IPTV testing involves multiple types of tests since IPTV systems transmit mixed media, the service can be content dependent, there are multiple media conversion (encoding) processes,. There is also an inability to measure the quality of protected (encrypted) content, and hidden distortion due to error concealment techniques.

Mixed Media

Mixed media is the combining of media of different types. An example of mixed media is the combining of video, audio, and text graphics on a video or television monitor. The challenge that this can cause is in the way each media type is processed as it is distributed through the network. Video and audio processing functions can result in different amounts of delay or quality resulting in acceptable quality on one type of media while another type of media has an unacceptable quality level.

Content Dependent

Content dependency factors are a set of conditions such as rapid motion graphics, that can influence the display or perception of media. Content dependency causes some types of content to look good while other types of content look bad given the same network performance impairments. This means that the user's perceived quality can vary on the same network depending on the content that is sent through the network.

Multiple Media Conversions

Multiple media conversions are the process of changing information from one format to another format. There may be several conversion processes along the content flow path in IPTV systems and one or more of them may degrade the quality of the media.

IPTV media conversion commonly uses lossy media compression. Lossy compression is a process of reducing an amount of information (usually in digital form) by converting it into another format

(such as MPEG) that represents the initial form of information. Each time the media is converted, additional distortion occurs.

The content producer (such as a studio) provides the media to a content distribution system (such as a satellite distribution system), usually in high-quality uncompressed form. Content distributors may compress the media and send it to broadcasters (such as IPTV systems). When it is received by the IPTV systems, it is decoded and re-encoded for local distribution. The re-encoding process may be in another compressed format (such as MPEG-4). The encoder may change the media format from variable bit rate (VBR) to constant bit rate (CBR). Each of these conversions can add distortion to the media signal.

Figure 1.3 shows how content may be converted multiple times between its high-quality format and when the media is received by the viewing device. This example shows that the media is compressed and encoded into MPEG-2 before it is distributed via a satellite system. When the satellite signal is received at the cable head end, it is decoded, switched with other video sources, and re-encoded into MPEG-4 before it is distributed to the viewer.

Content Protection

Content Protection is the end-to-end encryption system that prevents content from being pirated or tampered with in a communication network (such as in a television system). Content protection involves uniquely identifying the content, assigning the usage rights and scram-

Figure 1.3, IPTV Multiple Conversions

bling and encrypting the digital assets prior to play-out or storage (both in the network or end user devices), as well as delivering the accompanying rights to allow legal users to access the content. When content is encrypted or uniquely encoded, it is usually not possible to analyze the underlying media.

Error Concealment

Error concealment is a process that is used by a coding device (such as a speech coder) to create information that replaces data that has been received in error.

 Error concealment is possible when portions of the signal output of the coder have some relationship to other portions of the signal output and that relationship can be used to produce an approximated signal that replaces the lost information period (lost bits). Error concealment methods (such as repeating the last frame of video when a frame is lost) can influence the ability to accurately measure the effects of distortion (such as packet loss).