A Short Guide to
HDMI Cables and HDMI Connectors

First we had DVI, soon HDMI followed and with it a full range of HDMI cables, HDMI connectors, adaptors, splitters, etc.  To many, HDMI is still an unfamiliar term. This needs no longer be the case! This short guide  will explain all there is the need to know about HDMI.

HDMI Basics

Introduction:

Originally released in December 2002, ver. HDMI 1.0 -- short for High-Definition Multimedia Interface -- is the first industry supported interface that carries uncompressed all-digital audio and video over the same interconnect.

By end 2003, we started to see the first HDMI-enabled consumer products on stores shelves. Since then, HDMI saw an exponential rise in the rate of annual growth of HDMI-enabled products, and is fast becoming the 'de facto' standard for HDTV connections. It is estimated that today, there are some 100 million HDMI-enabled devices, and this figure is expected to double by end-2008!

The HDMI standard was founded by leading consumer electronics manufactures - Hitachi, Panasonic, Philips, Sony, Toshiba, Thomson and Silicone Image. Because of its HDCP (High-Bandwidth Digital Content Protection) compliance, which provides copyright protection, it is also fully supported by major motion picture producers like Fox, Warner Bros, and Disney, as well as by system operators like DirecTV and EchoStar.

HDMI - Main Benefits:

The principle idea behind HDMI is the use of a single interconnect instead of several cables when connecting an audio/video source, such as a set-top-box or DVD player and an audio and/or video monitor, such as a plasma TV, over a single HDMI cable.

The use of a single HDMI cable that replaces the maze of cabling behind home entertainment centers is the primary advantage brought about by this new standard. This is not the case with DVI; DVI supports digital video only - hence, a separate audio cable (TosLINK or SPDIF) is needed to transport digital audio when a DVI cable is used.

But there are other advantageous as well:

• HDMI transfers uncompressed digital audio and video leading to the best digital quality possible since there is no degradation of the signal through compression/decompression processes.

• It is fully backward compatible with DVI - all that is required is a DVI to HDMI cable adaptor. This opens HDMI to a wide range of DVI-equipped products from a variety of manufacturers.

• HDMI supports two-way communication between the video source and the display device, thus enabling new functionality  such as automatic configuration - e.g. of remote controls of devices connected by HDMI.

• HDMI technology has been designed to accommodate long copper cables at lengths up to 15 meters (50 feet) without the use of HDMI amplifiers or repeaters.

Note however... Actual performance constraints vis-à-vis cable length, are similar to DVI in view that HDMI uses the same encoding protocol over the same twisted copper pair. As we have already stated in our DVI Cable Guide, the use of twisted copper pair to carry high bit-rate digital data without error correction can lead to severe problems with signal degradation over distance. It is for this reason that the HDMI specifications standard does not state a maximum cable length - but rather indicates the expected supported cable length using affordable high quality HDMI cables.

What type of video and audio formats HDMI supports?

The HDMI specs support standard NTSC and PAL, enhanced, and high-definition video formats (720p, 1080i, and 1080p up to 60Hz), plus 8-channels, of 192kHz, 24-bit uncompressed digital audio on a single HDMI cable. And still, there is bandwidth to spare to accommodate future enhancements and new requirements. 

Note that support for 1080p/60Hz is not mandatory and therefore equipment manufacturers may opt to implement only 1080i.

Also, beginning with HDMI ver. 1.2 (released August 05), HDMI now includes support for One-Bit Audio format used in SACD (SuperAudio CD's) Direct Stream Digital.

HDMI and DVI - Differences and Similarities

We thought that it would be appropriate at this point to bring about the main similarities and differences between these two relatively new digital standards. Both are closely related in that HDMI uses the same digital encoding scheme used by DVI-D in the transport of digital video.

This explains why all that is required to hook up a DVI device with HDMI-enabled equipment, is a simple DVI to HDMI cable adaptor with a DVI-D plug on one end and an HDMI connector on the other. In this case however, digital audio will have to be carried separately since DVI does not support audio over the same interconnect. This would not be the case in an 'all' HDMI to HDMI cable, where the digital audio signal is carried along with the digital video over the same interconnect.

A second major difference between these two standards is that HDMI does not come with the may different flavors associated with DVI -- DVI-A, DVI-D, DVI-I, DVI dual-link, etc. To a certain extent, this simplifies matters when it comes to HDMI cables - but you would still find single-link and dual-link HDMI using different HDMI cable connectors.

Note that the situation with the dual-link HDMI is similar to that of dual-link DVI in that while we have a defined dual-link HDMI standard, yet as of the time of this article (March 2006), there is still no dual-link HDMI-enabled consumer equipment.

The fact that HDMI is equivalent to DVI-D, implies that it is limited to that format only. In other words, there's no way to adapt an analog VGA signal to go in through an HDMI connection as one can with a DVI-I interface.

Other important differences include:

• HDMI connectors are substantially smaller than the 37.0 mm wide DVI plug.

• The DVI standard was originally developed to be used by PCs, while HDMI was developed by some of the major electronic manufacturers to be used specifically in consumer electronic products such as DVD players and digital televisions.

How does HDMI transport digital video and audio over the same HDMI Cable?

We will not go into the actual architecture of how this is done, yet there are a few basics worth knowing about how HDMI transport data and in particular, how this transport mechanism relates to the maximum data throughput or bandwidth, that can be carried over an HDMI cable connection.

HDMI uses the same unique TMDS encoding protocol used in DVI, to transport video and audio information over the same interconnect. TMDS stands for transition minimized differential signaling; it conveys data by transitioning between 'on' and 'off' states while utilizing an advanced encoding algorithm to minimize the transitions necessary to transport data between the HDMI 'source' e.g. an HDMI-enabled digital satellite TV set-top box, and an HDMI-enabled 'sink' or monitor e.g. a digital television.

Minimizing the required transitions avoids excessive electromagnetic interference (EMI) levels on the interconnecting HDMI cable. At the same time, additional operation is performed to minimize long strings of '1' and '0' which otherwise can cause detection errors. In this process, incoming 8-bit data is encoded into a 10-bit transition-minimized, DC-balanced word.

The HDMI standard specifies both single-link and dual-link HDMI. A single HDMI link is made up of 3 TMDS data channels using three separate differential pairs on the HDMI cable to transport audio and video data at a maximum pixel clock rate of 165MHz; this is equivalent to 165 million pixels per second. A fourth differential pair, called the TMDS Clock provides the pixel clock for timing the data stream.

Single-link HDMI is fully compatible with single-link DVI-D while dual-link HDMI is compatible with dual-link DVI-D.

Video Transport: Video information is transmitted as a series of 24-bit pixels - 8 bits each for each of the primary colors; these are encoded using the TMDS protocol into three 10-bit words per pixel clock period (i.e. each pixel is made up of 30bits). This means that the effective maximum data throughput  is 4.95Gbps (165MHz x 30-bits) over a single-link HDMI cable, or 9.9Gbps over a dual-link HDMI interconnect; these rates determine the maximum video signal resolution that can be transmitted over single-link and dual-link HDMI cables.

How much bandwidth do you need? To better understand how this 'pixel clock' relates to resolution, let's consider the highest HDTV standard defined today: 1080p 60Hz HDTV video signal. Lets also assume a typical 16% overall blanking interval; this represents the portion of the video signal that occurs at the end of each horizontal line (horizontal blanking) as well as at the end of each frame or field (vertical blanking).

The bandwidth required for a given resolution is governed by the refresh rate and blanking interval of the monitor; bandwidth can be calculated using the following formula:

Where the Blanking Period is the sum of the horizontal and vertical blanking intervals.

This means that in our case, the required bandwidth is equal to 1920 x 1080 x 60 x [1 + 0.16] = 144.4MHz or 144.4 million pixels/sec.

Considering that a single-link HDMI can transfer up to 165 million pixels per second, it is clear that HDMI has more than enough bandwidth on a single link to accommodate even the highest HDTV format available today for consumer electronic products. In the case of a dual-link mode, an HDMI cable connection can handle a bandwidth of up to 330Mhz.

Audio Transport: Audio can be from two to eight channels, using sample rates up to 192KHz. Multi-channel digital audio is time multiplexed into the same  TMDS data streams used for video; this is possible as audio requires a much lower data (max. 192Kbps), and therefore, it can be easily 'stuffed' in between empty spaces available on the data channels.   

The HDMI standard also includes two other important channels - these are the DDC and the CEC.

DDC - Display Data Channel, is used to enable the source device to interrogate the receiving device about its configuration and capabilities. This is done by reading the E-EDID (Enhanced Extended Display Identification Data) data from the receiving device. Data is transferred using I ² C signaling with a 100kHz clock.

CEC - Consumer Electronics Control Channel, is optional and allows the control of several audiovisual devices that the user might have. It is mainly used for remote control functions. It uses the industry standard AV Link protocol transmitted over a one-wire bi-directional serial bus.

HDMI Cable Connectors

As already indicated earlier on, HDMI cables can come with two kinds of connectors: a compact Type-A 19 pin connector, and a slightly bigger  29-pin Type-B HDMI connector. This second connector allows for the use of dual-link HDMI configuration, which doubles the maximum transfer rate that can be used, up to a maximum pixel clock rate of 330Mhz.

Schematic drawings showing the relevant HDMI plug details are given below; just click on the respective image to enlarge.

Type-A HDMI		Type-B HDMI

Notes:

Transmitting devices with a Type-A HDMI connector can be connected to receiving devices with a Type-B plug using an HDMI cable with a Type-A plug at one end and a Type-B plug at the other.

It is not possible to connect a transmitting device with a Type-B plug to a receiving device with a Type-A plug.

Further more, as already mentioned earlier on, HDMI and DVI-D enabled devices can be interconnected through the use of a DVI-D to HDMI cable converter for both single-link and dual-link applications.

This means that in addition to standard straight Type-A to Type-A, and Type-B to Type-B HDMI cables,  you can also come across three other different types of HDMI cable connectors:

• DVI to HDMI Cable adaptor with single-link DVI-D connector on one end and HDMI Type-A connector on the other.

• DVI to HDMI Cable adaptor with dual-link DVI-D connector on one end and HDMI Type-B connector on the other.

• HDMI to HDMI cable adaptor with a Type-A connector on one end and a Type-B connector on the other.

For the respective detailed connection pin-outs covering HDMI type A and type B connectors as well as DVI to HDMI cable adaptors, please check out the following link: HDMI Connectors Pin-out Details

Finally, keep in mind that if you are connecting a DVI-enabled device that does not support HDCP, then connecting your DVI gear to an HDMI port will force the HDMI-enabled device to downgrade high definition video content to standard resolution.

For more information about HDMI, check out the HDMI website at http://www.hdmi.org; HDMI also implements High-Bandwidth Digital Copy Protection (HDCP) mechanism developed by Intel. To discover more about HDCP, check the following HDCP website at http://www.digital-cp.com.

For a full range of affordable high-quality HDMI Cables from major suppliers, available at amazon.com, please check out the following link: HDMI Interconnects

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