Multimedia Systems

1 January 2017

Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion. Raw video can be regarded as being a series of single images. There are typically 25, 30 or 50 frames per second. There are two types of video recording systems/formats. Also, the term video (“video” meaning “I see”, from the Latin verb “videre”) commonly refers to several storage formats for moving pictures. These are Analog Video and Digital Video.

The world we sense is full of analog signal; electrical sensors such as transducers, thermocouples, microphones convert the medium they sense into electrical signals. These are usually continuous and still analog. These analog signals must be converted or digitised into discrete digital signals that computer can readily deal with. Special hardware devices called Analog-to-Digital converters perform this task. For playback Digital-to-Analog must perform a converse operation. Analog Video Analog Video is usually captured by a video camera and then digitised.

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Digital Video

Digital video is a type of video recording system that works by using a digital rather than an analog video signal. 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 (FPS). 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 second half contains only the even-numbered lines. Those halves are referred to individually as fields. Two consecutive fields compose a full frame. If an interlaced video has a frame rate of 15 frames per second the field rate is 30 fields per second. All the properties and formulas discussed here apply equally to interlaced video but one should be careful not to confuse the fields per second with the frames per second. Properties of compressed video The above are accurate for uncompressed video. Because of the relatively high bit rate of uncompressed video, video compression is extensively used.

In the case of compressed video each frame requires a small percentage of the original bits. Assuming a compression algorithm that shrinks the input data by a factor of CF, the bit rate and video size would equal to: BR = W * H * CD * FPS / CF VS = BR * T / CF Please note that it is not necessary that all frames are equally compressed by a factor of CF. In practice they are not so CF is the average factor of compression for all the frames taken together. The above equation for the bit rate can be rewritten by combining the compression factor and the color depth like this: BR = W * H * (CD / CF) * FPS

The value (CD / CF) represents the average bits per pixel (BPP). As an example, if we have a color depth of 12bits/pixel and an algorithm that compresses at 40x, then BPP equals 0. 3 (12/40). So in the case of compressed video the formula for bit rate is: BR = W * H * BPP * FPS In fact the same formula is valid for uncompressed video because in that case one can assume that the “compression” factor is 1 and that the average bits per pixel equal the color depth. Bit rate and BPP As is obvious by its definition bit rate is a measure of the rate of information content of the digital video stream.

In the case of uncompressed video, bit rate corresponds directly to the quality of the video (remember that bit rate is proportional to every property that affects the video quality). Bit rate is an important property when transmitting video because the transmission link must be capable of supporting that bit rate. Bit rate is also important when dealing with the storage of video because, as shown above, the video size is proportional to the bit rate and the duration. Bit rate of uncompressed video is too high for most practical applications. Video compression is used to greatly reduce the bit rate.

BPP is a measure of the efficiency of compression. A true-color video with no compression at all may have a BPP of 24 bits/pixel. Chroma subsampling can reduce the BPP to 16 or 12 bits/pixel. Applying jpeg compression on every frame can reduce the BPP to 8 or even 1 bits/pixel. Applying video compression algorithms like MPEG1, MPEG2 or MPEG4 allows for fractional BPP values. Constant bit rate versus variable bit rate As noted above BPP represents the average bits per pixel. There are compression algorithms that keep the BPP almost constant throughout the entire duration of the video.

In this case we also get video output with a constant bit rate (CBR). This CBR video is suitable for real-time, non-buffered, fixed bandwidth video streaming (e. g. in videoconferencing). Noting that not all frames can be compressed at the same level because quality is more severely impacted for scenes of high complexity some algorithms try to constantly adjust the BPP. They keep it high while compressing complex scenes and low for less demanding scenes. This way one gets the best quality at the smallest average bit rate (and the smallest file size accordingly).

Of course when using this method the bit rate is variable because it tracks the variations of the BPP. Video can be recorded and transmitted in various physical media: in magnetic tape when recorded as PAL or NTSC or SECAM electric signals by video cameras or in MPEG-4 (Moving Pictures Expert Group-4 format) or DV (Digital Video) digital media, when recorded by digital cameras. PAL, short for Phase Alternate Line, is an analogue television encoding system used in broadcast television systems in large parts of the world such as Europe, Asia, Australia, parts of Africa (Kenya etc) etc. ).

NTSC, named for the National Television System Committee, is the analog television system used in most of North America, most countries in South America, Burma, South Korea, Taiwan, Japan, Philippines, and some Pacific island nations and territories. SECAM (Sequentiel couleur a memoire, French for “Sequential Color with Memory”), is an analog color television system first used in France, Russia, parts of Africa etc. It is, historically, the first European color television standard. There is also 3D-video, digital video in three dimensions apart from 2D-video. Characteristics of video streams

Number of frames per second Frame rate, the number of still pictures per unit of time of video, ranges from six or eight frames per second (frame/s) for old mechanical cameras to 120 or more frames per second for new professional cameras. PAL and SECAM standards specify 25 frames/s, while NTSC specifies 29. 97 frame/s. Film is shot at the slower frame rate of 24photograms/s, which complicates slightly the process of transferring a cinematic motion picture to video. The minimum frame rate to achieve the illusion of a moving image is about fifteen frames per second.

Interlacing Video can be Interlaced (Interlace is a technique of improving the picture quality of a video signal without consuming extra bandwidth. Interlaced video was designed for display on CRT televisions. ) or Progressive(Progressive or non-interlaced scanning is a method for displaying, storing or transmitting moving images in which all the lines of each frame are drawn in sequence. ). Interlacing was invented as a way to achieve good visual quality within the limitations of a narrow bandwidth.

The horizontal scan lines of each interlaced frame are numbered consecutively and partitioned into two fields: the odd field (upper field) consisting of the odd-numbered lines and the even field (lower field) consisting of the even-numbered lines. NTSC, PAL and SECAM are interlaced formats. Abbreviated video resolution specifications often include an i to indicate interlacing. For example, PAL video format is often specified as 576i50, where 576 indicates the vertical line resolution, i indicate interlacing, and 50 indicate 50 fields (half-frames) per second.

In progressive scan systems, each refresh period updates all of the scan lines. The result is a higher spatial resolution and a lack of various artifacts that can make parts of a stationary picture appear to be moving or flashing. A procedure known as deinterlacing can be used for converting an interlaced stream, such as analog, DVD, or satellite, to be processed by progressive scan devices, such as TFT (Thin Fluorescent Tube) TV-sets, projectors, and plasma panels. Deinterlacing cannot, however, produce a video quality that is equivalent to true progressive scan source material. Display resolution

The size of a video image is measured in pixels for digital video, or horizontal scan lines and vertical lines of resolution for analog video. In the digital domain (e. g. DVD) standard-definition television (SDTV) is specified as 720/704/640? 480i60 for NTSC and 768/720? 576i50 for PAL or SECAM resolution. However in the analog domain, the number of visible scanlines remains constant (486 NTSC/576 PAL) while the horizontal measurement varies with the quality of the signal: approximately 320 pixels per scanline for VCR quality, 400 pixels for TV broadcasts, and 720 pixels for DVD sources.

Aspect ratio is preserved because of non-square “pixels”. New high-definition televisions (HDTV) are capable of resolutions up to 1920? 1080p60, i. e. 1920 pixels per scan line by 1080 scan lines, progressive, at 60 frames per second. Video resolution for 3D-video is measured in voxels (volume picture element, representing a value in three dimensional spaces). For example 512? 512? 512 voxels resolution, now used for simple 3D-video, can be displayed even on some PDAs. Aspect ratio Aspect ratio describes the dimensions of video screens and video picture elements.

All popular video formats are rectilinear, and so can be described by a ratio between width and height. The screen aspect ratio of a traditional television screen is 4:3, or about 1. 33:1. High definition televisions use an aspect ratio of 16:9, or about 1. 78:1. The aspect ratio of a full 35 mm film frame with soundtrack (also known as the Academy ratio) is 1. 375:1. Ratios where the height is taller than the width are uncommon in general everyday use, but do have application in computer systems where the screen may be better suited for a vertical layout.

The most common tall aspect ratio of 3:4 is referred to as portrait mode and is created by physically rotating the display device 90 degrees from the normal position. Other tall aspect ratios such as 9:16 are technically possible but rarely used. Pixels on computer monitors are usually square, but pixels used in digital video often have non-square aspect ratios, such as those used in the PAL and NTSC variants of the CCIR (Consultative Committee on International Radio) 601 digital video standards, and the corresponding anamorphic widescreen formats.

Therefore, an NTSC DV image which is 720 pixels by 480 pixels is displayed with the aspect ratio of 4:3 (which is the traditional television standard) if the pixels are thin and displayed with the aspect ratio of 16:9 (which is the anamorphic widescreen format) if the pixels are fat. EDITING VIDEO Editing videos typically involves: removing frames, inserting frames, mixing audio with video, giving special effects on video, adding transitions between two clips, superimposing clips and adjusting transparency, applying filters to the clips, and adjusting volume of audio.

We can also fade-in and fade-out the audio in various frames. In order to edit videos, it is helpful to understand the concept of timecode. Timecode, a unit of measurement, can be used as an address of a frame, and it can also be used as a unit to measure the duration of a video clip. The timecode used by SMPTE (Society of Motion Picture and Television Engineers) has become a standard and is in the form Hrs: mins: secs: frames. For example, a clip with duration of 00:02:31:15 plays for 2 Minutes, 31 seconds, and 15 frames. At the rate of 30 frames per second (fps), 15 frames will take half a second (0. seconds) to play.

This clip will, therefore, play for 2 minutes and 31. 5 seconds. Thus, a fraction of a second is measured in frames, and to know the time with millisecond accuracy, we need to know the fps of the clip. The same code can be used to address a particular frame. For example, 00:1:30:12 is the address of the 12th frame that comes after 1 minute and 30 seconds of playing a clip. There are many software packages which support such editing. Some that are well known among these are Adobe Premiere, Adobe After Effects, Strata Avid Video, Asymmetric M Digital Video.

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