Video interfaces connect the monitor to the Image decoding device. Concerns over image quality and video protection has led to significant changes in display interface technology gradually. Although the VGA interface is still used in PCs, the future is moving to digital interfaces. 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.
Type of connector:
There are three main types of HDMI ports, each with 19 pins. The standard size (Type A) commonly found on TVs, projectors, set-top boxes and laptops. Smaller HDMI (Type B) is called mini- and mainly found on some laptops and tablets. The smallest is micro-HDMI (Type C) found on tablets and smartphones.
DisplayPort is available in two different types with 20 pins. One of them is the full-size version and the other is smaller called Mini DisplayPort which is used in Apple laptops.
Supporting the quantity of monitors (video and audio stream):
In video surveillance, using more than one display is preferable. Also, many gamers use two or more monitors simultaneously. HDMI can handle single video stream and a single audio stream, so it can support only one display at a time.
On the other hand, DiaplayPort can connect to multiple monitors up to four with a 1920-by-1200 pixel resolution. By using multiple DisplayPort interfaces, connecting as many as six display to one output is feasible.
The maximum cable length supported by each interface is an important subject. Since high data rate signals will be degraded over lengths, so cables should be kept short. A maximum HDMI cable length has not been specified, and a wide range of lengths is available, but manufacturers limit them to 15 meters (about 49 feet). For instance, by HDMI cables as long as 50 feet, the quality of video data has been degraded. In order to cover a long distance, a signal booster or an active cable to amplify the signal might be needed.
For DisplayPort, 3840- by- 2160 pixels (4K) video over a length up to 2 meters by using a passive cable can be achieved. Also, running a passive cable up to 15 meters is possible, but resolution will be limited to 1080p (full-HD), as defined by the standard.
DisplayPort does not support Ethernet data and the standard does not have an audio return channel, while by using the latest version of HDMI, 100 Mbps HDMI Ethernet Channel (HEC) can be implemented. HEC is used primarily for audio control.
The Ethernet channel can be used to provide an Internet connection between devices, especially when HDMI devices are used for streaming video or other content from the Internet.
Advantages of HDMI in video surveillance
HDMI cables have several advantages that make it superior over some interfaces like DVI and component video, especially in video surveillance systems.
HDMI includes all components of both video and audio data in one cable, so there is no need for extra cables. Because, everything is contained in one compact cable package. HDMI transmission line could be used instead of at most 13 similar transmission lines, thus the problem of cable clutter will be solved effectively.
On the other hand, like many other modern data transmission cables, an HDMI cable is composed of a series of twisted pairs. It has been proved that this cable manufacturing technology, is superior over other cables even huge coaxial cables for reducing noise and interferences. Also HDMI guarantees HDCP, thus video information is protected from copying during transmission, confirming its benefit in video surveillance.
Why we should consider DisplayPort in addition to HDMI in video surveillance?
In video streaming, choosing the types of video connection to stream the content to the display is an important issue. Over the years, delivering this content through HDMI, DVI, and VGA cables has been the most popular method.
In recent years, a new technology called DisplayPort has been developed. It has slowly been adopted by motherboard and displays. By the newest version DisplayPort 1.2, achieving to 4K60p resolution (3840×2160@60Hz) is possible, while HDMI only provides this resolution at half the frame rate or less.
The most important advantage of DisplayPort 1.2 over the other standards is a Multi-Stream Transport (MST) technology, in which DisplayPort can transfer multiple video streams from one video output by using 21.6 Gbps bandwidth. Therefore, achieving up to 4 displays with 1080p resolution from a single DisplayPort output is possible, confirming the benefit of DisplayPort for video surveillance.
The main digital connectors used on computers and display devices are DVI and HDMI. DisplayPort is a newer connector mainly designed to replace VGA and DVI ports. The first choice to connect a device to display is HDMI which is commonly used in the most NVRs. The cables are cheap and it also carries audio. On the other hand, HDMI has some limitations and is not a perfect choice for all purposes. In most cases HDMI is fine, but for really high resolutions and frame rates, other options might be taken into consideration.