that4301 THAT Corporation, that4301 Datasheet - Page 10

• Manufacturer Part Number: that4301
• Description: That Analog Engine Ic Dynamics Processor
• Manufacturer: THAT Corporation

Page 10
choice with this circuit) and observe THD at the sig-
nal output. Set the trim for minimum THD.
RMS-Level Detector
VCA. An input resistor (R
put voltage to a current within the linear range of the
4301. (Peak detector input currents should be kept
under 1 mA for best linearity.) The coupling capacitor
(C
preceeding stages (and from offset voltage at the in-
put of the detector). Any dc current into the detector
will limit the low-level resolution of the detector, and
will upset the rectifier balance at low levels. Note
that, as with the VCA input circuitry, C
tion with R
detector.
mined by the capacitor attached to C
the size of the current in pin I
2 MW and the negative power supply, –15V). Since the
voltage at I
14 produces 7.5 mA in I
rored with a gain of 1.1 to the C
available to discharge the timing capacitor (C
combination produces a log filter with time constant
equal to approximately 0.026 C
circuit shown).
bel) value of the input signal envelope, plus a dc off-
set of about 1.3 V (2 V
capacitor to be used for the timing capacitor, usually
an electrolytic. The capacitor used should be a
low-leakage type in order not to add significantly to
the timing current.
buffer the voltage at C
set, resulting in an output centered around 0 V for in-
put signals of about 85 mV. The output voltage
increases 6.5 mV for every 1 dB increase in input sig-
nal level. This relationship holds over more than a
60 dB range in input currents.
Control Path
gain as signals rise above a threshold. The output of
the RMS detector represents the input signal level
over a wide range of levels, but compression only oc-
curs when the level is above the threshold. OA
configured as a variable threshold detector to block
envelope information for low-level signals, passing
only information for signals above threshold.
threshold and 0 below threshold. Neglecting the ac-
tion of the THRESHOLD control (R
ated resistors (R
RMS detector output drive the output of OA
tive. This forward biases CR
3
, 47 mf) is recommended to block dc current from
The RMS detector’s input is similar to that of the
The time response of the RMS detector is deter-
The waveform at C
The output stage of the RMS detector serves to
A compressor/limiter is intended to reduce its
OA
1
is an inverting stage with gain of 2 above
T
6
is approximately 0 V, the circuit of Figure
will set the lower frequency limit of the
11
and R
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
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T
T
and remove the 1.3 V dc off-
BE
will follow the logged (deci-
10
T
6
. The current in I
, 10 kW) converts the ac in-
). This allows a polarized
), positive signals from the
2
, closing the feedback
T
T
(determined by R
/I
T
T
12
T
pin, where it is
(~35 ms in the
) and its associ-
(C
3
4
, 10 mf) and
in conjunc-
T
1
is mir-
4
). The
nega-
1
is
7
,
loop such that the junction of R
of the threshold detector) sits at -(R
the circuit of Figure 14, this is –2 RMS
signals from the RMS detector drive the output of
OA
CR
0 V, and no signal level informaion is passed to the
threshold detector’s output.
OLD control, is provided. Via R
up to ±39.2 μA of current to OA
requiring the same amount of opposite-polarity cur-
rent from the RMS detector output to counterbalance
it. At 4.99 kΩ, the voltage across R
duce a counterbalancing current is ± 195 mV, which
represents a ±30 dB change in RMS detector input
level.
85 mV, the center of the THRESHOLD pot’s range
would be 85 mV, were it not for R
provides an offset. R
summing junction, which would be counterbalanced
by 37.4 mV at the detector output. This corresponds
to 5.8 dB, offsetting the THRESHOLD center by this
much to 165 mV, or approximately -16 dBV.
the signal level above the determined threshold, at a
constant
[R
COMPRESSION control (R
ates the signal passed on to OA
OA
OA
the gain of OA
trol lets the user vary the above-threshold gain be-
tween the RMS detector output and the output of OA
from zero to a maximum of unity.
is exactly equal to the output scaling constant of the
RMS detector. Therefore, at maximum COMPRES-
SION, above threshold, every dB increase in input
signal level causes a 6.5 mV increase in the output of
OA
gain. With this setting, the output will not increase
despite large increases in input level above threshold.
This is infinite compression. For intermediate set-
tings of COMPRESSION, a 1 dB increase in input sig-
nal level will cause less than a 1 dB decrease in gain,
thereby varying the compression ratio.
the COMPRESSION pot to better suit common use. If
a linear taper pot is used for R
tio will be 1:2 at the middle of the rotation. However,
1:2 compression in an above-threshold compressor
is not very strong processing, so 1:4 is often pre-
ferred at the midpoint. R
that 1:4 compression occurs at approximately the
midpoint of R
9
1
1
/R
2
2
2
In order to vary the threshold, R
The output of the threshold detector represents
The gain control constant of the VCA, 6.5 mV/dB,
The resistor R
. In this case, the junction of R
, from the wiper of the COMPRESSION control to
‘s output, is R
, which in turn causes a 1 dB decrease in the VCA
positive, reverse biasing CR
Since the RMS detector’s 0 dB reference level is
8
] 6.5 mV/dB). This signal is passed on to the
of
13
1
‘s rotation.
. Therefore, the COMPRESSION con-
16
14
about
/R
10
is included to alter the taper of
15
adds an extra –7.5 mA to OA
(0.5), precisely the inverse of
14
13
warps the taper of R
), which variably attenu-
Document 600069 Rev 02
13
13
2
1
9
2
. Note that the gain of
11
, the compression ra-
‘s summing junction,
and CR
and forward biasing
(383 kΩ), R
9
8
9
10
/R
mV/dB
and CR
12
required to pro-
, the THRESH-
(2 MW), which
8
) RMS
2
OUT
(the output
. Negative
2
OUT
rests at
12
(from
. For
adds
13
1
so
‘s
1