the tv picture tube that displays the picture, large glass vacuum tube
CRT is made of
inside front is covered with a phosphorescent substance that glows when struck by a beam of electrons. in back, narrow neck, electron gun (cathode ray that is heated) emits a beam of electrons.
CRT: relation of voltage, electron beam and brightness of glow
higher the voltage activating the gun, the stronger the electron beam, the brighter the glow
direction of electron beam is controlled by
deflection yoke, group of large electromagnets surrounding middle of CRT
electron beam has to go back & forth to make successful horizontal sweeps of the CRT, makes 525 sweeps top to bottom. sweeps odd lines then even lines (2 fields of 262.5 lines each) gives us 60 fields and 30 frames for each second
computers and some formats of digital TV, no interlaced fields, each frame is canned by line, computers don't have bandwidth limitations of broadcast
need for interlaced scanning
used in the National Television System Committee system. when the CRT's electron beam struck a phosphor on the face plate, it caused the phosphor to glow temporarily. if CRT scanned from 1 all the way to 525 top of pic would be dark.
limitations of old TVs
phosphor coating of CRT and bandwidth. if we tried to scan 525 lines for 60 frames it would take more bandwidth. the future will use progressive
in analog video,
the bright images on the face of CCD are changed into higher voltages and the darker images are lower voltages. then reproduced on CRT. quality is limited and is easily degraded.
process that uses computer technology and language to create, store and transmit video images. computer only manipulate numbers.
smallest part of a computer memory
bit. a bit may have a charge. if charged=1 if no charge=0. combine numbers of bits in a unit=byte. 8 bit processing combines 8 bits in one byte. 2^8=256 possibilities. we can assign them to be letters, punctuation marks, numbers, math signs, etc.
the video signal from 0 to 100 on the waveform monitor is
around 0.7 V strong. A to D converters divide the brightness range into 256 levels (8 bit) 0.7=strong=255 CRT 0.0=weak=0
process of grabbing a piece of video information and holding it. more times you sample a video, the more accurate.
process of changing the individual sample points into numbers. brightness levels and sample points can make up graph.
to display a picture a ____ is needed
D to A converter. the electron gun can shoot a stream of electrons on to the phosphors on face of CRT. numbers to D to A to voltages
stand for CODe/DECode. defines a digital standard. includes sampling frequency, process levels, encoding ratios, compression standards, etc
CODEC tells you if
it is possible to go from digital out to digital in. if two pieces use same CODEC, they will talk digitally. if equipment use different CODECs they won't talk and have to use analog connections
plasma display screen
made up of 2 pieces of glass sandwiched together. back has ridges going vertically down the glass. in space between ridges, alternating columns of R,G, & B phosphors, data electrode, front piece has horizontal ridges to seal off each pixel, also has rare gas trapped within pixel departments.
each color of each pixel has an electrode on the back, plasma display screen
individual pixels have a
red component with trapped rare gas (helium, neon, and xenon) that has a data electrode on back and transparent scan and common electrodes on front. red, green and blue compartments together make one pixel.
in plasma display screen, an electronic charge is sent to
scan and data electrodes to activate a color by electrifying rare gas inside pixel segment. gas with electrical charge gives off invisible ultraviolet light that causes phosphor to glow as long as it is active.
to turn off phosphor in plasma display,
the charge is drained through common electrode.
liquid crystal display, traits of liquid and solids. when voltage is applied to liquid crystals they turn within the liquid. many layers sandwiched together. used for small and medium screens because very expensive.
layers of LCD screens
a filter layer with R,G & B filters for each pixel, layer of glass that has a transparent contact for each pixel element's color, liquid crystal substance, another glass layer with transparent contact for each pixel elements color, then a light source (often fluorescent)
normally liquid crystal elements are ___
flat so no light can get passed to colored filters. when charge goes from front glass plate to back one, crystal elements rotate onto their edges allowing light to pass.
for LCD screens: mixing red, green and blue is controlled by
controlling the electrical charge making crystals rotate
digital videotape recorders
most commonly used digital recorders can play back analog or digital signals. could be a cheap miniDV (small cassettes) to full sized DV (big/professional), lack of uniform compatibility between makers. being replaced by disc-based systems that use DVCODEC
Digital video servers
compress many videos and audio onto hard drives. they have several channels so users can do different things at the same time.
more reliable and interface with computers better than VTR (tape takes real time to download and breaks down)
digital video takes up a lot of bandwidth but video compression fixes by throwing away information, lossless or lossy. all compression loses a little quality
eliminates redundancy and saves bandwidth by keeping only key info. info from 1 pixel could have a code to repeat for 63 pixels. entropy reduction, entropy encoding, and algorithms.
takes information thats unimportant and throws it away
uses shorter bit strings for common colors and shades
used in video compression, transmitting end and receiving end must use some formula.
for frames that are alike, system only sends the info that is different instead of the whole frame, needs sophisticated equipment, always a compromise (loss of information)
compression is described by
its compression ratio. bandwidth it takes to carry one complete uncompressed video signal. 4:1 compression= 4 complete signals in one bandwidth. higher ratio=greater expectation of poor quality.
MPEG Compression Standard
created the current standard for video compression, MP standard is for music files, MPEG 2 are for video, includes spatial & temporal. if compressed material fit in MPEG protocol it could be decoded by decoder no matter what algorithm was used to compress
the digital studio
starts with digital cameras and signal remains digital form to transmitter. only piece of analog is CRT, but final display is on plasma. very expensive.