THAT4311 Datasheet, PDF (11/12 Pages) List of Unclassifed Manufacturers – Low-voltage, Low-power Analog Engine Dynamics Processor

Rev. 08/30/01
Preliminary Information
Page 11
this case, OA2's feedback is provided through R9,
and the sensitivity at this point is 12.2 mV/dB, since
the output of the RMS detector is multiplied by
-R9/R8, or a gain of -2.
A threshold control is provided to vary the
threshold above or below -10 dBu. The output sensi-
tivity of the RMS detector is 6.1 mV/dB. This is con-
verted to a current by R8, and the sensitivity at the
summing node of OA2 is
6.1^mdBV
4.99kW
= 1.2mA
The wiper of R12 can swing between -2V and +3V
relative to the summing node of OA2 which is at
VREF. If we want the threshold to swing as high as
+30 dB, then the value required for R11 can be cal-
culated as
R11 =
2V
1.2_d^m_BA ´ 30dB
»
51 kW
when rounded to the nearest 5% resistor value.
Using this value and knowing that the pot's swing in
the other direction is 3V, we can calculate the thresh-
old swing in the negative direction to be
3V
51kW
1.2_d^m_BA
» -49 dB
Since the zero dB reference level of the detector is
-10 dBu, the threshold can be adjusted from 20 dBu
to -59 dBu.
The output of the threshold detector represents
the signal level above the determined threshold, at a
constant of about 13mV/dB (from [R9/R8]
6.1mV/dB). This signal is passed on to the COM-
PRESSION control (R13), which variably attenuates
the signal passed on to OA1. Note that the gain of
OA1, from the wiper of the COMPRESSION control to
OA1’s output is R16/R15 (0.5), precisely the inverse
of the gain of OA2. Therefore, the COMPRESSION
control lets the user vary the above threshold gain be-
tween the RMS detector output and the output of
OA1, from zero to a maximum of unity.
The gain control constant of the VCA (6.1mV/dB)
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 6.1mV increase in the output of
OA1, which in turn causes a 1dB decrease in the VCA
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 1dB increase in input sig-
nal level will cause less than a 1dB decrease in gain,
thereby varying the compression ratio.
The resistor R14 is included to alter the taper of
the COMPRESSION pot to better suit common usage.
If a linear taper pot is used for R13, the compression
ratio will be 1:2 at the middle of the rotation. How-
ever, 1:2 compression in an above-threshold com-
pressor is not very strong processing, so 1:4 is often
preferred at the midpoint. R14 warps the taper of
R13 so that 1:4 compression occurs at approximately
the midpoint of R13’s rotation,
The GAIN control (R18) is used to provide static
gain or attenuation in the signal path. This control
adds between 120mV and -180mV of offset to the
output of OA1, which is approximately a -20dB to
+30dB change in gain of the VCA. The gain control
signal is passed into OA1 via R17, but this signal is
also passed back to the threshold amplifier (OA2) via
R10. This arrangement results in the threshold be-
ing fixed relative to the output. In other words, as
the gain is increased, the threshold is lowered to
keep the threshold of compression or limiting at the
same output level. This is particularly important in
limiters, since it keeps the gain control from interact-
ing with the threshold.
C5 is used to attenuate the noise of OA1, OA2,
and the resistors R8 through R16 used in the control
path. All these active and passive components pro-
duce noise which is passed on to the control port of
the VCA, causing modulation of the signal. By itself,
the THAT 4311 VCA produces very little noise modu-
lation, and its performance can be significantly de-
graded by the use of noisy components in the control
voltage path.
Overall Result
The resulting compressor circuit provides
hard-knee compression above threshold with three
essential user adjustable controls. The threshold of
compression may be varied over a range from about
-58dBu to +20dBu. The compression ratio may be
varied from 1:1 (no compression) to ¥:1. And, static
gain may be added between -20 and +30dB. Audio
performance is excellent, with THD running below
0.1% at middle frequencies even with 10 dB of com-
pression, and an input dynamic range of over 105dB.
Perhaps most important, this example design
only scratches the surface of the large body of appli-
cations circuits which may be constructed with the
THAT 4311. The combination of an accurate, wide
dynamic range, log-responding level detector with a
high-quality, exponentially-responding VCA produces
a versatile and powerful analog engine. These, along
with its on-board opamps, allow a designer to con-
struct, with a single IC and a handful of external
components, gates, expanders, de-essers, noise re-
duction systems and the like.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com