gain control ic for audio signal processing hr 0777
TRANSCRIPT
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gain control 1C
for audio signal processing
Asamulti-purposed 1C,
theNE570analogcompandor
fulfillsmany
audioprocessingneeds
Tw o ICs recentl y i n troduced by S igneti cs , the
NE570and NE571, permitthedesignof efficient and
prac ti ca l aud io -s igna l con tro l func tions w i th a
minimum overall parts count. These devices are
primarily designed to act ascompandors; the com
plementary processes of compression and expan
s i o n . 1^ They are both dual-channel ICs and either
portion can be used individually as a compandor.
However, asw ill beseeninthisarticletheyarealso
well suited to a variety of other tasks useful to the
amateur.
basic device operat ion
Each channel of the 570and 571 consists of the
functional componentsshown in f ig. 1A. Packaged
ina 16-pin DIP, the only items common to the two
signal channels arethe power supply, ground con
nections,andaninternal1.8voltbiasregulator.
The three principal components of each section
are a AG cell, fu ll-wave rect if ier, and an output
amplifier.TheAGcellisusedtocontrolthegainover
a rangegreaterthan 80 dB. The controlvoltagefor
this cell is generated by rectifying an input signal
(RECTIN). Thefinal output is thendeveloped bythe
bufferedo utpu tamplifierfrom thescaledsignalcu r
rent supplied by the AG cell. The 570 and 571 are
identicalelectrically, butthe570isselectedfo rlower
inherent d istor tion and a h igher supply voltagerange.
TheAGcell,asshowninf ig.1B, consistsofanop
amp, A1, andtransistorpairsQ1-Q2andQ3-Q4. The
input signal is first converted by R2 into a current
thatdrives A1. Thefeedback forthisop amp is via
thetransistorpair Q1-Q2. Therefore, theam ount of
currentinthispairisthesameasthecurrentthrough
R2. In addition todriving Q1-Q2, theopamp isalso
connected to Q3-Q4. Unlike Q1-Q2, this transistor
pair does not have a constant-current source. By
scaling the Q3-Q4emittercurrent, their outpu t is a
linear product of the input signal from A1 and thescaled current. This circuit is a linearized transcon
ductance multiplier4-7 which cancels the inherent
non-linearity and temperature sensitivity of the dif
ferential pairs, greatly enhancing the usefulness of
thisgain-controltechnique.
The rec tif ie r port ion consis ts o f op amp, A2,
class-B transistors Q5-Q6, a pnp current mirror Q7,
and an npn cur rent mir ror Q9. When rectifying a
signal at the RECT IN terminal, Q5and Q6 produce
pulses of current proportional to the positive and
negative input signal swings. The output currentof
Q6 is used directly, while theQ5cu rrent is mirrored
byQ7.Thus, thedrivetoQ9 isa positivegoing,full-
wave rectified pulsating dc. These pulsesarefiltered
byanexternalsmoothing capacitor attached to the
C r e c t terminal.
The output stage is a s imple inverting op amp
similarin performance toa741. Various optionsare
possible byuse ofeitherR3, externalinput, orfeed
backresistors. Theoverallcircu itgainisun ity (AG IN
to OUT) , with R3 connected as a feedback resistor
and70/iA rectifiercurrentintoQ9.
In addition, the T HDT RIM terminalallowsasmall
offset to be introduced into the AG cell to null its
distortion. The two inputop amps (A1 and A2) are
connected tothe internal1.8vo ltregulator. Eachop-
amp input should be capacitively coupled while the
inputimpedance isdetermined by R2or R1, respec
tively. Circuitoperation isverystableand immune to
power-supply var iations. A single supply voltage
from +6to +1 8volts(571)or +6to +24volts(570)
can be used, though the following applicationswil l
usea +15vo ltsupply.
basic com pando r c i rcui ts
The570and571 canbequitesimp lyconnectedfo r
theirbasicfunctions ofexpansion and compression,
asillustratedin fig.2. Thesecircuitswillnotbedealt
wi th in great detail because most amateurs wi ll
probably be more interested in some of the other
uses. Also, compandoroperationiscovered indetail
inotherliterature.1-3
Thegainthroughtheexpandorshownin f ig. 2A is
1.43 VIN, where VIN is the average input voltage.
By Walter G. Jung
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8 THD TRIM 13V+
570/571
(A SECTION)
J 5 (-) IN
j 6 R3
570/571
(BSECTION)
> 12(- ) IN
i II R3
f ig . 1 . Funct iona l d iagramo f the S ign et icsNE570/571.A s im pl i f ied schem at ic d iagramo f the device is shown in B. C ons tant cu r rent sources l -j
and I2 feedtransistorpairQ1-Q2.
The 570/571 circuit constants are set up such that
unitygainoccursatanrmsinputlevelof0.775volts,
or0 dBm in 600-ohm systems. The Cin and Coare
coupling capacitors, chosen for the desired low-
frequency rolloff. Cr ec t is selected for the desired
timeconstant(10ms)inconjunctionwiththeinternal
10-kilohmresistor(R5).
Resistors Ra , Rb , Rc and Cb are not essential to
basic operation, but are desirable. Rr furnishes
short-circuitprotectionfortheoutputandcapacitive
loadbuffering,whileRa andRcpolarizeCin andCa -
Cr isa powersupply bypass, typicallyanaluminum
electrolytic.
The compressorconfiguration in f ig . 2B also has
unity gain at 0.775 vol t (rms) input , but , a com
p lementary in /ou t characte ris tic . The main d i f
ferenceinthiscircuitisthattheAGcellisconnected
asafeedbackimpedanceviaCf ,andtheinputisap
pliedtoR3throughCin - Biasfortheo utputstageis
set up by the RC-decoupling ne twork , w i th the
valuesshownappropriatefor15-voltpowersupply.
In general, the OUTterminal should be biased to
one-halfthesupply voltage. Useo fa570or571 asa
compandor is not limited to the gains shown, but
maybeextendedtootherrangesbyuseofadditional
components.
t r imm ing techniques
Deviceperformancecanbeenhancedbyjudicious
trimm ing, as shown in f ig.3. Each technique is op
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tional, and can be applied in anycombinationwhen
the highestperformance isdesired. Themost useful
ofthethreemethodsisprobablytheT HD trim,which
minimizes the gain cell harmonic distortion. In this
case, a smallvoltage (0to 3volts) is usedto injecta
currentintotheT HDT RIM terminalthroughthe100k
resistor. By biasing the rectifier terminal as shown,
the inherent currentflow in the rectifieris compen
satedforandpermitsbetterlow-levelsignaltracking.Typically, thegain-controlsignalto A1 shouldnotbe
reflected in the output. The contro l feedthrough
trimmer will minimize that signal during periods of
lowinputvoltage.
appl icat ions
An interest ing and versat ile group of circuits, the
gated orswitched-mode amplifier, can be builtfrom
the 570/571. With thedevice controlled by external
logicappliedtothe RECT in input,theongainisnor
mallysettoanyvalueandtheo ff attenuationcanbe
in excess of 80 dB. Use of the 570 or 571 is ad
vantageousinthatallportionsofthefunctioncanbeperformedentirelywithinthe IC. Further, theo n/o ff
transition timescanbesetto a valuedetermined by
thetimeconstantfrom C r e c t -
Fig. 4A is a logic controlled amplifier configured
for a HIGH input to be on, and LOWoff. When the
control input is HIGH, CR1 is off and the current
V+
lOpF
rh
f ig . 2 . Schemat ic d iagramof the devicesconnectedasanexpander ,
A , an d a co m pre ss o r, B. Th e vo ltage ga in th ro ug h th e ex pan de r is
1 . 43 * V j j y , w h i le f o r t h e c o m pr es s or i t i s \ / 0 .7 /V j js j . - V j n is t h e
average input vo l tage.
developed by Rgain f lows into the rect if ier input,
which turnson theAG cell allowing thesignal tobe
amplified. Rgain can be selected for the desired on
stategain,whichisunitywitharectifiercurrentof70
nA. R1 and R3 also effect device gain, but R3 is
selected basicallyfor an optimum ou tput bias of7.5
Vdc. R1 canalsobeadjustedfo rgain, but asshown
thevalueallowsupto3voltsrmsinput/outputsignal
levels.
f ig . 3. B y a p p ly in g t h e d i ff e re n t t r im m i n g m e t h o d s , d i s t o rt io n
t h ro u g h t h e 5 70 /5 71 c an b e r e d u ce d . T h o u g h e a c h m e t h o d is o p
t iona l , theycanbeappl ied in anycom binat ion.
As can be seen in the contro l ch aracte ris tic s
plotted in f ig. 4C, the gain is unity (or its nominal
value, ifchosenotherwise)forco ntrol inputsgreater
than 3volts. Switching is quiteab rupt, with full at
tenuationbeingachievedatlevelslessthan 1.5volts.
This narrow t ransit ion width and the nominal dc
center of 1.8voltsallowsdirectcon trolfrom CMOS,
T T L, D T L, or o ther pos it ive log ic . The u lt imate
voltage ofthe HIGH stateis non-critical, due to the
100-voltratingofCR1. Unfortunate ly, thisc ircuit hasone inherentweak point. Gain issensitive to supply
voltage due to the connection of R g a i n - Thus, the
supply voltage should be stable whi le choosing
R g a i n for70/xAintotheRECT IN terminal.
A companion circui t w ith complem en tary contro l
characteristics is shown inf ig . 4B. In thiscase, the
gainisdeterminedbythecurrentdevelopedthrough
R g a i n in con junction w i th the in te rna l vo ltage
reference(1.8V). With a lowc ontrol input, thenor
mal currentwil l f low outthrough R g a i n - When the
control signal is high, CR1 is forward biased, inter
rupting the currentflow. Therefore, the output will
beattenuatedsinceQ3-Q4havebeenturnedo ff.
Both circuits can be tai lored for specific on-off
transitiontimesbyselectionof C r e c t - Thetimecon
stantissimply 10k-CREc r HOkilohmsistheinternal
resistor). Thus, the audible switching effect can be
smoothed,eliminatingthetransientsproducedbyan
asynchronousfast switch. The C r e c t value shown
yieldsnominaltimesof5milliseconds.
UseofC r e c t inasw itchedamplifierofthistypeis
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+15V
INPUT
f ig . 4 .The log i c con t ro l led amp l i f ie r s can be con f igu red to p rov ide
an o u t p u t w i t h e it h er a H IG H o r L OW i n p u t. T h is a ll ow s th e
a m p l if ie r s t o b e i n te r fa c e d w i t h m a n y d i f f e r e n t l o g ic t y pe s , T T L,
CMOS, DTL , e tc . The res i s to r Rq a IN shou ld be se lec ted to p rov ide
70 fiA a t un i ty ga in .Eachamp l i f ie r hasanon /o f f t ime determined by
the t ime constan t o f C r^C Tw i th in terna l 10k resistor .
optional and not absolutely necessary. However, .to
minimize noise pickup some capacitance wi ll be
founduseful. Also, theultimateo ff stateattenuation
willbelimitedtoabout60dBduetotheinternalbiascurrent. Thisfeedthrough errorcan beeliminatedby
connect ing a 1 megohm resistor f rom C r e c t to
ground to bleed awaythe error current. This allows
attenuationof80dBormore.
Fig.5illustratestwosectionsofa571 combinedas
a two-input multiplexer, for FSK orother uses. This
circuitoperation issimilartothe others, but is biased
and switched in a simpler manner. Gain of eachon
channelisunity,asdeterminedbyRgain-Theoutput
ofthe B channelAG cell issum med with channel A
by connecting the ( - ) IN terminals of the A and B
sections.Therespective channelsaregated o ff bya
low control logic input, which clamps the rectif iercurrent,switchingtheAGcelloff. Forfskoralternate
channel use, the CONTROLAand CONTROLBsignals
shou ld be comp lemen ta ry . T hus, the inpu t is
"instantaneously” switched between the A and B
inputs.
Control signal suppression can be optimized with
the CHOPPER NULL contro l, wh ich trims the control
signalc omponen tin theo utput. Suppression is bet
terthan60dBa ftertrimm ing. Responsetimeisquite
fast, and is actually l imited by the slew rate of the
output op amp rather than the AG cell itself . This
makes the sw itch ing in te rval a function o f the
signal's peak amplitude. For instance with a 4-voltpeak amplitude signal the 0.5 V//xs slew rate wil l
f ig . 5. Byprovid ingco mp leme ntarycon tro l s igna ls, the FSKgenera
t o r w i l l s w i t c h b e t w e en t h e t w o s ig n a l i n pu t s . T h e o u t p u t s , w h e n
ON,are summ ed throu gh the f i rs t ope rat iona l am pl i f ie r .
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al low switching in 8 ^s; lower amplitudes wi ll be
proportionally faster. Although the circuit is touted
as a multiplexer, it can also be used as a summing
switch,withbothsignalsonatanygiveninstant.
automat ic level control
Automatic level control isa relative ly common re
qu ir emen t in aud io s ignal p rocess ing . A 571
automatic levelcontrolc ircuitcan provideconstant,
h igh percentage modu la tion w i th vary ing inpu t
levels, yet without danger of overload i f properly
handled.8 This circuit ( f ig. 6) is adapted from the
570/571 data sheet. There is one additional feature
which may be useful, however: an optional resistor
allows thethreshold of level regulation to be varied.
IfRxisleftopen,thecircuitwillhaveitswidestrange
of gain control. As this resistor value is lowered, a
larger input signal is required for full output. The
general effect, for var ious Rx values, is shown in
f ig.6B.
Since the 570/571 device operates on the prin
ciplesofaveragelevelde tection , itwilleasilysaturate
on large crestfactor inputlevelssuch asspeech, or
large transient envelope changes. This condition is
highly undesirable, but, fortunatelyitse ffectcan be
negatedquitesimply. Sincethe resultof overshoots
are peak-to-peak amplitudes in excess of the regu
lated output level, it follows that appropriate peak-
levelclipping can effectivelycontrol theovershoots.
Inthiscase, thermsoutpu tamplitude is0.775voltor
2.2 volts p-p. This part icular level is conveniently
clippedw ith a pairo f reverseparalleled LEDs, which
willlimitto3.2vo ltsp-p. TheLEDscanbeconnected
asshown in f ig. 6with a series resistorRy, wh ich is
usedtoregulatetheclippedamplitude.
This clipping technique, while not a requisite part
of the automatic level control, greatly enhances its
regulationw ith nonsinusoidal signals. Useofthis cir
cuit with speech inputs wil l necessitatethe diodes,
which willtypicallybeclipping a good portion ofthe
time. This, of course, adds audibledistortion to the
output.Therefore,ausefulitemwiththeALC/clipper
isaspeechfilterto removethe superfluous highand
low frequencies. Thefilterw illalsogreatly attenuate
harmoniccomponentsgeneratedbyclipping.
The circuit in f ig. 6C is a bandpass speech filter
which usesa single IC.Thecircuitissimply a pair of
cascaded Sallen and Key10 highpass filters (3-pole
Bessel type) . The Bessel response is one of the
poorest intermso fcu toffsharpness, but goodfrom
500 IOOO 5000
FREQUENCY IN CYCLES PER SECOND
-4 0 -3 0 -2 0
INPUT,RELATIVE dB
f i g . 6 . Schem a t i c d iag ram o f an NE570 connec ted as an au tom a t i c l eve l con t ro l (A ). The speech f i l t e r (C) i s added to r emove the undes i r ed com pon
createdby thepeakc l ipp ing. Thea t tenuat ion curve (D) , thoug hno t sharp, providesexce l lent re ject ion o f the sharp pu lsesp ikesgeneratedby thec l ipp i
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a pulse response standpoint. This feature is impor
tantto minimize amplitude overshoots which could
occurwithseverelyclippedinputs.
A comm on 1458 (dual 741) op amp is used with
nearest5 per cent component values for the fil ter
elements. Iflow-powe roperation isdesired, the1458
canbe replaced directlyw ith a 358. Ifa 358 is used,
10 kilohm resistors should be added from each outputterminaltocom mon. W ith unitygain, thecircuit
candriveloadimpedancesgreaterthan10kilohms.
Oneveryeffective usefor the570and571 deviceis
an amplitude-regulated RC sine wave oscil lator.
Typically, such circuits use a Wien bridge or other
frequency-selective RC network, with some form of
amplitudestabilization to maintain cons tantand cor
re ct lo op g ain , and also t o g ua ra nte e o u tp u t
waveformpurity. A 570or571 is nearlyoptimum for
this type of circuit because it contains the required
functions of ampli fier, recti fier, and gain-control
circuits.
Two types of sine wave oscil lators are shown inf ig. 7. The oscillator circuit ( f ig. 7A) based on the
WiennetworkisformedbythecombinationofR1-C1
andR2-C2.Thisnetworkisplacedaroundtheoutput
amplifier ofsection A, which effectively makes it a
bandpassamplifierresonantat
/ = 72-rrRC
With equal values o f R and C, the inpu t/ou tpu t
voltageratiois2to1.
Tooriginateandsustainoscillations,the571Bsec
tion is used as an inverting amplifierw ith a nominal
gain of2. A slightlygreater initial gain is establishedbythecombinationofR6,R7,and R8,wh ichensures
startup.TheBsectionAGcellisconnectedasacom
pressor, which regulates this stage's gain at the
precisevaluerequiredtomaintainundistorted,stable
amplitudeoscillations.
Thereare two main steps taken to enhanceflex
ibilityofthecircuit.Aseparatedcfeedbackpath(R3,
R4, C4)isusedaround theA stage, to removevalue
restrictions on R2 due to bias considerations. This
a llows R2 (R1) to range f rom 10k to 1 megohm
without a major performance compromise. C1 and
C2 have an even greater range, from 1 /*F down to
100 pF. To minimize errordue to strays, the lowestvalue should be used. W ith the values shown, the
circuitis capable of reasonably low harmonic distor
t ion. For example, 0.03 per cent d istort ion was
measured at 1.6 kHz and THD (Total Harmonic
Distortion)can generally be heldbelow0.1 percent.
This will vary according to the specific frequency,
and the selected impedance of the Wien network.
The lowvalueof distortion isdueto thelightdegree
ofAGcellregulation.
+I5V
+I5V
f i g . 7 . The NE570/571 can be connec ted as a s ine -wave osc i ll a to r .
TheWien br idge type osci l la tor is sho wn in A. For R= R1=R2and
C= C1=C2 , the ope ra t ing f r equency i s 1 /27 rRC. Res is to r R shou ld
be l im i tedbetw een10kand1 meg ohm w i th Cbetwe en 1000pFand
1/xF. The no rma l f r equency range can be va r ied f r om 10 Hz to 10
kHz. Thepha se-sh i f t osci l la tor should beused to generate d iscre te
f requencieson ly. Dependingupon these lect ion o f par ts, the output
frequ enc yw il l be1/27rRCV3.
The circuit wil l operate as shown over the range
from 10 Hzto 10 kHz. Below10 Hz comp onentsize
becomesimpractical,andabove 10kHz slewlimiting
in theoutpu tamplifiercauses distortion to rise. The
circuit is useful as a fixed frequency oscil lator, butcan also be tuned if a matched dual pot is available
for R1-R2. Outputamplitudeissetby R6, and is op
timum at1.5 volts rms output,from section A. Ifa
higher output level is needed, section B outp ut can
alsobeused,at3volts.
The circuit of f ig . 7A maybe undulycom plexfor
some uses, so an a lte rnate and much s imp le r
sinusoidal oscillator is shown in f ig. 7B. Thiscircuit
is a form of phase-shif t osci llator, similar to that
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describedbyTobey,Graeme, andHuelsman.9A571
is well suited for a phase-shift oscillator because it
contains the necessaryinverting amplifierto sustain
oscilla tion. In thecircuitshow n, C1, C2, andC3are
the t iming capacitors, while R1 and R2 are the
resistorsfor the phase-shift network. R3 mustbeat
least 12 times the R1-R2 value for adequate loop
gain. AGC is provided by using the AG cell as acompressor.
Thiscircu itisnotsuitablefo rtunable use. Itshould
only be used as a spot frequency oscil la tor, by
varyingC1,C2andC3. ThisisbecauseR1 andR2are
related, bythedesign, to R3; in thisspecific caseR3
cannot bevariable becauseit is used tosettheo ut
putdcbiaspoint.
Although it uses a simple design, this cir cu it pro
ducesexcellentresults.A tthefrequency indicated,a
laboratorytes tindicatedaTHDof 0.01 per centat 3
voltso utput, which is remarkable in viewo f the cir
cuit's simplicity. Totake full advantage ofthis per
formance, an output buffer may be useful; for thisyou could simply use the remaining channel as a
simpleunitygaininverter.
conclusions
Thisdiscusssion hascoveredafe w usesfora new
and interesting chip. In the course of this article's
preparation several other potential uses suggested
themselves, such as phase comparators, phase-
locked loops, voltage-tuned oscillators, and others.
Unfortunately, space and time restrictions did not
permittheircompleteexamination.
references1.C.Todd, "A MonolithicAnalogCom pandor,"IEEE Journal o f
Solid State Circuits, VolumeSC-11, number6, December, 1976,
page754.
2. C. Todd, "The MonolithicCompandor— A HighPerformance
Gain Control Integrated Circuit," Aud io Eng inee ring So ciety
Preprint, number1100, May,1976.
3. W. G. Jung, C. Todd, "Operationand Usesfor the570/571 IC
CompandorChip," The Audio Am ateur, 4/1976,page3.
4. B. Gilbert , "A Precise Four-Quadrant Mult ip lier with Sub
nanosecond Response,” IEEE Journ al o f So lid State Circuits,
December, 1968,page365.
5.W . G.Jung, "GetGain Control of80 to 100 dB,"Electronics
Design,June21, 1974,page94.
6. W. G.Jung, "ICOpAmpC ookbook," HowardW . SamsCo.,
1974,page251 and451.
7. W. G. Jung, "Application of the Two-Quadrant Amplif ier
Multiplier in Audio Signal Processing," Journal of the AES, Volume23,num ber3,April, 1975,page207.
8.J. Fisk, W1HR, "Novel Audio Speech ProcessingTechn ique,"
ham radio,June, 1976,page30.
9. Tobey, Graeme, Huelsman, "Operational Amplifiers; Design
andApp lications," McGraw-Hill, 1971,page391.
10. R. P. Sallen, E. L. Key, "Practical Method of Designing RC
Act ive F ilte rs," IRETransactions, VolumeCT-2, 1955,page74.
ham radio
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describedbyTobey,Graeme, andHuelsman.9A571
is well suited for a phase-shift oscillator because it
contains the necessaryinverting amplifierto sustain
oscilla tion. In thecircuitshow n, C1, C2, andC3are
the t iming capacitors, while R1 and R2 are the
resistorsfor the phase-shift network. R3 mustbeat
least 12 times the R1-R2 value for adequate loop
gain. AGC is provided by using the AG cell as acompressor.
Thiscircu itisnotsuitablefo rtunable use. Itshould
only be used as a spot frequency oscil la tor, by
varyingC1,C2andC3. ThisisbecauseR1 andR2are
related, bythedesign, to R3; in thisspecific caseR3
cannot bevariable becauseit is used tosettheo ut
putdcbiaspoint.
Although it uses a simple design, this cir cu it pro
ducesexcellentresults.A tthefrequency indicated,a
laboratorytes tindicatedaTHDof 0.01 per centat 3
voltso utput, which is remarkable in viewo f the cir
cuit's simplicity. Totake full advantage ofthis per
formance, an output buffer may be useful; for thisyou could simply use the remaining channel as a
simpleunitygaininverter.
conclusions
Thisdiscusssion hascoveredafe w usesfora new
and interesting chip. In the course of this article's
preparation several other potential uses suggested
themselves, such as phase comparators, phase-
locked loops, voltage-tuned oscillators, and others.
Unfortunately, space and time restrictions did not
permittheircompleteexamination.
references1.C.Todd, "A MonolithicAnalogCom pandor,"IEEE Journal o f
Solid State Circuits, VolumeSC-11, number6, December, 1976,
page754.
2. C. Todd, "The MonolithicCompandor— A HighPerformance
Gain Control Integrated Circuit," Aud io Eng inee ring So ciety
Preprint, number1100, May,1976.
3. W. G. Jung, C. Todd, "Operationand Usesfor the570/571 IC
CompandorChip," The Audio Am ateur, December, 1976,page1.
4. B. Gilbert , "A Precise Four-Quadrant Mult ip lier with Sub
nanosecond Response,” IEEE Journ al o f So lid State Circuits,
December, 1968,page365.
5.W . G.Jung, "GetGain Control of80 to 100 dB ,"Electronics
Design,June21, 1974,page94.
6. W. G.Jung, "ICOpAmpC ookbook," HowardW . SamsCo.,
1974,page251 and451.
7. W. G. Jung, "Application of the Two-Quadrant Amplif ier
Multiplier in Audio Signal Processing," Journal of the AES, Volume23,num ber3,April, 1975,page207.
8.J. Fisk, W1HR, "Novel Audio Speech ProcessingTechn ique,"
ham radio,June, 1976,page30.
9. Tobey, Graeme, Huelsman, "Operational Amplifiers; Design
andApp lications," McGraw-Hill, 1971,page391.
10. R. P. Sallen, E. L. Key, "Practical Method of Designing RC
Act ive F ilte rs," IRETransactions, VolumeCT-2, 1955,page74.
ham radio