While quite ambitious, these posts are intended to give you a brief and relevant overview of HDMI. HDMI is a four letter word with explicative connotations for the custom installer. Most of us have had nothing but trouble with HDMI. All of us have to deal with it. This brief tour is intended to give you a working knowledge of how HDMI work and what to look for when it doesn’t. Enjoy.
What is HDMI?
HDMI is a digital standard that transmits uncompressed video, audio and control from point to point. Control presents itself in two distinct fashions in HDMI: device control (e.g., CEC) and signal path control. We’ll focus on the latter in the posts that follow. HDMI is not the encryption per se. That is HDCP. While related, they are independent.
Providing uncompressed video and audio, HDMI does deliver very high quality video. The differences are most able in low brightness scenes and higher resolutions. This uncompressed video comes at a price however – and that price is bandwidth.
The one element of HDMI transmission that you should understand more than all others is bandwidth. The bandwidth required to deliver HDMI at ever increasing resolutions and color depths is staggering. For example:
|1080p deep color||6.68Gbps|
|4k HDMI 30Hz||8.89Gbps|
HDMI 2.0 will push bandwidth requirements to 18Gbps! As clock frequencies approach these numbers all kinds of challenges arise: clock skew, jitter, coupling effects in the cables, EMI, transmission line effects and more…it’s a challenge.
HDMI uses the CEA 861 standards. Those standards have evolved over time along with HDMI. HDMI version 1.4 support version F of CEA 861. You don’t really need to track these details as HDMI devices are required to be backward compatible to the sRGB color space at 8 bits per component. The primary contributors to video bandwidth requirements are video sampling format, color depth and resolution.
Video sampling the process by which video systems sample and encode each of the color components of an image. Most major video formats use the YCbCr color space. You know its close cousin, the YUV color space from component video. (Y=green cable carrying luminance or black and white image etc)
Chroma subsampling is a related technique that takes advantage of the human eye’s lower acuity for differences in color than for luminance. Plainly put, it is a technique that encodes the luminance (black / white) for every pixel and the chroma (color)less often.
Commonly expressed as 4:4:4 or 4:2:2, subsampling works on 4 x 4 pixel grids. If we were to simply encode all components of the image, we would be using 4:4:4 sample and it would look like this:
All three components are sampled for all 16 pixels. While easy to imagine this working very well, it is typically a waste of bandwidth since the human eye is not very good at detecting color details.
A very common subsampling technique is 4:2:2. In this subsampling technique, every grouping of 4 pixels is only sampled for luminance. The display averages the chroma for every other group of pixels to create color.
This is considered very high end video and it reduces the samples from 48 to 32 with little to no affect on the video quality. One of the most common subsampling techniques is 4:2:0 which is used by MPGEG, H.26x, DVD and BluRay. There are other subsampling formats. You can go to the Wiki Link to learn more.
Color depth is the bit depth utilized to quantify the color of a single pixel. Simply put, an 8 bit color depth will represent 256 gradients of color whereas a 24 bit color depth will represent 256 gradients per component (3 total) or over 16 million colors. Deep color is 30/36/48 bit representation for three colors. When bit depths get this high, it is common for the extra bits to represent color intensity.
Resolution when combined with frame rate are a primary driver of video bandwidth requirements. From the table above you can see going from 1080i to 1080p (interlaced to progressive) doubles bandwidth.
4k resolution is 3820×2160 or 8.3 megapixels vs. 1080p at 1920×1080 or 2.1 megapixels. The bandwidth requirements shown above do not quadruple (2.1mp to 8.3mp) because HDMI v1.3 supports 4k up to 30Hz where as the 1080p bandwidth spec shown above is 60 Hz. HDMI v2.0 supports 4k at 60Hz
From a practical perspective, you will rarely see or interact with color subsampling methods or settings as those are part of the content you are transmitting. You will have access to and control of resolution and color depth settings in most cases. These three elements drive video bandwidth and higher video bandwidth is more challenging to deliver from point to point. Period.
HDMI unlike DVI, does support audio. From the beginning HDMI has supported 8 channel, 192Khz, 24 bit audio. By version 1.3, HDMI supported Dolby TrueHD and DTS-HD Master Audio. HDMI 1.4 added audio return channel and HDMI 2.0 will add several new audio features.