HDMI: Guide to the HDTV Connection of the Future
Background:
As the HDTV market continues to heat up, consumers need to be educated on the latest technology. One of the terms that you, the consumer, will surely come across when selecting an HDTV is: HDMI. For most, this is an unfamiliar term and we will explain the details of HDMI in this article so that you, the consumer, have the knowledge to make smart 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 world’s largest consumer electronics manufacturers: Hitachi, Matsushita, Philips, Silicon Image, Sony, Thomson, and Toshiba. HDMI is the latest digital HDTV interconnect standard. The notable differences between HDMI and previous HDTV interconnect standards (component video and DVI) are:
-HDMI is fully 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 video and audio signals, eliminating cable clutter.
-HDMI incorporates content protection called HDCP (High Definition Content Protection)
What does an HDMI connector look like? HDMI looks like a USB cable. The compact size and high integration (transmits audio and video) make the HDTV installation experience truly “plug and play”.
Why should you use HDMI?
-HDMI is fully digital, so image quality is “perfect” from source to screen
-HDMI is a digital video and audio connection. This will minimize the wiring in your system.
-HDMI is fast becoming the de facto standard for HDTV connections.
How does HDMI carry digital video? The video portion of HDMI is carried out by 3 separate differential pairs. Each pair carries 1 of 3 uncompressed native digital R, G, B signals from the source (DVD player, set-top box) to the sink (HDTV display). A unique protocol, TMDS (Minimized Differential Signaling Transmission), is used to transport the digital data. Each pixel is represented by 24 bits (8 bits each for each of the primary colors). The TMDS protocol then “computes” and adds an additional 2 bits to the video data stream to create a digital stream with minimal transitions (lower EMI, less interference) and also minimize long strings of ‘1’ and ‘0’ that can cause errors detection.
A fourth differential pair, called the TMDS clock, provides the pixel clock to time the data flow. The maximum TMDS single link pixel clock frequency is 165 MHz.
What is the data rate of a single link HDMI connection? The maximum pixel clock frequency 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 on the HDMI connector. There are 3 pairs of TMDS signals that carry all digital video and audio signals.
How is the digital audio signal transported? Multichannel audio is time multiplexed into TMDS data streams. Audio has a much lower data rate (192 kbps) and the additional 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 to control unauthorized copying of digital media. Encryption is done at the HDMI transmitter at the “source” (DVD player, set-top box) and decryption is done via the HDMI receiver (the HDTV screen). The secret keys for encryption are exchanged between the source and the display via an I2C bus (pins 15 and 16).
Is HDMI compatible with DVI? DVI is the predecessor of HDMI. HDMI and DVI are identical when it comes to video. Therefore, there is backward compatibility with video. However, DVI will not support digital audio. For example, if you have an older DVI connection on your source and an HDMI connector on your display, an HDMI to DVI cable is all you need to view the video. A separate audio cable (TOSLINK or SPDIF) will be required to carry digital audio.
What formats will HDMI support? HDMI is a high-speed digital connection and will support resolutions of 480i, 480P, 720i, 720P, 1080i, and in the future 1080P.
Conclution:
HDTV technology is changing rapidly. HD connections like HDMI will become the de facto standard for HDTV connections. We have outlined the important features of HDMI, so the consumer is well informed and ready to make smart purchasing decisions.