PART I The keyer combines pitch and envelope information in an

PART I The keyer combines pitch and envelope information in an
The keyer
combines pitch
and envelope
information in
an amplifier to shape
a realistic mode.
K names. In organs, they are called
EYING circuits go by many
simply keyers; in traditional electronic
synthesizers, they are often called
voltage-controlled amplifiers (vca's),
envelope shapers, or modulators.
Functionally, they are electronic mul­
tipliers. All of them do the same job­
-they combine pitch information with
the envelope information to·produce a
realistic note or note sequence.
Done properly, this sets the attack,
sustain, fallback, decay, and snubbing
of any note on an individual basis. En­
velope control can also introduce
such special effects as percussion, the
"bite" on the leading edge of a horn
sound, echos, tremelo, and noise
A very few older organs did not em­
ploy keyers. They simply applied and
removed the supply voltage to and
from oscillators or used the keys
themselves as off/on note controllers.
Envelope shaping ability is very lim­
ited by this means. At the other ex­
treme, some true computer compos­
ing circuits and programs completely
specify the amplitude and frequency
instantaneously. With these excep­
tions, virtually every other electronic
music system generates tones and en­
velope information separately. These
two signals are then routed into a cir­
cuit that provides a combination of the
desired loudness and frequency.
The keyer or vca might work on any
note in a monophonic _instrument, or
one individual keyer can be supplied
with each note in a polyphonic system.
We can use much more sophisticated
keying circuitry if we need only a few,
rather than one for each note. Keying
or envelope shaping can take place
either before or after the tinbre or
tone-quality circuitry, working either
with the raw frequencies as gener­
ated or the final highly structured har­
monic tone. Organs usually employ
fixed formant filters; and a polyphon­
ic instrument is usually keyed first
and voiced later. In synthesizers, the
note will more often be colored by a
patchwork of voltage-controlled
oscillators (vco's) and filters (vet's),
and then envelope shaping t11kes
Fig. 1. In a good keyer, pitch
·and envelope waveforms (above
A) are combined to produc(3 a.
rwte which is their mathematical
product (below A). Output of
poor keyer (B) has excessive
transients; (C) has envelope
in output; (D) has unequal
positive and negative swings.
of the tone or waveshape without be­
having like a low-pass filter.
To be useful in a circuit, the keyer
should have a medium-to-high tone
input impedance and a low output im­
pedance so that it can drive output
and timbre circuitry without difficulty
and does not load down the tone
source too heavily. The impedance at
the envelope input would ideally be
infinite so that large-value resistors
and economical small capacitors
could be used for attack-sustain-de­
cay shaping. This is particularly im­
portant on polyphonic instruments
where a hundred or more keyer cir­
cuits might be needed.
Keyer design is no simple task, since
it shares all the woes of any faithful
electronic analog or digital multiplica­
tion problem. Let us look at some
popular approaches to keyer design.
In this first part of our two-part article
on keyers, we will discuss diode,
differential-arriplifier, four-quadrant
multiplier, and gain block keyers. Next
month, JFET, MOSFET, Transcon­
ductance-amplifier and CMOS keyers.
Diode Keyer. The diode keyer is
by now, fairly obsolete. An ordi­
nary silicon diode has its small-signal
(50 mV or less) ac impedance set by
the direct current through the diode. If
there is no current, the diode is an
open circuit. For small direct currents,
the impedance presented to ac signals
is given by the ratio 26/1, where I is the
current in milliamperes. So, a diode
carrying a 0.5-mA current "looks" like
52 ohms to a low-level ac signal routed
through it.
In Fig. 2A, one capacitor is used to
couple a tone into the diode and use
the envelope information to set the di­
rect current through the diode. The
disadvantage of this circuit is that it
will thump as the envelope appears in
the output. A second diode and equal
positive and negative signal swings
(Fig. 28) from the envelope circuitry
overcome this disadvantage. The two
diodes are in series across the en­
velope circuit but in parallel with the
tone input. Two more diodes (Fig.
2C) eliminate thE;l extra coupling
Input impedance is low, output im­
pedance is high, and a wide v<;>ltage
swing into a medium resistive load is
needed for the envelope input. But if
the diodes are identical and if the con­
trol voltages are also identical, and if
the ac signal level is low enough, the
diode keyer can operate without in­
troducing intolerable distortion, and it
What a Keyer Does. Any keying cir­
cuit must simply control the gain of
the tones fed through it. In Fig. 1 are
shown some good and some bad
things keyers do. The keyer must be­
have as a linear, electrically bilateral
variable resistor. Ideally, it must intro­
duce no coloration or significant dis­
tortion of its own (Fig. 1A).
A keyer must be transient-free. This
means that no ringing or overshoot
can be permitted as in Fig. 1 B. It also
means that no feedthrough of the en­
velope information to the output can
be permitted (Fig. 1C). The result of
feedthrough is a loud thump and other
response-recovery problems later on
in the circuit. Any keyer circuit in the de output level changes as a
function of the envelope command is
bound to be a bad design and will
thump badly.
A keyer must treat positive and
negative signal swings equally, am­
plifying or attenuating them with
equal gain (Fig. 1D). And the keyer sys­
tem must have a wide enough fre­
quency response to follow faithfully
the envelope and pass all frequencies
Fig. 2. Simple
diode keyer (A)
thumps as envelope·
appears in output.
Two diodes (BJ
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most oif s-o M1u1.,_a... r.s
output thumpi'Yl{J.
Four-diode version NEG."lrlV&
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(C) saves one
\ (
capacitor and has
-io v-i
l01per distortion.
will not cost much to make. (A diode
keyer will always add some distortion. )
Various transistor schemes have
been used in keying setups. They are
basically diode keyers that use the
base-emitter junctions of transistors
as the multiplier elements. They ease
the loading and impedance problems,
but they can introduce thumping if
they are not properly designed. Today,
we have much better methods.
Differential Amplifier. In Fig. 3A is
shown a differential amplifier. It is the
most commonly used amplifying cir­
cuit at this time and is found in almost
all linear integrated circuits. A differ­
ential amplifier normally amplifies the
difference between two input signals.
In Fig. 3A, on� input is tied tq ground
!o provide single-ended-input opera­
tion. This circuit can be used as a
key�r by routing the tone signal into
input X and the envelope into input Y.
If a fixed voltage is applied to input
Y, some emitter current will appe<)r in
01 and 03. Input A goes to 01 and
comes out of the transistor's emitter.
The emitter signal drives 03, which is
operated as a grounde<;l-base stage,
and an amplified version of the input
signal appears at 03's collector.
The gain of the circuit in Fig. 3A is
obtained fror:n the formula IR/104
where R is the 03 collector load resis­
tance in kilohms and I is the emitter
current in 03, the 104 comes about
because 03 receives only hajf the
available current and because 01's
output impedance is equal to the input
impedance of 03. which yields a second 2:1 attenuation.
It is important to note that the gain is
directly proportional to the emitter
current. Change the voltage on input
Y, and the gain changes, and the
product of the envelope and tone
input signals is obtained. This type of
circuit is called a vca. It bilaterally and
at high speed gives the product of the
two input signals.
One obvious problem with this cir­
cuit is that the de drop across 03's
collector load resistor follows the en­
velope, resulting in two output
terms-the desired shaped tone anc:l
an undesirable thumping from en­
velope feedthrough. In Fig. 38, a sec­
ond loacj resistor, in the collector cir­
cuit of 01, ha� been added. This cir­
cuit has two outputs, one of which is
in-phase and the other out-of-phase
with the input. Both outputs bounce
up anc:l down together.
A good differential amplifier ignores
the common-mode up-and-down
bouncing of the input signals. It is only
the difference between the two signals
that matters. So, by simply adding still
another differential amplifier stage to
the one shown in Fig. 38, the output
stays at a fixed de level regardless of
the envelope and is a thump-free rep­
lica pf the (:Jesir�d signal.
Differential amplifiers are widely
used in synthesizer vca's. While many
similar devices exist, the RCA CA3000
series linear IC's offer many differen­
tial-amplifier possibilities. An ordi­
nary 741 or 5558 operational am­
plifier can be used to eliminate the
common-mode thumping on the last
stage. The differential-amplifier vca
otters good input and output imped­
ances, controllable gain, and large
signal swing. The envelope signal
must be referenced to a negative sup­
ply, and the input impedance might
be a pit lower than we would like it to
be. Dynamic range is good and distor­
tion is low, but the systern becomes
a bit complex if a separate circuit
is used for each polyphonic note. The
differential-amplifier vca is a very
good choice for monophonic synthe­
sizer circuits.
Four-Quadrant Multiplier. A four­
quadrant multiplier is a true electronic
multiplier that provides the product of
the envelope and pitch inputs directly.
No offsets are needed on the envelope
input, and the outpLJt is normally ref­
erenced to ground.
A typical four-quadrant multiplier
circuit is shown schematically in Fig.
4. The multiple paths through all the
differentially-arranged transistors
provide for automatic cancelling of
common-mode feedthrough and
thumping. As a sometimes handy
gimmick in some advanced synthesiz­
ers the phase of the tone signal is re­
versed by inverting the envelppe.
sEcON'.O cv,c-,,: -§MP.
llAl�IPIE.S PIFF.E�e:IVCl!f.
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our?ur.s .t!JIJT tC.-N'ateES
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. ....
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T#l.IMl"IN6r .
Fig. 3. In basic differential dmplifi� (A), envel<Ype causes thump in the output. An additional amplifier
can be added (BJ to eliminat(:! thump. Second stage can Qe an ordinp,-ry 741 operational c;,mplifier.
are too costly if you are considering
using one for each note in a poly.­
phonic system. Aside from this, ttre
four-quadrant multiplier is just about
the best you could hope for.
Gain Block. Several linear IC's offer
remote-controlled gain capability that
can be used as a keyer circuit.
Motorola's MFC6040 is a typical ex­
ample of such a circuit ( Fig. 5). It costs
about $1. Its circuit is one more variant
on the differential-amplifier theme
loom V
our PUT'
f-3.5V";..T.S= OA./
f 5.5' VtJ1..T5.:af!:,c
Fig. 5. Gain block
envelope shaper.
Fig. 4 (A) are typical connections for 4-quadrant
multiplier. Key internal circuitry is shou.m at (B),
Typical four-quadrant multiplier
IC's include Motorola's MC1595 and
Analog Devices' AD532. Alternatively,
you can use the much lower priced
Signetics 5596 as the key component
in a multiplier of your own design.
The only real disadvantage with the
four-quadrant multiplier is its cost.
None of these IC's is inexpensive.
Some exceed $20 each and obviously
with common-mode bucking. Typical
voltage gain, wide open, is 3:1 or 10
dB; attenuation can go as low as 70 dB
below full output.
The output swing of the MFC6040
can be up to 6 volts peak-to-peak. One
potential disadvantage of the circuit
is that the attenuation is somewhat
Now-the "Sentinels", a series of fine
monitor receivers joins the big, world­
wide-accepted line of SBE communica­
tions equipment. Single or dual band
models of these versatile, application­
oriented SBE scanners provide coverage
of the most widely used VHF/UHF bands.
For example, the six different receivers
available give FM reception and positive
scanning operation for business, indus­
8 channels-continuous "scanning".
trial (including RCC and other mobile tel­
Receiver locks on i.lrst channel
ephone), law enforcement, fire, highway
on which signal appears.
emergency, traffic advisory, marine-in­
Priority channel feature: Receiver
cluding intership I Ship-to-shore I Coast
shifts instantly to designated chan·
Guard/Weather - many other services.
nel whenever carrier appears on it
Sentinel I: Hl-144-171MHz, Lo, 30-51MHz.
Sentinel II : 144-171MHz.
Sentinel Ill: 30-51MHz.
Sentinel V: 450-470MHz.
Sentinel VI: Hl-450-470MHz,
Lo-144-171 MHz.
Sentinel VII: Marine, 156-164MHz.
regardless of existing channel status.
Channel lock-out buttons: Unwanted
Big audio volume-3 to 4 watts.
Fractional microvolt sensitivity
Bullt·in speaker
for external speaker
Crystals used
are readily available types not included).
plug-in telescoping anten·
na, mobile mounting bracket and cord,
plug sets for 12VAC and 115VAC.
channels can be locked out.
Manual advance button allows chan­
Write for Brochure
nels to be scanned manually.
Base or mobile (AC or DC). Use in
car or home/office. Operates both
on 12VDC fof mobile and 115VAC for
home station.
Ls e E t
LINEAR svsTEMs, 1Nc.
220 Airport Blvd., Watsonville, CA 95076.
,., ···1
Gain-block IC, JFET, MOSFET, CMOS and digital keyers
AST MONTH, Part I discussed
First, the output is a bilateral current
by controlling the current fed into pin
what a keyer does and several dif­
coming from a very high impedance
5 of the IC. Zero current provides zero
ferent types of keyers and vca's used
source. Second, the internal current
gain, while +100 µA provides a max­
in electronic musical instruments. We
gain is linearly variable from zero yp
imum useful gain.
continue here with descriptions of
other types of keyers, including the
digital vari�ty.
A Specia' Gain Block. The CA3080
is a special, inexpensive gain-block IC
made by RCA. It can serve either as a
voltage-controlled amplifier (vca) or
as a two-quadrant multiplier, making it
almost ideal for use in electronic mu­
sical instruments.
A typical circuit in which the CA3080
is used is. shown in Fig. 1. While the IC
looks like an ordinary operational am1·
plifier (the connections are about the
same as for the 741 op amp, in fact),
there are some important differences.
Fig. I. The CA3080 gain-block IC used as a vca . of unit in large quantities is about 50 cents.
input. One problem is that the cutoff
TONE o--VV1,.-----o,No7E
voltage varies quite a bit from one
J.FET to another. Thus, it may be
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necessary to adjust the envelope am­
plitude and off level to suit the particu­
lar JFET in use.
.ENV. 11-------'
The MO�FET. Enhancement-mode
MOS(metal-oxide silicon) FET's with
insulated gates can also be used as
Fig. 2. The JFET can be 11sed in the shunt
mode (A) or the- series mode circuit (B).
variable resistors.MOSFET's cost a bit
more than conventional JFET's, but
they have a number of distinct advan­
There are two inputs to the IC, in­
more and more negative, the equiva­
Shown in Fig. 3 are typical circuits in
verting(-) and noninverting(+). The
lent resistance increases until the cut­
which the 2N4351 MOSFET is used in
IC amplifies the difference voltage on
off voltage is reached, at which point
the shunt(A) and series (B) modes. If
these inputs and converts it to an out­
the JFET acts as an open circuit.
the substrate lead is permitted to float
put current. When an output load re­
A JFET must be used in a shunt
and the two-resistor feed back net­
sistor is put into the circuit as shown,
mode(Fig. 2A), or the signal into the
work is used exactly as shown, the cir­
the output current is converted to an
virtual ground of an operational am­
cuits can be operated with up to 10
output voltage. Therefore, the overall'
plifier in the series mode (Fig. 28)
volts of peak-to-peak audio signal.
voltage gain is set by the load resistor
must be summed to keep the control
A voltage must be applied to the
and the control current fed into pin 5.
or envelope voltage from appearing at
gate of a MOSFET to drive the device
Th�re are three important things to
(unlike the depletion-mode JFET that
the output.
remember when using the CA3080:(1)
The input impedance on the control
requires that a voltage be removed
always keep input signal levels below
line is very high because a reverse­
from the gate to turn it off). This per­
100 mV to prevent distortion and clip-
biased diode is being driven as an
mits the use of positive envelope and
7&>.IY'e /NPUT
control voltages.
The MOSFET remains cut off until
Fig. 3. Typical circuits using a MOSFET.
Shunt mode is show'n at (A); series at (B).
the envelope input signal �eaches +4
volts or so. Between +4 and +8 volts,
control of gain and resistance is more
c:>'-J rP.IJ r
or less linear.Any potential beyond +8
volts or so does not significantly
TONE 0--'V'l('v---11��
change the resistance.
The input impedance to the MOS­
FET is essentially infinite on the en­
velope line. However, the feedback re­
sistors reduce the impedance to about
6 megohms, a value low enough to
permit the use of small capacitors in
ping;(2) always current-limit the input
to pin 5 with not less than 100,000
the envelope shaping circuitry. At $2
ohms; (3) the voltage gain obtained
depends on the output load resistor.
The JFET. The junction field effect
t r ansistor (JFET) can serve as a
small-signal, electrically variable re­
sistor. JFET's are low in cost. Texas
Pe 0-"M...-.-:-+-�
Instruments' 2N3819 is a typical ex­
ample of such devices. Also, there is a
wide variety of custom JFET's for
variable-resistance applications, with
Siliconix offering several devices and
some good application notes.
The ac input signal to a JFET must
be kept very low in amplitude, prefer­
ably less than 10 mV peak-to-peak.
Grounding the gate input
Fig. +. Tltis 12-input
l'Oltage-controlled ampl!fiei·
uses CMOS logic and costs
only 25 cents per inp11t.
of an
n-channel JFET causes the device to
conduct heavily. As the gate is made
:Z CtJ40/6
i r f r
#GJn:; �
woe: YER.S,R>
Fig . .5. At
(A) is digitally
controlled analog multiplier;
(8) is economical but unipolar
DIA converter; (C) is
all-digital .5x.5 multiplier.
1/YPUT o-JW\r-,.._
S/�/V \;S
�� .J.2.
.>-+-� Ot'JTPl/T
.. __,,,
._ '-'
or so per device, the MOSFET tech-
tortion permissible at this point in your
ory can be in the form of a permanent
nique can be economically used on
system. Since each keyer works on
store, program card, or programma­
only one note, distortion changes the
ble information ·store. One benefit of
smaller polyphonic instruments.
The CMOS Technique. There are
harmonic structure of only the one
this approach is the ability to generate
note and does not intermodulate.
any envelope you want, including
many games you can play with the
The most important advantages of
CMOS digital-logic family of devices,
the CMOS keying approach include
impossible with conventional acous­
especi a i ly
very simple ci(cuit design, low parts
tical instruments. Precision resistors
the ind us try sta,nd ard
waveshapes that would be physically
are required for th is particular circuit.
CD4000 series. One obvious thing to
cost and, electronically, very high im-
do is to bias a hex inverter to obtain six
pedance on the envelope input lines.
n-channel MOSFET's, yielding six
(A fully custom version of the Fig. 4
keyers in a $1 or $2 package. The re­
circuit technique is used by one major
MC1408P-8 IC. An analog input cur­
sultant unit keyer cost wi II then be 15c
electronic organ manufacturer.)
to 30c, which is the pricing you must
While you are looking at CMOS de­
In Fig. 58, all switching is accom­
rent and a digital word are applied to
the inputs. The output current is a
aim for when considering a fully
v ices,
polyphonic keying system on a large
analog gate CD4016 IC. It cannot be
full value in one·.of 256 discrete steps.
but reasonably priced instrument.
used in a variable-gain mode, but it is
This circuit is also useful for changing
a digital to an analog envelope wave­
check out the quadrature
ratio of the input current from zero to
The only catch to the above is that
great for on/off control of electronic
ordinary CMOS hex inverters contain
music signals. Even in single-quantity
form, or for converting digital timbre
input-protection diodes that make this
prices, it costs only about 25¢ per
information into an equivalent analog
essentially impossible. But the new
waveshape. One limitation of the de­
vice is that the input current must be
RCA CD4049 or Motorola MC14049
IC's eliminate the problem. The circuit
Going Digital. So far, only analog
single-directional with respect to
for using these hew IC's is shown in
keying and control techniques have
ground; so, an MC1408P-8 cannot be
Fig. 4. It is simply the circuit shown in
been described. Digital techniques
used directly for keying operations.
Fig. 3 repeated 12 times for a full
can also be used in electronic music.
In Fig. 5C is a 5-bit by 5-bit (5X5)
octave's worth of keying (12 notes),
You will be seeing more EM digital
digital multiplier that provides a digital
word as the product of an input en­
accomplished with three low-cost in­
circuitry in the future. Let us take a
tegrated circuits.
brief look at some of the possibilities:
velope word and an input tone word.
The signals must be limited to very
In Fig. 5A, eight stages of CD4016
The five bits are derived from a pair of
4X4 multipliers and an exclusive-OR
low levels at the note inputs, pref­
CMOS IC switching are used to set the
erably to between 50 and 100 mV rms
gain of an operational amplifier to one
gate to take care of the sign bits.
from a 400-ohm source. Thanks to the
part in 256. The gain can be set to any
Though we would like to see more bits
operational amplifier, the output im­
of 256 d is
. crete values that are close
than th is, the cost rises considerably if
pedance from the system is low. The
enough that they appear to continu­
you shoot for greater accuracy.
output signal level is 2 volts peak-to­
ously change in amplitude.
There you have the keying and vca
peak. The resistors provide a lineariz­
The tone signal is fed to the input of
techniques commonly used in elec­
ing effect. Depending on your system,
the operational amplifier, and en­
tronic musical instruments. Good luck
however, you may be able to eliminate
velope information is derived from a
in applying these to your own instru­
the resistors. It all depends on the dis-
mask or a digital memory. The mem-
ment designs.
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