What is HDMI?
Do you remember the cables that you use to connect your DVD player with the TV? You could have used Composite audio/video cables or S-Video cables or Component video cables for HDTV. They all carry signals in the analog format (In component video, the signals are converted from digital to analog for transmission and reconverted to digital at the receiving end).
HDMI is the All-Digital interface where the signals are carried in digital format and hence there is no conversion required at either end. HDMI stands for High Definition Multimedia Interface. It is an all digital audio-video interface which carries signals in uncompressed format. It has been accepted by the consumer/ institutional electronics industry.
HDMI cables consist of 19 separate wires which carry high speed video, audio and other digital information. The digital audio and video data is encoded in to three colour channels and a clock channel.
Types of HDMI Cables:
Standard cables: They are also known as Category 1 HDMI cables. They perform at pixel speeds of 75 Mhz supporting bandwidth of about 2.23 Gbps. This is fine for carrying an uncompressed 1080i signal.
High Speed cables: They are also known as Category 2 HDMI cables. They perform at pixel speeds of 340 MHz supporting bandwidth of about 10.2 Gbps. They can handle the latest 1440p and WQXGA resolutions.
Advantages of HDMI:
¤ Higher Quality: HDMI enables loss-less transmission and better quality video at low brightness scenes at higher resolutions. So, the video quality is much better than their analog counterparts as there is no conversion involved either. High contrast details like text etc. are displayed more sharper.
¤ Intelligence: Two way communications between video sources are enabled by HDMI Interface, which enables automatic configuration (between 480p or 720p, 16:9 or 4:3 for example). So, external intervention to identify the best resolutions and audio formats is minimized. This is accomplished by using a standard known as EDID – Extended Display Identification Data.
¤ Authentication and Encryption: HDMI standard supports authentication to ensure that the devices are authorized to receive the content sent by the HDMI enabled sources. They also enable authentication to make sure that people cannot tap in to the cables to copy or pirate content sent through them.
¤ Signal Integrity: They enable digital signals to be stored, transmitted and viewed without changes from the original (unlike other media which require analog to digital conversion) and hence the signal degradation is not prominent. So, it is better to transmit HD content.
¤ Single Cable: This is truly a single cable solution as there is only a single cable that carries audio, video and control information. So, the complexity of implementing an audio video control system is lesser.
¤ Deep Colours: HDMI supports 10 bit, 12 bit and 16 bit (RGB or YCbCr) colour depths which can render over one billion colours in good detail.
¤ No Compression: Since HD signals are not compressed while transmission, there is no de-gradation in signal quality.
¤ Compatibility with DVI: Since it is backward compatible with DVI interface, DVI enabled PC’s can send HD content to display devices.
¤ Supports multiple audio and video formats: Multiple audio and video formats like standard stereo, multi channel surround sound, 720p, 1080i, 1080p, NTSC, PAL etc. are supported by HDMI standard.
¤ Hot Plug Detect: The sink device (display unit) can indicate its presence to a source with a hot plug detect signal to identify when a cable has been connected and to start authentication.
¤ CEC: Consumer Electronics Control is a communication link that enables devices connected via HDMI to talk to each other. For example, multiple DVD players from a single manufacturer can communicate with each other so that only one plays at a time. Some video sources can also send a power-off signal to turn off certain displays connected to it.
¤ Display port compatibility: Display port is a parallel technology for audio/video interface like HDMI and HDMI devices are compatible with Display Port interfaces.
¤ Dolby/DTS: HDMI supports Dolby/DTS-8 channel audio streams for highest quality.
Dis-advantages of HDMI:
¤ Distance Limitations: Maximum distance for HDMI Cat1 cables is up to 35 meters (approx) for full capacity and maximum distance for HDMI Cat2 cables is up to 10 meters (approx) for full capacity. Beyond this limit, they need extenders. There are extenders like UTP cable extenders for HDMI, HDMI cable extenders, Fiber extenders for HDMI and Coaxial extenders for HDMI. Even Switches, Distribution amplifiers, audio/video processors act as repeaters. But extending HDMI cables this way has its limitations.
¤ Switching Delays: Sometimes, blank screens can be caused due to authentication delays. There can also be screen flashing errors.
¤ Field Termination: There are limitations to field terminations of HDMI cables. They cannot be easily terminated in the field like their analog counterparts.
¤ Costly: HDMI cables are more expensive (per meter) than their analog counterparts.
¤ Multiple locations: They are difficult to run in multiple applications spanning various locations. There are more complexities in such situations than just the distance limitations.
¤ EDID: One such complexity is with EDID (Extended Display Identification Data) which is used in automatic identification of resolutions and audio formats. Both HDMI and EDID specifications offer limited guidance in multiple location deployment scenarios.
¤ CEC: The Consumer Electronic Format which enables the different devices to talk to each other becomes a problem when there is a central control system to control all the connected audio/video devices. It causes confusion when the devices themselves change their configuration as they might get undetected by the control systems.
¤ Multi-Channel Audio: While HDMI sources can transmit multi-channel audio sources, they cannot transmit multi-channel and standard two channel stereo signals at the same time (simultaneously). This becomes a problem with multi-room installations that has various devices supporting different audio formats.
¤ DVI Compatibility: HDMI devices are compatible with DVI interfaces but require a separate audio cable as DVI carries only video signals. It also needs a HDMI-DVI connector in such scenarios.
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