How to test HDMI and MHL interfaces

Nowadays set-top boxes, tablet PCs and smartphones are equipped with analogue or digital video interfaces such as the high-definition multimedia interface (HDMI) and the mobile high-definition link (MHL). Manufacturers test these interfaces at different stages in the value chain to verify their functionality and compliance with standards and thus ensure that they operate flawlessly when they reach consumers. Besides pure interoperability tests, manufacturers are also conducting an increasing number of application tests to see how equipment performs under realistic conditions.

The high-definition multimedia interface
When HDMI technology was first introduced in 2002, it triggered a paradigm shift within the consumer electronics industry. For the first time, equipment manufacturers and content providers alike embraced a digital video and audio interface standard.

Since 2002, more than a billion electronic devices equipped with HDMI have been manufactured. Prior to the introduction of HDMI, consumer electronics equipment typically used analogue interfaces to transmit video content, in spite of the known shortcomings compared to digital interfaces.

The main reason behind the reluctance among content providers such as Hollywood studios to make the transition from analogue to digital was that digital versatile discs (DVDs) or digital video broadcasting (DVB) could potentially allow lossless duplication of content by pirates, directly over external ports. HDMI succeeded in eliminating these concerns. This is because HDMI uses a special encryption mechanism – high bandwidth digital content protection (HDCP) – to secure content against duplication. If this is enabled on the source device (a Blu-ray player, for instance), the sink is checked to determine whether it supports HDCP. If it does, the source and sink exchange an encryption key that is used to decrypt the transmitted data.

HDMI's technical design also ensures that video and audio content is transmitted in high quality, with little interference. The data is transferred between devices via three high-speed channels known as transition minimised differential signalling (TMDS) lines. In the most recent version of the interface, these channels are specified to support a bandwidth of up to 3.4 Gbit/s each. The interface also incorporates control lines (figure 1).

Going forward, the focus will be on introducing ultra-definition (UD), which offers resolutions far higher than the 1920 x 1080 pixels currently supported by Full HD. This will take resolutions like 4K x 2K, already widespread in cinemas and studios for some time now, into people's living rooms. The demand is there: displays are getting bigger all the time, the first consumer video cameras capable of this kind of resolution are already available, and people are using TVs more and more as an alternative to personal computers.

Mobile high-definition link
Mobile high-definition link is a new standard closely related to HDMI. This new interface technology is most common in smartphones. It allows content such as Internet-streamed media, movies and user-created video to be output to TV screens in HD resolution. MHL runs on the micro-USB port now included in many mobile devices. MHL consortium founders Nokia, Samsung, Silicon Image, Sony and Toshiba point to the new technology's following benefits for mobile applications:

???The micro-USB port can be used to transmit uncompressed HD video and audio.
???Mobile devices can charge while playing media.
???Eliminating the need for a separate audio/video port – combined with micro-USB's small size – enables manufacturers to produce even smaller smartphones.