ry: I I s

ry:
I
l5
I
s
95
t
I
il
ry:CX decoder
-40 dBV is boosted
everything below
-40
increasesfrom
signal
ZOO'S.ns the
so
dBV to O dBV, the gain reduces that
gain'
by 0 dBV there is 0 dB or unitY
Above 0 dBV, the gain continues to fall
so that a + I 2'dBV input is reduced by a
-6-dB gain for a *6-dBV outPut'
One oi the design goals of the CX system is to produce good sound quality
even when'a decoder is not being used
(however, with no noise reduction)' Because of this, compression is limited
below -40 dBV' If it weren't, tape hiss
Uoott"O bY more than *20 dB could
become audible above the record-surface noise. Likewise, the circuits that
control gain changes must be carefully
d".ien"J to minimize the perception of
thos"echanges. Since both the left and
rieht channels are varied by the same
cJntrol voltage, the stereo image does
not wander ibout as it would if both
were compressed indePendentlY'
The CX decoder is a l:2 exPander re-20 dBV' EverYverting to l:l below
is reduced 20 dB'
dBV
thing 6elow 20
es lhe signal increases to 0 dBV' the
qain increases to zero dB until once
igain a 0-dBV input gives a 0-dBV outoit. Above 0 dBV the gain contihues to
-F6 dBV to + 12
in"r"ur", restoring the
of the masreplica
dBV for an accurate
ter recording's dynamic range' In the
Drocessof restoring dynamic range, the
Lackground "surface" noise of the disc
is reJuced 20 dB (10 times lower)'
Gircuit Operation. Since both channels operate the same way, only the left
channll is shown in the schematic in
Fis. 2. Part numbers for the right chann"f u." the same but in the 200 seriesthat is, Rl in the left channel becomes
R20l in the right channel' If no 200-
TA
I
Fig. l. Wdveforms showing
the dynamic rangb of the
CX noise-reduction system.
Fit
th.
A
A
cx
OUT
FROM
TAPE
A-l
JZI
I
Il". I
C- -;
E
Rrl
t .t K
lntamal I
c3
.ltF
r23
.tr
- o IaPUT
E Dcv ar
C) Dcv ar o dB INPUT
AT
O dA INPUT
a Acv
-r5v
RIGXT
CHANilA
PARTS LIST
#il
D t o o ,t o t ,t o 2 ,
1 0 3= l N 4 O O 2
ICIOO= UA78Ll5
lClol. LM32O
Fig. 2. Schematic diagram
of the left channel of
the decoder- see texl
fot explanation of
part numbers fof the
two channels. Power
supply is at left.
103'c105c 1,c201,c6,c7,c207,c208,c
1OpF, 25-V electrolytic
C2,C2O2-O.O1'1rF, SO-V, 5o/o Mylar ca'
pacitor
C3,C4-O.1-pF, 5O-V, 5o/oMylar capacitor
C5-1-rrF, 35'V, 1oo/otanlalum capacitor
C9,C2O9-43-pF, 16O'V' 50/o polvstyren€
i
capacitor
ELECTROI{ICS
POPULAR
c100,c10
clo2,c104
D1,D2,D20
1 N 9 1 4s i
D1OO,D1
Fl-trl-A lu
tc1,rcs-TL
rc2,tc3*TL
rc4-cA32t
ductance
lC1O0-pA7
|c101-LM(
J 1,J201,J2
jacks
LEDl-Two.
Q1,Q3-2N
o2,04,Q5,Q
R1,R201,R
pot
The lollowin
resislorg:
JAilUANY
T0S
'ffi
(€
o
Fig. 3. Actual-size toil pattem for
the printed-circuitboad is shown above.
T t-
Fig. 4. Componenttayout fot the printedcircuit bodrd is at right.
.,,.
"
-0loo-!
ttt-t^11::;
.rtl*lliil.:
'.,i
,!Re4J
sJJ*
CT r
-l+
cl
n*nVi.o
lnternal view of the author's prototype.
I
J
A
35-V olectrolytic
C 1OO,C
10l - lOOO-pF,
l-pF,ceramicdisc
C1O2,C1O4-O.
D1,D2,D202,D3,D203,D4,D5,D6,D71N914signaldiode
1O1,D1O2,D103- 1N4OO2rectifi€r
D1OO,O
/
Fl-t&-A fuse
lC1,|C5-TL072dualB|FETop amp
|C2,|C3-TLO74quad B|FETop amp
|C4-CA328O dual operationaltransconductanceamplifier
IClOO-pA78L1sAWC+ 15-Vregulator
- 15-Vr€gulator
lC101-LM32OLZ-15
- tre' RCA
J 1JzO1,J2,J2O2,J2O3,J4,J2O4
jacks
LEDl-Two-color LED (3 lead)
Q 1,Q3-2N3904 npn transistor
02,O4,Q5,Q6-2N39OOpnp transistor
R1,R2O1,R2,R2O2,R3,R2O3sO-kO trimpot
The followingare l/l-W, 5o/ocarbon film
resistors:
1982
JAI{UARY
1o-k0 r€sistor
R4,R2o4,R15,R37,R237R 12,R13-33-kO resistor
R6,R2O6,R8,R208,R9,R2O9-16O-kQ
resistor
R7,R2O7-9. 1-kO resistor
- 1s-kOresistor
R1O,R22
R 11.R23- 1.1-kOresistor
-91 -kOresistor
R 14,R31,R231
R 16,R17- 1sO-kO
rssislor
R18-3O-kO resistor
R19-2-MO resistor
R2O-2OO-kOresistor
R2 1,R32,R232,R36,R236-5.
1-kO resistor
R24,R3O,R23O,R38,R39,R239-30O
O resistor
R25.R27- 1.6-kOresistor
R26-3.3-kO resistor
- 47-kQresistor
R5,R2O5,R28,R29
R33,R233-68-kO resistor
R34,R234-2-kO resistor
R35,R235-36-kO r€sistor
S1,S2-2pdt push-push
switch
TR1-28-V, CT transtormer(SlG 241-328)
Misc.-wire, pc board.chassis.
Note: The followlng ls avallable lrom
Phoenix Systoms,91 Elm Street, Mancheater, CT 06040 (Tel: 20$6434484): complot€ klt of parte, P-82€X
at $69.q), Also avallable soparately:
2&V CT transtormer, P-51&T, $6.00;
otch€d and drllled pc board, P€2-8,
$9.q); RCA CA328O dual OTA, P{A
328O, 9a.OO;2pdt rp swltch, P-2POT,
$1.OO; and test record, F82-TR,
S1.oo.
Ordere lees than S1Oadd $1.(X)handllng. Connectlcut resldonts ploaso
add 71/zo/oaale3 tax. Forelgn resF
dents please add 1Oolotor 8hlpplng.
r'
CX decoder
Tape monitor switch Sl selectseither
seriesnumberis listedfor a component,
then that componentis commonto both the sourceor tape output to feed 1C1,
channels.For op amps, right-channel which forms an input buffer, with trimpin connections
are in parentheses.
mer R1 usedto set input levels.Capaci-
HIRSCH-HOUCKTESTS THE PE CX DECODER
tor C2 and resistorR6 high-passthe signal (-3 dB at 100 Hz) beforeit is rectified. Op amp IC2, with diodesD2 and
D.3, boosts the signal by a factor of
about 20 and then full-waverectifiesit.
Transistor QI and diode DI set the
THECX decod€rwas adjustedtor operation in a system using an ADC Astrion
cartridge,a CarverC-4000preamplifier,
Phase Linear 40O pow€r amplifi€r,and
several speakers that included a KEF
1 0 5 . 2a n d P o l k 1 2 4 .
Noneof the signal-processing
circuits
of the preamplifierwere used duringour
listeningtssts with the decoder. The
sourcs materialincludedabout a dozen
records (including both classical and
popular music, instrumentaland vocal)
prepared by CBS to d€monstrat€the
sysl€m.Severalhad th9 same programs
on both sides, with one side unprocessed and the other with CX encoding,
simplifyingth€ evaluationof the sysl€m's p€rformance.Portions of all encoded records were played without d€codingto check their compatibility.
The benchmeasur€ments
made on th€
CX decoder consistedof its fr€quency
rssponse at sev€ral signal levels, harmonic distortionas a tunctionof output
level (withth€ CX functionop€rativ€and
b y p a s s e d )a, n dt h e i n p u t / o u t p ut tr a n s f e r
characteristic at s€veral fr€quencies
( 1 O 0 ,1 , O O Oa ,n d 1 0 , O 0 O
H z ) .T h e n o i s e
r€ductionol th€ circuitwas measuredby
driving it with th€ output of an RIAAequalizedpreamplifierwhos€ input was
terminatedby a 1,Coo-ohm
r€sistor.Output of the decoderwas displayedon our
H-P 3580A spectrum analyzer (log
sweep mode).The analyzeroutput was
plottedon an H-P X-Y r€cord€r,with the
CX decoder both active and bypassed.
Tort Rorultr.The "O dB" relerencelevel lor our transf€r characl€risticmeasurementswas the point at which the
LED on the panelchangedfrom greento
red. As the inputsignalwas decreased,
output fell at a doubled rat€ (2O dB ol
output change for each 10 dB input
change)in th€ first 20 to 30 dB ot signal
reduction.Below that, there was a transition to a linear slope that continued
down to our low€r meagurementlimit of
-50 dB (inpul)which correspondedto a
-70-dB output l€vsl. The expansion
modecontinuedaboveO dB, at a slightly
r e d u c e ds l o p e ,s o t h a t a + 1 O - d B i n p u t
p r o d u c € da n o u t p u to f + 1 5 t o + 1 8 d B ,
dependingon th€ frequency.
Frequencyrespons€ of the decoder
system rolled off at low lrequonciasto
- 3 d B a t 11 0 H z a n d - 1 5 d B a t 2 0 H z .
This efiectcould be se€n in the action of
the LED indicator,which requiredabout
3 dB moreinputat 10OHz than at the two
high€r trequenciestor its color transition. The decod€r respons€is built in to
complem€nta boost in th€ €ncodingproceas us€d on the record.
In the CX mode, the distortion rose
smoothlyfrom0.030/o
at O.1 volt outputto
about 0.50/0at th€ clipping point of 9
12
-20-d
fers thr
IC3A t
time c
respect
point fc
volts.
volts,
ano c(
or thir
ing dis
urable
reach
andS
TheI
system
| --
-20
DECIBELS
-lo
lnput/ output transler characteilstic.
rl
,- Li
I
iil
43
9t
200
gto
430
2K
( LYO GS W P )
FREAUENC
4.3K
g.lK
20(
Noise reduction using a Hewtett-Packard 358@Aspectrum analyzer.
POPUI.AR
ELECTRO}IICS
Distdti,
terminated h
JAIIUARY
O82
- 20-dBV threshold.Transistor bnfQ2
fers the full-waverectifiedoutput, while
IC3A sets the first attack and release
time constantsat 1 ms and 10 ms,
respectively.Op amp IC-IB buffers this
point for the next set of time constants.
Small-signal changes are controlled by
R19 and CJ for a two-second time constant, while large-signal changes cause
D5 or D6 to conduct for faster response.
For large-signal releases,D6, with R20
and CJ provide a 200-ms time constant.
For large attacks, DJ with R18 and Cj
provide a 30-ms time constant.
The leading edges of large attacks are
passed by C4, RI6, RI7, and Q3, which
form a 30-ms high-pass. This rather
complicated network delivers excellent
volts. At normal signal levels of 1 to 2
volts, the distortionwas less than O.2olo
and consistedentirelyof either second
or third harmonics.With the CX decoding disabled,the distortionwas unmeasb€low 1 volt,
urable(less than O.0O3o/o)
between 2
reachinga peak of O.0560/o
and 3 volts.
The noise-reduction
benefitsof the CX
system are illustrateddramaticallyby
the spectrum analysis. The noise was
attenuatedby typically 16 to 18 dB over
the tull trequencyrange of 20 to 20,OO0
Hz and beyond.lt is noteworthythat the
CX system reduces hum and rumbleas
much as it do€s higher fr€quency
noises.
groove sections. Playing those, it wa8
not possibleto hear any noise.whatsoever with an ear pressed againsta sp€aker, unless the volume wa8 set to an
unreasonablyhigh level. In such cases,
the first note of the r€cord€d program
usually blew our speaker prot€ction
fuses.Usingth€ highestpracticallist€ning volumesetting,the CX system produces a totally silent backgroundfrom
an unmodulatedgroove.
Most criticismof th€ CX procees(from
comp€titorsand certain recordingengineerg)concernsiis supposed"compatibility" with undecodedplayback.Our listeningtests have convincedus that it ls
compatibl€,in that sense. Listeningto
any ol the CX recoids at our disposal
without decoding(and, of course, without knowingthat they were CX-encoded)
we doubt that anyonewould be able lo
id€ntify them as being CX-encoded.
True,th€ir dynamicaare somewhatcomoressed, but that is tru€ ot most gtandard records as well. Th€ir nois€ l€vels
are no dift€rentfrom thos€ of ordinary
records. Th€ recording quality of the
sampleswe heardvaried widely;th6 CX
proc€ss has no €ffect on this. Some of
th€m were gup€rb,others w€re very mediocre,and most were in betw6€n.
Of course,wh€n the CX is turned on,
these recordsall soundbotter than without decoding.Their more naturaldynamics can be especially appreciat6d by
comparisonto the compressed sound
that is heardwithoutdecoding.Sinc€we
have all been hearingthat compregs€d
sound for years, it s€€ms perfectly normal untilthe €xpansionproc€ssremoves
it. W6 nev€rheardany "pumping"or other signs of incorrectcompanderoperation. The absenceof noise is not always
immediatelyobviousdue to maskingby
the program,but duringqui€t passages
it is striking. Untortunately,there is
alwaysthe mast€rtap€ hisgto be heard,
since most of the demonstrationrecordg
were apparently derived from analog
tape masters. Unless you play at earsplitting levels, though, even this is
unlikelyto be audible in a typical home
installation.
We were even mor€ impressedby the
almost total eliminationof audible rumnois€s
ble, hum,and other low-frequency
by the CX decoder. To a surprising
degr€€,this can mak€ it possibl€to get
better, quiet€r sound from an in€xpensive turntablethan can be realiz€dwith a
much more expensiveturntabl€and conventionalrecords.
The CX decoderis, in our view, a highly worthwhileadditionto any musicsystem. The kit pric€, not much more than
half the cost ot many manufacturedCX
decoders, makes this an ev€n greater
O
bargain.
User Comment. Some of the demonstration records we used had silent
FREOUENcy(Hr)
Frequency responsasat 3 input levels.
z
F
F
o
o
C O N T I N U O US
SI N E - W A V O
E U T P U TI N V O L T S
Distottion with decoder in and out of system. Output is
terminatedin lflF load (1o kilohms, |OOOpF). Clippingat I volts.
JAI{UARY
1982
13
CX decoder,
OQt"elqn
tndash
Tbchnics
DBX NoiseReduction
ffi,rz_y
AL'T-EC
Bookshelf
Speakerc
SONY
Light Weights
$43...
t02B
CIRCLENO.48 OI{ FREEINFORMATION
CARD
CompuServe
CDr'IONEEII
F
transient responsewith a l-ms large-signal attack time and low distortion drie to
a two-second small-signal release time.
Op anip IC.IC buffers the output of this
network. Op amp IC3D and transistor
Q4 convert the control voltage into a
current suitable for varying the gain of
IC4. IC4 (RCA CA3280) is an operationai
transconductance
amplifier
(OTA). The output current of an OTA
is the product of the differential input
voltage and the control current, for linear gain control over a wide range. Op
amp IC5 converts the current output of
the OTA back into a voltage for interfacing with the final output. Trdnsistors
Q5 and Q6 form a differential pair and
sense the control voltage. Signal levels
below 0 dBV will light the two-color
LED green; above 0 dBV the LED will
flash red. Switch ,S2 can be used to
bypass the decoder circuitry if desired.
lt
D
c
RN
ra&
While pc construction
Gonrtructlon.
is recommended, satisfactory results can
be obtained from other methods, as long
as you follow the original layout closely.
The finished assembly should be
mounted inside a shielded box. A fullsize etching and drilling guide is provided in Fig. 3. Its components placement guide appears in Fig. 4.
When mounting the components on
the printed circuit board, take note of
device orientation. The cathodes of all
diodes will be marked by a band, while
pin I of the ICs will be indicated by a
dot. Observe polarity markings on the
electrolytic capacitors.
Performance may be degraded if
wider tolerance components are substituted; likewise, high-leakage capacitors
can alter time constants.
Gallbratlon and Use. For best results.
the CX decoder should be calibrated to
youi cartridge/preamp
combination.
Center all trimpots and patch the decoder into your tape monitor loop. Plug the
power cord into a switched outlet. With
a suitable test record, adjust trimmer
R1 so the level LED just turns red for a
3.54-cm/s at I kHz test tone. With an
ac voltmeter connected to the left output, adjust the output level trimmer, R2,
so that the signal with CX switched in is
3 dB louder than with CX switched out.
Repeat adjustments for the right channel. Connect either a tone-burst generator or a record with high transient information to the right input only. With a
sensitive voltmeter connected to the left
output, adjust R3 for minimum feedthrough. Connect the signal source to
the left input and adjust R203. The
input level will only have to be re-calibrated if you change cartridge or
preamp.
Cue up a CX record and enjoy.
o
PON'L^REITflNO{IGS
{flftilff