HDMI Interface - A Beginner's Guide
HDMI Cables
Starting around 2003 we saw a rapid adoption of the Digital Visual Interface (DVI) across the digital consumer market. This included DTVs, high definition set -top boxes and computer graphics boards. By the end of that year, well over 500 consumer electronics products featured a DVI connection, with approximately 80% of DTVs shipped to the US using that technology. Later in the year, HDMI also emerged as a digital transmission format, but addressed some specific needs tailored to the consumer electronics market:
- HDMI could carry both uncompressed high definition video (with support for SDTV and HDTV color spaces) along with all existing multi-channel audio formats and even device control data in a single connector
- HDMI offered "link intelligence" enabling devices to automatically "discover" each other and recognize resolutions & formats
- By design, HDMI was intended to be a smaller, more consumer-friendly connection (we would argue this last point due to lack of strain relief and any mechanism to secure the connection in place)
- HDMI would be fully compatible with DVI
The HDMI Working Group was announced in April of 2002 and included Hitachi, Panasonic, Silicon Image, Sony, Thomson, and Toshiba. The group's charter was to define the next-generation digital interface specification for consumer electronics products.
In excess of six million HDMI-enabled consumer electronics devices were shipped in 2004, up from a mere 250,000 in 2003. There are expectations that there will be 125 million HDMI-enabled devices in the market by 2007. According to IDC and Silicon Image estimates, there will be over 15 million HDMI-enabled digital televisions at the end of 2005, growing to nearly 50 million units at the end of 2007.
Specifications, Versions, and Capabilities
HDMI version 1.0 met the goals of the HDMI Working Group and provided a true one-cable solution for uncompressed HD video and multi-channel audio including Dolby Digital and DTS bit streams (more on format support later).
HDMI 1.1 was a relatively minor update. The primary feature was to add some packets of audio-related content protection information. These packets were required by DVD-Audio in order to permit DVD-Audio content transmission on HDMI. HDMI 1.0 had the audio and video bandwidth and capabilities and HDCP already had the content protection capabilities, but there was some data that the DVD-Audio folks wanted to send to HDMI/HDCP sinks to tell them not to send the DVD-Audio content elsewhere.
As for HDMI 1.2, several companies have requested enhancements to the HDMI spec that are being considered by the HDMI Founders, but these items are, by agreement, not permitted to be discussed publicly until the specification is released. The HDMI Founders designed the HDMI specification to be dynamic. As such, HDMI has plenty of extra bandwidth to accommodate future audio and video requirements, and the Founders are committed to evaluating and updating the specification to accommodate new audio and video formats that may be introduced in the foreseeable future.
Let's Talk Bandwidth
Everyone would agree that if you want the optimal audio and video reproduction quality it is generally best to transfer an uncompressed audio-video stream from the source. Transferring uncompressed audio-video data, however, requires a lot of bandwidth (you're looking at giga bits per second) over a single cable. Interface standards like IEEE 1394 (400/800 M bps) transfer only compressed audio-video data, thereby potentially compromising the picture quality.
A single HDMI link, on the other hand, is capable of transferring up to 24 bits of user data at 165 Mpixels per second, resulting in a massive bandwidth of nearly 4 Gbps . This is enough to support the 1080p resolutions of today's newest high-definition displays while still leaving room to transport up to 8 channels of high-resolution audio with 24 bits of resolution and a sampling frequency up to 192 kHz - all across a single HDMI cable. This is well beyond the maximum specifications of even DVD-Audio, which tops out at 6 channels and a sample rate of 96 kHz. The fact is, the HDMI standard includes extra headroom to allow for future upgrades to audio formats.
TDMS Encoding & Signaling
Like DVI, the HDMI specification is based on TDMS encoding and signaling technology. A TDMS link consists of a single clock channel and three data channels. Eight (8) bits of video data are converted into a 10-bit transition-minimized, DC balanced sequence through the use of an advanced data-encoding algorithm implemented on each of the three data channels. This allows for a very strong transmission, while also minimizing the potential for E M I (electro-magnetic interference) over copper cables. Because of the use of the advanced encoding algorithm, data recovery on the receiving end is very reliable, enabling transmission over fairly long cable runs.
With the exception of the connector itself and the optional control signal, the HDMI physical layer is the same as that for DVI. Compliant DVI sources and displays will be interoperable with HDMI devices through the use of a simple, passive DVI-to-HDMI converter cable or converter. This protects consumers' investment in DVI-enabled CE products.
With a single-link clock frequency of 165 MHz, all existing HDTV video formats can be readily supported, and there is more than sufficient bandwidth to support future video formats such as 1080p at 60Hz (making HDMI ready for the next-generation video formats that have finally started to emerge in 2005). As with DVI, an optional but rarely used second link can be used to double the bandwidth to support any resolutions above UXGA (162 MHz). This second link is only expected to be used in high-resolution PC applications and utilizes a different size connector (we'll expand on this later).
Supported Color Space and Video Formats
HDMI pixel encoding includes support for RGB 4:4:4 as well as digital TV's YCbCr 4:4:4 amd YCbCr 4:2:2 color spaces. The two 4:4:4 encoding formats are both 8-bit per component sampling for 24-bit per pixel delivery. The 4:2:2 encoding format uses up to 12-bits per component for greater color depth.
HDMI can support all existing and planned PC or TV video formats. Several formats were specifically established in order to jump-start compatibility between products and media whose resolutions were different:
- SDTV: 720x480i (NTSC), 720x576i (PAL)
- EDTV: 640x480p (VGA), 720x480p (NTSC progressive), 720x576p (PAL progressive)
- HDTV: 1280x720p, 1920x1080i (1920x1080p is supported but was not initially defined when the spec was penned)
All SD formats are available in 4:3 as well as 16:9 aspect ratios while HD formats are available in the 16:9 ratio only.
