, send a test signal of your choice to
each channel input, and verify that the signal is passing through the unit by
auditioning or measuring the signal at each output.
After you are satisfied that signal is passing through the unit, proceed to the
next section for an explanation of the Front Panel Controls.
Installation 3-5

4.0 FRONT PANEL CONTROLS
The front panel layout of the Aphex Aural Exciter III provides identical sets
of controls for two independent channels of Aural Excitement, as shown in
Fig. 4-1.
Front Panel Controls 4-1
Each channel contains six rotary controls (located left to right), for setting
NR THRESHOLD, TUNE, PEAKING, NULL FILL, HARMONICS, TIMBRE,
and MIX parameters. In addition, each channel also has an identical set of
four in/out pushbutton switches (located left to right), for selection of NR
MODE, Ax, SOLO, and SPR. Besides these controls and switches, the
Model 250 also includes a single two-channel RELAY bypass pushbutton
and a unit POWER on/off switch.
The following sections contain details on the function and use of each control
and switch. Furthermore, a block diagram, as shown in Fig. 4-2, shows a
simplified signal path to help you understand the effect of each adjustment
or selection.

Notice that the signal coming from the input is divided into two paths,
known as the "Main Path" and "Ax Sidechain". The "Main Path" contains
very little processing and basically serves as a signal tap and summing point
for the Excitement circuits. In the "Ax Sidechain", the signal tap is altered
and enhanced via the rotary controls, and then is added back to the original
signal on the way to the output connector.
As a suggestion, periodically refer to the block diagram as you become
familiar with the controls and switches described below. When you have
completed this section, move on to APPLICATIONS for details on typical
settings.
4.1 NR MODE Switch
The NR MODE is a new feature which reduces noise that becomes enhanced
by the Exciter circuits. This NR MODE switch, located above each accompa
nying NR THRESHOLD control (described in section 4.2), gives you a choice
of either A or B noise reduction. Two LED's, labelled A and B, show which
mode is active, depending on the position of the switch. In addition, each
mode also has three associated LED's that indicate the present threshold
status for a chosen noise reduction mode.
4.1.1 NR MODE A
NR MODE A is a linear 2.5 to 1 downward expansion circuit that controls the
"Ax Sidechain" signal. This primary mode of noise reduction controls the
noise that becomes enhanced by the "Ax Sidechain" circuits. The associated
red, yellow, and green LED's, labelled -20, -10, and 0 dB respectively, give
an indication of how much the "Ax Sidechain" signal is being attenuated
when you set the NR THRESHOLD Control.
As a demonstration. Fig. 4-3 shows the effect NR MODE A has on reducing
noise as a percentage of the MIX signal at different levels below a given
THRESHOLD setting. The expansion ratio of 2.5-to-l permits the sidechain
MIX to follow the signal level below a threshold setting. When the higher
frequencies of the input drop below threshold, the MIX will drop at a pro
portional 2.5 to-1 ratio. In this way, the original signal-to-noise is not af
fected, even with a great deal of enhancement. This mode is most useful
when you enhance complex program material such as composite mixes.
4-2 Front Panel Controls

When selected, the NR MODE B is a variable shelving equalization circuit
that also controls the "Ax Sidechain" signal. The associated red, yellow, and
green LED's correspond directly to a -, 0, or + change in a variable high pass
shelving circuit when you set the NR THRESHOLD Control. The yellow (0)
LED indicates no shelving equalization, while the red (-) and green (+)
LED's signify a shelving cut or boost of the high frequencies, respectively.
Fig. 4-4 shows the action of NR MODE B (variable shelving equalization)
for three different MIX Control settings (refer to Section 4.8) at different
levels below a given THRESHOLD setting. Notice that a maximum MIX
Control setting produces the greatest shelving drop as sidechain signal falls
below a set threshold. When NR MODE B is active, the Model 250 will actu
ally "erase" audio (broadband) noise while, at the same time, restore bright
ness and intelligibility to a seriously noisy audio track or source.
Front Panel Controls 4-3
4.1.2 NR MODE B

4-4 Front Panel Controls
4.2 NR THRESHOLD Control
This rotary control provides a threshold setting range from +30 dB to -60 dB
for a chosen NR MODE (described in section 4.1). When audio program is
present at the channel input, the NR MODE LED's provide an indication of
noise reduction action for a given threshold setting. The actual position of
this control will vary with program content (i.e. dynamics, composition, etc).
For NR MODE A, an optimum noise reduction setting is achieved when the
yellow (-10) LED and green (0) LED flashes alternately with each audio pro
gram peak and pause. At this point, the noise reduction circuits provide the
greatest downward expansion of the "Ax Sidechain" signal, as the program
signal drops below the NR THRESHOLD setting (refer to Fig. 4-3).
For NR MODE B, a proper noise reduction setting is achieved when the red
(-) and yellow (0) LED's alternately flash between audio program pauses or
breaks. At this point, the shelving circuit will cut the high frequencies, as the
program signal drops below the NR THRESHOLD setting (refer to Fig. 4-4).
As you adjust the other parameters (i.e. TUNE, PEAKING, etc), you will
need to trim the NR THRESHOLD control to achieve the optimum setting.
Refer to Section 5, entitled APPLICATIONS, for some example settings.