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Understanding HDMI Specs

by Jamey Calloway
Liberty Product Manager

Understanding HDMI cable specifications can be confusing. The following are a few terms you may have seen on HDMI cable spec sheets: 225MHz, 2.25Gbps, 10.2Gbps, Category-1 74.25MHz , Category-2 340MHz, 1080p24, 1080p60, 4K x 2K, 8-bit, 12-bit, 36-bit deep color, HEAC, and many more. Without an adequate understanding of these terms, it is nearly impossible to compare one cable to another or determine if a particular cable will support the video resolution that your application requires.

The HDMI connector interface consists of 19 conductors. Twelve of these pins (including the shields) comprise the four differential twisted pairs that make up the TMDS (Transition Minimized Differential Signaling) clock channel and three TMDS data channels (CH0, 1 and 2). These three channels carry digital audio, video and auxiliary data. The TMDS clock frequency is specified in megahertz (MHz), such as 340MHz. 10 bits of digital data are transmitted during each clock cycle on each channel. For example, a 340MHz TMDS clock frequency supports transmission of 3.4Gbps (340 Million x 10/bits) on each of the TMDS channels. So, the total data throughput is 10.2Gbps (3.4Gbps x 3/Channels). Since the HDMI specification supports TMDS clock frequencies of 25MHz up to 340MHz, the actual data rate may vary from 750Mbps to 10.2Gbps total throughput. Some vendors specify the cable performance by the TMDS clock frequency while others specify the data throughput per TMDS channel or the total throughput of all three channels. Since data is carried on all three channels, total throughput is most relevant. It is important to note that a cable’s electrical characteristics, such as attenuation, cross-talk, return loss, skew, as well as other factors, affect its ability to carry high frequency signals. These characteristics are determined by its mechanical construction, such as wire gauge, pair twist rate, conductor type and insulation material.

Now that we have explained how to determine the actual data carrying capacity of the cable, the next step is to calculate how much data bandwidth is required for a particular video format. HDMI supports three video pixel formats: RGB4:4:4, YCbCr4:4:4 and YCbCr4:2:2. A Pixel color depth of 8 bits per TMDS channel must be supported by HDMI devices. “Deep Color” depths of 10, 12, or 16-bits per channel are optional; however if a source/display device supports Deep Color mode, 12-bit resolution must also be supported. In order to calculate the total pixel color depth, we must multiply the pixels per channel by three TMDS channels (Ex. 8-bits x 3/Ch = 24-bits). Deep color provides for a much more brilliant display. For example, 24-bit color supports 16.384 million colors per pixel whereas 36-bit resolution supports 68.7 billion colors. The TMDS clock must run at a 50% faster rate for 36-bit color than 24-bit. In addition to the color resolution, the data requirement is determined by the total number of horizontal and vertical pixels and the number of frames displayed per second (fps). The EIA/CEA-861 standard defines the actual video formats and timing requirements. For example, we can calculate the video and audio data rates of a Blu-ray player output at 1080p24 with 8 channel audio as follows