Aloqa kanali

Teletext Telematn

Although relatively unknown in North America, teletext is routine throughout Europe. Teletext uses the vertical blanking interval (see the section The picture signal: Wave form) to send text and simple graphic information for display on the picture screen. The information is organized into pages that are sent repetitively, in a round-robin fashion; a few hundred pages can be sent in about one minute. The page selected by the viewer is recognized by electronic circuitry in the television receiver and then decoded for display. The information content is mostly of a timely, general interest, such as weather, news, sports, and television schedules. Graphics are formed from simple mosaics. The British Broadcasting Corporation (BBC) developed teletext and initiated teletext transmission in 1973. The BBC ended the service in 2012, but teletext is still used in several European countries.

Shimoliy Amerikada nisbatan noma'lum bo'lsa-da, telematn butun Evropada odatiy holdir. Telematn rasm ekranida ko'rsatish uchun matn va oddiy grafik ma'lumotlarni yuborish uchun vertikal bo'shliq oralig'idan foydalanadi (Rasm signali: To'lqin shakli bo'limiga qarang). Axborot aylana tartibida takroriy yuboriladigan sahifalarga ajratiladi; bir necha yuz sahifalar taxminan bir daqiqada yuborilishi mumkin. Tomoshabin tomonidan tanlangan sahifa televizor qabul qilgichidagi elektron sxemalar tomonidan tan olinadi va keyin ko'rsatish uchun dekodlanadi. Axborot mazmuni asosan ob-havo, yangiliklar, sport va televidenie jadvallari kabi o'z vaqtida, umumiy manfaatlarga ega. Grafika oddiy mozaikadan yaratilgan. Britaniya radioeshittirish korporatsiyasi (BBC) telematnni ishlab chiqdi va 1973 yilda telematn uzatishni boshladi. BBC xizmatni 2012 yilda tugatdi, biroq telematn hali ham Yevropaning bir qancha davlatlarida qo'llaniladi.


Reception Reception Qabul qilish qabul qilish

At the television receiver the sound and picture carrier waves are picked up by the receiving antenna, producing currents that are identical in form to those flowing in the transmitter antenna but much weaker. These currents are conducted from the antenna to the receiver by a lead-in transmission line, typically a 12-mm (one-half-inch) ribbon of plastic in which are embedded two parallel copper wires. This form of transmission line is capable of passing the carrier currents to the receiver, without relative discrimination between frequencies, on all the channels to which the receiver may be tuned. Television signals also are delivered to the receiver over coaxial cable from a cable service provider or from a videocassette recorder. In addition, some television receivers have an input that bypasses the tuner and detector so that an unmodulated video signal can be viewed directly, in effect making the television receiver into a video display terminal.

Televizion qabul qiluvchida tovush va tasvir tashuvchisi to'lqinlari qabul qiluvchi antenna tomonidan qabul qilinadi, bu esa uzatuvchi antennada oqayotganlarga o'xshash, ammo ancha zaifroq oqimlarni hosil qiladi. Ushbu oqimlar antennadan qabul qilgichga ikkita parallel mis simlar o'rnatilgan, odatda 12 mm (yarim dyuym) plastmassa lenta orqali uzatiladigan uzatish liniyasi orqali o'tkaziladi. Elektr uzatish liniyasining ushbu shakli qabul qiluvchiga sozlanishi mumkin bo'lgan barcha kanallarda chastotalar orasidagi nisbiy kamsitmasdan tashuvchi oqimlarni o'tkazishga qodir. Televizion signallar, shuningdek, simi xizmat ko'rsatuvchi provayderdan yoki videokassetali magnitafondan koaksiyal kabel orqali qabul qiluvchiga etkaziladi. Bunga qo'shimcha ravishda, ba'zi televizor qabul qiluvchilarida tyuner va detektorni chetlab o'tadigan kirish mavjud, shuning uchun modulyatsiyalanmagan video signalni to'g'ridan-to'g'ri ko'rish mumkin, bu esa televizor qabul qilgichni video displey terminaliga aylantiradi.