Saturday, November 24, 2007

HDMI: Guide to HDTV Connection of the Future


As the HDTV market continues to heat up, consumers are in need of being educated on the latest technology. One of the terms that you, the consumer, will surely run into while selecting a HDTV set is: HDMI. For most this is an unfamiliar term and we will explain the details of HDMI in this article so you, the consumer, is armed with the proper knowledge to make intelligent purchasing decisions.

What is HDMI: HDMI is an acronym for High Definition Multi-Media Interface. The HDMI specification (now at 1.2) was created by some of the largest consumer electronics manufacturers in the world: Hitachi, Matsushita, Philips, Silicon Image, Sony, Thomson, and Toshiba. HDMI is the latest digital HDTV interconnection standard. The notable differences between HDMI and the earlier HDTV interconnects standards (component video, and DVI) are:

-HDMI is all digital unlike analog component video cables

-HDMI supports multi-channel audio in addition to digital video. (DVI only supports digital video)

- HDMI is more compact in size and carries both audio and video signals therefore eliminate cable clutter.

-HDMI incorporates content protection called HDCP ( high definition content protection)

What does a HDMI connector look like: HDMI looks similar to a USB cable. The compact size and high integration (carries both audio and video) makes the HDTV installation experience truly "plug and play."

Why should I use HDMI:

-HDMI is all digital, so picture quality is "perfect" from source to display

-HDMI is both a digital audio and video connection. This will minimize cabling in your system

-HDMI is rapidly becoming the de facto standard for HDTV connections.

How does HDMI transport the digital video: The video portion of HDMI is carried by 3 separate differential pairs. Each pair transports 1 of 3 uncompressed native digital R,G, B signals from source ( dvd player, set top box) to the sink ( HDTV display). A unique protocol, T.M.D.S.( transmission minimized differential signaling), is used to transport the digital data. Each pixel is represented by 24 bits ( 8 bits each for each of the primary colors). The T.M.D.S. protocol then "calculates" and stuffs 2 extra bits to the video data stream in order to create a digital stream with minimum transitions ( lower EMI, lower interference) and also minimize long strings of '1' and '0' which can cause detection errors.

A fourth differential pair, called the TMDS clock provides the pixel clock for timing the data stream. The maximum TMDS single link pixel clock rate is 165 MHz.

What is the data rate of a single link HDMI connection: The maximum pixel clock rate is 165MHz and each of the 3 TMDS video streams carries 10 bits. Therefore the aggregate data rate is 3 x 10 x 165MHz = 4.96Gbps.

How many pins are included in the HDMI connector: There are 19 individual pins in the HDMI connector. There are 3 pairs of TMDS signals which carry all the digital audio and video signals.

How is the digital Audio signal transported: The multi-channel audio is time multiplexed into the TMDS data streams. Audio is much lower data rate (192kbps) and the extra time is used to demux the audio signals.

What is HDCP: HDCP is an acronym for High-Bandwidth Digital Content Protection. HDCP is an encryption method developed by Intel in order to control unauthorized copying of digital media. The encryption is carried out in the HDMI transmitter found in the "source" ( dvd player, set top box) and decryption is carried out by the HDMI receiver ( the HDTV display). The secret keys for encryption are exchanged between the source and display over an I2C bus ( pins 15 and 16).

Is HDMI compatible with DVI: DVI is the predecessor to HDMI. HDMI and DVI are identical as far as video is concerned. Therefore, video backward compatibility exists. However, DVI will not support digital audio. For example, if you have an older DVI connection on your source and a HDMI connector on your display, a HDMI to DVI cable is all that is needed in order to view the video. A separate audio cable ( TOSLINK or SPDIF) will be needed to carry the digital audio.

What formats will HDMI support: HDMI is high speed digital connection and will support resolutions of 480i, 480P, 720i, 720 P, 1080i and in the future,1080P.


HDTV technology is changing rapidly. HD connections such as HDMI will become the de facto standard in HDTV connections. We have outlined the important features of HDMI, so the consumer is well informed and ready to make intelligent purchasing decisions.

Jeff Su is product development manager at Jeff is a graduate of the Georgia Institute of Technology (BSEE, MSEE) and has over 10 years of experience in CATV systems, RF, and microwave designs. He may be reached at

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