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. 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 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; these are 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 4.95Gbps (165MHz x 30-bits) over a single-link HDMI cable, or 9.9Gbps over a dual-link HDMI interconnect; these rates determine the maximum video signal resolution that can be transmitted over single-link and dual-link HDMI cables.
Considering that a single-link HDMI can transfer up to 165 million pixels per second, it is clear that HDMI has more than enough bandwidth on a single link to accommodate even the highest HDTV format available today for consumer electronic products. In the case of a dual-link mode, an HDMI cable connection can handle a bandwidth of up to 330Mhz.
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 (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.
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 I 2 C signaling with a 100kHz clock.
CEC – Consumer Electronics Control Channel, is optional and allows the control of several audiovisual devices 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.
