CA1110548A - Circuit for suppressing noise caused by scratches on a gramophone record - Google Patents

Abstract

ABSTRACT:
Circuit for suppressing audio frequency noise in a signal originating from a gramophone record, com-prising a signal processing section arranged between an input and an output, provided with a delay unit and a noise suppressor coupled thereto, the circuit also com-prising a control signal portion connected to the input, provided with a noise detector, the audio signal deriving from the record being suppressed inaudibly during a short constant period of time at the occurrence of noise.

Description

l~ S~ `

PHN.8?32 S(~IIM/T'`I~`¦LOOP
10-7-l97S

"Cireu;t ~or suppressing no:ise caused by seratehes on a g~ramopholle reeord"

The :inventi.on relates to a eireult for suppressing~
audio frequoney noise in a signal or:ig~nating ~rom a g~rcmlophone recordS eomprising a signal proeessing section arranged bet1~reen an input and an ou-tpu-t, providecl ~ith a 5 delay ullit and a noise suppressor arrangemellt eoupled thereto, the circuit also comprising a eontrol signal sec-tion comlected to the input, provided with a noise detec-tor Such a circuit is described in the artiele "~udio 10 d:ise seratch ~ilter" by M.R. Sach ar.d J.M.~ullingham, published in "Eleetronics Let-ters" Deeember 9, 1976, volunle 12, No. 25, pages 656-657.
.In the pri.or art ei.reuit the.signal delayed in the delay unlt is passecl -to -the output via -the noise suppressor 15 :in the ~bsence of noise. A pul.se-sl-.aped interferellee,

-2- ~ 5~8 PI-IN.8932 10-7-'l97~
for e.~EIllp1e cau3ed by a sc:ratch on -the gramo~hone record, is detc,cted :.iIl -tl-le noise detec-tor l-lnd generatas a control si,gnal ~h-i.ch act;ivates t,ha no:ise suppressor. Durillg l.ll-3 act:ivati,oll1>or,ic,cl o:E~ tlle noise suI)p:essor a ,switolling 5 over -is e:rl`octod :in th-? no:ise sup1)ressor, tlle de]ayec1 lla.l. n~ .'i ecl. 'tO thc Otl't:l':)ll'l; bO:i.rlg :rel- l.E~C:eCl l:)y the non-~d().lil,ye/l in~ t; ~ g11E~1~ Tll:is sw:i,tc1~ g-over 9 t7~ ;0 ~ W11C-,`re i.ll 1,1l~ :inl~)llt :i,s conrloc-ted to l;lle ouLpllt; V:la l:hc noi.sa 9Llp p:rQs~or, :i~, ma:i.l-~t;a:ined fox 70 msec. whcracl:r`ter a s~it:-1O Ch'.i.llg l:).lC'k to l;lle c~r:iginE~J. colldil,:i,oll au-toTIlat:icc-llly ta'lces p.Lace, tl1,e norl-(le1ayed s;,grlE~:I. be:ing replaced again by -the delayod s:igllal. Dur:ing khe per:iocl wherei.n the input is connec-ted -to -the outpu-t via the noise suppressor lnte~I`erence than occurring camlot be suppressed and are, 15 consequen-tly, audible :in the audio frequenc-~ output si,g-nal.
:[t is' an object O:r the lnventioll to improve the l;no~n circu:i.t, a substall-tiE~lly fu1.:L suppresslon of the noise be:ing nchleved and the activation pe:r:iocl of the 20 no:ise suppressor bein~ much sn1aller than that of the kno~n nolse suppressor.
- According to the invent:ion, a c:ircu:it of the type defined in the preamble is chEIracter-i~ed in that the no;se suppressor compr:ises a mult:iplier arranged between the 25 dela)r unit and the output, ~lich is connected -to the noise detec-tor v:La a func-tion gene:rator, which functioll genera-tor generates, at the occurrence of noise, a control s:ig-nal which varies -the signal originating from the record during a first period of time f`rom a working level to a 30 suppression,level, at which suppression level the signal, has a minimal value, the signal being m,aintained at -the suppression,level:during a second period of time and - varying during a third period of t:ime from the suppression level to the operating level, said period of time having 35 values of at ].east substanti,ally 0.5 msec and 3 msec., the time delay of the delay unit being at least equal to the firs-t peri.od of time.
From t;he article "An ~stimation of A,nnoyance CE~used PI]N.~932 by Dr-,po~l1;s in M.1gnetically Recorded Music" by B.L.Cardozo and G.Donlhurg, publis11ed i.n "Journal oi` the Audio Engi-neering Society", October 1968, volume 16, No. 4, pages 1l26-1l~9 it is kno1~n that signal interruptions not exceecl-5 :i.ng a ~lurat:i.on of approxi.mate:ly 10 msec and wherein boththe reduction and the increase in the sign.al amplitude oecurs gradually over a certain period of time, are in--audible, irrespoetive of t.he value of the signal ampli-tude variation.
I`he invelltion uses this phenomenon, whi.cll i.s known se, by providiIlg a eircuit produei.ng such signal interruption arti.fieially and substi.tutes them for the generally pu]se-s1-1aped interferences present in the ori-ginal signal, whieh interferences may be produeed during 15 playing of a reeord, for ~xample owin.g to damage or con-tamination of the record.
Experiments showed that with the above-mentioned periods of the signal reduction time -the signal suppression time and the signal increase ti.me, such interferenees are 20 in practice suppressed to such an extent that they are substantially inaudible.
A preferred embodiment of a circuit according to the invention is characterized in that the fùnction gene-rator comprises a easeade arrangement of a multivibrator : 25 eonneeted to th.e noise deteetor, an integrating network for integrating the output pulses of the monostable mul-: tivibrator and an amplitude limiter eonnected to the con-~ trol input of the noise suppressor for limiting the OUtpllt ; pulses of the integrating network to one amplitude value.
A square pulse havin.g a eonstant duration and am-plitude can be obtained by means of the monostable multi-vibrator at eaeh interferenee detected by the noise de~
teetor. By integrating this pulse in the inte~rating net-worl~ ancl ].imiting it thereafter in the amplitude limiter 35 a control pul.se of a suitable shape i.s obtai.ncd for ae1lieY-: ing th.erewith a signal gain variation in aceordanee with the variation mentioned above.
A f`urther embodiment of th.e circuit accord:ing to ..... .

PIIN.8932 the i.n~elItion ls characterized in that the integrating networI; comprises an operational amplifier shunted by a capacitor ~hicII amplifier is connected, via a resistor, to an outpu~ of the monostable multivibrator, an output 5 being coupled to the common junction of the first and second zener diodes included in th.o amplitude limiter and arranged in series between a supply voltage and ground.
The invent.ion will be further explai.ned with.
reference t~ the ~igures as shown in the drawing. Herein:
Fig. 1 shows an embodiment of a circuit according to the invention Fig. 1a shows a preferred e.mbodiment of a control pulse for suppressing the stereo signal at the occurrence of noise in the circuit shown in I~ig. 1.
Fig. 2 shows a practical implementation of a .full-wave rectifier and a smoothing filter as used in the cir-cuit of Fig. 1.
Fig. 3 shows a practi.cal implementation of a con-verter for converting a pulse frequency into a d.c. vol-20 tag.e as used in tho circuit of Fig. 1.
Fi.g. 4 shows an other embodiment of a circuit according to the invention wherein the noise suppression is performed digital].y on PCM signals representing an audio frequency stereo signal. ~.
25~- Fig. 1 shows a circuit according ~to the invention having a signal processing section 9 arranged between in-puts 1 and 2 and outputs 3 and 4 and a control signal section 10 included in a forward control loop. The signal ~; : processing secti.on 9 comprises two identioal parallel .
30 signal paths, each signal path.including a cascade arrange-ment of preamplifiers 5 and 6, connected to the inputs 1 : and 2, respectively, delay circuits 7a and 7b and control].ed amplifiers 8a and 8b functioning as multipliers being con~
nected to the outputs 3 and 4. Tho dela~ circuits 7a and 35 7b and the controlled amplifiers 8a and 8b constitute a delay unit_7 and a suppressor 8, respectively.
The control signal seotion 10 comprises prea.mpl:i-fi.ers 13 and 14, connected to the inputs 1 and 2, respec-~ 541~8 PHN.8932 10-7-'l978 ' "' t~ e~.y, a cl:i.:f`:l'eren-t:ial ampli.I`ie:r 29 connected to OtltpUtS
90 allcl 91 o.f` the prca~ l.:i.rier~s 13 and 111, as we:L:L as a sum al1lp].:i:[`-ie:r 21 ~llso eoup:Le(l to tlle output~s 90 and 91, _a :res:isto:rs l9 arlcl 20, res11eet:ive].y, of a s~ ;-rlal adder 5 c:ircll:i.l; 2~ o (I:i r`:rol~:nt:i.a:L nllll?L:i.:ri.er 2~ is couplccl _.a a d:i.:l:`:l`o:l:elll;:i"l,or 53 tv a cl:i.:L`.I~'orellee :Lnl)u-l: l~ o:L` a dynam:i.e Cc~llll~)r~c;c-:L~ -I1, -tl~ m~ r b(~ coImc~ct,~d I;o t:~ SUIll s igll;-lL i~ lit 15 tllo:l:ol~:f. V:i,a a~ )l:L:Ei.els 3() and 35 an oul:pl:ll;33 o:t` tlle cly:tlalll:i.e eomp:resso:r 11 :is colll)locl to a monoskclble '10 nlll:l.-t:i.v:ibra-tor 36 wh:icll i.s connected to a conver-ter 37 eollvert:i.ng a pulse :[`reqlleney into a d.e. voltage. Tlle ampl:i:t`:i.er 30 :is prov:i.ded ~ith a fixecl threshold voltage.
The eonverter 37 :is shunt,ed by a serles arrange-ment, included in a connee-t:ing line 17, of a coupling ca-i5 pac:itor 38 and a res:i.stor 39. The output 33 o:E` the dynamiccompressor l1 i9 coupled to an input li0 oI` a noise detec-tor 32 v.ia the connect:;ng l:ine l7. The eonver-ter 37 is also -eom~ected to -the ln.put 40. ~n output 18 of the no:i.se de-tee-tor 32 is coupled to a -f`unct:ion generator 31~. ~n out~
20 put of the :l~unetion generator 3l~ is eonnee-ted to a eontro:L
inpllt ll9 o:~ the noise suppressor 8.
The le:[`t-hand and right hand audio signals (L, and R, respeeti.vely) of an audio :f`requency s-te.reo signal re-eorded on a gramophone reeord and originating from a 25 scanning element, not sho~n, o:E~ for example a gramophone, are appli.ed separately to the inputs 1 and 2. In the sig-nal proees.c;ing section 9 these audio signals are first ampli:.ied to the same extent by means of -the preamplirier 5 and 6, -thereafter delayed :in the delay unit 7 ove'r an 30 equal peri.od of time and, finally, the noise is remo~ed in the noise supressor 8 by a signal suppression during the occurrenee O-r noise prod-uced, for e:~ample, by a scratch iIl the gramophone reeord. The cont;:rol signal for those si.gnal suppressions is applied to the noise suppressor 8

3.5 ~r-Ja t:h.e con-t:rol inpu-t 49. Th:is control signal is supplied by the cont,rol signal section 10 afl;er detection o.t: noise in the audi.o signcals L and R, applied to the inputs 1 and 2.

G ~105~8 PIIN.~932 ~ eside~s ampl:itu~e vari~tions owing to the dyn.am;c raIlge the (l.i.stu:l-bed stereo signal also shows pulse-shapecl nmp:lit;udc vari.lL;ions supc,r:i.lnposed thereon, W}l:iCh are of a con~s:i.d(?:rt~ l.y sllc,r-tor du:r.~a-l;:i on l;llall the :l~i.rs t-menl;i.oll.ed 5 ;.lm~ lclo v<ari.at,i.olls. 1`1'ley Illay be proclllced by, :L`or ox.amp:Le~
scrat;cllllo:i..;o on tlle g:rtlll1c)pllo:l1.o reco:lcl ancl tJlore:rore be U~ t~-~d . IlO~lO'Ve:[`, t'lley mny a].SO der:ive f`rOIII mllS:iCEl l :i.n-s~:rl.llrlenl;c7~ suc:ll as, for exall1p:le, t:rull1pets OI` pe:rCl:lSS:i.O}l i.]:lst:rtlllle~ ;, :in wh:ich case they are des:irecl. lhe desl.red 10 arlcl ~ aM.-ted -plllse-s1lapecl aml~l:i.tude Val.i.at:iollS are calJ.ed mus:ic pu~.ses anc1. noi.se pulses, respect:ively, :in the fu.r-the.r course of -thi.s description.
The cle-tec-ti.on of noise pulses is done in two select:ion stages.
!5 ~[n the f:i.rst selection s-tage the p-u:l.se-shaped an1-plitude var-iat:i.ons, that is -to say the noi.se arld music plllses, are sepa.:rated fronl th.e arnpli.tlld.e va.riat:ions caused by -tlle dynarn:ic xange Or the sound signal. :[n the second selection s-tage the no:ise pulses are separated :f:roln the 20 muslc pll] 9CS, ~or this use is made of tile featllre that the t:ime intervals between -the noise pulses are g~enerall.y much larger than those between the music pulse~s.
The first selection stage can be realised by means of`, for example, an amplifier having a thres}lold, 25 which ampl:ifies only those signals wllose amplitude exceeds a g:iven threshold voltage. This th:reshold vo].tage can be made to vary wit1l a cer-tain slowIless with the dynamic range of the stereo signal, so that only the noise and music pulses superimposed the:reon exceed the -threshold 30 voltage. Al-terna-tively, it is possible to give the thres-hold voltage a fi.xed value and to compress -the disturbed stereo sigIlal wi.th a certain slowness so that only ths noi.se and IIIUSiC pulses superimposed on the dynamic range exceed th;s fixed threshold value.
The circuit shown in the l~igure uses the latter possibili.ty for said first se]ection, starting from -the di:Cfererlce signal L-R forlmed in -the diffe:renti.al am-pli.fier ~9. ~or~ -the arti.cl.e "Audio disc scratch fi]ter" by M.R

,- '~7~
PIIN.8932 Sach a1Id J'.~l,I',u~]i.nghalll, pub.l:i.s1~.cd in Electronies Letters da-tecl J)ecelJIber ~th l976, volume 12, pages 656 and 657 dis-c:Loses t}lat -t:he cIi:f:fereIlce sif~;na.l.:l,-l~ Or a ste~'eo ~ rllal, o.r:i.g:in<~l;:il1c :rronl a g,rnlllopIlone :reco:rcl~ :i.s elli:inellt:l.y sl1,:i.l;Qble Lj :r'Or tll( Cl~ ;eC~ t i.OII O:f` IlO:iSe CflllSO(I by sc:r.~t(:IIos OIL the :I`~ Vlll'l`(, ~.r~ ,llollc~ .r~ co:l:~d .
Ir~ l.C~ Ll:r`~ ;oilt Ci:l.cl:ll t; tll:i'i Cl:i:r':~ cc~ .
:i.'i l.`:i.:rst rli.f'r'Cr'ell.ti.c~l;ed iIl tlle di:r:retel~ at;Or 53. ~s a.
~:'es~ ; tl)e alnpl:i l;ucle O:r -the mlls:ic aIlcI no:i.se pulc.ies :is :in-l0 cr(3ased relal;:ive to i,he amI)l:i-tude ~a:r:iat:ionLs caused by th(? dy-nam:i.c Or the stereo signnl. To th:is encl the di.f:E`e-r(3IIt:i.ntor ha.s a time constant of 0.14 ins(?c. in a prQe-t;:ical :implen1en-tat:ion. ThereLf-ter LIle di:L`f`ereIIce s:ignQl L-R is compressed in a coIltrollable alilpli.f:ier 16aL of -the cdyncLmic 15 com]-ressor 1'1. The control s:i.gnal requirecl :Cor the COill-press:i.on sholIlc1.'be a-.representat:ioll o:[~ t;lIe d-yIlmlli.c raIlge of -the ulldi.stlIrb(3d stereo si.gnal.. ~n fLccept..able approxi.-mQt:i.oIl Or tthe dynfLIllic range of tIle d:istu:rb(3d stereo si~-nal :is foullc1.:i.n tlIe sum s:i.gnQl. I,-~R because the:re:i.n the 20 noi.se pulses f~re mllch smalle:r relfl-t,ive to the o-ther music s:igna:Ls than :in each of the s:ign.als L9 R ailcl L-~.
TI-le swn s:i.gnal L~I~ is forrned in the si~nal ad.der ei.rcu:i.t 2l and appl:ied to a control].able ampli:~ier 16b O:r the dynaml.c corllpressor 1~ wh:icIl controllable ainpl:ifier 25 16b is negatively fed back -to a high extent. The negati~re feedbacI~ of t'he controllable amplif:ier IG-b i.s constituted by a cascade a:rraIlgemellt consi.stlng Or an amplifier stage 23, a full-wave rectifier 24 and a smoothi.ng filter 25, arranged between an output 22 and a control input 2S. The 30 time constant o~ the smoothing filter 25 is chosen to be so high tha.-t the brie~ pulse-shaped amplit1lde var:iations, that is to sa~r the noise and music pulses, canno-t be followed.. The contro] signa.l applied to -the control input 28 consequently represents f`airly accurQtely the d-yIlamic 35 range of the stereo signal and con-trols, w.i-th an increas~
ing or decreasiIlg ampli-tude o:f the surm sign.al, -the gain of the two amp3.i.:~'iers 16a and. 16b -to a smaller a.ncl larger ga:in, re 5pe C tive]y.

S~8 PHN.8932 ~ .5 a result both -the loud and the sof-t passages in tlle c1~ `fc]-ence signal L-~ are adjusted to substantlally the s ame alll p 1. :i tuc1e level.
I1ecau.9e t]le no:i.se and rnusi.c pu:l.ies are s11bstan-5 t:in:Ll.5~ not pres(311t i1~ the col1t30l .si~rl1al they are nolie~ n:i.nuted ~`.ron~ e d:i.:rl`e:rence s~ ,rna:L L~ and aro eon-.';0~l1.l0l1l;ly ('t)O:iIl~-") Ill.l:inta:LlleCI, A:f'l;er tl1:is clynam:ic coml?ress:i.on t]-l.e cl:l.I`.re:rQ1lce s:ig 11aL l,-:I'l :i'3 ~ S.'ied tO an nmp:L:i.f`:ier 3~), 11av:in~,r a th.res'ho].d, 'l0 ~ orQ-i11 sia:i(l:r`:i.rs-t sel.eeti.ol1 of the rn1~ i.c ar1d tho no:i.se puLses frc)lrl t;]1e d:i.frere1lce .si.gnal L-R :i9 e:rrectec1. To th:is e~nd the thleshold vo:L-tage O:r the ampLi.:r;er 30 :is ehosen so that 031:Ly music and noise pu:Lses can exceed thls threshol~
voltage. ~ ey are alllplii`ied in an ampl:i.:r:ier 35 and there-f-ter acti~a~te the monos-table mult:i~lbrator 36.
For tho second select:i.on~ namel-5r the clis-t:ine-tion be-t~een t.he noise nnd the mus:i.c pulses, the star-ting, po:i.n-l:
:i.s -the ]cno~n fact -th.-t :in gQ11e3-al nois~3 pulses occu.r in-c:identa:l.l5r, tha-t is ra11domly and grenerally regu]arly, an.cl 20 m~sic pulses i.n ser:ies of f:ive pulses or more ~:ith 1 :rre-quency oP ~1pprox:ilrlatcly 120 ]1~, or higl1er. So -the pulse frequency of -the ou-tput pu].ses of the monostable mul-t:i-v-i~
brator 3~ contains :i.n3rorn1a-tion about the pu:Lse -type. This pulse I'requency is converted in -the converter 37 :into a 25 negative d.c. voltage. In a practical :implementation th~
converter 37 hcls SllCh a charge time CO11Stant that the d.c.
voltage obta;.ns a maximum negative value ~ith Q pulse series O r 5 pulses or more within a time of 20 msec and such a di.scharge -time constant -that the d.c. voltage ex-30 ponent:i.ally decreases from the maximum value to a minimum value during 200 msec in case pulses are absent.
Li.1~e -the firs-t selec-tion also -the second selection can be realised by rneans of an amplifier having a -thresho].d9 ~hich only amplif:i.es -those slcgnals whose amplitude exceeds 35 a g:i.ven threshol.d voltage.This -thresho.Ld voltage may be gi.ven a f-iYed value. ln that case it is possible to firs-t :i.ncrease, i.n a control]a'ble a.n1p].iI'ier, using -the d.c.
- voll,age o:` the converter 37 as a con-trol si.gnal, -the no:ise .

4~3 PMN .8932 10--rl--1978 pulse'j iIl the olltput sigrlal Or the clynamic compressor 11 O:r thc~ nlonocl-tal)le nlult:iv:;'brator 36, cons:i.do:rably re'lative to the mu~C;ic p~lses occurr;ng :in thi.s oul;put signal and to pa~s~Y the:rc~a.r`tcr these nolse and mus:ic p-ulses t;o -t:he 'j a~llp:l.:i.L`i.er haV:i.ll.,'; a :E`:i.xod. t:h:re~;}lol.d. The th.resllo].d vo:ltage t).r t;~ fl ~ I..:i .L i ~I:L' ll..lV:i llt',` ~:~ tilrCsllCI l.cl SllOIl1Cl l~c CllOSC`11 to ll~a\/e ;1 vn llle L-e t;~lee.ll tho anllo:l.:i. t~l(lc-~ vn:lale o:t` 'tllf? no:i.se .;o~; ltl(1 I;ll~lt: ~)I' t;]lt~ :i.C ~ l.se~3.
I~:l.I,c~ al;:ive.ly it .is L)o~ss.i.l):l.e to havo tlle throsl).{)lcl 'lO vol-ta~!;e va:ry ~:il,h the cl.c. v()l.tngc~ o:E` the converter 3'7 ancl to apply the noi,se al~cl l~lUS:i,c pu:l.ses sucll. as they occur :i.n tlle outpllt S~ rlLal o:E` the dyn.llrl:i.c conlprecisc-r 11 or -the monos-t.1ble nllllt:i.vlbrator 36, d:i,rectly to th:is amp:L:i:t`:ier havlnr, a var:iable thresholcl. Thi.s selec-tion metllocl ls usecl 15 i.n the presellt c:i.rcu:it. The d.c. vol-tage o:E` the conver-ter 37 ls appl:ied V~.a the illpllt. 40 Or the llO.iYQ detec-tor 32 to the bclsc o:t` a trans:isto.r ll1 i,nc:Ludecl :in the no:i.se de-tecto:r 32 and :t`uncl;ionlng as an amp:l;.:-`:if?r hav:ing a thros-hold. ~t the same -ti.me the d:i:L':f.`erence ~;lgnal. L~ o:E` the 20 output; oJ~ l;lle dynalll;,c compressor 11 ls ap-pli,ed to the i,n-put 40 v:ia l;he comlec-ting :I,:ine 17. Tho eln:itter o:L` the trans:istor ll1 i,s comlected to ground, :its eollector is connected to a collector resistor on the one hand and to an output 1~ on the other halld.
The threshold vol-tage of t,hc~ transis-tor 41 is determllled by i,ts base-emitter bias vol.tage, which :is in-creased i:t` -th.e ne,gat:ive va:Lue of -the d.c. voltatfre of the conver-ter 37 increases. Music pulses ~7hi.ch are applied on the one hand via the connec-ting line 17 to the base o:~
30 the translstor l~l produce on the other hand such a nega-tive d.c. voltage at -the output o:E the converter 37 that they e3llnot rellder transistor 41 conductlve. No:ise pulses on the con-trary procluce substan-l;i.ally no negative d.c.
voltage at the output of the conver-ter 37 so that tht?y 35 can render the translstor 1l 1 conduc-tive via the connectlng l lMt? 37 -Hol~rever, no:ise p-ulses may also OCCU1'` during a ser:ies o:L` music pulses. Sllch. nol~e pul~ses clo nc)t reMder .. `

-'10~

PIIN.8932 transisto3 rll condl1ct-ive and are~ therefore, recognised as dl!sirec]rl1tlsic pulses. In pract:ice these noise pulses are l~ard1.y fl~ld:ible dur:ing suc11 music s:ignals T.1le noise ~ Lsos at ~ ol.ltpllt 18 O:r -tlLe r~ se detector 32 initilte

5 a Illol-lostab].e mu].ti.v:i.b:ral:or 43 o:f the :E`uncl;:ion gonerltor ~1l 'I`l)c ~uliL`orlll sq~1iarc Olltp~1t; pll:Lses o:E` t;:ll:is monosl11:~:l.c l3 ~ :illtocr:l~c~ in ~ ;O~ ~ti11~ t1c,~t~To~ c 50 Co1l~:.i.Sti:ng of a so:r.i.os arr~:l.ngelllerlt o:r` a va:r:i..lb1.e :rc?
s:i~sl;()r 11~l co~ ect:ed l;o t;he morlosta1~1e m~1l t~; vibrr 1. -to:r 1~3 'I() nl1cl nl~ operal;:i.()n.l] alllpl:i:E`:ier 1l5 ~sL-1~ln-ted by a capac:itance 1l6 :into tr:ianc~ Lar pulses. Tlle slope -thereo:E` is c1eter-n1:ined by the tilllL? constant o:E` I;he v1:r:i.able resisto:r 4.LI and the caplc:itallce 1l6. The ampl:i.-tllde o:f these triangu:l.lr p-ul-ses is lim:i.-ted in a lim:iter c:ircui t 51 o:E` tlle :E`uncti.o 15 generator 311, cons-tit;uted by a serles arrangerl1ent of Ze:ner di.ode~c.; 1l7 and.1l8. The common jurlctio1l o:E`-these zener di.ocies is comlect;ed -to both an OUtp~lt of` the :inte-g:rat:ing. nei;work 50 and the con trc)l :i.npu-L. 1,9 Of the noise suppressol 8. F:ig. 1a sho~s -the shape o-E` the control 20 plllses~ where;.n the period of -t-i.me in w1:1ich -the tra jec~-tories n, b and c are plssed may amount to 1 to 2 msec :in practicc. A ma~i~ rl1 supprcssio.n of -the stereo T--:ignal is ef:~ected in no:ise suppressor 8 durillg the t:rajec-tory b. ~Uring the trajector:ies a and c, respectively, the 25 gain of the stereo signa1 is recLuced and increasecL, res pectively, in the controlled amplifiers 8a and 8'b of the noi.se suppressor 8.
The dura-tion of the c1e:Lay reali.sed in the clelay uni-t 7 should be at least equal to -the period elapsing 30 'between the detection of noise and the occurrence of the trajectory b in the con-t;rol pulse derived -therefrom.
In a c:ircuit sho~Tn in I~`ig. 1; tested in practice, the ~mplif`i.ers 5, 6, 13, 14a 21', 29, 23, 35 and 45 were provided wi.th operational ampli.fiers of`-the type TCA 680 35 the amplif`i.er 30 with 2 BC 550-type transistor and the mol].os-table mult;ivi.brators 36 and /~3 wi-th a 11~ 4528-type integra1;ed circuit. 'I.`he dela-y urlit 7 compr:ised an i.n-te--~ratec1, di~ni.tal, cloc1~~colltro].led5 dual charge transfer .

4~
PIIN.8932 StOl'e (buli:c--t--b:rigade delay line) of tl~e type TD~ 1022, w;.t11 ~h:i.cll a signa:l. de.1.ay per ehannel o~ bet~een 3 a.nd 1.5 m~sec col:llcl bo real:isecl uncle:r the eonl:ro:l o~ a e].oek :Ero~ e:rlcy ndjl~s-tflble bet:wec,n R5 ICIIr~ and 170 '~IIY~ C
5 rlo:ise sl:lp~ essor 8 a.q t~e:l.:L as -i;l-Le eont;:rc-:L'I.cLt).l.e aml)J.~ :icrs I Gn a~lcl 1 Gl~ ~ore ~ lnl.~lllo~ ed 'b~lneal1s o:l` il l;y1?c 'I`C~ 730 :int,Og:ln~l;od dI~ CO:~It;:r(:)I L~ :lLe alnl-).l.:i.:f`:i.e:l-. 'I`:l.~e -t.rnrls:ic:lor ~ c,.~.' t]1~ I;yl~)o :l~( 'j50. ~ 2 l~l~oc ~ lsc~ ellos :I`or l;:llo m():llosta1~:l.e Illu:Lt:iv-i~:rat:o:r 36, a 5 Ill~eC ~ Se 'I() wi.cll;ll:t`o:r the m0ll0st;ab].e m~1lt:iv:ib:l-a-tor 1l3.
'I`he :i.n-l;eg:rat:ing lIet;~or1~ 50 and the :L:im:iter ei:reu:i.t 5l ~.re:l-e d:i.men~s:ioned so thclt; t-he tra~jeetor:i.es a and e o:~ t]le eont-.rol pulse dep:ic-i;c-~d :in l~`i.g. la were appro~ima-te:Ly 'I.5 msee. The de:Lay o~ delay ei.rellit 7 '15 ~as also adjustecl to 1.5 msee ~ ig. 2 sho~Ts a de-tai.led pre:~erred en1bodiment o:~
the :~u:Ll-~ave :reet:i:~:ier 24 and -the smooth:illg :t`.ilter 25 w:tlereill element:s eo:rresponding to -the e:Lernents o:t-`-tlle eireu:i.-t sho~.n :i.n ~!'ig. 1 have been g:i.vell the same reference 20 nuIllerals.
The ~ull-~Tave rectl:~ier 21l eompr:ises operat:ional ampli:r:ier 6'1 and 62 eonneeted by means of their non-in-ver-t:in.g and inverting input, respeetively, t;o a signal inpu-t 31 via mateh-in.g resi.stors 63 and 61~. T:he ope:ra-t:ional 25 ampli.fl.ers 61 and 62 are both negativeiy ~ed baek ~roln their outputs to the respeetive invertirl&r inputs via diodes 65 and 67 respect:ively. The eathodes of diodes 65 and 67 are eonneeted to said inverting i.nputs on the one hand and on the other hand to an input 71 o:~ srnoothing 30 :rilter 25 ~]a resistors 69 and 70. The outputs of -the operational ampl.i:~iers 61 and 62 are eonneeted to the eathodes of the diodes 66 and 68, respectively, -through hieh they are eoup]ed to the inpu-t 71. The non-inver-t:irlg i.nput o:t` tIle operational ampli~ie:r 62 is connee-ted to a 35 re:~erence voltage.
The smoothing fi.lter 25 compri.ses a seri.es ar--rangeIneilt of a :res:is-tor 26 arld a smoo-tll:irlg capaeitor 27, arrangecl between the input 71 and ground. Th.e eonllect:i.o 4~
PTTN.8932 10-7-19'7~3 bet~een res:isior 26 and srnoothing capacitor 27 i.s eouplecl -to the control :input 28.
:I'n the case of a posi-t:i.ve polar:ity o:f l;he signal at I:l-e s:i.gna:L .inpu-l; 31 relative to I;he re:f.`ereIIce voltage 5 ;It; tllo n()~ ve:rt;:illg :il~pll-l; o:f tl-e oper.~tiollal. alll,r) L:i.f:ie:r

6;~ d.-i.odo 65 is collclllct;:in~r~;, sc) tl~ l; I;hc.~ ol,t~:rat:i.o:Lla.l. amE):L:i~
f'i.C?:L' 6 1 'i.':; Ml~ l;:ivel.~r :f`e(l.bt~cl; to a ~refl.t; extel~t and :i.l;~s ga:i.~ iS :llO~,';I:i.~;.Ll)l.`-3' 5111f~ LS pO~ i tj,~ro C;:ig:naL appea~rs, amp:l..i.r.i.otl aIld :i.n-vc:rted, at l;:he OUtp~lt o~` l;lle operat:i.ol~ :L
'lO a~ p.li:[`:i.e:L 62. I):iocle 67 :i.s cut-o:f`:f.` f-ulcl tht1 ouLpul; volta.ge, ~t]lat :i'3 to say tht3 ln~-ertecl posi t;~re i.n~l?LI:t siir~;nal of` the s:i.~rnal :inpll1; 31 appears a-t tlle inpl.lt 7l o:f.` smoo-tllin.g f':i:Lter 25 v_a tht? condllc-t;:ing d:iocle 68. D-iocle 66 is also cut-o:f.`:t` so that the outpLlt i.mpedance o:f.` the operational 15 arllpl:il`lt-3r 6l no :Longer loacls the OUt:pllt o:E` -the opera-tional amp:L:if`:ier 62.
:Cn -th.e c~lse of a negat:ivo pol~lri-ty o:f.` thc si.gnal.
at the SigJ~.al :inpu-t 31 relative to ~sa:i.cl re:t'erence vol.tage d:iode ~5 is cu t-~o:f.`f and dlodc 65 conducts. The outpuL si.g 20 Ilfll o:t' tl:le operat:i.onal an1plif`:ier 61, thf-~t is t;o say tht-3 negative i.nput signa:L of th.e signal :in.put 31 appea.rs, ampli.:l~:i.ed, a.t the input 71 o:L` smooth:i.Ilg f:ilter 25 v:La the cond.uct.ing diode 66. The cllode ~7 i.s then conducti.ng 50 thaL the operational amplif`ier 62 :is negatively :f`ed 25 baclc to a great exte~nt anc]. lts ampll:f`i.catioll ls negllgibl~-small. Diode 68 is cut-of:f' so -tha-t the ou-tput impedance of the operatlonal amplif:ie:r 62 does not ]oad t:he ou-tput of the operat:ional amplifier 61.
So a signal whose amplltude is equal to the dif-30 ference between the reference voltage at the non-:inver-ing input of the ope.rat.ional amp~l:`ier 62 and the ampli-tude of the :~ul:L-wave rectified swn signal L-~R appears at -the lnput 71 o:f` smoothlng f`ilter 25. This signa.L increases ~hen the amplit-u-1e of the sum s.ignal decreases and v:ice 35 ~ersa. In the smoot;hing :filter the signal is smool;hed to a cont:rol slgnal fo:r -the controllable aMpli:L`ier 16a ~nd 161~ 7 for whlch the smoothin.g fil.ter has a time con-;tant ~o:f' app:rc~j.mately 3~3 msec. I~rlth such a -I;inne constan-t the 54~ PMN,8~32 eonlro]. si.~ a.l. only reproc,ents amplitude variations OWillg to th~f3 dy:n.lln:ic rancgo of th~ s-tereo signal ancd substan-ti~ll.y no-t the no:ise alld mllsieal pulses.
I-IL 1 practi.ca:l :in~ l.elllen-tat:ion -the ol)erat:i.onal all1-5 P:l i:f`-j Ol S G I .-nnd ~)~) WerQ ; llt(`C';r'atQd n11d 0~ tlle~ t~rP~ )XO~
Illo di.C)tl(?s oP tlle tyl~e ~ .X 1'3, ne.r`orellcc! voll;cltf~e ~r~s 8 vc)~ vu:l.l.l~s o:r t.l~ o~ tol~s ~,3, ()11, f)9 ..~ 1 7 W~ e 10 IC.~, l;hc~ v,~ e of the res:i.sl;o:r 26 WaS 3iC'3 ~2. Whi I~
t~lO .~..nlc)c)l;ll:ing c~r)aci tor ;~7 htld a C.1l-aC:; I;LlnC~t~ 0 f 1/Ul~.
IO ! l~;g~ 3 9]~l0~5 a de-t;~ Led pr~!:t`er:red ~mbod:iment; of`
t1le COnV~:rtetr 37 ar:lallgQd bQ-tween an output 8,o of the mono-! stable mu:l(;:iY:ibl-ato:r 3G ELnd th~ :input 40 of the noise de-t;eeto:r 32, The eon~erter 37 COI11Prj SeS a SQr:ieS arrangen1ent 15 o:f a coupling cap~-Lcitor 81, a resistor 83 and a cli.ocle 8LI
wllose eathode is eonnee-teci to the res:i.s-tor 83. The anode o:t'-the diode 8/1 :is eonneetecl to g:round through a paralle].
I'C' melllber eons:ist:iIlg of a eapaeitor 85 and a potelltlollleter 8G, T]~e wiper of potent:i.ometer 86 iS COUP1eCI Vifl L m.1t-t~O c.h:ing rQ9:iStor 87 -to the :inpu-t 1~L0 O:r the noise d~tec-tor 32. T:l-~e eonnect:ion betwQen the coupling capLci tor 81 ar..cl reS1S tOr 83 :j S eonneeted to ground v:ia a ma-tehi.ng resis-tor 82.
The negativQ pulses a-t thQ output 80 Of -the mono-~5 stable mult:ivibra-tor 36 Charge the capacitor 85 negatively through the diod~ .'.4 which :is eonduetive for these pulses.
Th:i.s eapacitor 85 discharges via the body of poterltioinetel-86, the RC time constant being appro~imately 0,1 SQC in a prae-tical implementation, The RC member was rated so 30 that ftx.a pulse train of ~ to 6 pulses a maximum negative capacitor voltage of appro.~ima-tely -2,2 V is achieved wi-thin 20 msec. ~n adjustable portion of the eapaei.tor voltage is tapped off at the wiper of -the potent:iome-ter 86 aII1d applied as a varying threshold voltage to -the in-35 p-~t 1~o of noise cietee-tor 32 via mal;ehing resi.s-tor 87. The wiper was dd~justQd so that thQ threshold voltage of the noise detee-tor aeh.ieved a mi.nimuln value 7 thus increasing the se}lsl.tiv:ity o:f` -lhe noise deteetor -to a mFlx-imum v~alue PHN.8932 after no pulses had occurred for 200 msec.
:I:n -this ~racticl.L :implementL-t:i.oll the capac:Ltors ~1 ancl ~5 harl a c~apac:i.tallce value o,f 22/uF and I/u~, resr)oc:-l,:i.vo.Ly. Tho :resisl:0rs 8~, 83 anci 87 hacl cl value o:f ,'j '.3~ K ~., '~ lC~ 39 l~Q, rosl)ect,:ivol.y. 'ri1e 1)O tonL:i.omctcr T.~ IIIT.~ lm Vn.:LIIO 0f 100 k: ~). . r)lodo ~4 was o:f` the 'I~Y:E)~` IIA~: l3.
.i'`:i~,. Il S]lOltS a c:i.:rcl:l:i.t acco:rding to tllo :invellt:ion :in W11:;C1L t]l~ sul~-c:ilc~ .s wllos~ :fullcl;ioIl col~r~spoIld.s to IC) tl~nt o:~`-tho c~.rcu:il; shown -in :Fig. I ha-ve b~en grivell the 91111C re:fererlce nuTnerals.
~ d~g~:i~tal signal is appli.~d t:o the lnput 'I. Th.is d:i.gi.ta:L s:ign~1 may, for example, cons:i.st of a sequellce o:f cl:ig:ital code worcls representinT/r nuillbe:r values, cor:respon~
l5 d:i.ng -to samp:Ling va.lues of` an analog aud-io frequellcy si~.r-nal. In the case o:r a stereo s:ignal these code worcls may, :for e~aml:)le, alt;e:rnate]y i.nd-lcate sampl:ing v~l.lues o:f the :Le:ft-hand and r:ig]lt-hand auclio signnl.
Interferellcesin 1;his digi-tal s:ignal which are 20 audib:Le a:ftcr a diTgita]-~i;o-ancalog conversioll can be sup-pressed i.naudibly in accordance witll -the inventicn by multiply:inTg, in case such an. int;erference :is cletectedT -the num~er values of -the code words i Il a multiplier ~, which functions as.a noise suppressor, by weigh-ting factors which 25 decrease during said first periods in numer:ical value f'rom 1 to a low value, for example 0, are maintained during the second period at th:is low numerical value and increase durillg the -third period to the value 1.
Such weighting factors can be genera-ted in a 30 simple manner in the function generator 34, for example by mecalls of an R0~1 (read only memory)~ which is read under -the control of a clock at each interference.
To enable a simple detection of disturbed cocle words, one or more check bits a:re added to eclch code word ~5 duri.ng coding of the sampling values. The values of` these check bits ale deterlllined by the valuTes of the data 'bits alld that :in sucll a mailner that, f`or example, thT3 total num'ber o:f c,nes i:rl ea.ch code worcl ancl its check. bi.-ts remair!s .

111~ S~8 PIIN.8932 COll'; tnnt;.
~ y n(w COUIlti~ , f'or exalllp1e, the plural:ity o~
oneY :i.ll eac'h cc)de wo:rd allcl the assoc ir~tocl checl~ b:i-ts :in an acldcr 3~ :i.cll:r'~ c~:io~ls a~s .~ noise detoc-tor -tllo cllstllrb~cl c~ , cnll l)c~ cc~r~ cr ~ , p~ r~:Li ly c~ collse-C~ ;'i.-V(` (~ bc~(~ cc)c~e ~.1( :1~{~ c:c~o(l~ ,r:i.~c~ r ~ 80 ~ t lJley ~)eCOtllO ;-u(:l:i blo a~s nc)i~so cLr tor a cl:i.~,:i.ta:L-to-r111;~1LOg C'O~lVO.1`'3:iO11., the :r`uJ1c-t:ion ~;ello:rato:r 3ll :is ac-~.:ival,od a~lcl l~o-iso ~supp:l:ess:ion accorcl:ill~ to tl-e i~l-vol1-t:ion is per-'10 ro:rlllecl :in Inu:l t:il?:l.:ier S.
The de.1ay unit '~ mc~y be provi.cled in hnowll mr,~nnerWit}l sh:i:~-t re~isters. '1`heIl tlle delay time sllould be at :Least cql.-a1 -to -L;he ti.m~ dur.ing ~hich 1ac;-t-melltioned plura1ity o~ code ~orcls occurs and ro said :~:irs-t periocl 'l5 o:~ ti~

Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A circuit for suppressing audio frequency noise in a signal originating from a gramophone record, com-prising a signal processing section, arranged between an input and an output, provided with a delay unit and a noise suppressor coupled thereto, the circuit also comprising a control signal section connected to the input, provided with a noise detector, characterized in that the noise suppressor comprised a multiplier arranged between the delay unit and the output, comprising a control input which is connected to the noise detector via a function generator, the function generator generating at the oc-currence of noise a control signal which caries the signal originating from the record during a first period of time from a working level to a suppressing level the sig-nal having a minimum value at this suppressing level, being maintained during a second period of time at the suppressing level and being varied during a third period of time from the suppressing level to the working level, said periods of time having values between at least sub-stantially 0.5 msec and 3msec, the time delay of the delay unit being at least equal to the first-mentioned period.

2. A circuit as claimed in Claim 1, characterized in that the function generator comprises a cascade circuit of a monostable multivibrator connected to the noise de-tector, an integrating network for integrating the output pulses of the monostable multivibrator and an amplitude limiter connected to the control input or the noise sup-pressor for limiting the output pulses of the integrating network to one amplitude value.

3. A circuit as claimed in Claim 2, characterized in that the integrating network comprises an operational amplifier, shunted by a capacitor and connected via a resistor to an output of the monostable multivibrator of and coupled by means of an output to the common junction of first and second zener diodes included in the ampli-tude limiter and arranged in series between a supply voltage and ground.