Testing video applications in cellular networks
According to International Data Corporation (IDC), almost 500 million smartphones were sold worldwide in 2011. This trend will gain even greater momentum as the current rollout of Long Term Evolution (LTE), the fourth generation (4G) mobile communications, progresses. A large number of devices already support mobile multimedia reception and are equipped with video/audio interfaces that enable lossless transmission of high-definition content to TVs.
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 multicast 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 R&S CMW500 and an R&S 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 analyzer, 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 4
shows the T&M interface for image difference analysis on the 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 analyzes picture errors according to the relevant metric
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 center, R&S VTE video tester and R&S VTS compact video tester as shown on figure 5
are 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.
Figure 5: The Rohde & Schwarz video tester family for interoperability and application testing on AV interfaces
About the author
Harald Gsoedl is product manager Broadcasting at Rohde & Schwarz – www.rohde-schwarz.com
– he can be reached at email@example.com
Courtesy of EETimes Europe
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