United States Patent [111 3,557,305 [72] lnventor Bert H. Dann [56] References Cited Altadena, Calif. UNITED STATES PATENTS [211 App], No. 711,072 [22] Filed Mar. 6, 1968 2,850,627 9/1958 Moore et al. .............. .. 178/7.1DC [45] Patented Jan. 19, 1971 2,995,621 8/1961 Freedman et a1 ........ .. l78/7.5mtr_2‘1_n_s.v I73] Assignee Bell & Howell Company 3,267,387 8/1966 Felix ..................... .. 330/19 Chicago, Ill. 3,413,562 11/1968 l-lartin et al... 330/20 a corporation of Illinois 3,435,359 3/1969 Sennhenn .................. .. 330/29 Primary Examiner-Richard Murray Assistant Examiner-George G. Stellar Attorney-Christie, Parker & Hale ABSTRACT: There is described a transistorized direct-cou [541 DC RESTORATION AND WHITE CLIPPING pled amplifier circuit which receives a composite video color CIRCUIT FOR VIDEO RECORDER signal, ampli?es the signal and at the same time establishes a 9 Claims, 1 Drawing Fig. ?xed DC level for the sync tips and slices the “white” peaks of [52] US. Cl .................................................... .. l78/7.3, the video signal at a predetermined level. The ampli?er in l78/7.1,178/7.5 cludes a feedback loop which includes a sync tip detector with [51] Int. Cl ....................................................... .. H04n 5/16, a relatively long time constant on the output for controlling l-l04n 5/18 the DC operating point of the direct coupled ampli?er. The [50] Field of Search .......................................... .. 178/7.1DC, ampli?er exhibits good linearity up to its predetermined signal 7.3DC, 7.5DC, 7.5trans., GGAMMA; 330/1 1, 19, level, after which the output remains substantially constant as 20, 29; 307/237 the signal increases beyond this level. ________ “kw/7525a "4“ g’ PATENTEU JAN 1 9 I97! 3557305 INVENTOR. _ 1244/” Affair/:74.’ 3,557,305 1 2 DC RESTORATION AND WHITE CLIPPING CIRCUIT phase for detecting color information. Such standard televi FOR VIDEO RECORDER sion signals are well known in the art. BACKGROUND OF THE INVENTION The composite video signal is AC coupled through acapaci tor 12 to the base of an emitter-follower stage including a l. Field of the Invention transistor 14. The collector of the transistor 14 is connected to This invention pertains to a circuit which finds particular the positive terminal of a potential source (not shown), while application in video tape recorders but is useful wherever DC the emitter is connected by an emitter resistor 16 to a negative restoration and “white" peak clipping are required, such as in terminal of the potential source through a relatively large re a television camera circuit. In any recording system, a com sistor 18. The large resistor 18 in series with the potential posite video signal is derived either by a conventional tuner source acts as a substantially constant current source. for detecting standard television broadcasts or from a camera The emitter-follower stage may be coupled through an im system which generates such signal. The composite signal pedance shown as a resistor 20, to the emitter of a common must be ampli?ed, the DC level of the sync tips must be ?xed, base ampli?er stage including a transistor 22. The resistor 20 and the “white“ peaks of the video signal must be limited be may be modi?ed to provide a frequency-sensitive charac fore the composite signal is applied to a modulator for provid teristic, such as a resistor and a capacitor in series to introduce ing a modulated carrier to record on the tape. high frequency compensation. 2. Prior Art The emitter of the transistor 22 is connected through an Each of the functions of a DC restorer, a video ampli?er, emitter resistor 24 to the constant current source resistor 18. and a “white" clipper are well known in the art. The present 20 The base is connected to an intermediate potential, such as invention has the advantage that all of these functions are ground, while the collector is connected through a load re combined in a single'circuit at a considerable savings in com sistor 26 to the positive terminal of the potential source. plexity and cost. Furthermore, the present invention avoids The output is derived from a voltage divider extending the conventional use of diodes which are normally back between the collector of the transistor 22 and the negative ter biased and driven conductive to establish a clipping level. 25 minal of the potential source, the voltage divider consisting of Diodes have the problem that the reactance varies as the back a variable resistor 28 and ?xed resistor 30. The output is ap bias voltage varies. This presents a problem, particularly in the plied to a second emitter-follower stage including a transistor processing of color signals, because such clipping circuits in 32 having an emitter resistor 34. The ampli?ed composite troduce large differential phase effects on the chrominance video signal is derived across the emitter resistor 34. component of the composite signal. The present circuit has The output signal is applied to a sync detector circuit in the advantage that it introduces very little deterioration of cluding a transistor 36 whose emitter is connected to a ?xed overall picture quality even at a high clipping level. reference potential, such as ground. The ampli?er output signal is applied to the base of the transistor 36 through a se SUMMARY OF THE INVENTION ries resistor 38 and the collector is connected through a load The circuit of the present invention comprises a direct cou 35 resistor 40 to the negative terminal of the potential source. A pled ampli?er having a transfer characteristic that is linear large capacitor 42 provides integration or averaging of the over a range of input levels but which abruptly levels off at output by the sync tip detector. This average level is coupled some predetermined input level. The composite signal is AC to the base of the ?rst emitter-follower stage transistor 14 by coupled to the ampli?er from a voltage source of the com means of a voltage divider circuit comprising a resistor 44 and 40 posite video signal. The output is coupled to a sync tip detec a resistor 46. tor circuit which provides a DC signal proportional to the de Typical values of the components of the circuit are as fol parture of the level of the sync tips of the ampli?ed output lows: signal from a predetermined level. This DC signal is applied to Resistor 4:4 _________________________ _ _ 100K the input of the ampli?er to control the DC level of the sync Resistor 46 ________________________ _ _ 39K 45 Resistor 40 ________________________ _ _ 39K tips of the output signal to maintain the sync tips at the Resistor 16 __________________ _ _ohms__ 150 predetermined reference level, and at the same time to ?x the Resistor 24 ____________________ _ _ do- _ 150 level at which clipping occurs on the “white” peaks of the Resistor 18 ____________________ _ _ do_ _ 3, 900 video signal. Thus the output has the sync tip level maintained Resistor 26 ____________________ _ _ do- _ 2, 200 constant by the circuit relative to a‘ ?xed reference potential, Resistor 28 ____________________ _ _do_ _ 8. 2K-13. 2K such as ground, and the “white" peaks of the composite input Resistor 30 ________________________ _ _ 27K Resistor 34 __________________ __ohms__ 820 are sliced or clipped at some level ?xed in relation to the sync tip level by the transfer characteristic of the ampli?er. The cir In operation, the potential at the junction of resistors 40 and cuit provides linear voltage gain at all signal levels in between. 46 is initially at some level determined by the value of the volt 55 age divider resistors 40, 44 and 46. During the portion of the DESCRIPTION OF THE DRAWINGS input signal that the negative-going sync tips drive the base of the transistor 14 more negative, the collector-to-emitter cur For a more complete understanding of the invention, rent through the transistor 14 is reduced, which in turn in reference should be had to the accompanying drawing creases the current from the constant current source through wherein the single FIG. is a schematic diagram of the circuit of the present invention. 60 the transistor 22, causing the collector of the transistor 22 to go more negative in phase with the sync tip. This in turn ' DESCRIPTION OF THE PREFERRED EMBODIMENT reduces the current through the transistor 32 causing the out put signal to go more negative in synchronism with the sync Referring to the drawing in detail, a composite video signal tip. However, because of the direct coupled character of the is applied to an input terminal 10 from a suitable voltage 65 ampli?er, the DC level of the sync tip at the output is ?xed in source. As shown by the input waveform of a portion of the relation to the DC level of the base of the transistor 14. composite signal, the sync tip is a negative-going pulse occur If, because of changes in the DC component of the com ring at the horizontal sync frequency. Between sync tips, video posite video signal, the sync tip level of the output attempts to information for one line scan varies between the “black” level change, this changes the average voltage across the resistor and the “white” level. The sync tips extend below the “black“ 70 40. Only during the sync tips of the output is the transistor 36 level and the “white" peaks of maximum amplitude in the turned on, so that the voltage across the resistor 40 is propor positive direction should be limited to the “white” level.