MULTIMEDIA TECHNOLOGIES LECTURE 08 VIDEO IMRAN IHSAN ASSISTANT PROFESSOR
VIDEO Video streams are made up of a series of still images (frames) played one after another at high speed This fools the eye into believing that it is observing a continuous stream Video (real-world pictures) Animation (Computer generated) 2
FRAME RATE This is the number of frames per second that are displayed PAL (Phase Alternating Line) in Hong Kong produces 25fps. When producing video for the web we can save data by using lower frame rates e.g. 20fps, 15fps etc Below 15fps playback becomes noticeably jerky 3
ANALOGUE VIDEO Video information that is stored using television video signals, film, videotape or other noncomputer media Each frame is represented by a fluctuating voltage signal known as an analogue wave form or composite video. Amplitude cathode Time 4
COMPOSITE ANALOGUE VIDEO Composite analogue video has all the video components: brightness, colour and synchronization. Then, combined into one signal for delivery 5
COMPONENT VIDEO Component video sends the signals separately (e.g. S-video, RGB) Advantages: Good video quality. Disadvantages: (i) Higher bandwidth compared with Composite video. (ii) Synchronization problem between different colour components. 6
HDMI The High-Definition Multimedia Interface (HDMI) is an all-digital audio/video interface capable of transmitting uncompressed streams. HDMI provides an interface between any compatible digital audio/video source, such as a settop box, a DVD player, a PC, a video game system such as the PlayStation 3 or an AV receiver and a compatible digital audio and/or video monitor, such as a digital 7
HDMI The HDMI 1.3 was released 22 June 2006. Increases single-link bandwidth to 340 Mhz (10.2 Gbps) Supports 30-bit, 36-bit, and 48-bit (RGB or YCbCr) color depths or over one billion colors, up from 24-bit in previous versions. Incorporates automatic audio syncing (lip sync) capability. Supports output of Dolby TrueHD and DTS-HD Master Audio streams for external decoding by AV receivers. 8
TYPE OF TV VIDEO SIGNALS There are four analog video formats PAL NTSC SECAM HDTV (may be mixed analog/digital) 9
PAL & NTSC PAL (Phase Alternate Line) Standard television format used in Hong Kong, China, Europe, UK, Australia and South Africa Consists of 625 scan lines drawn every 1/25 second with a 4:3 aspect ratio * PAL is interlaced at 50 cycles per second PAL uses the YUV color model * Aspect ratio is the comparison of width to height for a viewing area. Two common ratios - 4:3 and 16:9. NTSC (National Television Standards Committee) The American and Japanese standard Consists of 525 scan lines drawn every 1/30 second (To be precise, the frame rate of NTSC is 29.97) Has a 4:3 aspect ratio NSTC uses the YIQ colour model 10
SECAM & HDTV SECAM ( Sequential Color with Memory ) Sequential Couleur Avec Memoire Format in France. Used 625 scan lines and is interlaced at 50 cycles per second. HDTV ( High Definition Television) based on 720 scan lines or higher, progressive scan with 16:9 aspect ratio many standards 11
TV TYPE GEOGRAPHICAL VIEW 12
PROGRESSIVE SCAN Used in normal computer monitor. As shown in Figure, the electronic or optic beam of an analog video camera continuously scans the imaged region from the top to bottom and then back to the top. 13
INTERLACED SCAN A refresh rate of at least 50 Hz or 50 fps is required in order for us to perceive the smooth movement of a video. However, it is not feasible to display 50 fps in our analog TV. Thus, interlaced scan was introduced to solve this problem. In interlaced scan, each frame is scanned in two fields and each field contains half the number of lines in a frame. Field containing the first line and following alternating lines in a frame is called top field. Field containing the second line and following alternating lines is called bottom field. 14
Odd field Even field Interlace scan
Field 1
Field 2
On Screen
VIDEO ACTUAL SIZE NTSC uses 525 scan lines per frame. Why does the video capture card ask you to set the size to 640*480? Not all lines are displayed on the screen since some are used to carry control and other information. In practice, the number of visible lines per frame for NTSC monitor is 480 and 576 for PAL & SECAM. Thus, number of pixels per line is 640 for NTSC monitor and 768 for PAL & SECAM. 19
VIDEO ACTUAL SIZE 20
ASPECT RATIO 21
VIDEO ACTUAL SIZE 22
WHAT IS DIGITAL VIDEO? Digital video is the digitisation of analogue video signals into numerical format It creates the illusion of full motion by displaying a rapid sequence of changing images on a display device. Conversion from analogue to digital format requires the use on an ADC (Analogue to Digital Converter) A Digital to Analogue Converter (DAC) can be used to output digital video on analogue equipment 23
VIDEO CAPTURE TECHNOLOGY Video Capture hardware Graphics adapter and display Video Overlay- convert video into digital signal Video capture - convert video into digital signal Video source camera, VCR Compression hardware - compresses digital video data Compressed Digital audio Computer Storage 24
WHY DIGITIZING VIDEO The main advantage is not subject to generational loss because each copy is identical to the parent. Repeated recording does not degrade image quality. Video stored in computer ready to be processed and integrated to various multimedia applications. Direct Access is possible. Ease of encryption and better tolerance to channel noise. 25
FILE SIZE CONSIDERATIONS Several elements determine the file size, in additional to the length of the video. These including: Frame Rate Measured in frames per second Standard video movie 30 fps, movie film 24 fps Digital video at least 15 fps Frame or Image Size the width and height of each individual frame or image Determines the quality of the image displayed and the processing involved to display that image Standard full screen resolution is 640x480 pixels Video screen display is one fourth the size of the full window screen (320x240) 26
THE ACTUAL SIZE OF A 320X240 PIXEL VIDEO WINDOW. 240 320 27
FILE SIZE CONSIDERATIONS Color Depth Number of colors displayed on the screen at one time Ranging from 1 bit to 8 bit, 16 or 24 bit per pixel Quality directly related to frame size, image size and colour depth. Quality also depends on content. Motion picture needs higher frame rate. 28
CALCULATING THE SIZE OF DIGITAL VIDEO File size = frame size * frame rate * color depth * time (f * r * c * t) Where: Frame size = image size ( width * height in pixels) Frame rate = frames per second Color depth = measured in bytes time = time in seconds This does not include any sound data! 29
Eg: 1 second of video at a frame rate of 15 fps, image size, 320 x 240 pixels and color depth, 24 bits Solution: File size = (320*240)*15*(24/8)*1 = 3 456 000 bytes / 1 048 576 = 3.3 MB 30
QUIZ 1 minute of video at a frame rate of 30 fps, image size, 160 x 120 pixels and color depth, 65,536 colors Solution: File size =(160*120)*30*(16/8)*60 = 69 120 000 bytes / 1 048 576 = 66 MB 31
COMPRESSION Since the size of raw digital video is so large we need some means to compress the information Compression Techniques more in later chapters Video compression and decompression program, known as codecs 32
DIGITAL VIDEO FORMATS There are numerous video formats: M-JPEG MPEG AVI QuickTime Real Video DV 33
M-JPEG Motion-JPEG not surprisingly based on the JPEG still image format Stores every frame as a compressed bitmap image Typical compression ratios of between 2:1 and 12:1 Can be applied in hardware or as a software codec Moving Picture Experts Group standards for compressing motion video and audio signals using DCT (Discrete Cosine Transform) The MPEG formats are asymmetrical - it takes longer to compress a frame of video than it does to decompress it. (Why is it considered as a good feature?) 34
MPEG There are a number of MPEG formats: MPEG-2 is used for digital TV broadcasts and DVDs MPEG-1 is a format used for low quality video (generally displayed on computers) MPEG-1 Layer 3 is the popular encoding mechanism for MP3 audio files MPEG-4 is a new format for multimedia presentations on web. It is based on the QuickTime file format and optimized for delivery of video at Internet data rates 35
QUICKTIME A video and animation system developed by Apple Computer In QuickTime, a structure of time-based data is called a movie, hence the file extensions.mov or.qt The QuickTime application can create, display, edit, copy, and compress video data Can achieve compression ratios of 25 to 200:1 Besides processing video data, QuickTime can handle still images, animated images (sprites), vector graphics, multiple sound channels, MIDI music, 3D objects, virtual reality panoramas and objects, and even text 36
AVI VIDEO FOR WINDOWS Audio Video Interleave File with extension.avi Interleaving is a technique used to embed two or more things into the same stream of information In every chunk of information you will find some video data and some audio data 8, 16 or 24 bits 001001010010010101010011110101... 001001010010010101010011110101...... 001001010010010101010011110101... 001001010010010101010011110101... Video information Audio information 37
REAL AUDIO Part of the RealMedia architecture designed to support live and on-demand audio and video over the Web Offers very good performance with lower data rates and motion intensive video Supports animations, MIDI, text, images, vector graphics and presentations 38
DV In the late 1990s a new generation of entirely digital cameras and camcorders emerged, and with them a new video format, Digital Video (DV) It uses a spatial compression technique (each frame being compressed on an individual basis rather than being compared to adjacent frames) based on the DCT DV Compression DV can compress different parts of each frame to different ratios So, the blue sky in an image backdrop can be brought down to, say, 25:1, while the complex forest in the foreground, which needs more detail, is reduced to only 7:1 In this way DV can optimise its video stream frame by frame 39
USING VIDEO FOR WEBSITE If possible, avoid video altogether. It is too slow. If forced to use video, reduce the size of video by use a stationary camera single color background close-up shots When choosing the file format, consider the user's willingness to download additional software. 40
MEDIUM SHOT 41
CLOSE UP SHOT 42
EXTREME CLOSE UP THE BEST 43
TEXT ON A WEB PAGE IS EASIER TO READ THAN A CAPTION ON A VIDEO 44
VIDEO BIT RATE Bit rate is a measure of the rate of information content in a video stream. It is quantified using the bit per second (bit/s) unit or Megabits per second (Mbit/s). A higher bit rate allows better video quality. For example VideoCD, with a bit rate of about 1 Mbit/s, is lower quality than DVD, with a bit rate of about 5 Mbit/s. HDTV has a still higher quality, with a bit rate of about 20 Mbit/s. Variable bit rate (VBR) is a strategy to maximize the visual video quality and minimize the bit rate. On fast motion scenes, a variable bit rate uses more bits than it does on slow motion scenes of similar duration yet achieves a consistent visual quality. For real-time and non-buffered video streaming when the available bandwidth is fixed, e.g. in videoconferencing delivered on channels of fixed bandwidth, a constant bit rate (CBR) must be used. CBR bitrates are very easy to use in calculations 45
EXAMPLE The file size can be computed as: Filesize (KByte) = (Video bitrate+audio bitrate) x sec/8 e.g. If you have a MPEG 2 file that has video bitrate 8000 Kbit/s and audio bitrate 384 Kbit/s and it lasts for 3 minutes, the amount of HDD space it takes can be calculated easily: (8000+384) x 180 / 8 = 188 640 KB = 182.22 MB 46
ANOTHER EXAMPLE e.g. Given that the video and audio bitrates of VCD is 1150 Kbit/s and 224 Kbit/s respectively. Calculate the required storage space of a PAL video which has 1000 frames. fps in PAL = 25 The length of video clip is 1000/25 = 40 sec File size = (1150+224) x 40 / 8 = 6870 KB = 6.71 MB 47