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How to test HDMI and MHL interfaces

Posted: 18 Mar 2013 ?? ?Print Version ?Bookmark and Share

Keywords:high-definition multimedia interface? mobile high-definition link? 3D video?

With the introduction of LTE, additional video services will become available. There are plans to introduce not just reliable high-definition video telephony, but also Internet-streamed video on demand (VOD), and TV-like enhanced multimedia broadcast multi-cast services (eMBMS).

During development and quality assurance, the ability of multimedia-ready mobile equipment to support these video services is assessed in application tests under deliberately degraded conditions. In the test lab, various types of interference are applied to the simulated transmission path between the base station and the mobile device that are likely to occur during real-life use. The tests simulate RF interference, apply noise to the signal, and simulate IP faults like packet loss and packet delay.

The actual application tests then detect the types of picture errors that typically result from transmission errors. Examples of the kinds of problems that arise include blocking, freezing and even the loss of whole pictures in a sequence.

A typical setup for an application test like this might consist of a base station simulator such as the Rohde & Schwarz CMW500 and an Rohde & Schwarz VTE video tester. The base station simulator sets up a valid LTE connection and makes the audio/video data available for download through its built-in media server. An optional fading simulator also allows RF interference to be added. The smartphone decodes the data it receives and outputs the pictures on its display or plays the audio over its speakers. The audiovisual data is transmitted without loss or external impacts over the phone's HDMI or MHL port to the video analyser, where it can be reviewed.

The transmission errors simulated cause picture errors. To record interference in reproducible form, T&M labs use image difference algorithms. These algorithms compare, in real time, an ideal reference against the image sequence being tested.

The difference between the images is then computed, both graphically and numerically. Figure 4shows the T&M interface for image difference analysis on the Rohde & Schwarz (R&S )VTE. Support for common industry metrics such as peak signal-to-noise ratio (PSNR) and structural similarity (SSIM) means that image evaluations are reproducible and can be automated.

Figure 4: Image difference analysis detects and analyses picture errors according to the relevant metric. (CTS).

The PSNR and SSIM are computed for individual images. However, to assess the visibility of errors occurring in moving images, time masking effects can be considered by setting thresholds. This means that visible errors in a moving sequence can serve as a trigger. To ensure that the AV interfaces and video processing in multimedia devices functions flawlessly, video content and protocols must be tested at every stage. Reproducible and conclusive test results are best achieved with T&M instruments that can be upgraded to support future applications.

The R&S VTC video test centre, R&S VTE video tester and R&S VTS compact video tester as are touted as capable of conducting the kinds of interoperability and application tests required during the development, quality assurance and manufacturing of multimedia devices with AV interfaces. Their modular hardware and software designs also mean they are capable of being expanded to support new interface standards as these emerge.

About the author
Harald Gsoedl is product manager at Rohde & Schwarz.

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