CA1129989A - Method and apparatus for pcm-encoding ntsc color television at sub-nyquist rate - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A sub-Nyquist sampled PCM NTSC color television signal obtained directly from a PCM encoded color television signal sampled at four times the color sub-carrier frequency by selecting every other sample in each line of the 4Fsc sampled television signal and introduc-ing a one 4Fsc sample displacement every two sequential television lines. A sub-Nyquist sampled signal may also be obtained from an NTSC color television signal having a color subcarrier frequency Fsc by generating a sampling signal having a frequency of 2Fsc, the sampling phase of which shifts by 180° every alternate/time sequented televison line, and sampling the NTSC color television signal in response to the sampling signal. Whichever of these two techniques is used to obtain the sub-Nyquist samples, the 4Fsc samples can be reconstructed when comb filters are used to remove alias components. The process of converting 4Fsc encoded signals to sub-Nyquist and back to 4Fsc can be repeated without impairing picture quality beyond that introduced during the first conversion - reconversion process.

Description

~.%~
C-14~6 BACKGROUND OF T~E INVENTION
....
This invention relates generally to television apparatus, and more par~icularly to a method ~nd apparatus for digitally encoding and processing an NTSC color ~ S television signal.
; In digital television ~ystems, ~t i9 necessary to r~duce the bit rate of the digital television signals, ~` usually pulse code modulated (PCM), whenever a trans-mission channel or a digital store i~ limited in capaci~y.
One way of doin~ this i~ ~o lower the PCM encoding fre-quency~ F8; however, ~he Nyqui~t ~ampling limit is soon ;~ ~. . .
.. ~ reachQd, and furthsr redu~tion in sampling frequency ~` ,i .: results in ~eat distor~ions due ~o "alias components"
wh~n the low~r ~ideb~nds of F~ pverlap the ba~eb~d video frequenci~s;~since the ba~eband vid~o bandwidth, Fv, for the NTSC system of ~elev1~ion i8 4.2 MHZ, the , ~:
Nyquist sampling 11mit is reached when F8-2FV, i.~., F~8.4 MHz.
~, : It is:known ~rom appli~ant'~ Pat. No. 4,065,784 0 that N~SC color t elevlBion ~ignal8 can be dlgitally encoded at sub-Nygui t r~es by placing ~he alia~ ~omponen~ into thoee parts o~ the spectxum 40t normally oocupied by the :; : luminance or chrominance ~ompo~ents of ~h~ vid~o signal.
~ In th~ patented 3y~temt F8 ~i8 ~xactly 2F~C+ l~Fh or :; ~ 25 2FBC- 1f4Fh, where FsC i~ the NTSC color ~ubcarx~ær .~ fr~quency ~nd Fh 1~ ~he l~ne-~an frequency. ~o~t of the. alia~ ~lgnal~ in the ~hu~-enaod~d ~ignal ~re removed ~rom the b~eband vldeo by aomb ~ ering ~e~wean ~F~-FV) and ; ~

It has recently been proposed that the NTSC
~; color television signal be encoded at a sampling rate four times the color subcarr~er frequency (i.e., 4FSc) in order to maintain picture quality and ea~e digital processing. Encoding at this sampling frequency results ~; in a bit rate of 114 Mb~, which may be exce~sive fox certain applications. Although sub-Nyquist encoding is a way to reduce the bit rate, the quarter line frequency ~-; of~sot de~crib~d in tbe aforementioned patent makes it ~,~ 10 difficult and e~pen~ive to obtain the correct ~amples from a 4F8C encoded PCM television signal.
It is the `primary object of ~he present inven-tlon to provlde a method~nd apparatu~s for obtaining a sub-Nyqui~t~ ~ample~d~ignal~directly from 4F8C sample3 without th~ne~d for in~erpolation, from~which the 4F
samples~can og~in~be~r dily~reconstructed,~u~lng comb fil~ers to remove alia~ oo~ponents.
Ano~her~o~ieot of the in~ention~ i9 bo~provide a~method and apparatu~ for obtaining fxom an analog NTSC color~television signal a sub-Nyquist encoded PCM
4F~c eample~ con bo F~adily r-constructed.

Briefly, acccrding to one aspect o~ th~ inven-tion, a ~ub-Nyqui~t sampled NTSC co~or ~alevi~ion signal having a 3~p11ng ~raqu~ncy of twice the color suboarrier, that 18, 2F~c, ln which the re~ultant alia~ compon~nts - are int~rl~ed between the luminance and chrominance ;, ,~, ~
pe~ks of ~h~ ~n~rgy ~p~otrum to allow th~ir removal from the ba~eband video by com~ filtering, is obta~ned by retaining every other ~a~ple of a PC~-encoded ~ignal sampled at four time6 the color ~ubcarrier fre~uency (i.e., 4F~c~
~nd introducing one ~ample dlsplacement every two se~uential television lines~ Thi~ i~ accompli~hed without the need for interpolation ~y in~roducing a 180 phase ~hift in the 2~8C samplina frequency ~very altern~e time 6e~uential television llne. Using comb flltering to remove alias compon~nts, the 4FBC s~mples can be reconslr~cted, and the process of convertlng 4F8C enc~din~ to ~ub-Nyqulst :~ ~nd ~ack to 4FSC can be repeated ~3 necessary witho~t impa$r~ng the ~elevision p~c~ure b~yond that introduced during the first ~onversion - reconversion proces~.
According to another aspect of the invention, instead of obtaining the ~ub-Nyquist ~ampled ~ignal from the 4F~c samples, means ~re provided for sampl~ng an analog NTSC ~olor television ~ignal ~t a sampling frequency :~ 2F8C to ob~aLn a PC~-encoded ~ignal in which the alias ~omponant~ nre interlaced between the lum~nan~e and 2D chro~inance peak~ of the energy speotru~. By using c~ ~
f$1tering to rem~ve alias c~mponents, 4F~ ~amples can be ~: constructed fr~m th~ 2~c sample3, ~nd the ~hu~-produced :~ ~ 4~c ~amples can be sampled ln the manner de~r~ed ~n :: the preceding par~sraph, lf desir~d, ts ~onvert ba~k to ' ~: 2F3c aamples, ;~ More particularly, there is provided:
.j :
A method of digitally encoding an NTSC c~lor .~ television signal having a color ~ubcarrier frequency FSc and a line scan frequency Fh, comprising the steps of:
generating a sampling signal having a frequency 2FsC, the sampling phase of which shiftq by 180 every alternate time sequential television line;
., ~

i ~

sampling said television signal in response to ~- the sampling signal; and converting the sampled television signal into digital form.

There is also provided:
Apparatus for digitally encoding an NTSC color television signal having a color subcarrier frequency FsC
and a line scan frequency Fh, comprising, in combination:
means for generating a sampling signal having a fre~uency 2FsC and the sampling phase of which shifts by 180 every alternate time seguential television line;
:; means for sampling the television signal in respon.~ie to said sampling signal: and - means for converting the sampled television ~,i signal into digital form.

i There is further provided:
:` ', .
~ ~ 6y6tem for encoding ~ compo~ite NTSC color ., . . ~
.~ televi6ion ~ignal including luminance ~nd chrominance component~ wherein the chrominance component is formed :~ 20 by modulating ~ ~bcsrrier wave having ~ frequency F~c with chrominance information~ the frequen~y F~c being :
~: ml2 of ~he line 6CaD frequency Fh of ~id televi~ion ignal, where ~ is ~n odd number, thc phase of said ~ ubcarrier wave ~hifting by 180 between two adjacent `i~ lines, said encoding system comprising:
:means for producing sampllng pulses h~ving a ~ repetition fr~quency 2Fsci .,~ means for~ ~mpling~nd encoding said television ignal in response to said:sampling pulses;
~:: 30 means ~or controlling 6aid sampling pulses ~o ;~ : cause samples of said television ~ignal to be taken at .~ points about 180 of 6aad subcarrier wave ~way from each ~.. .
~ other in ~ach ~c~nning line ~nd also ~bout 90 of ~aid B
-4a-.~ ;,, . ~" . ~ , . ., , . . . ~ . . . . ....
: ~ :

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cubcarrier wave away from the ~mples taken in ~lternate time ~equential ~cennin~ linec; and means for converting the sampled television signal into digital form.
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RIEF DESCRIPTION OF T~E D~ INGS

O~er c~b~e~t~, features and ~dvantage$ of the invention wlll become apparent, ~nd lts con truc~on and , .:

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operation better understood, from the following description, taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a diagram show~ng the foldover of the lower encoded video sideband onto the baseband video when Fs is less than 2FV;
FIG. 2 is a diagram showing the spectral characteristic of the main Y and C energy components of an NTSC color television : signal uithin the chrominance sidebands;
FIG. 3 is a diagram showing desired spectral character-~ 10 istics for a sub-Nyquist sampled NTSC color television signal;
:~, FIGS. 4, 5 and 6 are block diagrams of three different forms of comb filters useful in the practice of the invention;
FIG. 7 is a diagram showing the frequency response of ` one form of co~b filter;
15FIG. 8 is a diagram showing the frequency response of - -another form of comb filter;
FIG. 9 is the spatial sampling pattern on the image ~ plane of a sub-Nyquist sampled PCM NTSC color television signal :
derived from a 4FSc sampled signal;
: 20FIG. 10 is a block diagram of a system for sampling an analog NTSC television signal at a sub-Nyquist frequency 2FsC;
FIG. 11 is a set of ~aveforms at different points in -~ the system of FIG. lQ useful to understanding its operation;

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FIG. 12 i~ a bloc~ diagram of a ~ystem for deriving a sub-Nyquist sampled signal from ~ 4~sc sampled signal;
; FIG. 13 is a block diagram of the equivalent digital implementation of ~he filter shown in FIG. 4;
and ~IG. 14 i~ a block diagram of a ~igital-to-; digital ~onverter for conv~rting from sub-N~ist to 4F
:, ~c DESCRIPT N ~

; Beore pro¢eeding to th~ description of the , sub-N~quist encoa~ng sy~em according to the inve~tion9 it wlll be u~eful to r~viQw the problem inherent in sub-Nyqui~t eDcoding oE televi~ion signal~, and the ~ignificant ~p~ctral chara~eris~lcs of the NTSC color televi8ion 3ignal . ~ was ment~o~ed ~arlier~ wh~n in .~ :
PCM ~ncoding NTSC television ~ignals i~ i~ attempted to ; r~duc~ the sampling freguency, ~B7 b~l~w t~e ~ qu~st limit, beating 4r "aliasing" dist~rtion occur~ du~ to the lower ~idebands of F5 overlappi~g the ~aseband video . ~0 signals, a~ ~hown ln FIG. 1 r For NTSC television, ~ ~ . Fv~ 4.2MHz7 consequently, the Nyqui~t ~ampling limit is :~: reached when F8= 2F~, or wh~n F8~ 8.4~8z.
In the NTSC ~olor tel~vlsion ~ignal, ~h~
pectr~l ~ne~gy o~ the luminance (Y) signal i8 essentially . 25 o0n~red ~t h~nmonics of the lin~ scanning frequancy Fh;
e., ~F~, where n i8 an ln~ger. The chromln~n~e ~C~
gn~l ~pe~tr~1 energy peAks at odd harmonics o~ 1/2Fh;
: -6-~ C-1406 i.e., (n~ l/2)Fh. Thus, the luminance and ~he chrominance energy bundleR are frequency interleav~d as ~hown in FIG. 2.
To encode the PCM NTSC color television ~ignal at a sub-tJyquist rate and subsequently remove the alias co~po~ents, the enc~ding fre~uency F~ should be ~ho.~en to ~requencyi~erlace the alias co~ponents between the desired luminance and chro~inance components, ~s ~hown in FIG. 3O Included are the peal: baseband frequency com~onents of the luminance, YB, and chrominance, CB, ~ignal~, ~nd the alias luminance components, YA, and alias ohrominance component~, ~A. T~ sub-~yquist system described in the ~forementioned pa~ent uses an encodinq .~
- frequency of 2FSc~ l/4Fh to Prod~ce the fre~uenoy spestrum 1~ Bhown ~n FIG. 3. Unfortun~tely, ~uch samples are not readily ohta~nable rrc~m a 4~8c enc~ded PC~ televi ~n ` ~ signal that may be~ome the recom~ended standard in the : broadc sting l~du try.
,, -Applicant has ~hown in his aforementioned patent :~ 2~ th~t w~en th~ 3ub-Nyqui~t 6ampling fr~quency ~electod or N~SC cQlor teIevision ~i~nals produc~s the spectral : frequency respon~e shown ~n FIG. 3, the luminance and chro~lnance information c~n be recovered and the unde3ire~
~ .
alla~ comp~nent~ rejected by means of suit~ble tran3versal comb filtering. However, ~or clarity of under-stand1ng of th~ present method for PCM-encoding N~SC color telev~sion 8~gn~ t ~ ~ub-Nyqul~t rate, the technique ~5~ ~e~tlng ~lia~ signals by ~ean~ of comb filtering ~la ~ brl~fly revi~wed. ~ ~e~n ln FIG. 3, the ~enters . ~ -7-~ ?~

of the alias energy bursts are separated by frequency intervals of 1/2Fh. The comb ilter, therefore, should have it8 maximum xesponse~ (teeth3 or minlmum ra~ponses ~: (nulls) at frequency intervals of 1/2Fh. Such combfilter can ~e made by cGmbining video signals rom alter-nate time-~equ~ntial televiRion lines, For example, in a particular field, line R would be combined with ~` line (~ -2) or (Q+2). Since a tran~versal filter that ~ combines television lines can cause objectionable tran-: ` lQ 8i~nt6 and a loss of v~rtical re301ution in the television picture, it i9 desirable to combine as few line~ as ;~ possible to obtain the necessary comb filt~r frequency :~ ~ respor~se. The alias compon~nts can be removed from a Buh-Nyqui~t encoded NTSC color~televi~lon signal having the spec~ral frequens:y reEIpo~se shown in FIG. 3 by u~in~
either of ~he fol l owlng c~rb f llter algc~rithm~:
Add televl~ion line ,~ to ( ~ -2~ .

2. Add talevision l}ne k to ~ 2); or ~ 3. Add televicion line ~ to l/2[(~ -2)~ +2)~.
: ~ ~: 20 All line~are:from a 3ingle:~ield ~o avoid the need for interfield ~torage ~n:*h~ de~oder. Block diagrams ~f three pos~i~le ~omb fllters are ~hown in FIGS. 4, 5 and ~ ~ ;
6, which re3pectively correbpon~ to ~IGS,~ ~ 6, 7 and 8 o f the aforamentioned patent. Tha fr~qu~n~y re~ponse ~, Of t~e, f11tQr~ o FIGS. 4 ~nd 5 i~ shown in FIG. 7, ; and the frequ~ncy response of the FIG. 6 fllter i~ ~hown -~n FIG. 8~ It will b~ noted in both FIGS. 7 and 8 that : ~h~re ~re ~ull~ ~t ~he alias peak amplltude frequency component~, And the k~eth are centered at the peak .J~
C-14~6 amplitude fre~uency components of the ba~eband signals.

A~ has been previously noted, the sub-Nyquist ~ampling frequency 3elected for NTSC color television ::
signals shou~d re~ult in the spQctral frequency response shown in FIG. 3.in ord~r readily to recover the luminance and chrominance information and reject the unde~sred ~ alias components. A ~ub-Nyquist sampling frequency of : nFh will re~ult in luminance alias components overlapping the baseband main luminance components and chrominance 1~ alias component~ overlapping the baseband chrominance.
Sub-Nyquist ~ampling at nFh with 1/2Fh of~et will cause luminance alias component~ to overlap the baseband chromi-nance, and the chrominance alias aompooe~s to overlap the ba~aband luminanceO Thu~, the~e two sampling method~
1~ will not workO In the ~ystem of~th~ ~forementioned patent, a 1/4Fh off~et in the ~Fh sub-Nyqulst sampli~g rate places the alias components between the baseband ~. .
: luminance and ahrominance main frequency components, wh~ch operates sati~factorily. However, the ~uarter 2~ line ~requency off~et ma~s lt difficul~ and expen~ive to obksin ~he correct ~ample~ from a 4FeC ancoded PCM tele-c: - vi~ion ~iynal, which may ~ecome the recommend~d standard .; ; in the broadca~tlng inclustry, thus pre~enting the need ~:: for readily obtainL~g the ~ub~Ny~uist sampled signal from a 4F~a ~ampled ~ignalO The fact that 4F8C=9lOFh ~uggests that th~ ~ub-Nyquist sampl~ng freguency should ba 2F~C, ~: or 4~5Fhr which di~tat~ th~t means other than a 1/4Fh ,~ frequ~ncy off~et mu~t be provided to interl~ce the resul-tant aliaR componen~ betw0en ~he luminance and chrominance ~nergy pe~k~.
. _n ~?~

:

This iQ accomplished according to the pxe3ent invention by introducing a 180 pha.se shift in the ~ampling frequency every alternate time ~equent~al t~levisio~.~ line. The effect of this will be ~een from examlnation of FIG. 9 which de2icts khe pattern of the spat~al picture samples on the im~ge pla~e, the dots repr~senting ~ampling po~ts taken at the rate of 14.32MHz, or four tim~ the color sub-carrier frequency, which are spat~ally aligned on ~uccessive video scan lin~s (~ -2), (~ 1), etc., and spaced ~ ' or 70 nanoseconds apart. Th~ circle~ repre~ent samplin~ points o~ the sub-Nyquist sampled PCM N~SC color television ~ignal derived from the 4FSc ~mpl~d sigRal; it is seen that the circles ar~ spread apart ~ , or nominall~ 140 nanoseconds, ~ : sc alon~ each scan line,~ ~nd ara displaccd one sample interval every two saquentiai~tel~vision lines~ That i~, the cird e~
ln lin~s (~ -L) and ~ are di~placed to the~right one ple in~erval with~re~pec~ to the cirales~in the two tel~visi~n lines~preceding and fol~owing them. In sum~ary, h~ sampling frequ~ncy~of the ~ub-Ny~ulst samples is xactly 2F~c W~th~appropriate pha~e shifts at the starts of differ~nt tel2vision:1ines. It w~ll be ~ppreciated that with thi~ t~chni~ue the 8ub-Nyqui t ~ncoded signal can h~ obtained~from the 4F~c PCM ~ncoded ~gnal without any n-ed for i~terpclat~rs; thP proc~s i8 one of ~imply rets~ni~g the Rppropriate on~6 of the 4FSC samplQs.

Wlth proper precautions it is possible to~-go back and rth b~ween the ~8c encoded Bignal and khe 2F8C ancodQd ~ignal ~n unl~mlted number of tlme~ without any uxther ~L~?J~

.
.;
vidco degradation than that caused hy the first trans-la~ion from 4F8C to 2FSC, aq long as the sam~ sam~les . from the ~ame frames are ~elected. This ~uarantees that - the ~ub-Nyqui~t di~ital stream will always consist of . 5 a selected ~et (as per FIG. 9~ of unprocessed original 4FsC sam~les It is interesting to note that lf it iq assumed that no two sample~ in the sub-Nyquist digital stream ~re separated by less than the period l/2F8c, the resultant long term average sampling frequency is 2F~c-l/4Fh.
~, Thus, there is a 1/4F fre~uency of~set, except ~hat here, unlike in the patent~d ~ s~em, this ~requenc~ ofset i~
. .
not continuous but is a result o ~he sample displacements - ~ that are introduced evexy two ~elevi~ion line~.
: .
FIG. 10 show3 in block diagram form a system . ~or PCM-encoding an N~SC color television signal at the ub-Nyquist rake:of ZF ~ An input NTSC ~ideo si~nal in a~alog ~orm, which has prefer~bly been ~ubjected to comb ~iltering for the rea~ons and in the manner discussed hereinaftes, iæ recelved Gn ~ line 30 and ~oupled to the nput o~ an ana~og-~o-dig~tal converter 32, to sync ,, .
stripper 34, and to a color ~ubcarrier r~generator 36.
- The ~vl~r ~bcarrier r~:g~n0ra~r i~ of conventional . d~sign an~ regenerates ~he oolor subcarri~r which, in . 25 ~he NTSC ~olor tele~ision 8y6tem, iq 3.58MHZ. The regener-.

~ ~ ~ted ~olor zubcarriar ~ignal 15 applied to and control~
, :, . a clock g~nerator 38 which produce~ ~wo ~rains of clock ", pul~e~ both h~vin~ a ~r~quenay of 4F8C, but with one ~80 , , \

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out of phase with reYpect to the other, as shown by waveforms ~A) and ~B) of FIG. 11. The ~F8C pulse train is applied as one input to an AND circuifi 40 ~nd the 4F
pulse train is applied to the clock input of a "D-type"
~lip-flop circuit 42, the Q termlnal of which i5 connected as a second input to AND circuit 40O The sync stripper 34, of conventional desi~n, produces horizontal dri~e pulses (waveform C of FIG. 12) in synchroni~m w~th the horizontal sync pulse of successive television lines, which are applied to the clock input of each of two flip-flop circuits 44 and 46, both of which ara of "D-ty~en.
The ~ output of flip-flop 44 is aonnected to the data (D) input ~o as to produce a pulse signal at half the : ~ horizontal drive frequency, illustrated in waYeform (D) of FIG. 12, which is~ appli~d to the clock terminal of .
- ~ ~ a:fourth flip-flop circuit 48. The Q ou~put of flip-flop 48 : is connected~to its data inpu~, and the Q output (w~veform E) is applied as one lnput to an AND circuit : 50, the o~her inpu to which i5 the Q output of flip-flop : 20 42 (waveform G~. The Q ou~put of flip-flop 42 l~ave-:
form F) re~e~s flip-flop 48. The output of AMD ~IrcuI~
40, the re~ult of ANDin~ waveform~ (A~ and ~G3, ~ho-m . ~ . a8 wave~orm (H~ of FIG~ 8 applied to the clo~k terminal o a ~ifth D-type flip-flop 52, ~he Q output -~ 25 terminal of which is ~onnec'ced to its dat ter~nal.
~: . With the described connect~ons, fl$p-flop 52 produ~es at its ~ output ~he clo~k 5ignal ~hown ~8 wav~form lI~ of FIG, 12 hav~ng ~ fr~qu0n~y of -~2-'~", , ~-140~

~7~

2F~C, which is applied to ADC 32. The ADC, wh~ch in the pr~ent embodiment iB a PCM encoder, ~mplas the analo~ video signal under control of this clock si~nal, - which shifts sampling phase every 2~, or every alternate televi~ion line; this gives the sampling pattern shown by the clrcles ln FIG. 9.
The balance of the circuit illustrated in ~IG. 10 is provided to minimize fli~ker at vertlcal color tran~ltion6 by forcing the first sampling points of corresponding lines on odd field~ to be 3uperimposed, and the ~ampling point~ of all corresponding lines on ev~n fields to be sup~rimposed. ~hls relationship i8 ~ .
guaranteed in th~ system of ~IG. 10 by r~etting of th2 clock pha~e ~t the ~tart of each frame. This i8 accomp-lS lished by apply$ng succes~ive fr~me pulses, from ths sync stripper 34.
, to the clock terminal of a flip-flop 5~, the data in~ut termlnal of which i~ ~et "high", and the "re~et" inp~t of which i8 aonnected to the Q
ou put of previou~ly ment~oned flip-flop 46. The Q
~ 20 ; output of flip-~lop 54 18 applied as one i~put to an AND circuit 55, the oth~r ~nput to which i9 the output of an AND circuit 58 havlng the ~-drive ~lgnal (waveform ~ : C) and ~he Q outpu of flip-flop 44 a~ lnput~ e output : ~ ~ of AND clrcuit 56 i8 appli~d to the l~Bet" tormlnal of . 25 ~l~p-flop ~6, th~ dat~ input of whl~h is ~et "low", nd to the ~lock t~rmlnal ~f:whloh i8 applied the H-drive 31~n~ ith the de~cribsd oonn~ctions, t~e Q output 9f fl~p-flop 46 ~pplied to the ~t~ ~ermlnal of flip-flop 44 on~ure~ that the same ~patial ~ample~ are selected on adj~cent ~rame~.
. -13-, ~f~
: C-1406 It i~ important ~o the proper operation of the encoding system of FIG. 10 that the a~alog NTSC color televlsion ~$gnal be free of energy component~ at frequencies ~n+l/4)Fh within the spectxum extending from (2F8C-FV) to Fv~ becau~e othexwise the alia3 components ~ g~n~rated by sub-Nyquist encoding would overl~p the base-: band video ~ignal main ~pectral energy components and ~ would be inseparable. Thi3 problem can ~e avoided by : co~b filtering the televi~ion ~ignal prior to sub-Nyquist encoding to remove all tn+l/4)Fh energy component~, with : care exerci~ed, however, to usa ~he proper type of comb ; filter in order to avoid ex~es~ive loss of vertical re801ution. If pre-encoding comb filtering i8 employed, one should u~e o~ly the two-line comb filter6 ~hown in ;: 15 FI&S. 4 and S, u~ing either prior to sub-Nyquist ancoding ~; a~d the othex for p~t-encoding filtering.
~ .:
FIG~ hows in block diagram form a system or obtaining a ~ignal a~ the sub-Nyqui~ rate of 2F~C from ~; an NTSC color television si~nal PCM ~ncoded at 4FSc rate.
; 20 A digi~ 1 MTSC color video signal, PC~ e~coded at 4FSc, ;~ whlch has preferably been subject~d ~o comb filtering for ~-the r~a~on~ ~nd:ln ~he manner previou~ly discu~ed, is r~ceived on llne:31 and coupled to the input of AND gate 33, sync strippcr 35, ~nd 4F8C olock reg~n~r~tor 3~
~t ~hould be no~d that the PCM ~ignal a~ llne 31 could ~:
-~ con61st of either a s~rial bit strea~ at 4F9CX N bit rata, \
where N i~ tho numb~r of bits u~ed to quantize each ; . ,. ~
v~d~o ~ampl~, or ~ 4F8~, ~ parallel blt~, PCM encoded - ~lgnal~ In th~ latter ~se, line 31 would actually conal~t of N lin~g and gate 33 would represent N A~D gat~s.

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Clock regenerator 39 produces two train~ of clock pul~es both having a frequency of 4F8C but with one 180 out of phase with re~p~ct to the other, aR ~hown by waveform~
(A) and (B) of FIG. ll.
The digital ~ync str~pper 35 performs analogous functions to those of ~ync ~tripper 34 of FIG. lO. The AND gate 33, controlled by gating signal (I) of FIG. 11, allow~ every othex 4F8C Bample to pas~ through; in addition, it provides the ex~ra sample di~placement every ~H, or every alternate TV line, to give the sampling pAttern shown by the circles of FIG. 9.
All of the other circuit~ of the ~ystem of ~IG. 12 are the same circuits aæ in FIG. lO, a~d the wave-~orms of FIG. ll and thelr d~scription given ln ~he operation of the circu~ts of FIG. ~O also apply to FIG. 12.
Whether the BUb-~yqUi8t 6amp1ed signal ls derived from 4F~c sampl-~ u~ing the ~y3t~m of FIG. 12, or obtain~d by sampling of an analog NTSC video ~ignal using the 0 system of Fl~. lO, 4P8C ~ample~ may be r~generated in a number of way~, one example of whlch i~ ~hown in FIG. 13. ~his 18 a digi~al implementatlon of the filter ,~
circuit of FIG. 4 for ~ub-Nyqu~t to ~uper-Nyq~ist dlg~tal-to-digi~al oonver~lon. The ~ub-Ny~uist ~ignal, at a ~r~quency of 7.~6Mw~s~c. i~ rsc~lved on line ~0 ~nd npplled ~o ~ digital d~lay devi~e 62 having a d~lav ~ Of~ two tel~vigion l~n~s, to one t~rminal 64a of a ~witchin~
~vl~e ~ch~m~lc~l~y shown a~ 64, a~d nl~o to the input t~m$nal of ~ low-pa~ fllter ~nd lntsrpolator 6~. Tha J'?f~

switching device 64 has a switching rate of 4F~c thereby to alternately ~w~tch between undelayed video samples (which may be designated line ~ ) and video samples delayed by two televislon lines (that i8, from line ~-2). This oper-ation of addlng the digital bit ~tr~ams from two tele-vision line~ (the com~ filter) really consists of insert-ing the curr~nt ~amples of line ~. between the video ~ampl~s from line (~ -2) whexeby th~ 4F8C sampling rate .
.~ is r~obtain~d. The resulting signal, however, will exhibit a comb filter characteri~tic throu~hout the whole ~idQo ba~eband; thi~ problem i8 ove~come by applying - tho 4Y~c slgnal to a high pa88 filter 68 having character-i~tics to llmit tha comb filtex respon~e ~o the fr~quency :~ 15 ~ abov~ (2F~C-F~)~ Th~ non-comb-~iltered lower ,:
:~ vld~o ba~eband i:s obtai~ed by low-pa~s filtering the signal ~: ~ ~f line ~ from dc to ~2FS~-FV) in low pa~s filter 66 and ~:~ doubling its ~ampling rate by ~an~ of a linear pha~e interpolator of ~DGwn con~truction. The ~UtpDt of filter ~ 20 and interpola~or 66 is added in a ~umming devi~e, diagram-:~ m~tically shown at 70, to the high pa~sed si ~ al from filter S8 to con~truct the super-Nyquist sampled video slgnal which, in thls case, has a ~ampling frequency of 4F~ or a ~ampling rate of 14~3 Mw/~ec.
r It has ~aen ~hown in BBC Researah D~partment Report 1977/21 ~ntitled "Digital Vid~o: Multlple sub-Nyqui~t cod~ng" by J. ~. Stott and To J. Phillips, that ~ -rep~at~d ~ub-~yqui~t encodlng of P~L television sl~nal6 ~;~ doe~ not ~igniflcantly lmpair the ~levi~ion pict~re beyond . th~t i~troduced by the first ~ampling operation. Since . .

~,,a~9 the berein described ~ub-Nyquist technique for an NTSC
color television signal can sati~fy all the requirements set forth in the BBC report for a PAL signal undergoing multiple sub-Nyquist encoding, the analy~iq given therein S for the PAL signal can be readily ex~ended to the NTSC
signal to prove tha~ no extra dagradation due to re-sa~pling occurs. Indeed, i~ can be raadily sh~wn that ~: the xe-sa~pllng process can be performed indefini~ely without any degradation beyond that introduced by ~he first conversion. Thi3 will be more readily understood from conaideration of the block diagram of FIG. 14, which i~ of the block diagram of FIG. 13 re-drawn to ~ exact equivalent. In the~arrangement shown in FIG. 14, the televi~ion line lQ -2) delayed by 2~ by : , :
: ~ 15 delay device 74, is high paBS ~iltered above (2FSc-Fv) and i~ added in ~umming device 80 to the non-delayed television line R which is low-pass filtered o (2F5C-Fv).
The low pa~s filter lncludes~an interpolator or generat-: ing video samples in television line ,~ that are time coinci~
dent with the ~amples from televi~on line IQ -2). The ~ nondelaycd d1gltal~bit s~ream of line ~ is al~o fed ,~" ~ ' directly to an~output switch 82 having a ~witching rate f 4F~c or 14.3MHz. The ~uper-Nyqu~t 8~ ~al i~ regen- ~.
erated at ~witch 82 by taklng the ~ssenti~lly-unproce~3ed ~5 sample~ from lin~ twhich are also original ~amples ~` !`
o the 4F~c 3ampled v1deo) and in~Rrt~-ng between those .
~: ~mpl~s the low-pa~ed/high-p~sed combination of ~`~
.: - and l~-23 ~amplæs. The resultant ~uper-Nyquis~ signal truly a 4F8a PCM MTSC color televi310n signal which ha~

~ 9~ C-1406 undar~one a comb filter process ~qual to that shown in FIG. 4. It will now ~e clear that one can repeat the sub-Nyquist codin~ proce-~s indefinitely as long as the same sampl~s from th~ ~ame frames are always selected, since this guarantee~ that the sub-Nyquist digital ~tream will always cons~st of a ~elected s~t of unproce~s~d 4F8C samples.
Test~ o~ thi3 sub-Nyqui~t sampling systam with a variety of NTSC color tele~ision ~ignals ~ave good 1~ re~ults. Using the ~ystem of PIG. 10 or FIGo 12 to obtain a sub-Nyqui~t ~ampled signal, the 4FBC sa~s were regenerat~d u~ing the ~ystem of FIG~ 14. The re~ults were be~er than those obt ined using th~ 2F8C - 1/4Fh or 2F8C ~ 1/4Fh sampling rate~ taught by appl~cant 9 ~
13 Pat. NoO 4,065,784. The impro~ment r~sul~ from the ; elimin~tion of the 7.~z flick~r~at:vertioal color tr~n~ition~ by the use of the frame resetting pulse ~hat fvrc~ the ~ampling po~n~s on ad~a¢ent frame~ to b~ ~uperi~po~ed. If the frame puls~ from the~sync strlpper i~:not us~d, results ~quivalent to those ob-- taln~d ~lth the previou~ ~ystem are obtained~
; ' ~ It ~hould be noted that the effe~t of ~ub-: Nyqul~t s~mpling on the luminAn~e de~ails i8 dep~ndent on the ~ngla batween th~ p~cture deta~l and th~ scannin~
: 25 ~ llne~. V~rtical luminanc~ trans~lons are normal~y not l~pa~ rad ~lnco thelr ~requ~ncy compGnants do not xt~n~ i~to the pa~sband of th~ comb filter. W~th diagonal tran81~10n5 ~ how~va~, th~ comb ~ilt~r redu~e~ th~ amplitude - - ~

~?~9~ C-1406 of wante~ frequency components above f8 fv. and the corre~pGnding alias aomponents are not completel~ removed.
In general, however, a small deterioration of diagonal transitions does not appear subjectively objectionable.
Horizontal transitions are virtually unaffected because their energy components do not normally fall within the nulls o~ the comb .~ilter.
The effect of the sub-Nyqui~t sy~tem on chromi-nance wa~ judged lmp~rceptible witll most br~adcast ~ignals. Only highly saturated colors generate one or two lines o~ wrong chrominance at sharp vertical color :~ tran~itions. Thi~ problem i8 particularly noticeable with 100% saturated spllt ~ield color bar~. Other ~tatio~ary pictures including scene~ from lides No. 1 to 15 o the SMPTE Television Color R~ference slide set w~re ~udgedno~obj~tionably imp~ired~

.~ ~ . . .
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: ~:

., , ~, '~

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of digitally encoding an NTSC color television signal having a color subcarrier frequency Fsc and a line scan frequency Fh, comprising the steps of:
generating a sampling signal having a frequency 2Fsc, the sampling phase of which shifts by 180° every alternate time sequential television line;
sampling said television signal in response to the sampling signal; and converting the sampled television signal into digital form.

2. The method according to claim 1, including the further step of comb filtering the television signal before sampling to remove therefrom any (n + 1/4)Fh energy components, where n is an integer.

3. Apparatus for digitally encoding an NTSC color television signal having a color subcarrier frequency Fsc and a line scan frequency Fh, comprising, in combination:
means for generating a sampling signal having a frequency 2Fsc and the sampling phase of which shifts by 180° every alternate time sequential television line;
means for sampling the television signal in response to said sampling signal; and means for converting the sampled television signal into digital form.

4. A system for encoding a composite NTSC color television signal including luminance and chrominance components wherein the chrominance component is formed by modulating a subcarrier wave having a frequency Fsc with chrominance information, the frequency Fsc being m/2 of the line scan frequency Fh of said television signal, where m is an odd number, the phase of said subcarrier wave shifting by 180° between two adjacent lines, said encoding system comprising:
means for producing sampling pulses having a repetition frequency 2Fsc;
means for sampling and encoding said television signal in response to said sampling pulses;
means for controlling said sampling pulses to cause samples of said television signal to be taken at points about 180° of said subcarrier wave away from each other in each scanning line and also about 90° of said subcarrier wave away from the samples taken in alternate time sequential scanning lines; and means for converting the sampled television signal into digital form.

5. Apparatus in accordance with claim 4, wherein said color television signal is applied to both said means for sampling and encoding and said means for producing sampling pulses, and wherein said means for producing sampling pulses comprises:
a sync stripper connected to receive said color television signal and producing horizontal synchronizing signal Fh, means connected to receive said color television signal and producing two signals 4Fsc displaced in phase from each other by 180°;
means connected to receive said horizontal synchronizing signal Fh from said sync stripper for producing a signal Fh/2, and means for combining said signal 4Fsc and said signal Fh/2 to produce sampling pulses Fs having a repetition frequency 2Fsc, the phase of which shifts by about 180° every two scanning lines.

6. Apparatus in accordance with claim 4 or claim 5, wherein said color television signal is applied to said sampling and encoding means through comb filtering means for removing therefrom any (n+1/4)Fh energy components, where n is an integer.