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Vision systems information
This section contains information on the following:
1. Sources and devices
There is a variety of audiovisual (AV) components. The most frequently encountered include video and still picture cameras, presentation software, video players, projectors, monitors, video monitors and screens.
Cameras
The variety of formats and storage media for both video cameras and still cameras is so huge that several articles could be written on this alone. Traditional analogue, still and video cameras produce either photos developed in a processing lab or darkroom, or videos stored on tape and played back on a VCR. Digital photos and digital video are stored on digital media and played back on DVD players and computers. While this is a very simplified explanation, it outlines the most common camera options.
Presentation software
Software packages designed to produce animations and slides, and organise data for presentations. They usually use templates or styles, which provide backgrounds, colours, textures, borders and fonts into which data is inserted. The software usually provides a selection of transitions or crossfades between screens, slides or animations and a variety of data presentation modes. The most frequently encountered version of this type of package is PowerPoint.
Video players
While there are many other formats VHS (Video Home System) video players and DVD (Digital Versatile Disc) have almost total market dominance.
Projectors
See Projectors
Monitors and screens
There are two main types of monitor that most people are familiar with: television (TV) monitors (CRT or cathode ray tube displays), and graphics or computer monitors. Other types include LCD (Liquid Crystal Display) and plasma monitors, which are becoming more commonplace. Each of these types of screen has benefits and drawbacks, but each is ideally suited to its purpose. TV monitors play standard analogue video and digital video from DVD. Computer monitors are designed to display graphics, even if a little pixelated or jagged at times. LCD monitors are excellent flat panel substitutes for small monitor situations whereas plasma screens are spectacular for large home theatre type displays. If, however, the presentation is intended for a large audience, a projector and screen combination is required. Some screens can be used for both front and rear projection and others are designed only for front projection so the presentation requirements must be considered first when setting up a system.
2. Signal types
The best way to understand which components can be hooked together is to understand what kind of signal is running through the line. If you have two pieces of equipment, one putting out and the other receiving the same signal type, they can ‘communicate’ with each other if joined by an appropriate cable. The following descriptions of the common signal types are a helpful reference. Some of the signal types can be found on almost every piece of gear, whereas others are not so common.
RF (Radio Frequency) modulated television
RF is the type of signal that comes through the air by antenna or through a cable TV connection. In standard-definition broadcast and analogue cable, a composite video signal and accompanying audio are mixed at the transmitting end with high-frequency radio waves, and are broadcast through the air or distributed through a cable system. RF is used as a distribution medium because it propagates through the air very well, making it suitable for over-the-air broadcast. As many video signals can be modulated at different frequencies, it is possible to have many ‘channels’ available simultaneously without them interfering with one another.
Composite video
Composite video is a single signal which carries both the chrominance (colour) and luminance (brightness) components of a video signal, along with sync information, on a single wire. Unlike an RF signal, a composite video signal does not need to be demodulated to be understood by a video display. Composite video signals do not carry any audio content. This must be handled separately.
S-video
S-video is a format which splits the chrominance and luminance out onto two separate lines, ‘C’ and ‘Y,’ each requiring its own cable. The sync pulses are carried on the luminance line.
Component video
This ordinarily refers to ‘Y/Pb/Pr,’ also known as ‘YUV,’ video. In Y/Pb/Pr component video, there is a luminance channel, ‘Y,’ which carries the luminance along with the sync pulses, and two colour-difference channels, which carry signals representing blue minus luminance (B-Y, or Pb) and red minus luminance (R-Y, or Pr). From these signals, the display device separates out the sync information and reconstitutes the red, green and blue components of the picture. Similar to s-video, which requires two signal-carrying wires instead of one, component video requires three signal-carrying wires to convey the whole signal.
RGB and its variants: RGBHV ,RGBS and RGsB,
The original ‘component video’ was RGB, which appears in three principal varieties, each requiring a different number of connections:
- The most common type is RGBHV, with five lines: one for red, one for green, one for blue, one for the horizontal sync and one for the vertical sync. RGBHV is the standard used in VGA and other analogue PC computer monitors.
- RGBS has four connections and differs from RGBHV by having the vertical and horizontal sync combined on a single channel.
- RGsB, or ‘sync-on-green’ places the sync information on the green channel. Although this is similar to Y/Pb/Pr component video they are not compatible.
DVI and its several flavours: DVI-D, DVI-A, DVI-I
DVI is somewhat confusing because the term is identified both with more than one signal type and more than one connector type. DVI-A is nothing but RGBHV in a strange connector and isn't digital at all. DVI-I isn't really a signal type but refers to a connector type which combines DVI-A and DVI-D. DVI-D is a parallel digital standard: a nasty little tangle of wires in a nasty little plug which consists of up to seven balanced lines (all other common video standards are run unbalanced) carrying the video itself, and five miscellaneous conductors carrying other information. Because this is a digital rather than an analogue signal, it can only be converted to another format through a device that is equipped to decode the digital bitstream and render it in analogue form.
HDMI
HDMI is similar to DVI and is a standard intended to be backward compatible with DVI and employing the same encoding/decoding scheme.
SDI
SDI is serial digital video. Unlike DVI, it is run in an unbalanced line. It is used primarily in professional production environments.
3. Cables and connectors
All of the unbalanced analogue and digital standards, from RF down through SDI, are run in 75-ohm coaxial cables. This fact, in itself, seems to confuse people. It is widely assumed that ‘coax’ is something used for RF, or for SPDIF digital audio, and that composite video or component video are run in a different type of cable suited particularly for those formats. In fact, the differences are minor: RF is frequently, but not always, run in cables using copper-coated steel conductors for higher strength and lower cost; SDI is generally run in ‘precision’ video cables because its wide bandwidth requires very tight impedance tolerance; but these cables are all "coax." Even s-video is only apparently an exception. A round s-video cable is just a round jacket over two miniature coaxes, one carrying luminance and the other chrominance. DVI and HDMI are run in cables which are particular to their own applications.
Here are some common connector types, and what they are mostly used for:
The RCA (phono) plug and jack is the most common connector type on consumer gear for composite and component video, as well as for both digital and analogue audio. RCA jacks colour-coded yellow on a device are usually composite video inputs or outputs. Where there is a single RCA jack on the back panel, labelled ‘video’ or something similar, it is almost certainly composite. Component video is usually represented by three RCA connections colour-coded green (Y, or luminance), blue (Pb) and red (Pr). RGBHV will usually, though not always, be colour-coded red, green, blue, yellow (horizontal sync) and white (vertical sync). |
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The BNC plug and jack is the standard connector for most video signals on professional gear, and is appearing increasingly on high-end consumer gear as well. It will be labelled similarly to the RCA, indicating composite video (one connection), Y/C s-video (two connections), Y/Pb/Pr (three connections), or one form or another of RGB. The most common confusion with BNCs, is that people often assume the female connector is a male; the problem is that both the male and female connectors have what looks like a pin in the centre. On closer inspection, however, it is clear that a female BNC's ‘pin’ is actually a receptacle for the male pin. A panel-mounted BNC will ALWAYS be female; a cable-mounted BNC will almost always be male. |
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BNC Male
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Female BNC
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The F-connector is the screw-on type connection used for most antenna and cable TV connections. F-connectors are rarely used for anything other than RF; the one notable exception being that they were used as digital audio connectors on some laser disk players.
The 4-pin mini-DIN plug is the common s-video plug on consumer gear and is often considered a poor choice for video as it has a tendency to unplug itself at the slightest urging.
The HD15/mini dSub 15/VGA connector: An increasing number of devices are appearing with 15-pin connectors. There are about as many names as pins for this connector, which is well known as the plug used with most PC computer monitors and consequently is often called a "VGA" plug. Since VGA is an RGBHV-type video signal, however, this usage is a bit confusing. This same plug is used not only for RGBHV, but also for RGBS, RGB sync-on-green, and Y/Pb/Pr Component video. Because the plug can be used with so many different video standards, it's very important to know before using a 15-pin connector on a device, what sort of video it can put out or take in.
DVI Connectors come in a few types; the most important, in general, are DVI-I and DVI-D. The difference between the two is that a DVI-I connector has extra pins at one end, which carry most of the analogue video signal. A DVI-I cable can be used for either a digital or analogue signal, because it contains both the digital and analogue pins. A DVI-D socket, however, which is designed to take a DVI-D plug, will not normally have anywhere for the analogue pins on a DVI-I plug to plug into. It is therefore important when buying a cable to be sure that it will actually plug into the equipment it is intended for.
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HDMITM (High Definition Multimedia Interface) was created as a digital interface standard for the consumer electronics market. The HDMI protocol combines high-definition video, multi-channel audio and inter-component control in a single digital interface. This lone interconnect has the ability to transmit uncompressed digital video and up to eight channels of audio from source to display. |
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4. Guidelines for presentations
While the proper layout of a presentation room or multimedia theatre can involve complex geometry, there are some simple guidelines that can be used.
For optimum viewing, the audience should be seated within the shaded area indicated in this diagram. They should be within ±30º of a line perpendicular to the screen’s centre. They should be seated no closer than twice the image height. If a room is too wide to allow all viewers to fall within these guidelines, consider multiple displays. The distance to the furthest viewer depends on the content of the presentation. For general purpose (e.g. entertainment) the last row should be no further than eight times the image height. This should be adjusted to six times the image height for corporate or data presentations (e.g. spreadsheets), or four times the image height for critical applications (e.g. control rooms, CAD drawings, very fine detail, etc. |
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To prevent obstruction of the screen by the heads in front, the bottom of the screen should be 1200mm above the floor. This can be adjusted somewhat if the seating is staggered, or if the floor is raked. Allow a minimum of 150mm between the top of the screen and the ceiling. The screen should never be positioned too high – bear in mind ergonomic figures regarding comfortable head tilt and vertical eye movement range: no viewer should have to rotate their head more than ±30° from straight ahead, or tilt their head more than 25° from horizontal.
Choosing the type of projection
Front projection
Front projection can generally provide a wider viewing angle than rear projection. Controlled lighting is important and ambient light must be kept off the screen to eliminate a reduction in contrast ratio. Front projection requires either a projection room at the rear of the theatre, or a platform or mount for the projector(s) in the theatre. It is important to remember that having the projection equipment in the theatre is a potential noise source.
Rear Projection
Rear projection generally has a narrower viewing angle than front projection and is better for long, narrow rooms. Material can be viewed with higher levels of room illumination, and is less immune to ‘wash out’ by ambient lighting. Rear projection is available in both flexible and rigid materials although a rigid screen is more expensive than a front projection screen, and has size restrictions. It also requires space behind the screen. Mirrors may be used to reduce the required depth.
When choosing the type of projection it is necessary to determine the purpose of the presentation. If it is a ‘canned’ presentation either front or rear projection can be used. If a live talker is interacting with the presentation, rear projection is best so that the speaker can interact with the images without being in the projector’s light path. In addition, the higher allowable room illumination allows for note taking, etc.
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