6. Does the monitor match the PC?
Early methods of communication between PC and monitor relied on monochrome digital signals but were soon superseded by low resolution colour versions such as CGA (colour graphics adaptor) and EGA (enhanced graphics adaptor). In 1987, the VGA (Video Graphics Array) was introduced with a resolution of 640*480 pixels, which was followed in 1990 by the SVGA standard with 800*600 pixels. Both used analogue data signals but maintained the digital control lines of their predecessors. With increased screen sizes, further increases of resolution were needed, starting with the IBM developed XGA as shown in the table below.
The output ports provided by the computer to supply data to the monitors have progressively increased their performance to accommodate the higher speed data flows required by the interfaces. Below is a list of the most common of these.
Other analogue video standards have originated in the television and video equipment fields. Composite video, is in essence, a television RF (Radio Frequency) signal before being combined with sound and the RF carrier. Just two wires are needed, signal and ground, using a phono connector. Of the three analogue standards discussed, composite video produces the lowest quality of image. No audio is provided.
S-video (for super or separated video) carries the video data as two separate signals, for colour and luminance. A resolution of 700*486 pixels is available, again with no audio. Cables with DIN connectors with at least 4 pins are commonly used. As S-video has effectively the two component parts of composite video, the colour and luminance can be combined is a simple adaptor to form composite video. The reverse conversion is only possible with electronic manipulation. Due to lack of bandwidth, S-video is not suitable for high definition pictures.
Component Video is the analogue standard that has found favour for use with DVD players, high definition displays, video projectors as well as with computers. RGB colour signals are carried by three separate cables, each having their own phono connector. Video projectors and other professional video applications in this country utilise BNC connectors, whereas Japanese equipment tend to use “D” Type connectors. The line and frame synchronising signals are sent during the blanking periods of one or more of the colour signals and typically use Green for this purpose. Some variants of the system have separate synchronising lines, requiring a total of five cables. Audio is not inherently included but can be accommodated with two additional cables for stereo. Computer video cards typically label component video as “TV Out” or “VIVO” (Video in Video Out). These variations in the encoding of the RGB signals generate a significant lack of standardisation increasing the risks of improper picture generation on the monitor. Transmission of the component video signal is more robust than for the DVI and HDMI digital systems, enabling cables of up to 60 metres to be used without the use of boosters. There is a gradual deterioration in picture quality with length of cable, which is contrasted against the behaviour of digital signals, which experience a sudden occurrence of picture breakup at a specific cable length. Although the quality of the digital systems is better in theory, it is often a matter of conjecture as to whether the viewer would, in practice, see much difference.
Of the two common digital formats, the DVI (Digital Visual Interface) was the first to gain popular acceptance in computer displays. Introduced in 1999, it is the only widespread video standard that includes analogue and digital options within the same connector. Audio is not included. The connector is specific to the standard and has between 19 and 29 pins. Three main designs of connector are available. DVD-D (digital only), with 19 pins provides digital signals, DVD-A (analogue only) is analogue having 23 pins and DVD-I (Integrated, digital an analogue) has 29 pins. As the analogue signals of a VGA connector and the analogue section of a DVI connector use the same signals, it is possible to use a straightforward adaptor to convert between the two without having any intervening electronics. Because there is uncertainty as to which version of the connector is to found on the video interface and the monitor, the following summary may be useful in establishing which cable is needed to connect the two.
- If both connections are DVI-D, a DVI-D cable is needed.
- If both connections are DVI-A, a DVI-A cable is needed.
- If one is DVI and the other is VGA and the DVI is analogue compatible (i.e. is either DVI-A or DVI-I), a simple adaptor can be used without any electronics.
- If both connections are DVI-I, any DVI cable can be used but a DVI-I is recommended.
- If one connection is DVI-A and the other is DVI-D, it is not possible to connect them with a single cable. An electronic converter is required.
For displays of 1920*1200 pixels, cable lengths of up to 4.5 metres are possible. For 1280*1024 pixels, up to 15 metre cables are generally attainable. For longer distances, boosters are recommended. These are either self-powered or will need an external power supply. As already mentioned, digital signals are subject to sudden degradation if excessive cable lengths are used. This is known as the “digital cliff” and will occur at a specific distance for a given system and cable quality. There is no gradual deterioration as in analogue systems.
In 2003, a video/audio interface with a compact connector was introduced to provide high definition digital video with up to 8 audio channels and other features such as remote control. This HDMI (High Definition Multimedia Interface) system can support 2560*1600 pixels. The 8 channel audio is transmitted at a 192 KHz sample rate with 24 bits per sample. Digital Dolby is supported. Cable lengths of up to 5 metres are achievable for cheaper cables and better cables can be a maximum length of 15 metres. Degradation, when it occurs, is often in the form of instability or blinking. The connectors have 19 pins and are hot-pluggable. As the DVI digital signals are compatible with HDMI, a DVI-D or DVI-I cable may be used to drive a HDMI monitor, or vice versa, using an adaptor. In this case, but audio and remote control features would be lost.
A major feature of HDMI is the incorporation of High-Bandwidth Digital Content Protection (HDCP), which prevents the unauthorised user from viewing HDCP protected content. This has made it particularly attractive to software suppliers and some 800 companies in the video and PC fields have now adopted the format. Sales of the interface are set to exceed those of DVI in 2008.
Increasingly, monitor manufacturers are offering both types of digital interface and so one is faced with the decision of which to use. For normal screen sizes, the picture quality is the same. Both use the same encoding scheme and simple adaptors are available to convert between the two. The principle advantage of HDMI is that it also carries high quality audio. Also, the connectors are smaller and the cables are of smaller cross-section. The analogue Component Video is also capable of good quality reproduction though it achieves it with different colour information. Cable lengths can be as long as 60 metres, producing a gradual reduction of quality with increasing length.
The Apple range of computers deserves an individual mention as they have used a variety of video interface standards, often combining them to form proprietary connectors. Apple’s standard DVI connector, which can provide VGA via a short connector cable, also accommodates composite and S-Video on the Power Mac G5 and the Mac mini. A mini-VGA connector may be converted to normal VGA via a short cable and is available on the white iBook, eMac, iMac G4 and G5 as well as the first generation 12-inch Power Book G4. Later models also support composite and S-Video. A micro-DVI connector is used in the MacBook Air to cater for the restricted space available. This is also used on later 12 inch PowerBook G4, Intel based iMacs and MacBooks. A full DVI connector is supplied on the MacBook Pros.
Looking to the future, the current display standard most able to be upgraded to higher resolutions, is HDMI. This is the only one that has inbuilt audio capability. The Video Electronics Standards Association (VESA) has proposed a licence free new standard called DisplayPort which has been designed to succeed DVI which includes DRM (Digital Rights Management).