High-Definition Multimedia Interface 101
High-Definition Multimedia Interface or HDMI (see abbreviationfinder). It is an uncompressed encrypted digital Audio and Video standard supported by the industry to be the replacement for the scart. HDMI provides an interface between any digital audio and video source such as a DTT tuner, a Blue-ray player, a Tablet PC, a computer (Microsoft Windows, Linux, Apple Mac OS X, etc.) or a receiver A / V, and compatible digital audio / video monitor, such as a Digital Television (DTV).
- The HDMI connection is designed so that no copies can be made (allowed or not) of the streamed audio and video content, in accordance with HDCP 10 specifications. To do this, every manufacturer of equipment with HDMI must request an authorization code from the consortium, which, if it were to manufacture equipment that would allow copying, would be withdrawn and included in a “black list” so that from now on the HDMI equipment from other manufacturers does not transmit audio-video content to them.
The HDMI specification does not define a maximum cable length. As with all cables, the signal attenuation becomes too high after a certain length. Instead, HDMI specifies a minimum power level. Different materials and qualities of construction will allow cables of different lengths. In addition, the highest performance requirements must be met to support the higher resolution video formats and / or the frame rates of the HDTV standard formats.
Signal attenuation and interference caused by cable crosstalk can be compensated for by using an Adaptive Equalizer.
HDMI 1.3 defines two categories of cables: Category 1 (Standard or HDTV) and Category 2 (high speed or higher than HDTV) to reduce confusion about which cables support which video formats. Using 28 AWG, a cable of about 5 meters (~ 16 feet) can be manufactured easily and inexpensively to Category 1 specifications. Higher build quality (24 AWG, tighter construction in terms of tolerances, etc.) can reach lengths of 12 to 15 meters (~ 39 to 49 feet). In addition, it activates cables (fiber optic or dual Cat-5 cable instead of the standard copper) that can be used to extend HDMI to 100 meters or more. Some companies also offer amplifiers, equalizers, and repeaters that can string together various HDMI cable standards, not activate.
High definition HDMI and optical media players
Both introduced in 2006, Blu-ray Disc and HD DVD offer new hi-fi audio features that HDMI needs to get the best results. Dolby Digital Plus (DD +), Dolby TrueHD, and DTS-HD Master Audio use higher bit rates that exceed the capacity of TOSLINK. HDMI 1.3 can carry DD +, TrueHD and DTS-HD bit streams in compressed format. This capability would allow pre-processing or audio / video reception with the necessary decoder to decrypt the data, but having limited utility for HD DVD and Blu-ray.
HD DVD and Blu-ray enable “interactive audio,” where the content of the disc tells the player to combine multiple audio sources together, prior to output. Consequently, most players will handle internal audio decoding, and simply LPCM audio output. The LPCM Multichannel can be carried over an HDMI 1.1 (or higher) connection. While the audio / video receiver (or preprocessor) supports multiple channels of LPCM audio over HDMI, and supports HDCP, the audio playback is the same at HDMI 1.3 resolution. However, many of the cheaper AV receivers do not support HDMI audio and are often labeled “HDMI passthrough” devices. Xbox 360.
It should be noted that not all the features of an HDMI version can be applied to products adhering to that version, as certain HDMI features, such as Deep Color and xvYCC support, are optional.
|HDMI review||1.0||1.1||1.2 / 1.2a||1.3 / 1.3a / 1.3b|
|Maximum signal bandwidth (MHz)||165||165||165||340|
|Maximum TMDS Bandwidth (Gbit / s)||4.95||4.95||4.95||10.2|
|Maximum Video Bandwidth (Gbit / s)||3.96||3.96||3.96||8.16|
|Maximum Audio Bandwidth (Mbit / s)||36.86||36.86||36.86||36.86|
|Resolutions possible on a simple HDMI signal at 24 bits per pixel||1920x1080p60||1920x1080p60||1920x1080p60||2560x1440p60|
|Maximum Color Depth (bits per pixel)||24||24||24||48 *|
|Consumer Electronic Control (CEC) **|
|List of modifications of the CEC commands ***||(1.3a only)|
|Auto lip sync|
|8 channels / 192 kHz / 24-bit audio capability|
|Support for DVD-A|
|Support for SACD (DSD) ****|
|TrueHD bit stream|
|Master Audio DTS-HD bitstream|
|Full Blu-ray / HD DVD Video and Audio Resolution *****|
* = 36 bit depth is required for CE compliant Deep Color devices with 48 bit is optional.
** = CEC has been in the HDMI specification since version 1.0, but has only started to be used in “CE” products with HDMI version 1.3.
*** = Large number of additions and clarifications to CEC commands. An added CEC command is one that allows volume control of an AV receiver.
**** = SACD playback may be possible for old revisions if the source signal (like Oppo 970) is converted to LPCM. For those receivers that only have PCM DAC converters and not DSD converters, this means that there is no additional resolution loss.
***** = Even in the case of bitstream audio formats that come with a certain HDMI revision cannot be transported, this may be possible by decrypting the bitstream on the player and transmitting the audio as LPCM.
The main criticism of the HDMI connector is that it has been designed so that the equipment that uses it prevents the user from making a copy of the transmitted audio-video content, by encrypting said data.
Another criticism of HDMI is that the connectors are not as solid as the previous display connectors. Currently most HDMI-capable devices are using surface mount connectors and not through-hole or hardened connectors, making them more susceptible to damage from outside forces. Tripping over a cable connected to an HDMI port can easily cause damage to that port.
Additionally, HDMI is criticized, especially by installer systems, for its lack of any locking or warranty mechanisms built into its connectors (such as those built into DVI and BNC connectors). Like these, HDMI connectors are easily inadvertently disconnected, and worse, the plug and connector are more prone to physical or electrical damage. With third-party involvement in HDMI, locking mechanisms are available, but these are rare and expensive.
Closed caption problems
Although an HDMI display is allowed to define a “native mode” for video, which could extend the active line to cover line 21, most MPEG decoders cannot handle digital video formats that include extra lines and send only vertical bleaching. Even if it were possible, the closed caption characters would have to be encoded in some way to the pixel values of line 21. In this case it would be necessary to have a logical receiver on the screen to decipher the building codes and captions.
It is possible, although not standardized, that a certain part of the content in text format can be transmitted from the source to the destination using CEC commands or InfoFrame packages. However, since there is no standard format for this type of data, it is likely that this will only work between sources and destinations of the same manufacturer. That exception runs counter to HDMI’s standardization mission, which focuses in part on interoperability.
Of course, it is possible that in a future extension of the HDMI specification closed captions may be carried.