CA2108319C - Equalizing amplifier - Google Patents

Abstract

Equalizing amplifying circuitry for compensating frequency dependent losses occurring in a signal transmitted over a network such as a cable. The circuitry includes an equalizing network responsive to the signal transmitted over the cable, the equalizing circuitry having a predetermined frequency response characteristic. The output signal from the equalizing network is applied to an amplifier and an attenuator applies a portion of the the amplifier output as a positive feedback signal to frequency response characteristic modifying circuitry to vary the amount of current passing through the equalizing circuitry as a function of the amplitude of the positive feedback signal to thereby effectively modify the frequency response characteristic of the equalizing circuitry. Summing circuitry responsive to the signal transmitted over the network and to the output signal from the amplifier provides a corrected signal in which the frequency dependent losses introduced by the network are compensated for with a single control.

Description

-~ 2108~19 - l - Docket: 0618-26 i RovFn FQUA~ ~7r~G A~P~ r Field of ~hP InvPnti-~n This ;nvention relates to eq~qli7~ e.~ and. in particular, to such a~ ;el~ that are used to restore a subs~qntis1ly flat, for example, overall frequency ,e~0ll3c with respect to signals that have been l,a~ d through î~ u~n~;y de~n~le~l n~,twol~ incl~ ng ~are~;cC;~n lines, etc.

R-~ 1 of thP InvPntion Al~hough the present invention is h~.~t.. des~ d with respect to video eq~s~ nE rmrlifiçrs for compensating losses qe50c~ 1 with coaxi. l cable used by the profess;onql tele.;i,;on indu~hy, it is to be ul~d~ ood that the invention can be applied to any situation involving the ~.~n~ ;on of any type of signal over any type of Ç~,qucll.;y ~epe~e ~ n~,tw~J.L where it is desired to restore a co~ ,t~d (typically flat) overall fle~luell~iy .~onse.
Analogue TV signals are disl.;l.ut~,d in a system via coaxial cables.
Such cables have a signal loss cl,~.~ h . ;~I;c which is L~lu~ e~-depel.de .-.
with the loss at the higher video L~ u~ :es (e.g. S MHz) being much greater than the loss at low r.~ s (e.g. below 100 K~). When the length of such cables exceeds 30 feet, the effect of relatively greater attenuation of the higher fre4~ .. rs may become objecti- - ~bl- because of the loss of picture detail and color saturation. The loss of color occurs . . ~ --- 2 - Docket: 0618-26 because the NTSC color system employs the e ncod~ of color info.llldlion onto a 3.58 MHz subcarrier (4.43 MHz in the PAL system) and saturation is in propollion to mod~ tion ~...l l;l~.d~.
It is common practice to employ eql~qli7in~ ~mrlifiers to co,--~ nrht~, for such cable losses. The adju~llllc.ll of such prior-art ~ ,a to match a particular cable length is a co-~, '~~c procedule. It normally involves several adj..sl~ ~e .l~, each of which apply primarily to a di~r~ ,nl part of the rl~iquellC,y 9pCCI~ul~l but which typically interact. To aGcQ- .l~ h precise eqll~li7~tion ~ uil~s the availability and use of fi~quc~lc~ sweep g~,ne.~.tola and ~cso~ ed me~Curir~E cq~ I This pl~celul~i is further complicated by the dict~nce b~ n the ends of the cable, which l~,pl~,se.ll the ge.lc.ator and l..eas~;ng c~lu;l....e.~ loc~tione. In general, the proce.lu,e is quite ~l;f icult and only the largest and best inet~ t;ons have the ~. ce~C~ y e~u;r....~.~l to do this plope.ly.
An ~mrlifiPr which can eq~ i7ç a given fixed length of cable is shown in Fig. 1. In this ~ , the eqU~li7ing nelwol~ 2 provides high frequency slope adjusllllc.lt and co...r.;ces a comrle~ multi-section design with a s~zlate adjuallllc.~l of the higher fre~,e~lr:es and perhaps midfrequency adju~ t~i as well. The ~ S has a flat lei.~ol sc and adjuak~le gain (the gain may be set to zero). The resultant correction signal, which is oblained by al)pl~.;~t~,ly setting the gain of ~...pl;l;~.r 3, is added to the incolllillg signal via ~ ...ng circuit 4 to provide an output with co-rc~ d (flat) frequency l~i~onse.
At first, it may seem that, once the e~ A~ g n~,lw~l~ 2 is calibrated, various lengths of cable may be nGco.. od.. ~ed by ay~opl;~t.,lysetting the variable gain ~mrlifier 3, but this is not so. Consider the case where ~mrlifier 3 and n~,LwulL 2 have been calibrated for 500 feet of cable. If the cable length is in.,l~ased to lQ00 feet, it might seem that if -' 2108319 - 3 - Docket: 0618-26 the gain of ,mplifiP.r 3 were doubled, the cable co...i~n~-l;on would be correct. In fact, the high-Çle~luellcy colu~llsation would not be suffini~P.nt If, for example, for 500 feet of cable, the amount of 100 KHz signsl passed is 95% snd the amount of l0 MHz signsl is 70%, then it follows that for the 1000 feet cable, the output would at 100 KHz be 90.25% (.95 x .95) and at l0 MHz would be 49% (.7 x .7). Thus, as the cable length is increased, the input to the filter n~,two,L 3 beco~ s defi~ P~lt in high L~ en~:Ps; lhe~fo~e~ the filter shape must change in order to meet the lsrger demand for high L~t~ucr~cy correction. Obviously, a COI1~ filter ,~spù~lse will not work for both cases. There is a cqe~ ~ effect which lt~lUil'~,S a dirr~,ul equAAli7i~ ~ wulL for the longer cable. This is the reason for the mllltirlP. adju~ ; in the prior-art ~...pl;l~

S~ of thP. InvP.ntir)ll It is thus a ~;lllal y object of this invention to provide an equ~li7.ing s~l-plirlcr which exhibits ~u~ ;-l improv~,l.e.l~ in ease of use and accuracy of equ-Ali7~tion It is a further object to provide such an ~ for use in the eqU-li7A~tion of losses a~oc;-~t~,d with the coaxial cable used by the profescionql television ill.lu~
It is a further object of this invention to provide a single-control equ-Ali7er having a n~lwulL design~d to exhibit a colu~oullding effect versus the amount of correction l~.luil.,d.
The Çol~ ~oing objects may be rl~m~.nted by an aul~ ing and equAAli7.ing means in which the setting of only a single control is able to co...~ c~le for the frequency~ep~.n.l~ ~ loss of the t.~n~...;cciQn cable.
A further object of this invention is to provide an improved 4- Docket: 0618-26 equAli7ing ~mrlifi~r wh~reill the correction of the cable loss at any one rle.lue~ will cause the entire frequency l~s~n~nse to be flat, for example.
This is readily accomrlieh~ d without special test e4u;r....~ ~ (the sweep ~enerato,~ of the prior art, for e~A~ le) by feeding the cable with a correct normal color signal and observing the ~mrlifiPr output at the cable receiving end. For example, if the color burst ~I..pl;l~de (or some other indi~Ptor of high frequency loss, for example) has been diminished by the cable loss, the equAI;7çr can be ~ ~: sb~ to restore it. At this point, not only will the signal be correct, but the L~iquen~iy lwponse over the entire pecllunl will also be flat.
These and other objects will become a~p~e.ll after a reading of the specifi~tion and claims taken together with the drawing.

Rrjef nescri~rtioll of thP. Ola~

Figure 1 is a block diagram an eq~ i7ing ~mrlifiPr which iil~ at~,s problems ~eSociAtrd with prior art equ~li7p~rs~
Figure 2 is a block diaglalll of an illus~ative eq~sli7i~ amrlifip~r in accoldance with the invention.
Figure 3 is a block diagram of a further illu~ àliYe eql.~1i7ing amrlifier in accordance with the invention.
Figure 4 is a mr~ifi~d block diagram of the eq~lAli7ing ~mplifi~r of Figure 3 i~rl~ g further ~.;uil.~ aeeo~ L~ ..illh Figures SA and ~B, taken together, corS~ "~ a scl- - ~.A1;r di~.u, of the cilcuilly of Figure 4.

~ " 2108319 Docket: 06l8-26 n~t~ii~ Descr~ption of F~ef~,..ed F....l~o~
of thP InvPntinl~

Referring to Figure 2, an eqi~q1i7ing ~mp1ifiPr in acco.~ance with a first embodim~nt of the imention is illustrated wL~ in the input signal from a frequency d~ndf, ~1 r.~,lwol~ 30 such as cable to be eqrq1i7ed is applied to an input a ~p1;1~sr l. The output of the a~ sr is applied to a su---~i~ circuit or node 4 in a first branch and a s.~ node 7 in a second branch. The output of ~ e node 7 is applied to an e~ g circuit means or n~twolL 2 where equalizing l..,lwol~ is frequency depen~lP-n~ such that the i..,l~,..~ e thereof typically de~ ses with in~ g f~ uclic~. The output of the e~ ;~ ~Iw~.L is applied to a variable gain ~mr1ifiPr 3. The output of the v~;able gain ~mr1ifier is applied to ~.~....i~ node 4 and, as positive C.e '~a-lr to ~ g node 7 via an ~ ~. B, CQ~ ;n~ pot-ntigl dividing .~;sl~.~ 8a and 8b, where the employment of the positive f~ed~a-1~ is an iLIpolt~ll chara~ g feature of the invendon. The correcdon signal applied to s"....ni~e node 4 from the output of variable gain ~ 1;r;e~ 3 is ~ ed with the output from n~p~ e~ 1 in ~ ng node 4, the output of the ....... ;~ node being applied to ~ p1;1;~.~ 5 which preferably has a flat ,ponse and adjustable gain where the gain may be set to zero.
In operation, as the gain of ~...p1;r;. ~ 3 is inc~ased, ~'iti~n~1 high frequency energy is applied to n~lwol~ 2 due to the positive ~e~ll,a.~L
This results in a highfi~ ~n~;y boost at the input of n~p1;1~ 3, thus effectively ~ g the shape of the lcsponse curve of r.~lwu.h 2. The amount of a1~ ;on introduced by ~ tol 8 is such lo~ r with the design of eq~ 7in~ n~lwulL 2 as to give a flat .~spoase for the type of cable and range of cable length desired, for example, O ft. to lO00 ft. of - 6 - Docket: 0618-26 type 8281 (~mph~nnl, Inc.) cable. Too much a~t~....A1;on and there will not be enough HF boost, too little and there will be excessive HF boost or oscill~tion~
Hence, it can be seen that with the employment of the positive ~edl~c~ from the input of ~mplifiP~r 3 to sl.. il-~ node 7, a s~
in~p. o~e.llcnl in ease of use and accuracy of equ~li7Ation is achieved where the equ~li7~t;oll can be simply effected by adjusting the gain of variable gain ~mplifier 3 to thereby adjust the amount of positive feedbac~ this a~ljus~n Pn~ being effected by a single control conn~ct~d either locally or remotely to the variable gain AmplifiP,r.
Figure 3 is a block diagram of a further illu;~llalive e..,bo l; ...P ~-~ of the invention ~ ;n the input signal from the cable is applied to Amrlifier 1 and the output of the ~...pl;l~P~r is applied to s, ...~ node 4 and equ~li7~tiQn filter n~,lwolL 2. The output of equalization n~lwo.L 2 is applied to the neg~Live terminal of an operational A...~ 3 having a Ç~edl,~L resistor 10 also connected to the negative input thereof. The output of the amrlifier 3 is co~ p~l~d to a potentiometer 11, the output of the potentiorn~ter being connP!~;t~d to an A ll;fPr 9. The output of A.n~l;r;P,~ 9 iS co.~ t~ as a correction signal to s ~ node 4 and as a positive ~eJl~art signal to the positive input tenninal of Amrlifier 3 via hi _ ~or 8. The output of the ~u.. ;n~ node 4 is applied to an ~mrlifier 5.
In operation, when potentiometpr 10 is set near ~in;~ .... output very little energy is feed back to Amrlifi~r 3 Thus, the ~...p1;1~ fimctinn~
as a nonnal op-amp with gain at aparticular frequency prim~rily dep~ P.n~
on ratio of the value of resistor 10 to the value of the series i~ n~e of n~lwol~ 2 wLer~y the gain of the op-amp incl~ases with increasil1g frequency since the i.~ ed~ e of r~clwo-L 2 decl~ases with increasillg 7 Docket: 0618-26 frequency. When potentiometer 11 is set toward lllaX.illlUln output, the feedback caused by resistor 10 causes the sarne signal to appear at the negative terminal. This in turn causes the current through network 2 to increase, especi~lly at higher fre4uel-c:es and thus the output of ~...pl;l;er 3 similarly incleascs. In essence, the filter n~,lwo,L 2 is being used twice, once in response to the cable signal from ~mplifier 1 and once in lesponse to the positive fc~ signal.
It should be noted with respect to the Figure 2 embo 1;,..f .~ that filter n~,lw~lL 3 is also erR~ L~ly used twice, once in l~i~ol se ta the cable ~ signal and once in ,~sponse to the positive fee~har~ signal applied to ~u~ ..in,~ node 7.
As will be dese ;1 ed in further detail with respect to Figures 4 and 5A and SB, the equali7ir~ ~mrlifi~r of the present invention is preferably remotely controlled ~lthough it is to be und~.alood that the equ~li7i~
amrlifier may also be locally controlled. It should be noted that remote control of the equ~ tion run.,lion would not be prac~cal if more than one frequency .~onse function were controlled. The ~rnrlifier is typically used in co~ ec~ion with a remote l),~o~dcas~ truck (football, b~ce~?ll games, etc.) to receive external signals over various lengths of cable (remote Cd~ laS, feeds from other trucks, etc.) where every brc ad -~ct citU~tiOn wi]l be dirrt.~ L. Hence, the easily adapted ~mrlifi~r in accollldllce with the present invention is particularly adv~nt~ouc in that there is not time to use e'-'~o,~e me~h~s to co~ Lly flatten the frequency l~ollse.
~ 2~ferring now to Figures 4, 5A and 5B, there is ill~ tcd in further detail an equ~li7ing ~mplifi~r in acco~ ce with the present invention where the CilCUi~ inf l~ es circuits conce,llled with sync separation, video cl~...pil~ and remote control of gain and equ~li7~~ion ,~lthough some of the fo.~g~ing circuits are not required for the .

210831~
- 8 - Docket: 0618-26 equ~li7~tion of signals in general, they will be generally described he.~ ~. in col-n~cl;on with a ~ e~l,bodil-lent for the e~ on of coaxial cable typically used by the professional television in-lu,l. ;es in applications such as tho~e desçrihed above where for eY~rnrle the amplifiers are used in connection with a remote broadcast truck. In particular, the circuitry of Figs. 4, SA, and SB is de~i~n~d for use with 8182 (~mphçnol) type cable, the length of which may vary from 0 ft. to 1000 ft.
The following desc~irtion cim~ n~ously refers to both Figure 4 and Figures SA and SB where the re1~tio~chir bel~ the c~ desc ;l-ed hc.cinbefole with respect to Figure 3 are generally in~ir~tçd in Figures 4, 5A, and SB and are listed below togel]le. with equivalent el~..ç..l~; in Figures 5A and SB. Moreover, in Figure 4 the ~u~ . l;ally equivalent ek ..~ of Figures 3, SA and SB are generally in-lir~lt~o~l thereon.

FIG 3 FIGS. 5A ~n SR
Network 2 R76,52,61,78,79 CV2, C26,33,42,49 ,~mplifilo.r 3 U6 (EL2030) Pot 11, Amp 9 U7 (XT4122-20) Remote Controlled ~mplifiçr Su.. ;l-~ Node 4 R70,64 and U8 ;r~. ~ S U8 (EL2030) tor 10 R93 (below U6) r 8 R100,101 The block diagram of Figure 4 in.tiratPs the i.~t~,.cn~ ction of the 3 1 ~

- 9 - Docket: 0618-26 main funrtion~l areas. Figure 4 may be referred to along with the schem~tiC diagram of Figures SA and SB which provides further details when read with the following desc~ ion.
Illl.;,LlaLve values of valious c~ are given in Figs. SA and SB, it being understood these values are for pul~oses of illustration, there being no intent to be limited to these values. Moreover, the elements U3;
U2; Ul; (U8, U6) (EL2030 ~ r~ ); US; U9; and U7 are co.l.l.lc~;ially available from and l~s~cli~ly co.l~spond to Part Numbers 5Q0 103; 500-020; 500-034; 500-102; S01-001; 501-023; and 504-064 of Ross Video ~imihCl, Iroquois, Ontario, Canada.
The video input signal is capacitively coupled to input stage emitter followers Q3 and Q4 of input ~ 1. These feed â dirr~,.~,.~lial a..~ er (Ql and Q2) which p~vides good rejection of commrln mode hum. The push-pull signal from the coll~ctnrs of Q3 and Q4 is applied to the inputs of U4, a remote controlled gain stage. UlB buffers the control voltage for U4, as will be further ~es~ ed below. The push-pull outputs of U4 are in current form and pl~luce small signals across R32 and R66.
These signals are amrlifi~d by Q7 and Q8 and drive emitter follower Q9.
The signal at the Q9 emitter should p.~Ç.,.~ly have its back porch set to ground level for the a~pl;l~,~ circuits to function pro~.ly. To achieve this, a sync sepalalo~ clamp pulse former and fee~baclr clamp are used, the foregoing elernent~ being generally decign~ted at 16. The signal from Q9 emitter is applied over line 17 and ~mrlified with a gain of about 5 and inverted by U5B. It is then coupled to pin 2 of USA, wbich fun~ tion~ as a slicer as generally inr~ir~tf~d at 18 to sep~dte the sync. The sync ~.aveÇc.l.,l appears at pin 6 of U5A. Cl, R35 and USD, as indicated at 19, ~,ne~at~ a clamp pulse timed to sample the baclc porch. This pulse turns ~mplifi~r U9 on, causing it to make a COIII~A~ ;COII between the signal - lO- Docket: 0618-26 and ground level during the back porch. A correction signal is fed via Ql l to Q7 of input amp1ifiçr l, thus stabilizing the back porch level.
The signal from Q9 of input ~".p1;~çr 1 also passes through equA1i7Ation n~,lwolL 2 and then to ~IQ~p~ . U6, which cGlle~ponds to ~mp1ifi.or 3 of Fig. 3. As ~ cu~sed above, the output signal from U6 has a frequency response characteristic desigr~d to replace the cable a~t~ n..~ion of the video signal. In this regard, rer~;e.lce may also be made to U.S.
Patent No. 4,996,497 which is inCGl~o.,~te~ herein by lel;.,re~ce. The output signal from U6 is applied to remote control gain stage U7, where the correct amount of cable eq11~1i7-Ation may be set. As will be further dcsc~;bed below, potentiQm~ter ll and ~ 9 of Fig. 3 are ~ nd with mp1ifiçr U7 of Fig. SB and an equalization control signal is provided via either local potpntiom~tel ~ or remote pot~ntiompte~ 21 (Fig. 4). A
portion of the output signal is pGi~ ly fed back via p l- ntiAl divider 8 to the po~ , input of U6 as des~- i1 ed hc.~;nl~efo~c; to provide the requisite eqU~li7Ation in acco~ance with the invention.
A further feature of the invention is that the setting of potentiom~ter 22 (or potentiomet~r 21) required to eqllAli7e a given leng~ of cable varies S~.l9~ ;A11Y linearly with respect to the length of cable to be equalized where with the potçntiom~ter set toward ..-;.-;...u--- output, the length of cable is small. With the potentiom~t~r set at ~ output, lO00 feet of cable is equ~1i7çd and with the potentiomet~ set half-way, sul.,~ y 500 feet of cable is eq lAli7çd, etc.
UlA stabilizes equAA1i7Ati~n control span by providing te.~ alul~
co...l~n~A~;on of U7 over the ~lO00 feet cable range. UlC buffers a control .~iÇtl~ilce voltage which is ~ulL ;,IA..I;~A11Y equal to about ~e one-half setting of potP.nt;omPtçr ~ (or 21). UlD buffers the control signal from potentinm~r 21 (or 22). Potentiom~t~r RV6 sets the sensili~ily of the ~-" 210~3~

Docket: 0618-26 control signal applied from UID. Circuits U2 and U3 generally in~
at 20 in Fig. SB provide power regulation for the ~mrlifier.
The direct signal from Q9 of input ~mr1ifi~ r 1 and the equ~li7~til~n correction signal from U7 are co.llbincd in output driving ~mrlifier U8, which feeds six, for e~-a.l.i)le, outputs.
To further illustrate the co.l~,s~ dence between Fig. 3 and Figs. SA
and 5B, note the output from input ~mplifi~r 1 of Figure 3 occurs at point A in Figure SA; the output of eq~li7~tinn n~,lwolL 2 occurs at point B; the output of op-amp 3 occurs at point C; the output of remote controlled ~ pl;l~P~ U7 at point D; the output of attenuator 8 at point E; and the output of summir~ node 4 and ~ 5 at point F. Thus, the video signal oc~iu~ .;ng on line 12 of Fig. 5B, which is .~cei~,d from the cable to be eq~qli7e~, is applied to emitter followers Q3 and Q4 of ~mplifier 1 as in~ ted in Figure SA. The output of a~ ; ~ \ 1 oc~;u ~ e at point A is applied to equ~li7~ti-.n n~,lwo~L generally in~ieqt~d at 2 in Figure 5A. The output of the eqU~li7~ti~n n~.lwu.L oC~;u~;ng at point B is applied to the negdliv~ input terminal of operational ~ pl;l~er 3 where the output of the ~n.l lir.cr is also applied to the negative input terminal via resistor 10. The output of a...pl;l~" 3 is applied to remote controlled ~mplifier U7. A
portion of the output signal from ~mrlifier U7 is applied as positive reeJb~ to the ~ositive terminal of ~mplifi~r 3 via the i.l~ O~
d~ n~ted at 8 and co...~ ing resistors 8a and 8b. Pct~ntiometer ~
provides local equ~li7~tisn control, the output of potPntir~mpter 22 being applied through buffer UlD to ~mp1ifier U7 to control the amount of positive fçedbac~ to ~mplifier 3. Remote control of the e4u~ ;on is available from line 13 (Figs. 4 and 5B) and potenti~mPt~r 21 (Fig. 4) and is also applied to ~...1~1;1;. ~ U7 via buffer UlD. A switch 38 is illu~ aled in Fig. 4 to select the local or remote equqli7~tion although any hlown ~ ~1083~ 9 -12- Docket: 0618-26 means may be employed to effect this selPction The correction signal obtained from ~e output of AmrlifiPr U7 is applied over line 14 to the negative terminal of amrlifi~r S while the output from Amrlifiçr 1 is applied to the positive terminal thereof via line lS.
Referring to Figure SA, addition-Al co..~ro~ CV3, R9S and CS0 are p.ere~ d to trim the .e.,~onse at longer cable lengths to thus effectively remove what may be second-order effects.
It has been ac~u~ed throughout that the loss ~,h~..cte ;~I;c of all s~. ~17s of a given type of cable are s~ffic;~ntly similar for a pled~te.lllilled signal type and this in practice for ~lofc,s~:onql grades of cable has been found to be the case.
Thus, in accol.ldllce with the present invention, an equqli7ing a--~ er has been described whe~in the correction of cable loss at any one frequency will cause the entire fi~luc.lcy ~9t~0n5e to be flat, for ~r? ~. In particul"r, with the Amrlifisr of Figs. SA and SB, a su~alA ~ lly flat l~s~ollse from ~12 MHZ is obtaiAed. This is readily a~comrliqh~d without special test e~ e~ by feeding the cable with a correct normal color signal from a ler~..,.lc~ signal source 25 as in~i~qtP,~
in Fig. 3 where, for ~A~ le, the a...~ ,de of the back porch will be at the correct level. (Note that if it is known that the ;...~p~ de of the back porch, for çYAmp~, is not at the correct level as ~ ed from source 25, the gain of U4 (Fig. SA~ may be a~ .c~ed by a control signal applied from line 30 via buffer 29 to thus set the back porch at the correct level.) The output from ~mrlifie~ S at the cable l~.,eiving end can then be obs~,. ved with a signal observation device 26 such as an osçilloscope where the peak-to-peak voltage of the back porch can accordill~ly be observed.
Hence, for eY~mrle, if the color burst a...pl;~ ç (or some other indic~tor of high frequency loss, (for ,~ ) has been dimit~;~h~d by the cable ~ ~108319 -13 - Docket: 0618-26 loss, the eq~ i7er ~an be ~ljust~ by a~ljust;ng the gain of ~ gain amplifier 3 of Fig. 2 or ~ r U7 of Fig. 5A to restore the loss. At this point, not only will the signal output at ~ . 5 be correct, but ~e frequency l~,s~onse over the entire a~,llulll will also be flat.

Claims (42)

1. Equalizing amplifying circuitry for compensating for at least frequency dependent losses occurring in a signal transmitted over a network, said circuitry comprising:
equalizing circuit means responsive to the signal transmitted over the network, said equalizing circuit means having a predetermined frequency response characteristic which emphasizes at least one frequency component of said signal transmitted over said network with respect to other frequency components of the transmitted signal;
signal amplifying means responsive to the output signal from the equalizing circuit means;
attenuating means for providing a portion of the output signal from the signal amplifying means as a positive feedback signal;
frequency response characteristic modifying means for varying the amount of current passing through the equalizing circuit means at at least said one frequency component of the transmitted signal as a function of the amplitude of said positive feedback signal to thereby effectively modify the frequency response characteristic of the equalizing circuit means so that the output signal from said signal amplifying means is compensated for losses occurring in the signal transmitted over the network at at least said one frequency component of the transmitted signal;
and summing means responsive to the signal transmitted over the network and to the output signal from said signal amplifying means for providing a corrected signal in which at least the frequency dependent losses introduced by the network are compensated for.

2. Circuitry as in claim 1 where said signal transmitted over said network is a color television signal.

3. Circuitry as in claim 1 where said one frequency component of the signal transmitted over said network is a color subcarrier upon which color information is encoded.

4. Circuitry as in claim 3 where the color subcarrier frequency is 3.58 MHz.

5. Circuitry as in claim 1 including means for applying a reference signal to said network and means for observing at the output of said summing means an observation signal having a parameter which is related to the magnitude of said component having said one frequency whereby said frequency response characteristic of the equalizing circuit means can be effectively modified by the frequency response characteristic modifying means to adjust said parameter of said observation signal to a predetermined level at which time the frequency dependent losses introduced by the network will be compensated for.

6. Circuitry as in claim 5 where said network is a cable, said reference signal is a predetermined color television signal, said observation signal is the color burst signal of the predetermined color television signal, and said parameter of the observation signal is the magnitude of the color burst signal.

7. Circuitry as in claim 1 where the frequency response is substantially flat at the output of said summing means over the entire spectrum of the signal transmitted over the network.

8. Circuitry as in claim 7 where said frequency response is substantially flat from 0-12 MHz.

9. Circuitry as in claim 1 where said frequency dependent network is a cable whereby said frequency response characteristic of the equalizing circuit means is modified by the frequency response characteristic modifying means in accordance with the length of said cable.

10. Circuitry as in claim 9 where said signal transmitted over said cable is a color television signal.

11. Circuitry as in claim 10 where said one frequency component of the signal transmitted over said network is a color subcarrier upon which color information is encoded.

12. Circuitry as in claim 11 where the color subcarrier frequency is 3.58 MHz.

13. Circuitry as in claim 1 where said frequency response characteristic modifying means includes operational amplifier means having an impedance connected between the output thereof and the negative input terminal thereof where the negative input terminal thereof is responsive to the output signal from the equalizing circuit means and the positive input terminal thereof is responsive to the positive feedback signal.

14. Circuitry as in claim 13 where said signal amplifying means includes control means for varying the gain thereof and thus vary said amplitude of the positive feedback signal applied to the positive input terminal of the operational amplifier means to thereby vary the amount of said current passing through the equalizing circuit means at at least said one frequency component of the signal transmitted through the network.

15. Circuitry as in claim 14 where said control means for varying the gain of the signal amplifying means is the only control means for providing equalization correction to said corrected signal at the output of said summing means.

16. Circuitry as in claim 14 where said control means for varying the gain of said signal amplifying means includes means for varying said gain at a location remote from the location of the equalizing amplifying circuitry.

17. Circuitry as in claim 14 where said frequency dependent network is a cable whereby said frequency response characteristic of the equalizing circuit means is modified by said means for varying the gain of the signal amplifying means in accordance with the length of said cable.

18. Circuitry as in claim 17 where said control means for varying the gain of the signal amplifying means includes potentiometer means where the setting of the potentiometer means needed to provide said corrected signal at the output of said summing means varies substantially linearly with respect to the length of said cable to be equalized.

19. Circuitry as in claim 14 where said signal transmitted over said cable is a color television signal.

20. Circuitry as in claim 19 where said one frequency component of the signal transmitted over said network is a color subcarrier upon which color information is encoded.

21. Circuitry as in claim 20 where the color subcarrier frequency is 3.58 MHz.

22. Circuitry as in claim 14 including means for applying a reference signal to said network and means for observing at the output of said summing means an observation signal having a parameter which is related to the magnitude of said component having said one frequency the magnitude of said one frequency whereby said frequency response characteristic of the filter circuit means can be modified by the means for varying the gain of the signal amplifying means to adjust said parameter of said observation signal to a predetermined level at which time the frequency dependent losses introduced by the network will be compensated for.

23. Circuitry as in claim 22 where said network is a cable, said reference signal is a cable, said reference signal is a predetermined color television signal, said observation signal is the color signal of the predetermined color television signal and said parameter of the observation signal is the magnitude of the color burst signal.

24. Circuitry as in claim 13 including potentiometer control means connected between said output of the operational amplifier means and to the input of said signal amplifying means so that the amplitude of the positive feedback signal applied to the positive input terminal of the operational amplifier means may be varied to thereby vary the amount of said current passing through the equalizing circuit means at at least said one frequency component of the signal transmitted through the network.

25. Circuitry as in claim 24 where said potentiometer control means is the only control means for providing equalization correction to said corrected signal at the output of said summing means.

26. Circuitry as in claim 24 where said frequency dependent network is a cable whereby said frequency response characteristic of the equalizing circuit means is modified by said potentiometer means in accordance with the length of said cable.

27. Circuitry as in claim 26 where the setting of the potentiometer means needed to provide said corrected signal at the output of said summing means varies substantially linearly with respect to the length of said cable to be equalized.

28. Circuitry as in claim 24 where said signal transmitted over said network is a color television signal.

29. Circuitry as in claim 28 where said one frequency component of the signal transmitted over said network is a color subcarrier upon which color information is encoded.

30. Circuitry as in claim 29 where the color subcarrier frequency is 3.58 MHz.

31. Circuitry as in claim 13 where said impedance connected between the output of the operational amplifier means and the negative input terminal thereof is resistive.

32. Circuitry as in claim 1 where said frequency response characteristic modifying means includes further summing means responsive to the signal transmitted over the network and the positive feedback signal, the output of said further summing means being applied to said equalizing circuit means.

33. Circuitry as in claim 32 where said signal amplifying means includes control means for varying the gain thereof and thus said amplitude of said positive feedback signal to thereby vary the amount of said current passing through the equalizing circuit means at at least said one frequency component of the signal transmitted through the network.

34. Circuitry as in claim 33 where said control means for varying the gain of the signal amplifying means is the only control means for providing equalization correction to said corrected signal at the output of said summing means.

35. Circuitry as in claim 33 where said means for varying the gain of said signal amplifying means includes means for varying said gain at a location remote from the location of the equalizing amplifying circuitry.

36. Circuitry as in claim 33 where said frequency dependent network is a cable whereby said frequency response characteristic of the filter circuit means is modified by said means for varying the gain of the signal amplifying means in accordance with the length of said cable.

37. Circuitry as in claim 36 where said control means for varying the gain of the signal amplifying means includes potentiometer means where the setting of the potentiometer means needed to provide said corrected signal at the output of said summing means varies substantially linearly with respect to the length of said cable to be equalized.

38. Circuitry as in claim 36 where said signal transmitted over said network is a color television signal.

39. Circuitry as in claim 38 where said one frequency component of the signal transmitted over said network is a color subcarrier upon which color information is encoded.

40. Circuitry as in claim 33 where the color subcarrier frequency is 3.58 MHz.

41. Circuitry as in claim 33 including means for applying a reference signal to said network and means for observing at the output of said summing means an observation signal having a parameter which is related to the magnitude of said component having said one frequency the magnitude of said one frequency whereby said frequency response characteristic of the equalizing circuit means can be modified by the frequency response characteristic modifying means to adjust said parameter of said observation signal to a predetermined level at which time the frequency dependent losses introduced by the network will be compensated for.

42. Circuitry as in claim 41 where said network is a cable, said reference signal is a cable, said reference signal is a predetermined color television signal, said observation signal is the color signal of the predetermined color television signal and said parameter of the observation signal is the magnitude of the color burst signal.