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Video technology was first developed for cathode ray tube (CRT) television systems, but several new technologies for

video display devices have since been invented. Charles Ginsburg led an Ampex research team developing the first

practical video tape recorder (VTR)

In 1951 the first video tape recorder captured live images from television cameras by converting the camera’s

electrical impulses and saving the information onto magnetic video tape. Video recorders sold for $50,000 in 1956, and

videotape cost $300 per one-hour reel. However, prices steadily dropped over the years; in 1971, Sony began selling

videocassette recorder (VCR) tapes to the public. After the invention of the DVD in 1997 and Blu-ray Disc in 2006,

sales of videotape and tape equipment plummeted. Later advances in computer technology allowed computers to capture,

store, edit and transmit video clips. 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 (Europe, Asia, Australia, etc.)

and SECAM (France, Russia, parts of Africa etc.) standards specify 25 frame/s, while NTSC (USA, Canada, Japan, etc.)

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. Interlaced vs progressive Video can be interlaced or progressive.

Interlacing was invented as a way to reduce flicker in early mechanical and CRT video displays without increasing the

number of complete frames per second, which would have required sacrificing image detail in order to remain within the

limitations of a narrow bandwidth. The horizontal scan lines of each complete frame are treated as if numbered

consecutively and captured as two fields: an odd field (upper field) consisting of the odd-numbered lines and an even

field (lower field) consisting of the even-numbered lines. Analog display devices reproduce each frame in the same

way, effectively doubling the frame rate as far as perceptible overall flicker is concerned. When the image capture

device acquires the fields one at a time, rather than dividing up a complete frame after it is captured, the frame

rate for motion is effectively doubled as well, resulting in smoother, more life-like reproduction (although with

halved detail) of rapidly moving parts of the image when viewed on an interlaced CRT display, but the display of such

a signal on a progressive scan device is problematic. 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 total number of horizontal scan lines, i indicates interlacing, and 50 indicates 50

fields (half-frames) per second. In progressive scan systems, each refresh period updates all of the scan lines of

each frame in sequence. When displaying a natively progressive broadcast or recorded signal, the result is optimum

spatial resolution of both the stationary and moving parts of the image. When displaying a natively interlaced signal,

however, overall spatial resolution will be degraded by simple line doubling and artifacts such as flickering or

“comb” effects in moving parts of the image will be seen unless special signal processing is applied to eliminate

them. A procedure known as deinterlacing can be used to optimize the display of an interlaced video signal from an

analog, DVD or satellite source on a progressive scan device such as an LCD Television, digital video projector or

plasma panel. Deinterlacing cannot, however, produce video quality that is equivalent to true progressive scan source

material. Aspect ratio Comparison of common cinematography and traditional television (green) aspect ratios 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 601 digital video standard, 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. Color space and bits per pixel Example of U-V color plane, Y value0.5 Color model name

describes the video color representation. YIQ was used in NTSC television. It corresponds closely to the YUV scheme

used in NTSC and PAL television and the YDbDr scheme used by SECAM television. The number of distinct colors that can

be represented by a pixel depends on the number of bits per pixel (bpp)

A common way to reduce the number of bits per pixel in digital video is by chroma subsampling (e.g. 4:4:4, 4:2:2,

4:2:0/4:1:1)

Video quality Video quality can be measured with formal metrics like PSNR or with subjective video quality using

expert observation. The subjective video quality of a video processing system may be evaluated as follows: Choose the

video sequences (the SRC) to use for testing. Choose the settings of the system to evaluate (the HRC)

Choose a test method for how to present video sequences to experts and to collect their ratings. Invite a sufficient

number of experts, preferably not fewer than 15. Carry out testing. Calculate the average marks for each HRC based on

the experts’ ratings. Many subjective video quality methods are described in the ITU-T recommendation BT.500

One of the standardized method is the Double Stimulus Impairment Scale (DSIS)

In DSIS, each expert views an unimpaired reference video followed by an impaired version of the same video. The expert

then rates the impaired video using a scale rang from “impairments are imperceptible” to “impairments are very

annoying”

Video compression method (digital only) Main article: Video compression A wide variety of methods are used to compress

video streams. Video data contains spatial and temporal redundancy, making uncompressed video streams extremely

inefficient. Broadly speaking, spatial redundancy is reduced by registering differences between parts of a single

frame; this task is known as intraframe compression and is closely related to image compression. Likewise, temporal

redundancy can be reduced by registering differences between frames; this task is known as interframe compression,

including motion compensation and other techniques. The most common modern standards are MPEG-2, used for DVD, Blu-ray

and satellite television, and MPEG-4, used for AVCHD, Mobile phones (3GP) and

.

Past Exam Papers for KCPE and KCSE

We have an enourmous data quiz bank of past papers ranging from 1995 – 2017 .

Quick Revision Booklets

Candidates who would want their papers remarked should request for the same within a month after release of the

results. Those who will miss out on their results are advised to check with their respective school heads and not with

the examination council. .

Candidate benefit from our quick revision booklets which are comprehensive and how to tackle examination question

methods.

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