ARP 2600 - Envelope Follower Circuit; Envelope Follower Response Characteristics; Keyboard Controller Outputs

36
The
sample
command
input
is AC
coupled, and
will
trigger
reliably from
any sharp rising
waveform that:
a)
has
a
risetime of less
than
10
microseconds,
b)
has
an amplitude of
at
least
5
V,
and
c)
lasts longer than 20
microseconds.
The
sample
command input
impedance
is
not less than
50Kohm.
Sampling time is
less
than 25 microseconds.
Drift
is less
than 10 microvolts/second.
Output
impedance
is 1Kohm.
4.33
The
ENVELOPE FOLLOWER
generates
from any
audio-frequency
input
an
output
voltage
directly
proportional
to
the AVERAGE
of
the
input signal AMPLITUDE. Its
sensitivity is
such
that, with the
input
attenuator
wide open,
a
1 V
P-P square wave
will
produce a
+10V
out-
put. The
maximum
output
is
+1
0V.
The
risetime,
or
time
it
takes
for the
envelope follower
to respond
to
any sudden
change
in the
amplitude
of the signal input, is 10
milli-
seconds
to
50%
of
final
value and 30
milliseconds to
90%
of final
value.
Like ail
similar circuitry,
the
envelope
follower tends
to "ride"
on
low
audio frequencies as
if they
themselves
represented
changes
in
amplitude; this is
not critical
but
has
been
held to
a
ripple
of
Jess
than
1% P-P
down to 100Hz and
less than
10%
down
to
40Hz.
The
primary
use
of
the envelope
follower
is with
external
instru-
ments.
Essentially
it extracts
from any
input a
control
voltage
repre-
senting
the
amplitude-envelope
of
that
signal:
this voltage may
be
used
to
control
the
VCF,
VCA,
or any of the
VCO's.
It
should
in
general
be
enough
to
remember
that
the
envelope follower
output
is
an envelope
and
can be
used
in
the
same
fashion
as
the
output from
either
of
the
envelope
generators. Other
uses
may suggest themselves
to
the exper-
ienced.
The
input
impedance is
1
0OKohms
and
the output
impedance
is
1Kohm.
4.34
The
KEYBOARD
CONTROLLER
3604
has three
outputs: these
are
a
GATE
signal,
a
TRIGGER
signal,
and a CONTROL
VOLTAGE
signal. When
the keyboard is
connected
to
the
2600
these
signals
are
automatically
routed
to
the
functions they
are
most
often
used to
con-
trol.
The
GATE
and TRIGGER
signals are
fed
to
the
ENVELOPE GEN-
ERATORS
and the KEYBOARD CONTROL
VOLTAGE
(KBD
CV) is
^
ripple
w-
ENVELOPE
input
nw\/K
AND
OUTPUT
»*
"