A Short Guide to
HDMI Cables and HDMI Connectors

This was followed by HDMI ver. 1.1 in May 2004, ver. 1.2 in August 2005 - which was latter updated to ver. 1.2a in December of the same year.

But the version list did not stop there; version 1.3 followed in June 2006, again updated to ver. 1.3a in November of the same year, and then updated again to Ver. 1.3b in March 2007 and ver. 1.3c in August 2008.

In June 2009, a newer HDMI version was released labeled as ver. 1.4; the latter requires a different HDMI cable to support the added Ethernet channel defined in the new standard.

Meanwhile, the first HDMI-enabled consumer products started to appear on stores shelves in late 2003. Since then, HDMI saw an exponential rise in the rate of annual growth of HDMI-enabled products, and soon became the 'de facto' standard for HDTV connectivity. It is estimated that today, there are more than 200 million HDMI-enabled devices worldwide!

The HDMI standard was founded by leading consumer electronics manufactures - Hitachi, Panasonic, Philips, Sony, Toshiba, Thomson and Silicone Image.

Because HDMI is fully compliant with HDCP (High-Bandwidth Digital Content Protection), which provides copyright protection, the HDMI standard 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.

Article Content:

• HDMI - Main Benefits and supported cable lengths

• HDMI Versions - A comparison of supported features and AV formats

• HDMI and DVI-D - Differences and similarities

• HDMI AV transport mechanism basics

• Different HDMI connectors and different cable types

HDMI - Main Benefits

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

The use of a single HDMI cable that replaces the maze of AV cables often found 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 - meaning that a separate audio cable (TosLINK or SPDIF) is needed to transport digital audio when a DVI cable is used.

But there are other additional advantageous as well

This category difference in HDMI cables surfaced with the introduction of HDMI 1.3 - which defines two cable categories: Category 1-certified cables - tested at up to 74.5 MHz (720p60 and 1080i60), and Category 2-certified cables - tested at 340 MHz (1080p60 and 2160p30). Later in October 2008, Category-1 HDMI cables started to be marketed as 'Standard' while Category-2 HDMI cables as 'High Speed'.

Supported Cable Length

We have stated that the HDMI standard support longer cable lengths than DVI but…

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 indicated 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 specify a maximum cable length - but rather defines the expected performance parameters such as far-end crosstalk, attenuation, and differential impedance, that need to be met by an HDMI interconnect.

What type of video and audio formats HDMI supports - and what are the main differences between the respective HDMI versions

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. However, the actual supported image resolutions, audio standards, and features are dependent on the respective HDMI standard.

At this point, it is important to realize that some of the features specified by the various HDMI standards are not mandatory. Features such as support for 1080p 60Hz, Deep Color - which increase the precision of the supported brightness and color information, xvYCC for an extended color space that is 1.8 times that of standard sRGB,   and Super Audio CD DSD support, are optional and therefore manufacturers may opt not to implement these features. This means that a product having a specific HDMI version does not necessarily support all features listed for that HDMI version.

HDMI Version Comparison

HDMI Version 1 specifies the basic audio/video HDMI interconnect. It supports a maximum TMDS bandwidth of 4.95 Gbit/s - with a maximum allotted video bandwidth of 3.96 Gbit/s (165 MHz) supporting up to 1080p/60 Hz video 24 bit color depth, and 36.86 Mbit/s maximum audio bandwidth for up to 8 channels of 192 kHz 24-bit audio resolution.

In addition, HDMI version 1.0 also supports Blu Ray disc video and audio at full resolution and CEC - short for Consumers Electronics Control. The latter is often branded differently by different manufactures and enables the user to control HDMI connected devices via a single control unit.

However, while CEC was included with HDMI version 1, yet it wasn't before the release of Version 1.2a that all relevant CEC features, command sets, and CEC compliance tests were eventually specified to the full.

HDMI Version 1.1 added support for DVD Audio while HDMI Version 1.2 added support for One Bit Audio as used on Super Audio CDs for up to 8 channels, this apart from additional features related to supported color space by PC sources. And as stated in the previous paragraph, Version 1.2a fully specified for the first time the Consumer Electronic Control (CEC) support.

HDMI Version 1.3 has brought about significant enhancements to the original HDMI specifications - with the most important being increasing the single-link bandwidth to 340 MHz or 10.2 Gbit/s data stream.

It also supports Deep Color with 30-bit 2560x1600p75, 36-bit 2560x1600p60, and 48-bit 1920x1200p60 xvYCC color space compared to the original 24-bit sRGB or YCbCr specified in previous HDMI versions.

Other optional features include output of Dolby TrueHD and DTS-HD Master Audio streams for external decoding by AV receivers; it incorporates automatic audio syncing or auto lip sync capability; and as indicated earlier on, Version 1.3 has brought with it definitions for HDMI Cable categories 1 and 2 - this apart from defining a new mini HDMI connector (Type C) for portable devices.

HDMI versions 1.3a, 1.3b, 1.3b1, and 1.3c mainly updated the list of CEC commands, and added HDMI compliance tests.

HDMI Version 1.4 - released earlier this year - has so far completed the relatively long list of HDMI versions and with it, there came a number of major additions to the already important additions brought about by Version 1.3. The most important additions related to version 1.4 are the addition of a 100 Mb/s HDMI Ethernet Channel (HEC) between HDMI connected devices and an increase in the supported resolution to up to 4096x2160p24 or 3840x2160 at up to 30Hz.

In addition, Ver. 1.4 also introduces support for 3D over HDMI - supporting common 3D formats and resolutions up to 1080p, an expanded support for color spaces designed specifically for digital still cameras like sYCC601 and Adobe RGB, an Audio Return Channel for upstream audio transfers over the same HDMI cable, a Micro HDMI Connector – that is almost half the size of a standard 19-pin HDMI connector, sort of equivalent to a mini USB connector, and an Automotive Connection System for better in-vehicle HDMI use.

HDMI and DVI - Differences and Similarities

Before continuing further with our discussion on HDMI cables, it would be appropriate at this point to bring out 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 cable.

However, 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 cable 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 electronics manufacturers for use in consumer electronic products such as DVD players and digital televisions.

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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. As already indicated earlier on, this maximum bandwidth was further increased to 340 MHz with HDMI 1.3 and 1.4.

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 - but only at 165 MHz, 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 (ver. 1.3 and 1.4 supports up to 48 bit pixel information). 24-bit pixel information is 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 4.95Gbit/s (165MHz x 30-bits) over a single-link HDMI cable for up to HDMI ver. 1.2, or 9.9Gbit/s over a dual-link HDMI interconnect. In the case of HDMI ver. 1.3 and 1.4, maximum data throughput over a single link is 10.2 Gbit/s; 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 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:

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.

It is thus clear that even HDMI ver. 1 and ver. 2 have more than enough bandwidth on a single link to accommodate even the highest HDTV format most common today. This also explains why dual-link HDMI - like dual-link DVI, has remained practically non-existent

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 rate (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 - already referred to above.

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 I² C signaling with a 100 kHz clock.

CEC - Consumer Electronics Control Channel, is optional and as already expressed earlier on, allows the control of several audiovisual devices interconnected over HDMI 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.

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Different HDMI Cable Connectors

The various different HDMI versions have brought about different HDMI cable connectors. In total, there are four different HDMI cable connectors specified by the different HDMI version standards, as further summarized below:

Type-A and Type-B HDMI cable connectors: These are defined in HDMI 1.0 specifications. Type-A is a compact 14 mm 19-pin connector and is the most common HDMI connector present today. It is electrically compatible with DVI-D single-link connector.

Type-B is a slightly bigger 21 mm 29-pin connector for dual-link HDMI connectivity, allowing a maximum pixel clock rate of 330MHz. As with Type-A, Type-B HDMI cable connectors are electrically equivalent to dual-link DVI-D. However, as with dual-link DVI, despite being defined in the HDMI standard, Type-B HDMI connectors are non-existent in that no dual-link HDMI consumer equipment has been released so far.

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

Type-C is a mini HDMI cable connector defined in the HDMI 1.3 specification; it is intended for portable devices and is just 10.42 mm × 2.42 mm despite using the same 19-pin configuration. However, there are some differences with respect to the pins connections in comparison to standard Type-A connectors.

All positive signals of the differential pairs are swapped with their corresponding shield, the DDC/CEC Ground is assigned to pin 13 instead of pin 17, CEC is assigned to pin 14 instead of 13, and the reserved pin is 17 instead of 14.

Electrically, Type-C Mini HDMI connectors can be connected to a Type-A connector using a Type A-to-Type C HDMI cable.

Type-D is micro connector defined in the HDMI 1.4 specification that keeps the standard 19 pins of Types A and C but enclosed in a 2.8 mm x 6.4 mm package - smaller than a micro-USB connector.

And there is even more... Different HDMI Cable Types

When HDMI was first released way back in 2002, many thought that this standard would not come with the many flavors of cable connectors and cable types as instead is the case with DVI. Unfortunately, things are even more complicated!

Apart from the two different types of HDMI cable category certifications defined earlier on in our discussion for 'Standard' - supporting up to 1080i/60, and 'High Speed' - supporting date rates in excess of 1080p 60Hz, including Deep Color and all 3D formats of the new 1.4 specification, HDMI ver. 1.4 introduced three additional HDMI cables:

 

And in the remote eventuality that Type-B cable would ever be implemented on consumer gear, then there would arise even more additional HDMI cables consumers need to worry. Why? Electrically, 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.

In addition, while it is not possible to connect a transmitting device with a Type-B plug to a receiving device with a Type-A plug, yet the fact that HDMI devices can be connected to DVI-D equipment 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 DVI-HDMI cable connectors:

Note: 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. HDCP stands for High-Bandwidth Digital Copy Protection mechanism originally developed by Intel.

For more information on content protection, please refer to our HDCP guide; there we discuss also its implications on the end user as well as the issue of HDCP strippers. The HDCP website is available at http://www.digital-cp.com, while the official HDMI website is at http://www.hdmi.org.