The High-Definition Multimedia Interface (HDMI) is an industry-supported, uncompressed, all-digital audio/video interface. HDMI provides an interface between any compatible digital audio/video source, such as a set-top box, a DVD player, or an A/V receiver and a compatible digital audio and/or video monitor, such as a digital television (DTV).

HDMI supports standard, enhanced, or high-definition video, plus multi-channel digital audio on a single cable. It is independent of the various DTV standards such as ATSC, and DVB(-T,-S,-C), as these are encapsulations of the MPEG data streams, which are passed off to a decoder, and output as uncompressed video data, which can be high-definition. This video data is then encoded into TMDS for transmission digitally over HDMI. HDMI also includes support for 8-channel uncompressed digital audio. Beginning with version 1.2, HDMI now supports up to 8 channels of one-bit audio. One-bit audio is what is used on Super Audio CDs.

The standard Type A HDMI connector has 19 pins, and a higher resolution version called Type B, has been defined, although it is not yet in common use. Type B has 29 pins, allowing it to carry an expanded video channel for use with high-resolution displays. Type-B is designed to support resolutions higher than 1080p.

Type A HDMI is backward-compatible with the single-link Digital Visual Interface (DVI) used on modern computer monitors and graphics cards. This means that a DVI source can drive an HDMI monitor, or vice versa, by means of a suitable adapter or cable, but the audio and remote control features of HDMI will not be available. Additionally, without support for High-Bandwidth Digital Content Protection (HDCP) on both ends, the video quality and resolution may be artificially downgraded by the signal source to prevent the end user from viewing or especially copying restricted content. (While nearly all HDMI connections support HDCP, many DVI connections do not.)

Type B HDMI is similarly backward-compatible with dual-link DVI. The Type B HDMI connector includes all of the signals of the Type A connector, but also adds another set of RGB signals, (effectively dual link). This supports up to 330MHz of video bandwidth at 24 bits per pixel. However, it is unlikely to find  this connector in current applications. Dual link is primarily used for very high resolutions, beyond 1920 x 1080, or active 3D. In such cases, the DVI connector is preferred because it provides a more rugged construction.

Technical specifications

TMDS channel

• Carries audio, video and auxiliary data.
• Signalling method: According to DVI 1.0 spec. Single-link (Type A HDMI) or dual-link (Type B HDMI).
• Video pixel rate: 25 MHz to 165 MHz (Type A) or to 330 MHz (Type B). Video formats with rates below 25 MHz (e.g. 13.5 MHz for 480i/NTSC) transmitted using a pixel-repetition scheme. Up to 24 bits per pixel can be transferred, regardless of rate.   Supports 1080p60.
• Pixel encodings: RGB 4:4:4, YCbCr 4:4:4 (8 bits per component); YCbCr 4:2:2 (12 bits per component)
• Audio sample rates: 32 kHz, 44.1 kHz, 48 kHz, 88.2 kHz, 96kHz, 176.4 kHz, 192 kHz.
• Audio channels: up to 8.

DDC channel

• Allows source to interrogate capabilities of destination device.
• I²C signalling with 100 kHz clock.
• E-EDID data structure according to EIA/CEA-861B and VESA Enhanced EDID (V1.3).

Consumer Electronics Control (CEC) channel (optional)

• Uses the industry standard AV Link protocol
• Used for remote control functions.
• One-wire bidirectional serial bus.
• Defined in HDMI Specification 1.0.

Content Protection

• According to High-bandwith Digital Content Protection (HDCP) Specification 1.10.

Connector detail

Connector example: Molex 500254-1907

Type A Connector Pin Assignment

Pin	Signal Assignment	Pin	Signal Assignment
1	TMDS Data2+	2	TMDS Data2 Shield
3	TMDS Data2�	4	TMDS Data1+
5	TMDS Data1 Shield	6	TMDS Data1�
7	TMDS Data0+	8	TMDS Data0 Shield
9	TMDS Data0�	10	TMDS Clock+
11	TMDS Clock Shield	12	TMDS Clock�
13	CEC	14	Reserved (N.C. on device)
15	SCL	16	SDA
17	DDC/CEC Ground	18	+5 V Power
19	Hot Plug Detect

Distance limitations

A reported problem with HDMI is the maximum cable length. As with all cables, signal attenuation becomes too high at a certain length. For the standard HDMI copper cables at 28 AWG, some users have found signal performance degrades above a cable length of about 5 meters. For front projection televisions and computer hookups, this can result in lost data and the video device compensating in unacceptable ways.

The HDMI Web site, however, disputes the 5 meter limit. "HDMI technology has been designed to use standard copper cable construction at long lengths. In order to allow cable manufacturers to improve their products through the use of new technologies, HDMI specifies the required performance of a cable but does not specify a maximum cable length. Cable manufacturers are expected to sell reasonably priced copper cables at lengths of up to 15 meters."

One reported way to increase the distance limit is to increase the thickness of the copper cables, effectively decreasing impedance. 24 AWG wire is considered superior to 28 AWG. Another way is to use fiber optic or dual Cat-5 cables instead of standard copper. Some companies also offer amplifiers and repeaters that can string several HDMI cables together.

The Digital Visual Interface (DVI) is a video interface standard designed to maximize the visual quality of digital display devices such as flat panel LCD computer displays and digital projectors. It was developed by an industry consortium, the Digital Display Working Group (DDWG).

Overview

The DVI interface uses a digital protocol in which the desired brightness of pixels is transmitted as binary data. When the display is driven at its native resolution, all it has to do is read each number and apply that brightness to the appropriate pixel. In this way, each pixel in the output buffer of the source device corresponds directly to one pixel in the display device, whereas with an analog signal the appearance of each pixel may be affected by its adjacent pixels as well as by electrical noise and other forms of analog distortion.

Previous standards such as the analog VGA were designed for CRT-based devices and thus did not use discrete time. As the analog source transmits each horizontal line of the image, it varies its output voltage to represent the desired brightness. In a CRT device, this is used to vary the intensity of the scanning beam as it moves across the screen.

However, when using digital displays (such as LCD) with analog signals (such as VGA), there is an array of discrete pixels and a single brightness value must be chosen for each. The decoder does this by sampling the voltage of the input signal at regular intervals. When the source is also a digital device (such as a computer), this can lead to distortion if the samples are not taken at the center of each pixel, and there are also problems with crosstalk.

Technical discussion

The data format used by DVI is based on the PanelLink serial format devised by the semiconductor manufacturer Silicon Image Inc. This uses Transition Minimized Differential Signalling or TMDS. A single DVI link consists of four twisted pairs of wire (red, green, blue, and clock) to transmit 24 bits per pixel. The timing of the signal almost exactly matches that of an analog video signal. The picture is transmitted line by line with blanking intervals between each line and each frame, and without packetization. No compression is used and DVI has no provision for only transmitting changed parts of the image. This means the whole frame is constantly re-transmitted.

With a single DVI link, the largest resolution possible at 60 Hz is 2.6 megapixels. The DVI connector therefore has provision for a second link, containing another set of red, green, and blue twisted pairs. When more bandwidth is required than is possible with a single link, the second link is enabled, and alternate pixels may be transmitted on each. The DVI specification mandates a fixed single link cutoff point of 165 MHz, where all display modes that require less than this must use single link mode, and all those that require more must switch to dual link mode. When both links are in use, the pixel rate on each may exceed 165 MHz. The second link can also be used when more than 24 bits per pixel is required, in which case it carries the least significant bits.

Like modern analog VGA connectors, the DVI connector includes pins for the display data channel, version 2 (DDC 2) that allows the graphics adapter to read the monitor's extended display identification data (EDID).

Connector

The DVI connector usually contains pins to pass the DVI-native digital video signals. In the case of dual-link systems, additional pins are provided for the second set of data signals.

The DVI connector may also incorporate pins to pass through the legacy analog signals using the VGA standard. This feature was included in order to make DVI universal, as it allows either type of monitor (analog or digital) to be operated from the same connector.

The DVI connector on a device is therefore given one of three names, depending on which signals it implements:

• DVI-D (digital only)
• DVI-A (analog only)
• DVI-I (digital & analog)

The connector also includes provision for a second data link for high resolution displays, though many devices do not implement this. In those that do, the connector is sometimes referred to as DVI-DL (dual link).

DVI is the only widespread standard that includes analog and digital transmission options in the same connector. Competing standards are exclusively digital: these include a system using low-voltage differential signalling (LVDS), known by its proprietary names FPD (for Flat-Panel Display) Link and FLATLINK; and its successors, the LVDS Display Interface (LDI) and OpenLDI.

One oversight in DVI is that USB signals were not incorporated into the connector. This has been addressed in the VESA M1-DA connector used by InFocus on their projector systems, and in the now-defunct Apple Display Connector used by Apple Computer. The VESA M1 connector is essentially the VESA Plug & Display (P&D) connector, which was itself originally named the Enhanced Video Connector (EVC). The pinout of the Apple Display Connector is electrically compatible with the VESA P&D/M1, but physically the shell of the connector is a different shape.

Some new DVD players, TV sets (including HDTV sets) and video projectors have DVI/HDCP connectors; these are physically the same as DVI connectors but transmit an encrypted signal using the HDCP protocol for copyright protection. Computers with DVI video connectors can use many DVI-equipped HDTV sets as display.

Specifications

Digital

• Minimum clock frequency: 21.76 MHz
• Maximum clock frequency in single link mode: Capped at 165 MHz (3.7 Gbit/s)
• Maximum clock frequency in dual link mode: Limited only by cable quality (more than 7.4 Gbit/s)
• Pixels per clock cycle: 1 (single link) or 2 (dual link)
• Bits per pixel: 24

• Example display modes (single link):
  • HDTV (1920 × 1080) @ 60 Hz with 5% LCD blanking (131 MHz)
  • UXGA (1600 × 1200) @ 60 Hz with GTF blanking (161 MHz)
  • WUXGA (1920 × 1200) @ 60 Hz (154 MHz)
  • SXGA (1280 × 1024) @ 85 Hz with GTF blanking (159 MHz)
• Example display modes (dual link):
  • QXGA (2048 × 1536) @ 75 Hz with GTF blanking (2×170 MHz)
  • HDTV (1920 × 1080) @ 85 Hz with GTF blanking (2×126 MHz)
  • WQXGA (2560 × 1600) pixels (30" LCD)
  • WQUXGA(3840 × 2400) @ 41 Hz

GTF (Generalized Timing Formula) is a VESA standard.

Analog

• RGB bandwidth: 400 MHz at −3 dB

 

Connector

Pin numbers (looking at socket):

Pin assignments

Pin	Name	Function
1	TMDS Data 2-	Digital red - (Link 1)
2	TMDS Data 2+	Digital red + (Link 1)
3	TMDS Data 2/4 shield
4	TMDS Data 4−	Digital green − (Link 2)
5	TMDS Data 4+	Digital green + (Link 2)
6	DDC clock
7	DDC data
8	Analog Vertical Sync
9	TMDS Data 1−	Digital green − (Link 1)
10	TMDS Data 1+	Digital green + (Link 1)
11	TMDS Data 1/3 shield
12	TMDS Data 3−	Digital blue − (Link 2)
13	TMDS Data 3+	Digital blue + (Link 2)
14	+5V	Power for monitor when in standby
15	Ground	Return for pin 14 and analog sync
16	Hot Plug Detect
17	TMDS data 0−	Digital blue − (Link 1) and digital sync
18	TMDS data 0+	Digital blue + (Link 1) and digital sync
19	TMDS data 0/5 shield
20	TMDS data 5−	Digital red − (Link 2)
21	TMDS data 5+	Digital red + (Link 2)
22	TMDS clock shield
23	TMDS clock+	Digital clock + (Links 1 and 2)
24	TMDS clock−	Digital clock − (Links 1 and 2)
C1	Analog Red
C2	Analog Green
C3	Analog Blue
C4	Analog Horizontal Sync
C5	Analog Ground	Return for R, G and B signals