HDMI Basics

High Definition Multimedia Interface (HDMI) makes connecting my stuff much easier. A single full-duplex cable carries both high-definition multi-track audio and uncompressed video content, with a lot of bandwidth to spare for future innovation. It passes control signals as well, so when implemented widely, a single command component can control all HDMI-connected components. It communicates color space information between components. Version 1.4 carries an ethernet channel, eliminating a need for each component to provide a separate Internet connection. HDMI was developed by a consortium of Consumer Electronics manufacturers including Matsushita (Panasonic) and Toshiba. Adopters pay a fee to HDMI Licensing, LLC.

HDMI is based on DVI, but provides greater bandwidth and offers many extras. It allows media centers to keep A/V signals in the digital domain until the destination component. It implements High-bandwidth Digital Content Protection (HDCP), without which a media center could not freely process copyrighted HD content in the digital domain. All connected components need to be HDCP-compliant for this to work.

Versions

HDMI version 1.2a or 1.3a/b was implemented widely. The main enhancement of 1.3 is increased bandwidth, providing the ability to transmit the Blu-Ray Disk (BD) uncompressed multi-track audio formats: Dolby TrueHD and DTS-HD Master Audio. HDMI 1.3 also supports ‘deep color’ and automatic lip syncing.

HDMI 1.4 arrived in 2010, driven by anticipation of smarter, bigger TVs. It offers 3D video formats, and handles greater video resolutions to rival those of digital cinema projectors (4k support, 4000×2000 pixel resolution for wall-size displays). HDMI 1.4 further provides audio return from a TV with built-in tuner, automated real-time content type signaling between source and TV, support for wider-gamut color spaces such as Adobe RGB (a favorite of digital photographers), and ethernet connectivity.

HDMI 2.0 arrived in late 2013. It provides more audio and video capacity needed by enhanced media formats, supports 4k video resolution at 60fps, and provides a 21:9 aspect ratio, among several other higher-performance capabilities.

The illustration below, compliments of Audioquest, shows the complexity of the HDMI interface. HDMI cables contain 19 cores and the length of all cores must be controlled to fine tolerance (on order of hundredth of a millimeter). Thus, the cables cannot be terminated in the field; installers must establish the positions of their components and buy a cable that best approximates the interconnect distance. The quality of an HDMI signal degrades linearly with increasing signal bit rate and increasing cable length. Shorter is better in the sense that a shorter cable will have more usable bandwidth (headroom) than a longer one. Thus a cable certified for 1.5 Gbps at 5m will only pass 0.75 Gbps at 10m.

HDMI Physical Layer

Certification

Cable certification has changed over the HDMI development period. HDMI 1.2 certification was called Category 1 compliance, certifying the length of a given type of cable that could successfully pass a 0.75 Gbps signal (corresponding to 720p or 1080i video with 8-bit color depth). HDMI 1.3 was accompanied with a new Category 2 compliance based on a theoretical 3.4 Gbps signaling rate. In practice, compliance testing was limited to 1.65 Gbps, slightly exceeding the demands of the source and sink silicon available at the time, which topped out at 1080/30p with 8-bit color depth. HDMI 1.4 has re-normalized these compliance categories into Standard and High Speed compliance. There are two versions of each, one with embedded ethernet channel and one without. There is an additional automotive compliance category. High Speed certification does not appear to be a settled issue yet, but the indicated intent is for it to cover applications beyond 1080p bandwidth. Again, in practice, certification testing will top out at the highest rates supported by available silicon.

Mostly, cheapo HDMI cables from China have sufficed for current HDMI. In fact, virtually all HDMI cable is made in China. Most of today’s cables are offered with no evidence of certification. But as data rates and cable length increase, proof of certification will become necessary to ensure the consumer of appropriate quality.

To my knowledge, only Belden HDMI cable is made in the USA; it is made here and shipped in bulk for cable fabrication in China. It is said to differ in construction from Chinese cables because it uses bonded pairs rather than twisted pairs. The latter can create impedance inconsistencies that are magnified by cable physical twists and turns. I use a Belden 28 AWG 1m cable between the DVD and the AVR. This cable is certified at HDMI Category 2 for up to 3m; our simple 1m length and relatively lower 0.7 Gbps data rate should deliver a clean signal of audiophile quality, at least to my aging ears. This quality cable costs less than 1/10 the price of the rip-off brand names found at the usual places such as Best Shack or pretentious audiophile outlets. The Belden certification is important, because this is our most data-critical interconnect, high quality, multi-channel audio over HDMI. Signal quality is more important to audio applications than to video, because our ears are more sensitive to noise and jitter degradation than are our eyes.

Full-bandwidth applications of version 1.3, and most applications of the version 1.4 may require certified High Speed cables, meaning all existing cables would need to be upgraded, perhaps not easy if they are installed in-wall. For longer runs and in-wall use, converters are currently coming onto market, such as coax-to-HDMI and wireless HDMI, to lessen the cost and difficulty of custom installation.

Transmission Characteristics

There is only one clock in the HDMI interface and it is tied to the video signal. Audio data are packaged as ‘Data Islands’ inserted into the video blanking intervals of the video signal. This means that audio bandwidth depends on video bandwidth.

So what if the HDMI cable carries only DVD-Audio, say? Then if DVD player resolution is set to native 480i, there will not be enough audio bandwidth for multi-channel DVD-Audio or SACD, and they will revert to stereo at lower resolution. But if the video clock is pumped to at least 720p output resolution rate to accomodate multi-channel HD audio, then when video is played back at the same setting, the DVD-player will scale video to 720p, which may not be optimal or desired. Our DVD player has a remote button for switching HDMI video resolution, so manual switching is convenient when necessary.

HDMI signals may experience digital jitter up to 1000 times worse than CD audio jitter, since the audio data is embedded in clumps in the video signal. The clock signal is on a different conductor than the data; the data and clock jitter may become incoherent. The higher the clock rate, the more potentially degrading the jitter becomes to signal recovery.

Top-end receivers implement HDMI clock recovery and data stream buffering schemes to smooth the digital signal. HDMI presents a tradeoff: low cost and interconnect convenience at the expense of potentially degraded audio quality that may be detectable to golden ears. Quality source components and quality short cables are the consumers’ best chance for minimizing HDMI jitter effects on the cheap. Correcting after the fact adds more cost.

Compatibility of Implementations

Another glitch with HDMI is compatibility. Different components may have implemented different versions of HDMI. Even between similar versions, there can be incompatibility. Each time an HDMI connection is established, there is a handshake between all the interconnected components. This accounts for the pauses when beginning a video session. Part of the handshake involves ascertaining HDCP compatibility of all links in the HDMI interconnect chain.

The handshake allows the source component to learn the capabilities of various signal sinks via their Extended Display Information Data (EDID) chips, thus enabling the source to send compatible data formats. These handshakes sometimes fail because of implementation differences and device limits, resulting in no video. This has particularly been noted between some AVR components and some set top boxes. Such problems should be categorized as growing pains, however, and are not an indictment of the technology itself (other than its complexity).

Looking Ahead

It will be a while before sources of 4K video become available to realize the promise of HDMI 2.0. It will likely take three years for the prices of 4K OLED screens to reach the mass consumer support level. Expect the early adopters to be high-end PC gamers and video purists with money to burn. We content junkies who still get most of our video from Netflix and other streamers will be well-served for the near future by HDMI 1.4 connecting to an HDTV plasma screen. Compared to what I remember from a few years ago, the video quality of HDTV plasma is nirvana.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s