S ha tn hiu video1,Ly mu tn hiu video:Cng on u tin ca qu trnh bin i tn hiu tng t sang tn hiu s l ly mu (c ngha l ri rc tn hiu tng t theo thi gian). Do tn s ly mu l mt trong nhng thng s c bn ca h thng k thut s. C nhiu yu t quyt nh vic la chn tn s ly mu. Tn s ly mu cn c xc nh sao cho hnh nh nhn c c cht lng cao nht, tn hiu truyn i vi tc bit nh nht, rng bng tn nh nht v mch n gin.

1.1.Ly mu

cho vic ly mu khng gy mo, ta phi chn tn s ly mu tho mn cng thc Kachenhicop sa 2max (max = 5,5MHz i vi h PAL) ngha l sa 11MHz.

Trng hp sa < 2max s xy ra hin thng chng ph lm xut hin cc thnh phn ph (alias components) v xut hin mo, v d nh hiu ng li trn mn hnh (do cc tn hiu v ch nm trong bng tn video), mo sn xung tn hiu, lm nho bin nh (do hiu ng bc thang), cc im sng ti nhp nhy trn mn hnh.Tr s sa ti u s khc nhau cho cc trng hp: tn hiu chi (trng en), tn hiu mu c bn (R, G, B). cc tn hiu s mu, tn hiu Video mu tng hp. Cui cng vic chn tn s ly mu ph thuc vo h thng truyn hnh mu.

Trong trng hp ly mu tn hiu Video mu tng hp phi ch n tn s sng mang ph sc, khi chn sa c th xut hin cc trng hp sau y: + sa gp nhiu ln sc, v d sa = 3sc hoc 4sc (h PAL, NTSC ch dng mt tn s sc). H SECAM dng hai sng mang ph mu nn khng dng c mt tn s sa cho cc tn hiu hiu s mu. +sa khng c quan h trc tip vi sc. Trong trng hp ny ngoi cc thnh phn tn hiu c ch s xut hin cc thnh phn tn hiu ph do lin hp gia sa v sc hoc hi ca sc trong ph tn hiu ly mu. c bit thnh phn tn hiu (sa -2sc) s gy mo tn hiu Video (tng t) c khi phc li gi l mo iu ch cho (Intermodulation). Mo ny s khng xut hin trong trng hp ly mu v m ha ring tn hiu chi v cc tn hiu s mu. Trong trng hp ly mu tn hiu Video mu tng hp cho h NTSC, PAL thng th ngi ta chn bng hi bc 3 tn s ti mu sc : sa =3sc. saPAL = 13,3 MHz > 2maxPAL = 2x5=10MHz hoc 2x5,5=11MHz. saNTSC = 10,7 MHz > 2maxNTSC = 2x4,2=8,4MHz.

Nu chn sa= 4sc th cho cht lng khi phc rt tt. Tuy nhin, n s lm tng tc bit tn hiu s + Nu tn hiu truyn i tng thnh phn cht lng hnh nh thu c m bo tt hn do nh hng ca sng mang ph khi ly mu khng c. + Tn s ly mu ca tn hiu chi saY 2maxY v bng bi s ca tn s dng. + Tn s ly mu cc tn hiu mu sa(R-Y)(B-Y) 2max (R-Y)(BY) v bng bi s ca tn s dng. Kt hp iu ny vi thc t ngi ta chn: saY= 13,5 MHz sa(R-Y)(B-Y)= sc= 6,75MHz cho c 2 tiu chun: 625 50 v 525 60. Tuy nhin, s la chn sa theo nh l Kachenhicop th cha m phi tha thm cc iu kin sau: - Tn s sa phi ng b vi tn s qut dng fH. - Tn s sa phi ng b vi tn s qut mnh V. - Tn s sa phi ng b vi tn s nh fP, fP= 2V.

Ph ca tn hiu l tng

Hinh 2,hin tng chn ph

1.2.cu trc ly mu

Hinh 2.cu trc ly mu hai mnh lin tip

a/cu trc trc giao

b/Cu trc quincunx mnh

c/cu trc quincunx dng

2/lng t ha tn hiu video Qa trnh lng t ha tn hiu ri rc (sau khi ly mu) bao gm vic chia bin thnh nhiu mc (nhiu khong) v sp xp mi tr ca mu bng mt mc. Cc khong chia (khong lng t) c th u nhau v cng c th khng u nhau v ta gi l lng t tuyn tnh v lng t phi tuyn. Trong qu trnh lng t ha bin ca cc mu nm trong cng mt khong lng t (Q) s c bin bng nhau, bin ny c th l nm bc trn hay nm bc di ca mc lng t. Mi bc tng ng vi mt m s nht nh. 2 phng php lng t ha l:lng t ha tuyn tnh v lng t ha phi tuyn.

Sai s lng t ha

Trong : :sai s lng t. X(t):gi tr cc mu tn hiu trc khi lng t. X(t): gi tr cc mu sau lng t.

Cng tn hiu dither

3/M ha M ha tn hiu Video l bin i tn hiu lng t ha thnh tn hiu s bng cch sp xp s nh phn cho cc mc lng t ha v nh x ca cc mc ny thnh tn hiu c 2 mc logic 0 v 1. Theo l thuyt v thc nghim ta c th dng m 8 bit (tc 28 =256 mc lng t) m ha tn hiu Video. Nu s bit tng chnh xc ca b chuyn i tng nhng tc bit tng i hi knh truyn rng ng thi p ng ca b chuyn i thp. Cc m s dng trong truyn hnh s c th c chia thnh 4 nhm nh sau: + Cc m m ho tn hiu truyn hnh + Cc m truyn c hiu qu cao theo knh thng tin + Cc m thun tin cho vic gii m v ng b bn thu + Cc m x l s tn hiu trong cc b phn khc nhau ca h thng truyn hnh s

M s cp to tn hiu s trung tm truyn hnh, c dng tn hiu nh phn lin tc, cc bit 0 v 1 c th c biu din bng cc phng php khc nhau, c phn bit bng thi gian tn ti, cc tnh, mc pha chng hn NRZ, RZ, Biphase (hai pha) M s cp l m c s hnh thnh m bo v. M bo v dng tng cng kh nng chu ng nhiu cho tn hiu truyn trong knh thng tin. Ty theo cch sp xp s nh phn m ta c cc loi PCM (Pulse Code Modulation:iu ch xung m) tuyn tnh hay DPCM (Diffirent PCM: PCM vi sai). PCM tuyn tnh truyn s mc lng t (m nh phn) cn DPCM truyn tn hiu s bng lin hp cc ga tr lng t ca mt vi mu. Phng php iu ch PCM tuyn tnh 8 bit cho kt qu hnh nh thu c rt tt. Cc hin tng mo lng t gim ng k. Do vy i vi cc hnh nh yu cu cht lng cao ngi ta thng dng loi ny (Studio) nhng n c nhc im l tc bit ln, knh truyn phi c bng tn rng. Tn s ly mu ca ca tn hiu Video mu tng hp l 13,5MHz. Vi m ha ring tng thnh phn tn hiu chi c tn s ly mu l 13,5MHz v cc tn hiu mu c tn s ly mu l 6,75MHz