Creating Legato keyboard response using the Q105 Slew Limiter

Written by Brad Frohock 2/19/02


I've been experimenting with ways to fake a Legato playing mode with
the Q104 MIDI Interface, and would like to share my results.

On monophonic synthesizers, a type of Legato effect can be achieved
by not retriggering the keyboard GATE for a new note if the previous
note is still "down" when the new note begins. This "slightly
overlapped" playing style should result in a single GATE that begins
when the first note is played, and lasts for as long as the player is
playing legato/overlapped/smoothly. When the fingers are all lifted
from the keyboard, the GATE is finally closed, and a new GATE will
begin when the next key is played. In Legato mode, if you want a new
GATE every time you play a new note, you must play
somewhat "detached", or even staccato, on the keyboard. That is, you
must make sure you lift your finger off each key completely before
you press the next key. Many (legacy) monophonic synthesizers, and
even polyphonic synthesizers with monophonic patch modes, offer the
legato style as an option. Here is how I got something useable on
Roger's synth.

The Q104 MIDI INTERFACE is always in "multiple trigger" mode, and has
no Legato mode per se. When a new MIDI note is detected coming in,
the current GATE (if any is open) is closed, and a new GATE begins.
In other words, every new note triggers a new GATE no matter how you
play it on the keyboard. Even if you are playing very
smoothly/legato, the old GATE will close (its voltage will drop to 0)
and a new GATE will begin almost instantly for each new note played.
This "almost instantly" is the key to faking a very effective legato
mode with the Q104.

In order to fake legato mode, one approach that I have had great
success with is to process the Q104 MIDI INTERFACE with the Q105 SLEW
LIMITER. By applying a very tiny amount of slew to the GATE output
from the Q104 (not the PITCH output), with the Q105 direction switch
in the "DOWN" position, this processed GATE OUTPUT signal does not
retrigger a new envelope when playing legato. This happens because
the "lag" that the Slew Limiter introduces slows down the dropping
GATE voltage as the GATE is closing... and the new GATE is reopened
so quickly (when playing legato) that the voltage will start rising
again before it has had a chance to ever drop below 1.5V. The reason
1.5V is significant is because that is approximately the threshhold
for triggering the Q109 ENVELOPE GENERATORS. The EG will not go into
its RELEASE TIME cycle until the GATE voltage drops below 1.5V. As
long as the voltage stays above the threshhold, the envelope
generator never progresses past its SUSTAIN state. In other words,
as long as you are playing legato, the EG will not "start over" until
you lift all your fingers from the keyboard.

You may have to experiment with the right amont of slew for your
system, as slight variances may occur between systems and modules.
Too litle slew, and there won't be enough lag to keep the GATE open
when playing legato. Too much slew, and the GATE will stay open far
too long... causing notes to "hang on" or keep sounding long after
you have lifted all your fingers from the keyboard. The best way to
pick the right amount of slew is to start with the slew AMOUNT at 0,
then play legato/overlapped (like a fast trill for example) while
slowly nudging the slew AMOUNT above 0. When the Envelope Generators
begin to "stay open" consistently, you have found the right amount of
slew for your system. On my system, the SLEW AMOUNT = 0.5 (less than
1 on the dial). Now, playing legato will give you a single trigger
for the entire legato passage, and playing staccato will retrigger
the envelopes on every note.

Some patches will benefit from a legato mode more than
others. "Brassy" patches... that trigger a quick filter sweep "blat"
with each new note... will be more expressive with legato. You can
now play fast trills on the keyboard without retriggering a
new "blat" with each note in the trill. On the other hand,
sustaining sounds like "organ" patches will benefit less because they
are less dynamic, and so legato or detached notes may all sound
pretty much the same.

Legato "playing" on the keyboard is not the only way to realize a
Legato "effect". Anything that prohibits the GATE from dropping
below 1.5V while playing on the keyboard will accomplish the same
thing. For example, if you have a Q118 PEDAL INTERFACE, mix (with a
MIXER) at least 1.5V from the "0/+5 OUT" on the Pedal Interface
together with the (straight, unprocessed, not slewed) GATE OUTPUT
from the MIDI interface before sending that mix into an ENVELOPE
GENERATOR GATE input. Then you can depress a foot pedal to keep the
GATE open as long as the pedal is down. Pedal up, no legato. Pedal
down (after starting the legato passage), you get legato... but you
must lift your foot before you lift the last note in the passage.
This technique also will require some practice to master a legato
playing style, but might be easier or more natural for some. NOTE:
this is not the same as a "sustain pedal" effect... but it's close.

Faking legato in these ways would not be the ideal way to accomplish
a true legato mode from a MIDI interface. The best way to do it
would be to alter the software inside the Q104 to close the GATE only
when all MIDI notes have been released (all MIDI Note Offs
received). But as this would certainly add cost to the Q104 (a knob
or switch would also need to be added to the panel to turn Legato
Mode on/off) it is not likely to happen. And even less likely that
anyone will care. :) Some external MIDI-to-CV converters have
legato (or single vs multiple trigger) modes built into their
software, such as the Kenton Pro series and the Encore 8-channel
Expressionist. But these boxes are much more expensive than the
Q104, and are obviously not as convenient as a Q104 module on the
front (or back) of your system!

Legato style playing is probably not for everyone, and certainly not
for every style of music. However, you might give it a try and see
what it sounds like, and add this "fake" to your bag of tricks.


Brad