CORNFIELD ELECTRONICS Quadrant User manual

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Quadrant
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Apartlyechoing,partlyarpeggiating,possiblywarpedsynthfortheArduTouch.
**********************************************************************
IT IS RECOMMENDED THAT YOU USE ARDUINO VERSION 1.6.6.
Then uncomment the "#define BUILD_166" line in Model.h of the library.
Some features and/or presets may be omitted if BUILD_166 is not used.
(In this version of Quadrant all features are present, and only the
"FraraJaqua" preset is omitted.)
**********************************************************************
==== Audio/Control Flow Diagram for the Quadrant Synth ====
------------
| Tropes | --------.
------------ |
|
v
----------------
.-----------> | Echotron |
| ----------------
| | |
|------------------|-----|------------------.
| | | |
| v v v
------------ --------------- ------------
| Lead 0 | ---> | Mixer | <--- | Lead 1 |
------------ --------------- ------------
| |
| |
v v
[ Audio out ]
Lead 0 is duplicated in the opposite audio channel by Lead 1 (which
may be transposed and/or detuned from Lead 0). The note information
for Lead 0 is also fed into the Echotron. At the same time, a "trope"
(an arpeggiation pattern) can be applied to the echoed note. The
output from the Echotron is internally panned in stereo and then fed
into a mixer along with the audio output from Lead 0 and Lead 1.

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==== The Echotron Unit ====
The Echotron is a stereo synthesizer in its own right.
At the heart of the Echotron is a carousel of "slots", each of which
holds an instance of an echoing note:
---------------------------------------
| | <--- WARP FACTOR
| Note Carousel |
| | <--- DELAY TIME
| Slot 0 < Slot 1 < Slot 2 < Slot 3 |
| | <--- FEEDBACK
---------------------------------------
|
|
v
note out
The carousel is constantly "revolving", outputting the note held in
the current slot (the left-most slot in the above diagram), then
shifting the slots.
The speed at which the carousel revolves is controlled by the DELAY
TIME.
Each time a note echoes its volume will decrease by an amount
controlled by the FEEDBACK setting. If FEEDBACK is set to its highest
value (255) then the note will echo forever, or until all the slots in
the carousel have been used up, and a new incoming note "overwrites"
it.
Each time a note echoes its pitch can be raised or lowered by the
current WARP FACTOR.
The number of slots in the carousel is configurable and can be set
interactively via the Parameter Menu (see below) to between 1 and 4.
----- Tropes ------
The above description of the note carousel is a simplification. In
fact, each slot can contain not just a note, but also a pointer to a
"trope" (an arpeggiation pattern) in ROM. Each time the carousel
revolves it will play the next note in the arpeggiation pattern for
that slot.

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----- Note HOLD ------
You can instruct the Echotron to HOLD the next note played (see the
Parameter Menu below). When a note is on HOLD it will not decrease in
volume as it echoes, and it will not be overwritten by incoming notes
when the carousel is full. This is a very useful feature for setting
up a constant bass pulse and jamming over it. Better yet, if a trope
is active when you hold the note, then you can set up a whole bass
line or texture to be played over. And you can hold more than one
note :)
----- SAMPLE & HOLD ------
You can globally freeze whatever sound loop the Echotron is producing
by enabling SAMPLE & HOLD (toggled via the Parameter Menu, see below).
Then you can jam over it freely.
----- ISO-TEMPO -------
This feature allows the Echotron to maintain a constant tempo,
regardless of the number of notes in the carousel. So if there are 4
notes currently echoing in the carousel and ISO-TEMPO is on, then the
delay time will be 1/4 of what it would be if there were only 1 note
in the carousel. The "Telepromt" and "Frara Jaqua" presets use ISO-
TEMPO. You can toggle ISO_TEMPO via the Parameter Menu (see below).
----- ENVELOPE ------
A master envelope dynamically controls the amplitude of the notes
produced by the Echotron. This envelope is a traditional ADSR
(Attack/Decay/Sustain/Release) but with a twist: a Sustain Tsime
parameter is added. When the envelope for an echoing note reaches the
sustain stage it will automatically release after the
amount of time specified by the sustain time parameter. The various
stages of the master envelope can be controlled via the pots (see
below).
----- PULSE WIDTH ------
The notes produced by the Echotron use square wave oscillators. The
PULSE WIDTH of these oscillators can be modified via a pot (see
below). The pulse width controls the ratio between the positive and
negative edges of the square wave and can radically alter the timbre
of the tones.

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----- AUTO-WAH ------
The Echotron has a pair of internal autowahs (oscillating low-pass
filters) which are applied to the note output before it is panned.
Three parameters control the autowah behaviour: CUTOFF frequency,
oscillation FREQ, and oscillation DEPTH. All of these can be modified
via the pots (see below).
----- PAN CONTROL ------
A PAN control dynamically rotates the note output in the stereo field.
The DEPTH and FREQ of the panning can be controlled via the pots (see
below).
----------------------------------------------------------------------
Overview Diagram of the Echotron
Now that we have covered each component, here is an overall view:
|
[HOLD] notes | in [SAMPLE & HOLD]
\ |
\ v
----------------- < WARP >
Trope in ---> | Note Carousel | < DELAY > [ISO-TEMPO]
----------------- < FEEDBACK >
------------- | < # SLOTS >
| ENVELOPE | notes | out
| <Attack> | |
| <Decay> | v
| <Sustain> | -----------------------------
| <SusTime> | | AutoWahs |
| <Release> | | <CUTOFF> <FREQ> <DEPTH> |
------------- -----------------------------
| |
v v
-----------------------------
| <---- Pan Control ----> |
| <FREQ> <DEPTH> |
-----------------------------
| audio out |
v v

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----------------------------------------------------------------------
PLAYING THE KEYBOARD
----------------------------------------------------------------------
The Quadrant keyboard is a mono-touch instrument: you can play one
note at a time.
To raise the keyboard by an octave, tap the right button once.
To lower the keyboard by an octave, tap the left button once.
----------------------------------------------------------------------
USING THE POTS TO CONTROL PARAMETERS
----------------------------------------------------------------------
The pots can be used to control 18 different parameters, a pair at a
time.
By pressing (not tapping) the left and right buttons you can change
which pair of parameters the pots control. Each time a button is
pressed, its associated LED (red LED for left button, blue LED for
right button) will change state (from OFF to ON to BLINKING, back to
OFF etc.)
Below is a chart showing which pair of parameters the pots control
when the LEDs are in a given state:
red LED blue LED top POT bottom POT
------- -------- ------------------- -------------------
OFF OFF echo WARP echo DELAY time
ON OFF echo FEEDBACK lead/echo BALANCE
BLINK OFF panning FREQ panning DEPTH
OFF ON autowah FREQ autowah DEPTH
ON ON autowah CUTOFF echo PULSE WIDTH
BLINK ON [ reserved ] [ reserved ]
OFF BLINK envelope ATTACK* envelope DECAY*
ON BLINK envelope SUSTAIN* envelope RELEASE*
BLINK BLINK echo SUSTIME lead PORTAMENTO
* these pots can be configured via the Parameter Menu (see next
section) to control:
1) only the LEAD voices
2) only the ECHO voices
3) both the LEAD and ECHO voices

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----------------------------------------------------------------------
USING THE PARAMETER MENU
----------------------------------------------------------------------
You can control additional parameters by using the Parameter Menu.
To access the Parameter Menu double-tap the left button. The LEDs
will now start blinking on and off in alternation: this tells you that
the Parameter Menu is active. When the Parammeter Menu is active the
keys will no longer play notes but each key will select and modify a
particular parameter as described below. Depending on the parameter,
the pot positions may also be relevant, as described below. The
Parameter Menu is active until you press a key: then the parameter
associated with that key is modified, and the Parameter Menu is
automatically exited (restoring the prior user interface state).
You can exit the Parameter Menu without modifying any parameters by
double-tapping the left button a second time.
The list below describes what actions are taken when a key is pressed:
Key Action
--- -----------------------------------------------------------
C toggle Sample-and-Hold on/off
C# turn Sample-and-Hold off, and unhold all held notes
D hold next note played
D# [ reserved ]
E toggle lead voices on/off
F set # of slots in carousel based on pot 0 position *
F# toggle isoTempo or/off
G select trope based on pot 1 position **
G# [ reserved ]
A ADSR pots control envelopes of both lead and echo voices
A# ADSR pots control envelopes of lead voices only
B ADSR pots control envelopes of echo voices only
* turn pot 0 to desired position (all the way left = 1, all the way
right = 4) then press F
** turn pot 1 to desired position (all the way left = "no trope" )
then press G

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----------------------------------------------------------------------
GUIDE TO QUADRANT’S PRESETS
----------------------------------------------------------------------
Load presets by double-tapping the right button and then pressing any
"white" key.
The presets are as follows:
Key Name Performance Notes
--- ---- -------------------------------------------------
C Scaffold lead voices off (and should stay off)
D Farsy
E Teleprompt lead voices off (turn them on once 2 slots are
playing)
F Glacial play very slowly
G Blur
A ToneJack
B FraraJaqua lead voices off (turn them on once 4 slots are
playing) **
** FraraJaqua only available when compiled with build 1.6.6
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Typesofbuttonpresses:
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Tap: quicklytapabutton
Press: long‐pressabutton
Double‐Tap: quicklydouble‐tapabutton
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----------------------------------------------------------------------
About Quadrant's Tuning
----------------------------------------------------------------------
When you start playing notes on the keyboard you will probably notice that
the 'C' key plays an 'F'. And if you have a really good ear you may also
notice that the interval between C# and G# is not exactly a 5th.
Why is this?
To explain the 2nd observation first, Quadrant uses Harmonic Tuning (also
known as Just Temperament or "Helmholtz's scale") instead of the usual Equal
Temperament (which is the system default).
Harmonic Tuning is actually more "natural" than Equal Temperament (which was
created as a compromise so that keyboard instruments could play in all keys).
In Harmonic Tuning the relative frequencies of all notes compared to the
tonic ('C' in our case) are rational numbers:
Unison 1/1
Minor Second 25/24
Major Second 9/8
Minor Third 6/5
Major Third 5/4
Fourth 4/3
Diminish 5th 45/32
Fifth 3/2
Minor Sixth 8/5
Major Sixth 5/3
Minor Seventh 9/5
Major Seventh 15/8
Octave 2/1
The Echotron uses quantum square wave oscillators which can only be set to
frequencies which have integral wavelengths. The frequencies produced by
these oscillators conform more closely to a scale in Harmonic Tuning than one
in Equal Temperament.
But why does the 'C' key play an 'F'?
The short answer is that on the ArduTouch, whose audio rate is ~15.6 kHz, 'F'
(in octave 1) has a period of 360 samples. 360 is a number favored by the
ancients because it is divisible by a large number of integers: 1, 2, 3, 4,
5, 6, 8, 9, 10, 12, 15, 18, 20, 24, etc. It so happens that by using a tonic
frequency whose period is 360, we are able to use quantum oscillators to
closely approximate the Harmonic Tuning scale over several octaves. Low 'F',
so to speak, is the natural tonic frequency of the ArduTouch.*
Since Quadrant's tuning is only an approximation to an 'F' Harmonic tuning,
certain musical scales (such as E Major, F# Major on the ArduTouch keyboard)
will sound a bit sour. C Major / D Minor work best.
* Actually, on the ArduTouch a waveform with a period of 360 samples
generates a tone with a frequency of 43.578 Hz. In the equal temperament
scale, 'F' in octave 1 has a frequency of 43.654 Hz.