Dijital Video (Digital Video)
Digital Video Broadcasting
Sayısal Video Yayınlama, (İngilizce Digital Video Broadcasting kısaltması DVB olarak da bilinir), televizyon ve radyo yanında interaktif hizmetler (MHP, Mheg5, EPG, Teleteks, vs.) de aktarmaya yarayan sayısal televizyon yayın sistemidir. Yayın sistemini, içinde televizyon endüstrisinden 270'in üzerinde firmanın bulunduğu konsorsiyum (DVB Project) tarafından oluşturulmuş yayın standartları dizisi oluşturur. Oluşturulan standartlar Avrupa standart organizasyonları tarafından yayınlanmaktadır (ETSI, CENELEC ve EBU).
DVB yayını, sayısal görüntü ve ses sıkıştırma yöntemleri sayesinde yayın kanalı (frekans) başına analog yayından daha çok programı barındırabilir. Video sıkıştırma için standart çözünürlükte (PAL için 720x576i) genellikle MPEG-2 ve yüksek çözünürlüklerde de (HDTV 1920x1080’i ya da 1280x720p) genellikle daha yeni ve verimli olan H.264 yöntemleri kullanılır. Ses için Mpeg1, Mpeg2, AAC ve HE-AAC sıkıştırma yöntemleri kullanılmaktadır.
Yayın kalitesi DVB'de analog yanına göre çok daha yüksektir; kullanılan modülasyon ve hata önleme sistemleri sayesinde daha düşük yayın gücü ile daha geniş bölgelere hatasız olarak sayısal televizyon yayını aktarılabilmektedir.
DVB sistemi, şifreli yayınlar için de altyapı oluşturarak yayıncı kuruluşlar aracılığıyla paralı televizyon (Pay TV) ve İzle-ve-Öde (Pay-per-View) gibi iş modellerinin oluşmasını sağlamıştır.
DVB'nin birbirinden farklı aktarma yolları vardır.
DVB-S Uyduyla aktarma
DVB-C Kablo ağları üzerinden aktarma
DVB-T Sayısal Karasal Yayıncılık
DVB-H Mobil cihazlara aktarma
DVB-IPI IP bazlı bilgisayar ağları üzerinden aktarma
DVB-RC(S/C/T) Veri aktarımı için
DVB-SI Servis bilgilerinin aktarımı için
DVB-SH Uydu üzerinden mobil cihazlar için aktarımı için
Digital Video (English)
Digital video is a type of digital recording system that works by using a digital rather than an analog video signal.
Starting’in the late 1970s to the early 1980s, several types of video production equipment were introduced, such as time base correctors and digital video effects units (one of the former being the Thomson-CSF 9100 Digital Video Processor, an internally all-digital full-frame TBC introduced’in 1980, and two of the latter being the Ampex ADO, and the Nippon Electric Corporation DVE). They operated by taking a standard analog composite video input and digitizing it internally. This made it easier to either correct or enhance the video signal, as’in the case of a TBC, or to manipulate and add effects to the video,’in the case of a DVE unit. The digitized and processed video information from these units would then be converted back to standard analog video.
Later on’in the 1970s, manufacturers of professional video broadcast equipment, such as Bosch (through their Fernseh division), RCA, and Ampex developed prototype digital videotape recorders’in their research and development labs. Bosch's machine used a modified 1" Type B transport, and recorded an early form of CCIR 601 digital video. None of these machines from these manufacturers were ever marketed commercially, however.
Digital video was first introduced commercially’in 1986 with the Sony D-1 format, which recorded an uncompressed standard definition component video signal’in digital form instead of the high-band analog forms that had been commonplace until then. Due to its expense, D-1 was used primarily by large television networks. It would eventually be replaced by cheaper systems using video compression, most notably Sony's Digital Betacam (still heavily used as a electronic field production recording format by professional television producers) that were introduced into the network's television studios.
One of the first digital video products to run on personal computers was PACo: The PICS Animation Compiler from The Company of Science & Art’in Providence, RI, which was developed starting’in 1990 and first shipped’in May 1991. PACo could stream unlimited-length video with synchronized sound from a single file on CD-ROM. Creation required a Mac; playback was possible on Macs, PCs, and Sun Sparcstations. In 1992, Bernard Luskin, Philips Interactive Media, and Eric Doctorow, Paramount Worldwide Video, successfully put the first fifty videos’in digital MPEG 1 on CD, developed the packaging and launched movies on CD, leading to advancing versions of MPEG, and to DVD.
QuickTime, Apple Computer's architecture for time-based and streaming data formats appeared’in June, 1991. Initial consumer-level content creation tools were crude, requiring an analog video source to be digitized to a computer-readable format. While low-quality at first, consumer digital video increased rapidly’in quality, first with the introduction of playback standards such as MPEG-1 and MPEG-2 (adopted for use’in television transmission and DVD media), and then the introduction of the DV tape format allowing recording direct to digital data and simplifying the editing process, allowing non-linear editing systems to be deployed cheaply and widely on desktop computers with no external playback/recording equipment needed. The widespread adoption of digital video has also drastically reduced the bandwidth needed for a high-definition video signal (with HDV and AVCHD, as well as several commercial variants such as DVCPRO-HD, all using less bandwidth than a standard definition analog signal) and tapeless camcorders based on flash memory and often a variant of MPEG-4.
Overview of basic properties
Digital video comprises a series of orthogonal bitmap digital images displayed’in rapid succession at a constant rate. In the context of video these images are called frames. We measure the rate at which frames are displayed’in frames per second.
Since every frame is an orthogonal bitmap digital image it comprises a raster of pixels. If it has a width of W pixels and a height of H pixels we say that the frame size is WxH.
Pixels have only one property, their color. The color of a pixel is represented by a fixed number of bits. The more bits the more subtle variations of colors can be reproduced. This is called the color depth (CD) of the video.
An example video can have a duration (T) of 1 hour (3600sec), a frame size of 640x480 at a color depth of 24bits and a frame rate of 25fps. This example video has the following properties:
pixels per frame = 640 ○ 480 = 307,200
bits per frame = 307,200 ○ 24 = 7,372,800 = 7.37Mbits
bit rate (BR) = 7.37 ○ 25 = 184.25Mbits/sec
video size (VS) = 184Mbits/sec ○ 3600sec = 662,400Mbits = 82,800Mbytes = 82.8Gbytes
The most important properties are bit rate and video size. The formulas relating those two with all other properties are:
BR = W ○ H ○ CD ○ FPS
VS = BR ○ T = W ○ H ○ CD ○ FPS ○ T
(units are: BR’in bit/s, W and H’in pixels, CD’in bits, VS’in bits, T’in seconds)
while some secondary formulas are:
pixels_per_frame = W ○ H
pixels_per_second = W ○ H ○ FPS
bits_per_frame = W ○ H ○ CD