[nextpage title=”Introduction”]
HDMI (High-Definition Multimedia Interface) is today the best kind of digital audio and video connector available that promises to replace all connectors currently used by players, cable/satellite decoders, TV sets, videoprojectors and video monitors. The idea is to use a single cable instead of several cables when connecting your HD-DVD player to your TV set, for example. This new connection standard was developed by Hitachi, Matsushita (Panasonic/National/Technics), Phillips, Silicon Image, Sony, Thomson (RCA) and Toshiba.
The greatest advantage brought by this standard is the use of digital connection for both audio and video, bringing the best quality possible. DVI (DVI-D, actually) also provides digital connection between your devices and displays, but it doesn’t carry audio signal, meaning that you need an extra cable for the audio connection; and all other popular standards – component video and S-Video, for example – are analog connections. You can learn more about other video connection types available today by reading our Video Connectors tutorial.
There are three basic differences between HDMI and DVI-D. First, HDMI supports higher resolutions than DVI, including resolutions not commercially released yet (in theory it supports double the highest resolution available today in HDTV sets); second, DVI only connects video, audio connection must be done separately, while HDMI connects both audio and video; third, HDMI connector is far smaller than DVI’s.
It is interesting to notice that HDMI is fully compatible with DVI-D. It is possible to connect a device that uses DVI-D to another that uses HDMI, through a cable with a HDMI connector at one end and a DVI connector at the other.
Another important difference is that DVI standard was developed to be used by PCs, while HDMI was developed to be used by consumer electronics products, such as Blu-Ray, HD-DVD and DVD players, videoprojectors and HDTV sets.
HDMI also implements a copy-protection mechanism called HDCP (High-Bandwidth Digital Copy Protection), which was developed by Intel. You can learn more about it at https://www.digital-cp.com.
[nextpage title=”How It Works”]
In Figure 1 you see the block diagram of HDMI architecture. A “source” is any device with a HDMI output, while “sink” is any device with a HDMI input.
Figure 1: HDMI architecture.
Data transmission uses TMDS (Transition Minimized Differential Signaling) protocol, which was created by Silicon Image (who adopts PanelLink as commercial name for this protocol) and is the same standard used by DVI connection.
This standard codes an eight-bit data into a 10-bit signal and transfers them using differential transmission. Read our tutorial How Gigabit Ethernet Works to learn what is differential transmission, also known as cancellation.
Audio and video data are transferred using the three available TMDS data channels.
Video information is transmitted as a series of 24-bit pixels and 10 bits per pixel clock period are transmitted (pixel clock period, Tpixel, is defined as the time necessary to transmit one pixel; it equals to 10 times the bit transmission period, Tbit).
Pixel clock rate can vary from 25 MHz to 165 MHz. Screen formats that use transfer rates below 25 MHz (NTSC 480i standard [480 lines, non-interlaced scanning] uses a 13.5 MHz pixel clock rate) can use a pixel repetition technique to be transmitted.
This means that with HDMI it is possible to transfer up to 165 million pixels per second (using dual-link configuration, which we will be talking about in the next page, it is possible to double this rate). This rate tells us what is the maximum resolution that can be transmitted.
For you to understand better what is pixel clock and its importance, consider 720p resolution, which is widely used by HDTV sets. This resolution is in fact of 1280×720. Multiplying 1280 x 720 we have the number of pixels available on the screen. The number you found must be then multiplied by the number of frames per second (a.k.a. refresh rate or vertical frequency) so we can know the number of pixels per second (i.e., transfer rate) of this resolution. Assuming a refresh rate of 60 Hz (720p@60 Hz) we found that we need a link capable of transmitting 55,296,000 pixels per second or 55.3 MHz. Since HDMI standard can transfer up to 165 million of pixels per second, this connection is more than enough for this screen resolution.
If we make the math for the highest resolution available today, 1080p (1920×1080) at 60 MHz, we will see that it needs a 124.4 MHz transfer rate: HDMI can fit this.
So HDMI supports the highest resolutions available today for consumer electronics products and also has a dual-link mode with a 330 MHz transfer rate, supporting even higher resolutions not used commercially yet.
For more information on HDTV resolutions, read our HDTV Tutorial.
Video pixels can be coded under RGB, YCbCr 4:4:4 or YCbCr 4:2:2 formats at 24 bits per pixel.
YCbCr is the digital version of Video Component (analog version of Video Component, which is the most used one, is called YPbPr). These standards are also known as YUV. “Y” standards for luminance (or simply “luma”, which is the image without color information, i.e., black-and-white image), Cb is the difference between blue and luminance (B-Y) and Cr is the difference between red and luminance (R-Y). The three numbers represent the sample rate used to code Y, Cb and Cr signals, respectively. “4” means a 13.5 MHz rate, which is the sample rate used by NTSC, PAL and Secam systems. 4:4:4 standard means that all signals are being transmitted at the same rate. 4:2:2 means that Y signal is being transmitted at 13.5 MHz but Cb and Cr are being transmitted at 6.75 MHz. Obviously this mode offers a worse video quality, although being widely used.
Audio can be from two to eight channels, using sample rates up to 192 kHz.
DDC (Display Data Channel) channel is used to inform the transmitting device about the receiving device configuration and capabilities. This is done by reading the E-EDID (Enhanced Extended Display Identification Data) data from the receiving device.
CEC (Consumer Electronics Control) channel is optional and allows the control of several audiovisual devices that the user might have.
[nextpage title=”Connectors”]
HDMI has two kinds of connectors: type A, with 19 pins, and Type B, with 29 pins. This second connector is bigger and allows the use of dual-link configuration, which doubles the maximum transfer rate that can be used. With a type A connector it is possible to achieve a pixel clock up to 165 MHz (using the architecture shown in Figure 1) while with a type B connector it is possible to achieve a pixel clock rate up to 330 MHz (doubling the architecture shown in Figure 1).
Figure 2: Type A HDMI connector vs. DVI connector.
TMDS standard limits the cable length to 50 feet (15 meters).
Transmitting devices with a type A plug can be connected to receiving devices with a type B plug using a 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.
As we mentioned earlier, HDMI and DVI are compatible and there are converter cables available.
On the tables below we list the pinout for the type A and type B connectors and also the pinout for the DVI/HDMI converter cable. But first let’s talk something interesting about pricing.
Pricing
One interesting thing about HDMI is that it isn’t an open standard. Manufacturers willing to use it must pay royalties to HDMI Licensing, LLC: an annual fee of USD 15,000 and USD 0,15 per each device using HDMI sold. This rate drops to USD 0,05 if the manufacturer uses HDMI logo on the product and on its promotional material. If HDCP is implemented, the royalty fee drops to USD 0,04 per unit sold.
Also, if the manufacturers want to use HDCP copy-protection mechanism, they must pay an annual fee of USD 15,000 plus USD 0,005 per device key purchased, which must be paid to Digital Content Protection, LLC (a company owned by Intel).
More information about HDMI can be found on https://www.hdmi.org and more information about HDCP can be found at https://www.digital-cp.com.
Cables and Connectors Pinout
Type A HDMI Connector
| Pin | Signal |
| 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 | + 5V |
| 19 | Hot Plug Detect |
Type B HDMI Connector
| Pin | Signal |
| 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 | TMDS Data5+ |
| 14 | TMDS Data5 Shield |
| 15 | TMDS Data5- |
| 16 | TMDS Data4+ |
| 17 | TMDS Data4 Shield |
| 18 | TMDS Data4- |
| 19 | TMDS Data3+ |
| 20 | TMDS Data3 Shield |
| 21 | TMDS Data3- |
| 22 | CEC |
| 23 | Reserved (N.C. on device) |
| 24 | Reserved (N.C. on device) |
| 25 | SCL |
| 26 | SDA |
| 27 | DDC/CEC Ground |
| 28 | +5V |
| 29 | Hot Plug Detect |
Type A HDMI Connector x DVI-D
| HDMI Pin | Signal | Wire | DVI-D Pin |
| 1 | TMDS Data2+ | A | 2 |
| 2 | TMDS Data2 Shield | B | 3 |
| 3 | TMDS Data2- | A | 1 |
| 4 | TMDS Data1+ | A | 10 |
| 5 | TMDS Data1 Shield | B | 11 |
| 6 | TMDS Data1- | A | 9 |
| 7 | TMDS Data0+ | A | 18 |
| 8 | TMDS Data0 Shield | B | 19 |
| 9 | TMDS Data0- | A | 17 |
| 10 | TMDS Clock+ | A | 23 |
| 11 | TMDS Clock Shield | B | 22 |
| 12 | TMDS Clock- | A | 24 |
| 13 | CEC | N.C. | N.C. |
| 14 | Reserved | N.C. | N.C. |
| 15 | SCL | C | 6 |
| 16 | DDC | C | 7 |
| 17 | DDC/CEC Ground | D | 15 |
| 18 | +5V | 5V | 14 |
| 19 | Hot Plug Detect | C | 16 |
| 20 | Not Connected | 4 | |
| 21 | Not Connected | 5 | |
| 22 | Not Connected | 12 | |
| 23 | Not Connected | 13 | |
| 24 | Not Connected | 20 | |
| 25 | Not Connected | 21 | |
| 26 | Not Connected | 8 |
Type B HDMI Connector x DVI-D
| HDMI Pin | Signal | Wire | DVI-D Pin |
| 1 | TMDS Data2+ | A | 2 |
| 2 | TMDS Data2 Shield | B | 3 |
| 3 | TMDS Data2- | A | 1 |
| 4 | TMDS Data1+ | A | 10 |
| 5 | TMDS Data1 Shield | B | 11 |
| 6 | TMDS Data1- | A | 9 |
| 7 | TMDS Data0+ | A | 18 |
| 8 | TMDS Data0 Shield | B | 19 |
| 9 | TMDS Data0- | A | 17 |
| 10 | TMDS Clock+ | A | 23 |
| 11 | TMDS Clock Shield | B | 22 |
| 12 | TMDS Clock- | A | 24 |
| 13 | TMDS Data5+ | A | 21 |
| 14 | TMDS Data5 Shield | B | 19 |
| 15 | TMDS Data5- | A | 20 |
| 16 | TMDS Data4+ | A | 5 |
| 17 | TMDS Data4 Shield | B | 3 |
| 18 | TMDS Data4- | A | 4 |
| 19 | TMDS Data3+ | A | 13 |
| 20 | TMDS Data3 Shield | B | 11 |
| 21 | TMDS Data3- | A | 12 |
| 22 | CEC | N.C. | N.C. |
| 23 | Reserved | N.C. | N.C. |
| 24 | Reserved | N.C. | N.C. |
| 25 | SCL | C | 6 |
| 26 | DDC | C | 7 |
| 27 | DDC/CEC Ground | D | 15 |
| 28 | +5V | 5V | 14 |
| 29 | Hot Plug Detect | C | 16 |
| Not Connected | N.C. | 8 |
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