Testing hd multimedia content transmission systems

Since then, more than 1billion HDMI ready electronic devices have been produced. HDMI uses specialised high definition content protection. When this protection mechanism is active on the transmitter side (for example, a Blu-ray player), it verifies the receiver supports this mode before exchanging the encryption key that allows transmitted data to be decoded. The interface is also designed for high quality, low noise transmission of audio and video content over three high speed data channels, known as transition minimised differential signalling lines. HDMI version 1.4a – released in March 2010 – specifies a bandwidth of up to 3.4Gbit/s. It offers enhanced functionality, including home networking, 3d and improved resolutions. It also retains backward compatibility. HDMI 1.4a also implements a bidirectional 100Mbit/s Ethernet connection, providing a common data connection for all connected equipment. An external surround sound system is frequently used for audio output from a tv. This requires an additional S/PDIF connection between the tv and the sound system. HDMI integrates an audio return channel, used to transmit the decoded audio and serving to reduce cabling. The HDMI Ethernet and audio return channel function merges the concepts of the Ethernet channel and the audio return channel. Transmission via HDMI cable is made possible by shielding and twisting the hotplug and utility lines, thus creating a new data channel. HDMI 1.4a also defines new picture formats and timings that allow stereoscopic picture information to be transmitted to tvs: frame compatible formats are based on conventional resolutions that divide the two perspectives horizontally (side by side) or vertically (top and bottom). Frame packing formats – primarily used with Blu-ray players and offering optimum picture quality – generate sequential pictures in full resolution and complete the list. All formats (see fig 1) are used primarily for broadcasting 3d content. For some years, digital cinema projectors have been using resolutions beyond full HD, such as 4096 x 2160 pixels, but these resolutions have not reached the home. However, demand for higher resolutions has grown alongside bigger screens and the planned introduction of autostereoscopic screens (which make 3d glasses unnecessary). When processing 3d video signals, the software must convert motion estimation, scaling and deinterlacing to the new formats. The trouble spots in the various 3d tv technologies must also be identified through testing. No uniform test standards are currently available for 3d tv displays, primarily because the topic is new and the solutions are too varied. This was made clear when the first 3d tvs appeared in 2010. In active systems, faulty synchronisation mechanisms can cause ghosting, such as the crossover of information intended for the left eye to the right eye and vice versa. The fault lies with the slow response of lcd pixels; a pixel requires a frequency of 120Hz to change states. Brightness is frequently reduced in both active and passive systems; because of the very short exposure times, only a fraction of the transmitted light passes through the shutter glasses to the eye. Polarising filters in passive systems also absorb a significant portion of light. Highly accurate test and measurement instruments are required to provide precise and reproducible test results. When equipped with the DVSG-K10 AV signal generator option, the DVSG digital video signal generator can create all common primary 2d and 3d video formats in line with HDMI 1.4a, uncompressed and without interfering compression artefacts. Using the DVSG-B10 AV signal generator extension, users can output long 3d sequences: video memory is expanded to 4Gbyte, allowing, for example, sequences of 387 individual RGB pictures to be output at 1080p and 36bit colour depth. At lower resolutions or lower bit depths, the sequence length increases proportionally. A pattern library for various 2d and 3d picture formats is included as standard. This contains moving sequences for subjective picture quality assessment, as well as test patterns, which, when used with a Konica Minolta CS-2000 spectroradiometer, can analyse luminance, colour or crosstalk. The CS-2000 is set up at a measurement angle of 1° and at a distance corresponding to three to four times the screen height. The device is lined up vertically with the test point on the screen and measurement through shutter or polarising glasses is centred in order to simulate the position of a viewer's pupils. A positioning system facilitates the correct and reproducible alignment of the test setup at various test points. 3d test patterns for determining the luminance and colour information are derived from existing 2d test pattern templates. The test setup for 2d screens can perform traditional measurements, including contrast, gamma, gamut, white point, homogeneity and viewing angle dependency – the 2d templates from the EBU TECH 3321/3325 guidelines are suitable as test patterns. Meanwhile, Rohde & Schwarz' AVG pattern import software enables users to create their own 3d test scenarios from the uncompressed test patterns defined in the EBU guidelines. The software generates the appropriate 3d AVG files, based on picture pairs for the left and right eye. The import function for uncompressed PCM audio files with up to eight channels completes the range of functions provided by the software. The Windows application can run on the signal generator or on a pc. While special test patterns are needed to identify specific trouble spots in the latest generation of 3d tvs, it is possible to obtain an objective and reproducible comparison for crosstalk using the test setup in combination with the test pattern shown in Figure 2. Six grey blocks with a 20% gradation from 0 to 100% white on one eye are mapped to a greyscale with a 20% gradation from 0 to 100% white plus detection surface markers on the other eye. Display tests under real conditions The DVSG has a reference decoder option that lets users check how compressed 3d tv signals in typical transmissions are processed. They can play back live 3d signals containing complex scene cuts and pictures, blocking and other compression artefacts that were recorded anywhere on the basis of Mpeg2 transport streams. The DVSG, which supports Mpeg2 coded video and Mpeg4 advanced video coding, can also decode all current audio standards. This allows users to optimise display technology in the lab. Any errors experienced under critical application conditions can be reproduced. The introduction of 3d tv is still in its infancy: different technical approaches and the lack of binding test specifications make it difficult for producers of 3d tv equipment to perform a comparative evaluation of the quality of their products. However, new high performance test and measurement devices that can adapt flexibly to new test conditions will make this possible. Harald Gsödl is product manager, broadcast test and measurement, with Rohde and Schwarz.