Various methods including analog connector, hybrid digital plus analog interfaces and several digital interfaces have been developed up to now. However, there is some overlap in the features of display interfaces. We study several practical analog and digital video interfaces.
Analog display interfaces
S-video, standing for Separate Video, transmits video signals over a cable by dividing into two separate signals: one for color and another for brightness. Due to this separation, sharper and higher quality images than composite video can be achieved. S-video does not carry audio, so it commonly run with red and white RCA audio cables.
This video interface is superior over both composite and S-video. Because its three cables better preserve the various elements of the video signal including brightness and color, resulting in enhanced picture quality. Component video uses three RCA connections, Red, Blue and Green to send video signals. It carries visual data only, meaning that audio cables are still required. Hence, it is usually paired with stereo (red and white) RCA audio connections.
VGA or Video Graphics Array is the most popular video connection between computers and monitors. It is used on computer video cards, projectors and set top boxes. A standard VGA connection has 15 pins and supports display resolution of 640*480 pixels, while enhanced version SVGA is allowed for resolution of 1024*768. It is worth mentioning that the maximum resolution is limited by the connector bandwidth and cable quality and length.
In the VGA connector, due to tiny little pins into a small connector, building small coaxial cables is hard.
Digital display interfaces
Digital Video Interface (DVI) has been designed as a replacement for VGA (analog interface). However, in some cases DVI still supports analog displays. In fact, DVI is the only standard which can carry both analog and digital signals on one interface. If the display is analog, the DVI connection will convert the digital signal to an analog signal, otherwise no conversion will be necessary. It is worth mentioning that, the analog video bandwidth of DVI is higher than VGA, thus higher resolution can be supported compared to VGA. So, it can be applied by high resolution displays such as UXGA and HDTV.
DVI cables or ports with fewer pins are designed for lower resolution devices. In order to support maximum resolution, the DVI port should contain all the pins. Since DVI does not support High-bandwidth Digital Content Protection (HDCP) encryption by default, by using hardware which only includes DVI ports, the video protection is not guaranteed. This is the biggest drawback of DVI. Since HDCP protects video from illegal copying, this feature is very critical for video surveillance systems.
DVI has several variants: DVI-D (digital only), DVI-A (analog only) and integrated DVI-I which carries both digital and analog video. DVI connections are popular on computer video cards, monitors and projectors.
High-Definition Multimedia Interface (HDMI) transmits uncompressed digital HD video and audio data in a single cable, while other connections require separate cables for audio and video. For instance, a component cable connection uses three cables for video and two for audio, resulting in five cables totally. While using HDMI, the uncompressed audio and video information can be transmitted using one cable, thus cable clutter is eliminated greatly.
Since HDMI is a digital connection, it is more robust to interference and noise problem compared to analog connections. Since most processes are the digital such as DVD players, Blue-ray players and game consoles, so by using HDMI for these consumers the analog to digital conversion will be eliminated. So, it leads to better quality picture and sound compared to other connections. Also HDMI supports HDCP, meaning that it is practical for video surveillance systems. HDMI connection is the most popular display interface, found on every TV, AV receiver, Blue-ray disk player, DVR, laptops and digital cameras, because by using HDMI, there is no need to buy separate audio and video cables. Therefore, fewer cables and connections are used.
DisplayPort as a standard port to connect PCs, laptops, and other computers to video monitors has been called ultimate digital connection. It is a high definition AV connection which transmits audio and video data over a single cable with a locking connector. DisplayPort is a simple but high bandwidth interface which provides true digital images and clear sounds. It can even supply power.
One of the advantages of DisplayPort is that it works with older technology, as by using simple adapters, it is compatible with VGA, DVI even HDMI. DisplayPort enables high display performance, robustness, versatility, highest degree of system integration, and great interoperability among various device types. DisplayPort was originally developed as the next generation personal computer display interface and is now available on a wide range of tablets, notebooks, desktop computers and monitors. It is included on all newer Macs and many Dell, HP, and Lenovo computers and is also available on video graphics cards. DisplayPort can be used for consumer electronics but is not common.
DisplayPort is hot-pluggable, meaning that connections can be reconfigured without the need to restart the device. Also DisplayPort supports DPCP (DisplayPort Content Protection) and HDCP (High bandwidth Digital Content Protection). These two prevent illegal copying, so they are very critical in video security.
Comparing HDMI and DisplayPort
HDMI and DisplayPort are modern serial interfaces to transmit digital video over cables from one device to another. They are replacement for analog interfaces like VGA and S-video. DisplayPort is a newer connector can be found on Apple’s iMac desktops. It is compatible with HDMI signals, thus product interoperability is achieved.
The HDMI and DisplayPort both carry audio and video data on a same cable. They also support High-bandwidth Digital Content Protection (HDCP) to encrypt copyrighted video and audio data for the newest HD content. Therefore, data is protected from being played or copied during transmission over the interface.
Transmitting video and audio in high definition format (HD) must be conducted through interfaces with high data transfer rate. HDMI and DisplayPort are both high-speed digital interfaces, but they differ in several aspects. HDMI is mainly used for consumer electronics equipment, while DisplayPort is primarily developed for computer and peripheral video connections. In the following some of the main differences between these two, are studied.