4301P View Datasheet(PDF) - THAT Corporation

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4301P

THAT Analog Engine® IC Dynamics Processor

THAT Corporation 

Page 10

Rev. 04/10/02

choice with this circuit) and observe THD at the sig-

nal output. Set the trim for minimum THD.

RMS-Level Detector

The RMS detector’s input is similar to that of the

VCA. An input resistor (R6, 10 kW) converts the ac in-

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

(C3, 47 mf) is recommended to block dc current from

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, C3 in conjunc-

tion with R6 will set the lower frequency limit of the

detector.

The time response of the RMS detector is deter-

mined by the capacitor attached to CT (C4, 10 mf) and

the size of the current in pin IT (determined by R7,

2 MW and the negative power supply, –15V). Since the

voltage at IT is approximately 0 V, the circuit of Figure

14 produces 7.5 mA in IT. The current in IT is mir-

rored with a gain of 1.1 to the CT pin, where it is

available to discharge the timing capacitor (C4). The

combination produces a log filter with time constant

equal to approximately 0.026 CT/IT (~35 ms in the

circuit shown).

The waveform at CT will follow the logged (deci-

bel) value of the input signal envelope, plus a dc off-

set of about 1.3 V (2 VBE). This allows a polarized

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.

The output stage of the RMS detector serves to

buffer the voltage at CT and remove the 1.3 V dc off-

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

A compressor/limiter is intended to reduce its

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. OA1 is

configured as a variable threshold detector to block

envelope information for low-level signals, passing

only information for signals above threshold.

OA1 is an inverting stage with gain of 2 above

threshold and 0 below threshold. Neglecting the ac-

tion of the THRESHOLD control (R12) and its associ-

ated resistors (R11 and R10), positive signals from the

RMS detector output drive the output of OA1 nega-

tive. This forward biases CR2, closing the feedback

loop such that the junction of R9 and CR2 (the output

of the threshold detector) sits at -(R9/R8) RMSOUT. For

the circuit of Figure 14, this is –2 RMSOUT. Negative

signals from the RMS detector drive the output of

OA1 positive, reverse biasing CR2 and forward biasing

CR1. In this case, the junction of R9 and CR2 rests at

0 V, and no signal level informaion is passed to the

threshold detector’s output.

In order to vary the threshold, R12, the THRESH-

OLD control, is provided. Via R11 (383 kW), R12 adds

up to ±39.2 mA of current to OA1‘s summing junction,

requiring the same amount of opposite-polarity cur-

rent from the RMS detector output to counterbalance

it. At 4.99 kW, the voltage across R8 required to pro-

duce a counterbalancing current is ± 195 mV, which

represents a ±30 dB change in RMS detector input

level.

Since the RMS detector’s 0 dB reference level is

85 mV, the center of the THRESHOLD pot’s range

would be 85 mV, were it not for R10 (2 MW), which

provides an offset. R10 adds an extra –7.5 ma to OA1‘s

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 output of the threshold detector represents

the signal level above the determined threshold, at a

constant of about 13 mV/dB (from

[R9/R8] 6.5 mV/dB). This signal is passed on to the

COMPRESSION control (R13), which variably attenu-

ates the signal passed on to OA2. Note that the gain of

OA2, from the wiper of the COMPRESSION control to

OA2‘s output, is R16/R15 (0.5), precisely the inverse of

the gain of OA1. Therefore, the COMPRESSION con-

trol 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, 6.5 mV/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 6.5 mV increase in the output of

OA2, which in turn causes a 1 dB 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 1 dB increase in input sig-

nal level will cause less than a 1 dB 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 use. If

a linear taper pot is used for R13, the compression ra-

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. R14 warps the taper of R13 so

that 1:4 compression occurs at approximately the

midpoint of R13‘s rotation.

THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA

Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com

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