QuantAsylum QA401 User manual

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QuantAsylum QA401 User’s Manual
192Ksps 24-bit Stereo Audio Analyzer
Revision 1.704
January 2019

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Safety Notice
❖This device is not designed for working on potentially dangerous voltages.
❖This device is not designed for working on high energy circuits.
❖The maximum DC input voltage is +/- 5V into the device inputs.
❖The maximum AC input voltage is +/- 50V into the device inputs.
❖See additional safety notices throughout this document.
Limited Warranty
This product has a limited warranty for 6 months from the time of purchase. During this time, a device
failure that occurs under normal operating conditions will be replaced or repaired for free, not including
shipping. Generally, you will be responsible for shipping to us, and we will be responsible for shipping it
back to you.
Devices that have suffered a failure due to operation in excess of specified parameters can usually be
repaired for a nominal fee.
The contents of this document are provided “as-is” and may be changed or updated without notice. The
specifications on a particular product may also be changed at any time and without notice as we seek to
improve a product or improve availability of a product.
The limit of our warranty will not exceed the value of the product purchased under any conditions.

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S
Contents
Safety Notice.................................................................................................................................................2
Limited Warranty..........................................................................................................................................2
Legal..............................................................................................................................................................6
In the Box ......................................................................................................................................................6
Important Things to Know ............................................................................................................................6
Ground Reference.....................................................................................................................................6
BNC Input Voltages ...................................................................................................................................6
BNC Output Voltages ................................................................................................................................6
QA401 Output Voltage Offsets, Clicks and Pops.......................................................................................6
QA401 Analyzer Features..............................................................................................................................7
Analyzer Front Panel.....................................................................................................................................7
Front Panel Summary................................................................................................................................7
LEDs...........................................................................................................................................................8
BNC Inputs ................................................................................................................................................8
Understanding Differential Measurements..............................................................................................8
Rear Panel Summary.................................................................................................................................9
Electrical Characteristics of the Connectors .............................................................................................9
BNC Inputs.............................................................................................................................................9
BNC Outputs........................................................................................................................................10
USB Input ............................................................................................................................................10
Software Installation...................................................................................................................................10
X32 and X64 Installation .........................................................................................................................11
Calibration...................................................................................................................................................11
Verifying Calibration ...............................................................................................................................11
Calibration Adjustments .........................................................................................................................12
Attenuator Adjustments .....................................................................................................................13
QA401 Basic Controls..................................................................................................................................14

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Adjusting Knobs ......................................................................................................................................15
Adjusting Without a Mouse Wheel.....................................................................................................15
Context Menus........................................................................................................................................16
Control Overview........................................................................................................................................17
Run/Stop Button .....................................................................................................................................17
Display Options.......................................................................................................................................17
Axis..........................................................................................................................................................21
dBFS, dBV, and dBr Buttons................................................................................................................21
dBV Context Menu..............................................................................................................................21
dBR Button and Context Menu...........................................................................................................22
X Lin/X Log Buttons and Context Menus ............................................................................................23
Display Panning and Scrolling .................................................................................................................25
Titles and Markers ..................................................................................................................................25
Adding Markers.......................................................................................................................................26
Tracking Markers.................................................................................................................................28
Acquisition Control .................................................................................................................................28
Weighting................................................................................................................................................29
Windowing Functions .............................................................................................................................30
Measurement Types ...................................................................................................................................33
Definitions...............................................................................................................................................33
QA401 Measurement Types ...................................................................................................................34
RMS/Power.............................................................................................................................................35
THD..........................................................................................................................................................36
THD Context Menu..............................................................................................................................38
THD + N ...................................................................................................................................................40
Frequency Response...............................................................................................................................40
Generators ..............................................................................................................................................45
IMD Measurements ................................................................................................................................46
Power Supply Noise Measurements.......................................................................................................49
Settings........................................................................................................................................................50
General....................................................................................................................................................50

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Display.....................................................................................................................................................50
Other.......................................................................................................................................................51
Test Plugins .................................................................................................................................................54
Keyboard Shortcuts.....................................................................................................................................56
Troubleshooting..........................................................................................................................................56
Connectivity Issues..................................................................................................................................56
Appendix I: Test Plugins..............................................................................................................................61
Appendix 2: Visualizers ...............................................................................................................................63
Background .............................................................................................................................................63
Appendix 3: Software API ...........................................................................................................................65
Overview.................................................................................................................................................65
Appendix 4: Regulatory Notice ...................................................................................................................68
Overview & Safety ..................................................................................................................................68
Environmental.........................................................................................................................................68
Power Supply ..........................................................................................................................................68
Shock Hazard...........................................................................................................................................68
FCC ..........................................................................................................................................................68
Appendix 5: Specifications......................................................................................................................69

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Legal
This document and the associated computer codes, hardware design and hardware configuration files
are copyright © 2011 - 2019 by QuantAsylum USA LLC. All rights are reserved. You may share the
associated documents in PDF format freely. The EXE programs and code in the developer section is for
use only with QA products. The hardware and software designs are protected and the property of
QuantAsylum USA LLC.
In the Box
Your new analyzer should contain the following:
•QA401 Audio Analyzer
•Certain product variants may contain cables. But as a general rule, we’re moving towards NOT
shipping common cables with products any longer.
Installation software, this manual, and application notes for the analyzer are available on the web at
http://www.QuantAsylum.com. Inside the box you will find a card with a QR code that will take you to
our web page with the latest information on the product.
Important Things to Know
Ground Reference
The analyzer PC interface will share a ground with the PC. The audio inputs and outputs, however, are
isolated from the PC. This isolation is limited to 50V. Do not connect the QA401 to a product that has its
ground reference more than +/-50V from the PC ground.
BNC Input Voltages
The AC input to the analyzer should never exceed 56Vpp (single ended) or 28Vpp (differential). The DC
limits are lower. DC voltages above +/- 6V with the attenuator disengaged may permanently degrade
the performance of the device. In general, we recommend a DC limit of +/-5V regardless of attenuator
setting. If you need to measure power supply noise, see the section in the manual on how to safely do
so.
BNC Output Voltages
The BNC output connectors have a 47 ohm limiting resistor and thus have minimal protection. If you
accidentally connect a signal of more than few volts to the outputs, you may damage the output stage.
QA401 Output Voltage Offsets, Clicks and Pops
The QA401 output voltages will nominally have an offset of 1-2 mV. If you wish, you can trim those
offsets to microvolt levels through the Settings->Options->Other->Trim menu. However, be aware that
certain operations on the QA401 can generate clicks and pops and also modify offsets. This is especially
true when using external programs to control the QA401.

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Also, when changing the sample rate on the QA401, the codecs will be reset to ensure proper
application of their changed register settings. That will also generate a click or pop on the outputs.
Normally, these clicks and pops won’t present issues. But if you are working on very high-gain stages
with speakers or headphones attached, first take the time to familiarize yourself how the various mode
changes may impact your equipment under test.
QA401 Analyzer Features
The QA401 is our second-generation audio analyzer. Our goal in building the QA400 and the QA401 was
to leverage modern ADC and DAC codecs commonly used for audio analysis, while at the same time
eliminating some of the bigger problems that come with re-purposing audio ADCs for audio
measurement.
The QA401 Features:
•Stereo differential LEFT + RIGHT inputs
•Stereo differential LEFT + RIGHT outputs
•24-bit ADC and DAC
•Fully isolated from the PC
•Input attenuator and hardware-based overload protection
•USB powered
We hope you enjoy your purchase! Check back from time to time for new software updates, and drop by
the forum to say "hello" or ask a question!
Analyzer Front Panel
This section covers the various connectors on the analyzer and reviews the input and/or output
characteristics of these connectors.
Front Panel Summary
The front panel is shown in the picture below. From left to right, the following entities are explained.

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LEDs
Link LED This LED indicates the analyzer is connected to the PC and talking to the analyzer
application.
Run LED This LED indicates the analyzer is currently running (acquiring) data.
Atten LED This LED indicates the internal 20 dB attenuator is active. The attenuator is active when
the device is unpowered. However, try not to leave the QA401 connected to large
voltage swings while the device is unpowered. At higher voltages, it could damage the
unit.
BNC Inputs
There are two stereo inputs (Left and Right) and two stereo outputs (Left and Right). Each input and
output is differential. This means the magnitude of the + and –are signals are equal and opposite. For
example, if you were to look at the output signals on an oscilloscope, you’d see that when the L+ output
was sweeping a sine wave with a peak of +3V, the L- output would be sweeping a mirror image sine
wage with a peak at -3V. Differential signals are commonly used in noisy environments because
interfering signals that appear on both inputs simultaneously are “cancelled” and greatly attenuated.
However, for much of your audio work, you may prefer to use the device in single ended mode
especially if you are working on line level consumer audio type equipment.
If you wish to use the inputs single ended, then you could use a BNC terminator on the L- input, and
treat the L+ input as a single ended input. If you do not use the input terminator, then you will see some
thermal noise from the unused input resistor, which will raise the overall noise floor.
Understanding Differential Measurements
Differential measurements can create confusion even among very experienced engineers. Some
examples will help highlight the differences.
If you set the generator to 0 dBV and connect an Output+ to an Input+ and ground the Input-, then the
measured input will be reported as 0 dBV.
With the output set to 0 dBV, each output will measure 1Vrms on a DVM relative to ground (BNC outer
conductor). A differential measurement on a DVM (from Out+ to Out-) will measure as 2Vrms. This is
because the Out+ and Out- are 180 degrees out of phase with each other.
If you set the output to 0 dBV and connect both the Out+ and Out- connectors to the In+ and In-
connectors, then the QA401 measurement will show a peak of 6 dBV. This is because you are driving the
inputs differentially. This can be very confusing to first-time users: You are driving the inputs with 1Vrms
(0 dBV), and yet the QA401 is reporting 2Vrms (6 dBV). But this is precisely the same measurement
reported by the DVM when you placed the DVM across the outputs.
Keep in mind the QA401 inputs have no idea if you are driving a single input with 1Vrms and grounding
the other input OR if you are driving both inputs with 0.5Vrms. In both cases, you are hitting the ADC

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with the same differential voltage. That is, the differential input of the ADC is seeing 0.5Vrms on each
input in both cases.
Rear Panel Summary
The rear panel has a single USB connector. This is designed for high speed (480Mbps) USB connections.
The device consumes between 500 and 600 mA during normal operation. The device is not sensitive to
USB voltage variations.
Note that newer computers may employ very strict current sensing on the USB power flowing out of the
USB port. When the current exceeds a bit over 500 mA, the PC hardware might signal a fault.
If you suspect your PC has strict limits on the power, then you can use a USB Y connector. These are
connectors that plug into 2 USB ports and allow USB hardware to pull up to 1000 mA. One of the USB
ports has no data connection. It just takes power from the second port.
Alternately, most low-cost USB hubs that are self-powered do no sensing or limiting at all.
Electrical Characteristics of the Connectors
BNC Inputs
The 4 inputs pass through a 33uF series capacitor, followed a series 100 ohm resistor, and followed by a
resistor divider with a total impedance of 100K ohms. The corner frequency of this input network is
about 1.6 Hz.
The input DC blocking capacitor is polarized, with a 50V rating. The DC blocking capacitor can withstand
a negative voltage of 15%, or 7.5V, for 125 hours. In general, we recommend you limit the long-term DC
level (relative to ground) to just a few volts, keeping in mind that the capacitor will degrade over time
depending on the magnitude of the voltage.
The input stage will be clamped to the internal input rails (about +/-6.5V) through the 100 ohm input
resistor. The input attenuator is very fast. We’ve tested overload conditions of 70Vpp for hours on end,
with no impact noted to the performance of the device. During overload, the system will detect the
excessive input and engage the attenuator. After 1 second, the attenuator will be released, and the
input level again checked. This cycle will repeat indefinitely. Of course, the audible indication of the
relay clicking every second should alert you that the input voltage is too large and to manually engage
the attenuator.
IF YOU GOING TO APPLY A HIGH DC BIAS TO THE QA401 INPUTS, YOU MUST MAKE ABSOLUTELY SURE
THE ATTENUATOR IS ENGAGED. SEE THE SECTION ON MEASURING POWER SUPPLY NOISE COVERED IN
RMS MEASUREMENTS.
DO NOT EXCEED THE RATED MAX INPUT VOLTAGE OF THE Q401. IT CAN RESULT IN PERMANENT
DAMAGE TO THE QA401.

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NEVER USE THE QA401 TO MEASURE CIRCUITS THAT USE POTENTIALLY LETHAL VOLTAGES.
The default state of the input is that the attenuator is engaged. That is, when the device is unpowered,
the attenuator is active. When you first connect the device, regardless of UI setting, the attenuator will
remain active (attenuating). And when you close the QA401 application or unplug the QA401 hardware,
the attenuator will always re-engage.
DO NOT ROUTINELY LEAVE THE QA401 CONNECTED TO ACTIVE SIGNALS WHILE THE QA401 IS POWERED
OFF. ALWAYS DISCONNECT THE INPUT SIGNALS IF YOU ARE GOING TO POWER DOWN THE QA401.
BNC Outputs
The output op-amps have a 47 ohm series R in an 0603 form factor. If the output is accidentally
connected to a voltage more than few volts in magnitude, the 0603 resistor could act as a fuse and open
or the output op-amps could be damaged.
All 4 outputs are DC coupled, with a typical offset between + and –of a few millivolts. Beginning in the
1.5 release of the software, a trim function is provided to allow the user further null the offset. You
should be able to achieve a few 10’s of microvolts of offset between the differential outputs. See the
section on using the trim function for more information.
USB Input
The USB input is designed to operate with the maximum and minimum voltages specified by the USB
Implementers Forum, which is 4.75V to 5.25V. The noise performance of the analyzer should be
unchanged to down to 4V DC USB. Below that, certain regulators may begin to drop out and noise
performance might be impacted.
Do not exceed the 5.25V upper limit. Most hubs and equipment designed for USB will automatically
handle this for you.
Software Installation
Before plugging in your analyzer, you need to install the software. To do this, download the latest
installer package from QuantAsylum.com (see the Support → Downloads page) and run the installer.
Once installed, you may plug your analyzer into the USB connection on your computer.
The software no longer supports Windows XP, and we cannot help troubleshoot any issues you are
having with XP. Vista will probably work, but we cannot help trouble shoot any issues you are having
with Vista.
If you are running Vista or Windows 7 (32 and 64-bit version), the drivers should install automatically
when you plug your board in.

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X32 and X64 Installation
The drivers are signed, and on Windows 7 and later the installer will be able to auto-install the USB
drivers. This means that if you install the program and drivers, and then run the program and then plug
in the analyzer, everything should be taken care of automatically.
Calibration
The QA400 product, which was the predecessor to the QA401, required calibration by the user. This was
because the manufacturing window of the parts used could be fairly large (+/-25%). The QA401,
however doesn’t need calibration and will deliver outstanding accuracy out of the box.
Verifying Calibration
Calibration should be verified from time to time. The procedure is simple and requires a DVM with
trusted accuracy. Generally, quality DVMs will hold their stated accuracy for a decade or more easily,
because the drift and aging associated with the internal references used are very low. But quality DVM
manufacturers will also allow you send in your DVM and have it re-calibrated certified to be operating
correctly for a fee.
Using a DVM with a calibration that you trust, perform the following steps:
1) Use the Settings →Generate Fixed Tone menu option to generate a 0 dBV signal at 60 Hz. Note
the amplitude. It should be 1.000Vrms +/- 4% (0.96 to 1.04Vrms) on all 4 output ports of the
QA401. This is a single-ended measurement, with one input to the DVM taken from the center
of the BNC, and the other input to the DVM taken from the BCN outer conductor.
2) Repeat the measurement used in step 1), except this time generate a 1 KHz signal. Most DVMs
will show a very small reduction in reading (few %) between 60 Hz and 1 KHz. This is attributable
to the natural roll-off of the meter’s response. If your DVM shows more roll-off than a % or two,
then make a mental note that your DVM is not suitable for direct measurements at 1 KHz and
the calculations below should be adjusted accordingly. Most meters are specified at 60 Hz, and
the upper corner isn’t generally published by the DVM manufacturers. Thus, it’s important to
know.
3) Cancel the fixed tone generation and connect the + output L of the QA401 to the + input L of the
QA401. Short the –input.
4) Select File →Reset startup defaults. This will put the analyzer into a known state.
5) Set windowing to Flat Top, Atten Off, RMS Measurement, and dBV axis settings
6) Set Gen1 to 1 KHz, 0 dBV
7) Verify the input PeakL is 0 dBV +/- 0.1 dB
8) Apply the attenuator. Verify the signal is 0 dBV +/- 0.15 dB.
9) Activate the Right Channel and turn off the Left channel in the Display Options section of the UI.
Repeat the above steps for the right channel.
To verify the attenuator is working as expected, perform the following steps:

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1) Select File →Reset startup defaults
2) Select RMS Measurements and dBV Axis Settings
3) Enabled Gen1 for 1 KHz and -10 dBV
4) Note the Peak L amplitude. It should be -10 dBV +/- 0.1 dB
5) Engage the attenuator. You will see a notation “Atten: 20 dB” appear on the display. Note the
PeakL amplitude again. It should be within +/- 0.1 dB of the value noted in the previous step.
Note that the displayed value is -10 dBV again, because the device takes the attenuator into
account.
6) Repeat the steps for the right channel.
If your QA401 is operating outside of the windows specified above, then please contact
[e-mail] so that we can evaluate this together. Do not rely on the calibration
adjustments described below to try and adjust for errors that are larger than expected.
Calibration Adjustments
There is a file that resides in you’re My Documents folder under the QuantAsylum\QA401 directory. The
file is named CalibrationData.xml. The contents of this file are parsed at startup of the QA401
application. Once read, the file isn’t needed again until the next re-launch. So, if you ever make manual
changes to the file, you must re-launch the QA401 application to re-parse the file.
In general, there is no reason to make adjustments to the calibration file! This detail is provided here
for advanced users with unique requirements, or for users that are interested in eliminating the last %
or two of error. If you ever doubt the accuracy of the calibration file, just delete it and a new default
will be created.
There might be a few reasons for making changes to the calibration file. First, you might want to tweak
for different output impedances. As the QA401 has a 100K AC input impedance, if you are using a source
impedance that is 50 or 100K, then it might make sense to keep a calibration file around for Hi-Z
equipment and low-Z equipment. This could be done by renaming the file and swapping names as
needed before launching the QA401.
A typical file might appear as follows inside:

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Notice there are 4 numbers: two output numbers, and two input numbers. The output numbers
(OutputCal) are the value the output will be multiplied by before being sent to the DAC. If you want to
increase your output value by just a bit, you can increase the OutputCal figure by whatever amount you
wish.
Similarly, the InputCal is the amount the input will be multiplied by to get it where it needs to be to
deliver the precise dBFS level. You may also adjust these values as you see fit. As with the output value,
a single calibration value is used for both left and right. You can put in a separate value for left and right
if you see fit.
If you corrupt the contents of the file, the QA401 software will be unable to parse the contents and it
will create a new file, overwriting the corrupt file.
You can delete the file at any time, or re-calibrate at any time. Re-calibration will overwrite any existing
calibration file.
Attenuator Adjustments
The QA401 attenuator is built from 0.1% resistors, and should be nominally 20 dB. The software
assumes it is 20 dB precisely, but this can be changed. The real attenuator value may vary +/- 0.06 dB
depending on resistor variation. To fine-tune the attenuator, perform the following steps:
1) Select File →Reset startup defaults
2) Select RMS Measurements and dBV Axis Settings
3) Enabled Gen1 for 1 KHz and -10 dBV
4) Note the Peak L amplitude.
5) Engage the attenuator and again note the Peak L amplitude.
6) Take the un-attenuated reading and subtract from that attenuated reading. This is the
attenuator correction factor.
7) Close the QA401 application.
There is a file in the My Documents\QuantAsylum\QA401 directory called Default.Settings. Around line
53 of the file you will see the following:
The AttenVal is what we’ll adjust. Take the default value of 20 and add to that the attenuator correction
factor you calculated above. For example, if your correction factor was 0.08, then you would replace the

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20 with 20.08 as shown below. Close the file and restart the QA401 and verify the Peak value is the
same
Note that there is a single correction value for the attenuator. You must decide if you’d like to split the
difference between the left and right channel OR if you’d like to dial in one channel precisely. The
channel to channel variation is generally quite small (<0.02 dB).
QA401 Basic Controls
The QA401 application is shown below. On the left side of the screen is the display area, and on the right
side is the control panel.
If your screen height becomes limited, you can click and drag anywhere in the control region that there
isn’t a control. This will let you scroll the control region to hide some controls and reveal new controls.
This will work with both your mouse and with a finger if you have a touch screen.

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Adjusting Knobs
Knobs are used to permit quick and accurate adjustment of controls. If you are familiar with pro-audio
production software, then the knobs will already be second nature. In these environments, users must
deal with literally hundreds of adjustments shown on the screen simultaneously, and an enormous
amount of refinement has gone into making them useful.
If you have a mouse with a scroll wheel, then you can hover over a knob, click with your mouse and
move the mouse wheel to turn the knob. When you are done with the adjustment, move the mouse
away from the knob. Note that when the mouse moves over the knob and is clicked, the knob features
an LED that illuminates slightly. This is your cue that adjustment is then possible.
While adjusting the knob, there are a few keyboard shortcuts that can help.
If you hold the CONTROL key down while adjusting the mouse wheel, the knob will spin 10 times faster
than if no key were pressed. This makes it easy to quickly adjust something like the offset knob.
If you hold down the ALT key while adjusting the mouse wheel, the knob will spin 10 times slower than if
no key were pressed. This makes it easy to fine tune very precise settings.
Adjusting Without a Mouse Wheel
If you don't have a mouse wheel or if you are working on a laptop, then you adjust a knob by clicking on
the knob, and then moving the mouse up or down while holding the click. On a track pad, then you
would slide your finger up and down while holding the left mouse button. When you are done with the
adjustment, just release the click. The same accelerator keys (CONTROL and ALT) work using this
method too.
If you have a touch screen, then you can place your finger on the knob and slide it upwards or
downwards to adjust.
Notice that the knobs don’t have any indication of the value. The knobs are analogous to “rotary
encoders” used on equipment where the knob can spin forever in a clockwise or counterclockwise
direction. The actual values being adjusted by the knob are displayed on the analyzer display. This might
take a little getting used to, but it’s very quick with a little practice, and allows a large set of controls to
be placed in a small area.

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Context Menus
While perusing the QA401 menus, you’ll likely notice they are very sparse compared to other signal
analyzers. This is because the QA401 makes extensive use of context-sensitive menus. Functionality is
grouped under the various buttons that enable that functionality.
Take a look at the example below. Here we the dBV button, for example has a dot in the lower left
corner. If we want to bring up a context menu related to the dBV settings, we can click on that button
while holding the control key down. If a button doesn’t have a dot (such as the dBFS button), then it
means the button doesn’t have a context menu.
Control-click on the dBV button yields the following menu:
This is a very important concept related to the QA401 interface: Settings related to a particular function
are adjusted in a context menu, and that context menu is activated by control-clicking the button.

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Control Overview
There are a few basic control groupings on the analyzer, and these are covered below.
Run/Stop Button
The Run/Stop button is prominently located at the bottom of the control panel. This button starts and
stops the analyzer acquisition, although that can also be accomplished with the space key (see the
section on soft keys). In the first picture below, you can see the analyzer expanded to full screen, and
the yellow box highlights the Run/Stop button
Next to the Run/Stop button is the attenuator button. When this button is lit, it means it is active and
thus the 20 dB input attenuator is active.
Display Options
The buttons below determine what is shown on this display.

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The time button displays the transmitted and captured waveforms in the time domain. With the time
and input button pressed, we can see what was captured by the analyzer. Sometimes you can forget
that you are looking at the output data. The output data is exceptionally clean and might confuse you
into thinking that you are looking at test data from the device under test. For this reason, when you
have selected to look at the output data, you will be reminded with an on-screen indication that you are
viewing output data.
Below, we can see the input data in the time domain.
Similarly, we could push the output button and see the signal that was transmitted by the analyzer. If we
zoom in on the transmitted waveform (by dragging to the region we want to see using the left mouse
button), then we see the output waveform has 3 distinct regions.

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The same waveform above is shown below with some boxes highlighting the different regions.
The green box shows the ramp up period of the output waveform, the red region shows the constant
amplitude portion, and the blue region shows the ramp down region.
At this point, it’s important to highlight that the QA401 does not operate with constant stimulus tones.
The QA401 operates with stimulus bursts. The reason for this is to ensure that the full transaction with
the hardware can be treated as a discrete event with a clearly defined start and stop point. Each
transaction can start from a known state, and finish in a known state, and if various checkpoints along
the way are not met, then the transaction can be counted as flawed and the results can be rejected.

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The ramp up period of the waveform allows the various output audio stages external to the QA401 to
stabilize their DC operating points. This gentle ramp also ensures audible pops are avoided that might
harm power output stages.
The constant amplitude region is the region over which the FFT will actually be computed.
The ramp down region ensures again that a sine wave isn’t terminated mid cycle which might also result
in a click or pop, and it ensures the DC level is gently returned to zero.
Normally, you’ll be looking at the display with the following buttons pressed: Freq, Input and Left and/or
right. This will show you the captured FFT data on the incoming signal.
When the right button is pressed, as shown above, the display will show the right channel data in the
color red. Below, we see the left channel data shown in yellow, and the right channel data shown in red.
And of course, if we only wanted to show right channel data we would simply turn the left display off by
pressing the left button.